Internet X.509 Public Key Infrastructure -- Certificate Management Protocol (CMP)

Internet X.509 Public Key Infrastructure -- Certificate Management Protocol (CMP) Siemens

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sec LAMPS Working Group CMP Certificate Management PKI This document describes the Internet X.509 Public Key Infrastructure (PKI) Certificate Management Protocol (CMP). Protocol messages are defined for X.509v3 certificate creation and management. CMP provides interactions between client systems and PKI components such as a Registration Authority (RA) and a Certification Authority (CA). This document adds support for management of KEM certificates and use EnvelopedData instead of EncryptedValue. This document also includes the updates specified in Section 2 and Appendix A.2 of RFC 9480. The updates maintain backward compatibility with CMP version 2 wherever possible. Updates to CMP version 2 are improving crypto agility, extending the polling mechanism, adding new general message types, and adding extended key usages to identify special CMP server authorizations. CMP version 3 is introduced for changes to the ASN.1 syntax, which are support of EnvelopedData, certConf with hashAlg, POPOPrivKey with agreeMAC, and RootCaKeyUpdateContent in ckuann messages. This document obsoletes RFC 4210 and together with I-D.ietf-lamps-rfc6712bis and it also obsoletes RFC 9480. Appendix F of this document updates the Section 9 of RFC 5912.

Introduction [RFC Editor: please delete: During IESG telechat the CMP Updates document was approved on condition that LAMPS provides a RFC4210bis document. Version -00 of this document shall be identical to RFC 4210 and version -01 incorporates the changes specified in CMP Updates Section 2 and Appendix A.2. A history of changes is available in of this document. The authors of this document wish to thank Carlisle Adams, Stephen Farrell, Tomi Kause, and Tero Mononen, the original authors of RFC4210, for their work and invite them, next to further volunteers, to join the -bis activity as co-authors. ] [RFC Editor: Please perform the following substitution.

RFCXXXX --> the assigned numerical RFC value for this draft ]

This document describes the Internet X.509 Public Key Infrastructure (PKI) Certificate Management Protocol (CMP). Protocol messages are defined for certificate creation and management. The term "certificate" in this document refers to an X.509v3 Certificate as defined in .

Changes Made by RFC 4210 differs from in the following areas:

The PKI management message profile section is split to two appendices: the required profile and the optional profile. Some of the formerly mandatory functionality is moved to the optional profile.
The message confirmation mechanism has changed substantially.
A new polling mechanism is introduced, deprecating the old polling method at the CMP transport level.
The CMP transport protocol issues are handled in a separate document , thus the Transports section is removed.
A new implicit confirmation method is introduced to reduce the number of protocol messages exchanged in a transaction.
The new specification contains some less prominent protocol enhancements and improved explanatory text on several issues.

Updates Made by RFC 9480 CMP Updates and CMP Algorithms updated , supporting the PKI management operations specified in the Lightweight CMP Profile , in the following areas:

Added new extended key usages for various CMP server types, e.g., registration authority and certification authority, to express the authorization of the certificate holder that acts as the indicated type of PKI management entity.
Extended the description of multiple protection to cover additional use cases, e.g., batch processing of messages.
Use the Cryptographic Message Syntax (CMS) type EnvelopedData as the preferred choice instead of EncryptedValue to better support crypto agility in CMP. For reasons of completeness and consistency, the type EncryptedValue has been exchanged in all occurrences. This includes the protection of centrally generated private keys, encryption of certificates, proof-of-possession methods, and protection of revocation passphrases. To properly differentiate the support of EnvelopedData instead of EncryptedValue, CMP version 3 is introduced in case a transaction is supposed to use EnvelopedData. Note: According to , Section 2.1, point 9, the use of the EncryptedValue structure has been deprecated in favor of the EnvelopedData structure. offers the EncryptedKey structure a choice of EncryptedValue and EnvelopedData for migration to EnvelopedData.
Offer an optional hashAlg field in CertStatus supporting cases that a certificate needs to be confirmed that has a signature algorithm that does not indicate a specific hash algorithm to use for computing the certHash. This is also in preparation for upcoming post-quantum algorithms.
Added new general message types to request CA certificates, a root CA update, a certificate request template, or Certificate Revocation List (CRL) updates.
Extended the use of polling to p10cr, certConf, rr, genm, and error messages.
Deleted the mandatory algorithm profile in and refer instead to Section 7 of .
Added security considerations Sections , , , and .

Changes Made by This Document This document obsoletes and . It includes the changes specified by Section 2 and Appendix C.2 of as described in . Additionally this document updates the content of in the following areas:

Added introducing the Key Generation Authority.
Extended regarding use of Certificate Transparency logs.
Updated introducing RootCaKeyUpdateContent as alternative to using a repository to acquire new root CA certificates.
Added containing description of origPKIMessage content moved here from .
Added support for KEM keys for proof-of-possession to and , for message protection to , , and , and for usage with CMS EnvelopedData to .
Deprecated CAKeyUpdAnnContent in favor of RootCaKeyUpdateContent.
Incorporated the request message behavioral clarifications from Appendix C of to . The definition of altCertTemplate was incorporated into and the clarification on POPOSigningKey and on POPOPrivKey was incorporated into .
Added support for CMS EnvelopedData to different proof-of-possession methods for transferring encrypted private keys, certificates, and challenges to .
Added security considerations Sections , , , and .

Requirements The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

PKI Management Overview The PKI must be structured to be consistent with the types of individuals who must administer it. Providing such administrators with unbounded choices not only complicates the software required, but also increases the chances that a subtle mistake by an administrator or software developer will result in broader compromise. Similarly, restricting administrators with cumbersome mechanisms will cause them not to use the PKI. Management protocols are REQUIRED to support on-line interactions between Public Key Infrastructure (PKI) components. For example, a management protocol might be used between a Certification Authority (CA) and a client system with which a key pair is associated, or between two CAs that issue cross-certificates for each other.

PKI Management Model Before specifying particular message formats and procedures, we first define the entities involved in PKI management and their interactions (in terms of the PKI management functions required). We then group these functions in order to accommodate different identifiable types of end entities.

Definitions of PKI Entities The entities involved in PKI management include the end entity (i.e., the entity to whom the certificate is issued) and the certification authority (i.e., the entity that issues the certificate). A registration authority MAY also be involved in PKI management.

Subjects and End Entities The term "subject" is used here to refer to the entity to whom the certificate is issued, typically named in the subject or subjectAltName field of a certificate. When we wish to distinguish the tools and/or software used by the subject (e.g., a local certificate management module), we will use the term "subject equipment". In general, the term "end entity" (EE), rather than "subject", is preferred in order to avoid confusion with the field name. It is important to note that the end entities here will include not only human users of applications, but also applications themselves (e.g., for IP security) or devices (e.g., routers or industrial control systems). This factor influences the protocols that the PKI management operations use; for example, application software is far more likely to know exactly which certificate extensions are required than are human users. PKI management entities are also end entities in the sense that they are sometimes named in the subject or subjectAltName field of a certificate or cross-certificate. Where appropriate, the term "end entity" will be used to refer to end entities who are not PKI management entities. All end entities require secure local access to some information -- at a minimum, their own name and private key, the name of a CA that is directly trusted by this entity, and that CA's public key (or a fingerprint of the public key where a self-certified version is available elsewhere). Implementations MAY use secure local storage for more than this minimum (e.g., the end entity's own certificates or application-specific information). The form of storage will also vary -- from files to tamper-resistant cryptographic tokens. The information stored in such local, trusted storage is referred to here as the end entity's Personal Security Environment (PSE). Though PSE formats are beyond the scope of this document (they are very dependent on equipment, et cetera), a generic interchange format for PSEs is defined here: a certification response message MAY be used.

Certification Authority The certification authority (CA) may or may not actually be a real "third party" from the end entity's point of view. Quite often, the CA will actually belong to the same organization as the end entities it supports. Again, we use the term "CA" to refer to the entity named in the issuer field of a certificate. When it is necessary to distinguish the software or hardware tools used by the CA, we use the term "CA equipment". The CA equipment will often include both an "off-line" component and an "on-line" component, with the CA private key only available to the "off-line" component. This is, however, a matter for implementers (though it is also relevant as a policy issue). We use the term "root CA" to indicate a CA that is directly trusted by an end entity; that is, securely acquiring the value of a root CA public key requires some out-of-band step(s). This term is not meant to imply that a root CA is necessarily at the top of any hierarchy, simply that the CA in question is trusted directly. A "subordinate CA" is one that is not a root CA for the end entity in question. Often, a subordinate CA will not be a root CA for any entity, but this is not mandatory.

Registration Authority In addition to end-entities and CAs, many environments call for the existence of a Registration Authority (RA) separate from the Certification Authority. The functions that the registration authority may carry out will vary from case to case but MAY include identity checking, token distribution, checking certificate requests and authentication of their origin, revocation reporting, name assignment, archival of key pairs, et cetera. This document views the RA as an OPTIONAL component: when it is not present, the CA is assumed to be able to carry out the RA's functions so that the PKI management protocols are the same from the end-entity's point of view. Again, we distinguish, where necessary, between the RA and the tools used (the "RA equipment"). Note that an RA is itself an end entity. We further assume that all RAs are in fact certified end entities and that RAs have private keys that are usable for signing. How a particular CA equipment identifies some end entities as RAs is an implementation issue (i.e., this document specifies no special RA certification operation). We do not mandate that the RA is certified by the CA with which it is interacting at the moment (so one RA may work with more than one CA whilst only being certified once). In some circumstances, end entities will communicate directly with a CA even where an RA is present. For example, for initial registration and/or certification, the end entity may use its RA, but communicate directly with the CA in order to refresh its certificate.

Key Generation Authority A Key Generation Authority (KGA) is a PKI management entity generating key pairs on behalf of an end entity. As the KGA generates the key pair it knows the public and the private part. This document views the KGA as an OPTIONAL component. When it is not present and central key generation is needed, the CA is assumed to be able to carry out the KGA's functions so that the PKI management protocol messages are the same from the end-entity's point of view. If certain tasks of a CA are delegated to other components, this delegation needs authorization, which can be indicated by extended key usages (see ). Note: When doing central generation of key pairs, implementers should consider the implications of server-side retention on the overall security of the system; in some case retention is good, for example for escrow reasons, but in other cases the server should clear its copy after delivery to the end entity. Note: If the CA delegates key generation to a KGA, the KGA can be collocated with the RA.

PKI Management Requirements The protocols given here meet the following requirements on PKI management

PKI management must conform to the ISO/IEC 9594-8/ITU-T X.509 standards.
It must be possible to regularly update any key pair without affecting any other key pair.
The use of confidentiality in PKI management protocols must be kept to a minimum in order to ease acceptance in environments where strong confidentiality might cause regulatory problems.
PKI management protocols must allow the use of different industry-standard cryptographic algorithms, see CMP Algorithms . This means that any given CA, RA, or end entity may, in principle, use whichever algorithms suit it for its own key pair(s).
PKI management protocols must not preclude the generation of key pairs by the end entity concerned, by a KGA or by a CA. Key generation may also occur elsewhere, but for the purposes of PKI management we can regard key generation as occurring wherever the key is first present at an end entity, KGA, or CA.
PKI management protocols must support the publication of certificates by the end entity concerned, by an RA, or by a CA. Different implementations and different environments may choose any of the above approaches.
PKI management protocols must support the production of Certificate Revocation Lists (CRLs) by allowing certified end entities to make requests for the revocation of certificates. This must be done in such a way that the denial-of-service attacks, which are possible, are not made simpler.
PKI management protocols must be usable over a variety of "transport" mechanisms, specifically including mail, HTTP, MQTT, CoAP, and off-line file-based.
Final authority for certification creation rests with the CA. No RA or end entity equipment can assume that any certificate issued by a CA will contain what was requested; a CA may alter certificate field values or may add, delete, or alter extensions according to its operating policy. In other words, all PKI entities (end-entities, RAs, KGAs, and CAs) must be capable of handling responses to requests for certificates in which the actual certificate issued is different from that requested (for example, a CA may shorten the validity period requested). Note that policy may dictate that the CA must not publish or otherwise distribute the certificate until the requesting entity has reviewed and accepted the newly-created certificate or the POP is completed. In case of publication of the certificate (when using indirect POP, see ) or a precertificate in a Certificate Transparency log , the certificate must be revoked if it was not accepted by the EE or the POP could not be completed.
A graceful, scheduled change-over from one non-compromised CA key pair to the next (CA key update) must be supported (note that if the CA key is compromised, re-initialization must be performed for all entities in the domain of that CA). An end entity whose PSE contains the new CA public key (following a CA key update) may also need to be able to verify certificates verifiable using the old public key. End entities who directly trust the old CA key pair may also need to be able to verify certificates signed using the new CA private key (required for situations where the old CA public key is "hardwired" into the end entity's cryptographic equipment).
The functions of an RA may, in some implementations or environments, be carried out by the CA itself. The protocols must be designed so that end entities will use the same protocol regardless of whether the communication is with an RA or CA. Naturally, the end entity must use the correct RA or CA public key to verify the protection of the communication.
Where an end entity requests a certificate containing a given public key value, the end entity must be ready to demonstrate possession of the corresponding private key value. This may be accomplished in various ways, depending on the type of certification request. See for details of the in-band methods defined for the PKIX-CMP (i.e., Certificate Management Protocol) messages.

PKI Management Operations The following diagram shows the relationship between the entities defined above in terms of the PKI management operations. The letters in the diagram indicate "protocols" in the sense that a defined set of PKI management messages can be sent along each of the lettered lines.

CA establishment: When establishing a new CA, certain steps are required (e.g., production of initial CRLs, export of CA public key).
End entity initialization: This includes importing a root CA public key and requesting information about the options supported by a PKI management entity.
Certification: Various operations result in the creation of new certificates:
1. initial registration/certification: This is the process whereby an end entity first makes itself known to a CA or RA, prior to the CA issuing a certificate or certificates for that end entity. The end result of this process (when it is successful) is that a CA issues a certificate for an end entity's public key, and returns that certificate to the end entity and/or posts that certificate in a repository. This process may, and typically will, involve multiple "steps", possibly including an initialization of the end entity's equipment. For example, the end entity's equipment must be securely initialized with the public key of a CA, e.g., using zero-touch methods like BRSKI or SZTP , to be used in validating certificate paths. Furthermore, an end entity typically needs to be initialized with its own key pair(s).
2. key pair update: Every key pair needs to be updated regularly (i.e., replaced with a new key pair), and a new certificate needs to be issued.
3. certificate update: As certificates expire, they may be "refreshed" if nothing relevant in the environment has changed.
4. CA key pair update: As with end entities, CA key pairs need to be updated regularly; however, different mechanisms are required.
5. cross-certification request: One CA requests issuance of a cross-certificate from another CA. For the purposes of this standard, the following terms are defined. A "cross-certificate" is a certificate in which the subject CA and the issuer CA are distinct and SubjectPublicKeyInfo contains a verification key (i.e., the certificate has been issued for the subject CA's signing key pair). When it is necessary to distinguish more finely, the following terms may be used: a cross-certificate is called an "inter-domain cross-certificate" if the subject and issuer CAs belong to different administrative domains; it is called an "intra-domain cross-certificate" otherwise.
- Note 1: The above definition of "cross-certificate" aligns with the defined term "CA-certificate" in X.509. Note that this term is not to be confused with the X.500 "cACertificate" attribute type, which is unrelated.
- Note 2: In many environments, the term "cross-certificate", unless further qualified, will be understood to be synonymous with "inter-domain cross-certificate" as defined above.
- Note 3: Issuance of cross-certificates may be, but is not necessarily, mutual; that is, two CAs may issue cross-certificates for each other.
1. [RFC-Editor: Please fix the enumeration and continue with '6'.] cross-certificate update: Similar to a normal certificate update, but involving a cross-certificate.
Certificate/CRL discovery operations: Some PKI management operations result in the publication of certificates or CRLs:
1. certificate publication: Having gone to the trouble of producing a certificate, some means for publishing may be needed. The "means" defined in PKIX MAY involve the messages specified in Sections to , or MAY involve other methods (LDAP, for example) as described in or (the "Operational Protocols" documents of the PKIX series of specifications).
2. CRL publication: As for certificate publication.
Recovery operations: Some PKI management operations are used when an end entity has "lost" its PSE:
1. key pair recovery: As an option, user client key materials (e.g., a user's private key used for decryption purposes) MAY be backed up by a CA, an RA, or a key backup system associated with a CA or RA. If an entity needs to recover these backed up key materials (e.g., as a result of a forgotten password or a lost key chain file), a protocol exchange may be needed to support such recovery.
Revocation operations: Some PKI management operations result in the creation of new CRL entries and/or new CRLs:
1. revocation request: An authorized person advises a CA of an abnormal situation requiring certificate revocation.
PSE operations: Whilst the definition of PSE operations (e.g., moving a PSE, changing a PIN, etc.) are beyond the scope of this specification, we do define a PKIMessage (CertRepMessage) that can form the basis of such operations.

Note that on-line protocols are not the only way of implementing the above operations. For all operations, there are off-line methods of achieving the same result, and this specification does not mandate use of on-line protocols. For example, when hardware tokens are used, many of the operations MAY be achieved as part of the physical token delivery. Later sections define a set of standard messages supporting the above operations. Transfer protocols for conveying these exchanges in various environments (e.g., off-line: file-based, on-line: mail, HTTP , MQTT, and CoAP ) are beyond the scope of this document and must be specified separately. Appropriate transfer protocols MUST be capable of delivering the CMP messages reliably. CMP provides inbuilt integrity protection and authentication. The information communicated unencrypted in CMP messages does not contain sensitive information endangering the security of the PKI when intercepted. However, it might be possible for an eavesdropper to utilize the available information to gather confidential technical or business critical information. Therefore, users should consider protection of confidentiality on lower levels of the protocol stack, e.g., by using TLS , DTLS , or IPSEC .

Assumptions and Restrictions

End Entity Initialization The first step for an end entity in dealing with PKI management entities is to request information about the PKI functions supported and to securely acquire a copy of the relevant root CA public key(s).

Initial Registration/Certification There are many schemes that can be used to achieve initial registration and certification of end entities. No one method is suitable for all situations due to the range of policies that a CA may implement and the variation in the types of end entity which can occur. However, we can classify the initial registration/certification schemes that are supported by this specification. Note that the word "initial", above, is crucial: we are dealing with the situation where the end entity in question has had no previous contact with the PKI, except having received the root CA certificate of that PKI by some zero-touch method like BRSKI and or SZTP . In case the end entity already possesses certified keys, then some simplifications/alternatives are possible. Having classified the schemes that are supported by this specification we can then specify some as mandatory and some as optional. The goal is that the mandatory schemes cover a sufficient number of the cases that will arise in real use, whilst the optional schemes are available for special cases that arise less frequently. In this way, we achieve a balance between flexibility and ease of implementation. Further classification of mandatory and optional schemes addressing different environments is available, e.g., in and of this specification on managing human user certificates as well as in the Lightweight CMP Profile on fully automating certificate management in a machine-to-machine and IoT environment. Also industry standards like for mobile networks and for Rail Automation adopted CMP and have specified a set of mandatory schemes for their use case. We will now describe the classification of initial registration/certification schemes.

Criteria Used

Initiation of Registration/Certification In terms of the PKI messages that are produced, we can regard the initiation of the initial registration/certification exchanges as occurring wherever the first PKI message relating to the end entity is produced. Note that the real-world initiation of the registration/certification procedure may occur elsewhere (e.g., a personnel department may telephone an RA operator or using zero touch methods like BRSKI or SZTP ). The possible locations are at the end entity, an RA, or a CA.

End Entity Message Origin Authentication The on-line messages produced by the end entity that requires a certificate may be authenticated or not. The requirement here is to authenticate the origin of any messages from the end entity to the PKI (CA/RA). In this specification, such authentication is achieved by two different means:

symmetric: The PKI (CA/RA) issuing the end entity with a secret value (initial authentication key) and reference value (used to identify the secret value) via some out-of-band means. The initial authentication key can then be used to protect relevant PKI messages.
asymmetric: Using a private key and certificate issued by another PKI trusted for initial authentication, e.g., an IDevID IEEE 802.1AR. The trust establishment in this external PKI is out of scope of this document.

Thus, we can classify the initial registration/certification scheme according to whether or not the on-line 'end entity -> PKI management entity' messages are authenticated or not. Note 1: We do not discuss the authentication of the 'PKI management entity -> end entity' messages here, as this is always REQUIRED. In any case, it can be achieved simply once the root-CA public key has been installed at the end entity's equipment or it can be based on the initial authentication key. Note 2: An initial registration/certification procedure can be secure where the messages from the end entity are authenticated via some out-of-band means (e.g., a subsequent visit).

Location of Key Generation In this specification, "key generation" is regarded as occurring wherever either the public or private component of a key pair first occurs in a PKIMessage. Note that this does not preclude a centralized key generation service by a KGA; the actual key pair MAY have been generated elsewhere and transported to the end entity, RA, or CA using a (proprietary or standardized) key generation request/response protocol (outside the scope of this specification). Thus, there are three possibilities for the location of "key generation": the end entity, a KGA, or a CA.

Confirmation of Successful Certification Following the creation of a certificate for an end entity, additional assurance can be gained by having the end entity explicitly confirm successful receipt of the message containing (or indicating the creation of) the certificate. Naturally, this confirmation message must be protected (based on the initial symmetric or asymmetric authentication key or other means). This gives two further possibilities: confirmed or not.

Initial Registration/Certification Schemes The criteria above allow for a large number of initial registration/certification schemes. Examples of possible initial registration/certification schemes can be found in the following subsections. An entity may support other schemes specified in profiles of PKIX-CMP, such as and or .

Centralized Scheme In terms of the classification above, this scheme is, in some ways, the simplest possible, where:

initiation occurs at the certifying CA;
no on-line message authentication is required;
"key generation" occurs at the certifying CA (see );
no confirmation message is required.

In terms of message flow, this scheme means that the only message required is sent from the CA to the end entity. The message must contain the entire PSE for the end entity. Some out-of-band means must be provided to allow the end entity to authenticate the message received and to decrypt any encrypted values.

Basic Authenticated Scheme In terms of the classification above, this scheme is where:

initiation occurs at the end entity;
message authentication is required;
"key generation" occurs at the end entity (see );
a confirmation message is recommended.

Note: An Initial Authentication Key (IAK) can be either a symmetric key or an asymmetric private key with a certificate issued by another PKI trusted for this purpose. The establishment of such trust is out of scope of this document. EE) Key generation Creation of certification request Protect request with IAK -->>-- certification request -->>-- verify request process request create response --<<-- certification response --<<-- handle response create confirmation -->>-- cert conf message -->>-- verify confirmation create response --<<-- conf ack (optional) --<<-- handle response ]]> Note: Where verification of the cert confirmation message fails, the RA/CA MUST revoke the newly issued certificate if it has been published or otherwise made available.

Proof-of-Possession (POP) of Private Key Proof-of-possession (POP) is where a PKI management entity (CA/RA) verifies if an end entity has access to the private key corresponding to a given public key. The question of whether, and in what circumstances, POPs add value to a PKI is a debate as old as PKI itself! See for a further discussion on the necessity of proof-of-possession in PKI. The PKI management operations specified here make it possible for an end entity to prove to a CA/RA that it has possession of (i.e., is able to use) the private key corresponding to the public key for which a certificate is requested (see for different POP methods). A given CA/RA is free to choose how to enforce POP (e.g., out-of-band procedural means versus PKIX-CMP in-band messages) in its certification exchanges (i.e., this may be a policy issue). However, it is REQUIRED that CAs/RAs MUST enforce POP by some means because there are currently many non-PKIX operational protocols in use (various electronic mail protocols are one example) that do not explicitly check the binding between the end entity and the private key. Until operational protocols that do verify the binding (for signature, encryption, key agreement, and KEM key pairs) exist, and are ubiquitous, this binding can only be assumed to have been verified by the CA/RA. Therefore, if the binding is not verified by the CA/RA, certificates in the Internet Public-Key Infrastructure end up being somewhat less meaningful. POP is accomplished in different ways depending upon the type of key for which a certificate is requested. If a key can be used for multiple purposes (e.g., an RSA key) then any appropriate method MAY be used (e.g., a key that may be used for signing, as well as other purposes, MUST NOT be sent to the CA/RA in order to prove possession unless archival of the private key is explicitly desired). This specification explicitly allows for cases where an end entity supplies the relevant proof to an RA and the RA subsequently attests to the CA that the required proof has been received (and validated!). For example, an end entity wishing to have a signing key certified could send the appropriate signature to the RA, which then simply notifies the relevant CA that the end entity has supplied the required proof. Of course, such a situation may be disallowed by some policies (e.g., CAs may be the only entities permitted to verify POP during certification).

Signature Keys For signature keys, the end entity can sign a value to prove possession of the private key, see .

Encryption Keys For encryption keys, the end entity can provide the private key to the CA/RA (e.g., for archiving), see , or can be required to decrypt a value in order to prove possession of the private key. Decrypting a value can be achieved either directly (see ) or indirectly (see ). The direct method is for the RA/CA to issue a random challenge to which an immediate response by the EE is required. The indirect method is to issue a certificate that is encrypted for the end entity (and have the end entity demonstrate its ability to decrypt this certificate in the confirmation message). This allows a CA to issue a certificate in a form that can only be used by the intended end entity. This specification encourages use of the indirect method because it requires no extra messages to be sent (i.e., the proof can be demonstrated using the {request, response, confirmation} triple of messages).

Key Agreement Keys For key agreement keys, the end entity and the PKI management entity (i.e., CA or RA) must establish a shared secret key in order to prove that the end entity has possession of the private key. Note that this need not impose any restrictions on the keys that can be certified by a given CA. In particular, for Diffie-Hellman keys the end entity may freely choose its algorithm parameters provided that the CA can generate a short-term (or one-time) key pair with the appropriate parameters when necessary.

Key Encapsulation Mechanism Keys For key encapsulation mechanism (KEM) keys, the end entity can provide the private key to the CA/RA (e.g., for archiving), see , or can be required to decrypt a value in order to prove possession of the private key. Decrypting a value can be achieved either directly (see ) or indirectly (see ). Note: A definition of key encapsulation mechanisms can be found in . The direct method is for the RA/CA to issue a random challenge to which an immediate response by the EE is required. The indirect method is to issue a certificate that is encrypted for the end entity using a shared secret key derived from a key encapsulated using the public key (and have the end entity demonstrate its ability to use its private key for decapsulation of the KEM ciphertext, derive the shared secret key, decrypt this certificate, and provide a hash of the certificate in the confirmation message). This allows a CA to issue a certificate in a form that can only be used by the intended end entity. This specification encourages use of the indirect method because it requires no extra messages to be sent (i.e., the proof can be demonstrated using the {request, response, confirmation} triple of messages). A certification request message for a KEM certificate SHALL use POPOPrivKey by using the keyEncipherment choice of ProofOfPossession, see , in the popo field of CertReqMsg as long as no KEM-specific choice is available.

Root CA Key Update This discussion only applies to CAs that are directly trusted by some end entities. Recognizing whether a self-signed or non-self-signed CA is supposed to be directly trusted for some end entities is a matter of CA policy and end entity configuration. This is thus beyond the scope of this document. The basis of the procedure described here is that the CA protects its new public key using its previous private key and vice versa. Thus, when a CA updates its key pair it may generate two link certificates "old with new" and "new with old". Note: The usage of link certificates has been shown to be very use case specific and no assumptions are done on this aspect. RootCaKeyUpdateContent is updated to specify these link certificates as optional. Note: When an LDAP directory is used to publish root CA updates, the old and new root CA certificates together with the two link certificates are stored as cACertificate attribute values. When a CA changes its key pair, those entities who have acquired the old CA public key via "out-of-band" means are most affected. These end entities need to acquire the new CA public key in a trusted way. This may be achieved "out-of-band", by using a repository, or by using online messages also containing the link certificates "new with old". Once the end entity acquired and properly verified the new CA public key, it must load the new trust anchor information into its trusted store. The data structure used to protect the new and old CA public keys is typically a standard X.509 v3 self-signed certificate (which may also contain extensions). There are no new data structures required. Note: Sometimes root CA certificates do not make use of X.509 v3 extensions and may be X.509 v1 certificates. Therefore, a root CA key update must be able to work for version 1 certificates. The use of the X.509 v3 KeyIdentifier extension is recommended for easier path building. Note: While the scheme could be generalized to cover cases where the CA updates its key pair more than once during the validity period of one of its end entities' certificates, this generalization seems of dubious value. Not having this generalization simply means that the validity periods of certificates issued with the old CA key pair cannot exceed the end of the "old with new" certificate validity period. Note: This scheme offers a mechanism to ensures that end entities will acquire the new CA public key, at the latest by the expiry of the last certificate they owned that was signed with the old CA private key. Certificate and/or key update operations occurring at other times do not necessarily require this (depending on the end entity's equipment). Note: In practice, a new root CA may have a slightly different subject DN, e.g., indicating a generation identifier like the year of issuance or a version number, for instance in an OU element. How to bridge trust to the new root CA certificate in a CA DN change or a cross-certificate scenario is out of scope for this document.

CA Operator Actions To change the key of the CA, the CA operator does the following:

Generate a new key pair.
Create a certificate containing the new CA public key signed with the new private key (the "new with new" certificate).
Optionally: Create a link certificate containing the new CA public key signed with the old private key (the "new with old" certificate).
Optionally: Create a link certificate containing the old CA public key signed with the new private key (the "old with new" certificate).
Publish these new certificates so that end entities may acquire it, e.g., using a repository or RootCaKeyUpdateContent.

The old CA private key is then no longer required when the validity of the "old with old" certificate ended. However, the old CA public key will remain in use for validating the "new with old" link certificate until the new CA public key is loaded into the trusted store. The old CA public key is no longer required (other than for non-repudiation) when all end entities of this CA have securely acquired and stored the new CA public key. The "new with new" certificate must have a validity period with a notBefore time that is before the notAfter time of the "old with old" certificate and a notAfter time that is after the notBefore time of the next update of this certificate. The "new with old" certificate must have a validity period with the same notBefore time as the "new with new" certificate and a notAfter time by which all end entities of this CA will securely possess the new CA public key (at the latest, at the notAfter time of the "old with old" certificate). The "old with new" certificate must have a validity period with the same notBefore and notAfter time as the "old with old" certificate. Note: Further operational considerations on transition from one root CA self-signed certificate to the next is available in RFC 8649 Section 5.

Verifying Certificates Normally when verifying a signature, the verifier verifies (among other things) the certificate containing the public key of the signer. However, once a CA is allowed to update its key there are a range of new possibilities. These are shown in the table below.

	Verifier's PSE contains NEW public key	Verifier's PSE contains OLD public key
Signer's certificate is protected using NEW key pair	Case 1: The verifier can directly verify the certificate.	Case 2: The verifier is missing the NEW public key.
Signer's certificate is protected using OLD key pair	Case 3: The verifier is missing the OLD public key.	Case 4: The verifier can directly verify the certificate.

Verification in Cases 1 and 4 In these cases, the verifier has a local copy of the CA public key that can be used to verify the certificate directly. This is the same as the situation where no key change has occurred.

Verification in Case 2 In case 2, the verifier must get access to the new public key of the CA. Case 2 will arise when the CA operator has issued the verifier's certificate, then changed the CA's key, and then issued the signer's certificate; so it is quite a typical case. The verifier does the following:

Get the "new with new" and "new with old" certificates. The location to retrieve theses certificates from, may be available in the authority information access extension of the "old with old" certificate, see caIssuers access method in Section 4.2.2.1 of , or it may be locally configured.
1. If a repository is available, look up the certificates in the caCertificate attribute.
2. If a HTTP or FTP server is available, pick the certificates from the "certs-only" CMS message.
3. If a CMP server is available, request the certificates using the root CA update general message, see .
4. Otherwise, get the certificates "out-of-band" using any trustworthy mechanism.
If received the certificates, check that the validity periods and the subject and issuer fields match. Verify the signatures using the old root CA key (which the verifier has locally).
If all checks were successful, securely store the new trust anchor information and validate the signer's certificate.

Verification in Case 3 In case 3, the verifier must get access to the old public key of the CA. Case 3 will arise when the CA operator has issued the signer's certificate, then changed the key, and then issued the verifier's certificate. The verifier does the following:

Get the "old with new" certificate. The location to retrieve theses certificates from, may be available in the authority information access extension of the "new with new" certificate, see caIssuers access method in Section 4.2.2.1 of , or it may be locally configured.
1. If a repository is available, look up the certificate in the caCertificate attribute.
2. If a HTTP or FTP server is available, pick the certificate from the "certs-only" CMS message.
3. If a CMP server and an untrusted copy of the old root CA certificate is available (e.g., the signer provided it in-band in the CMP extraCerts filed), request the certificate using the root CA update general message, see .
4. Otherwise, get the certificate "out-of-band" using any trustworthy mechanism.
If received the certificate, check that the validity periods and the subject and issuer fields match. Verify the signatures using the new root CA key (which the verifier has locally).
If all checks were successful, securely store the old trust anchor information and validate the signer's certificate.

Revocation - Change of CA Key As we saw above, the verification of a certificate becomes more complex once the CA is allowed to change its key. This is also true for revocation checks as the CA may have signed the CRL using a newer private key than the one within the user's PSE. The analysis of the alternatives is the same as for certificate verification.

Extended Key Usage for PKI Entities The extended key usage (EKU) extension indicates the purposes for which the certified key pair may be used. Therefore, it restricts the use of a certificate to specific applications. A CA may want to delegate parts of its duties to other PKI management entities. This section provides a mechanism to both prove this delegation and enable automated means for checking the authorization of this delegation. Such delegation may also be expressed by other means, e.g., explicit configuration. To offer automatic validation for the delegation of a role by a CA to another entity, the certificates used for CMP message protection or signed data for central key generation MUST be issued by the delegating CA and MUST contain the respective EKUs. This proves that the delegating CA authorized this entity to act in the given role, as described below. The OIDs to be used for these EKUs are: Note: Section 2.10 of specifies OIDs for a Certificate Management over CMS (CMC) CA and a CMC RA. As the functionality of a CA and RA is not specific to any certificate management protocol (such as CMC or CMP), these EKUs are reused by CMP. The meaning of the id-kp-cmKGA EKU is as follows:

CMP KGA:: CMP key generation authorities are CAs or are identified by the id-kp-cmKGA extended key usage. The CMP KGA knows the private key it generated on behalf of the end entity. This is a very sensitive service and needs specific authorization, which by default is with the CA certificate itself. The CA may delegate its authorization by placing the id-kp-cmKGA extended key usage in the certificate used to authenticate the origin of the generated private key. The authorization may also be determined through local configuration of the end entity.

Data Structures This section contains descriptions of the data structures required for PKI management messages. describes constraints on their values and the sequence of events for each of the various PKI management operations.

Overall PKI Message All of the messages used in this specification for the purposes of PKI management use the following structure: The PKIHeader contains information that is common to many PKI messages. The PKIBody contains message-specific information. The PKIProtection, when used, contains bits that protect the PKI message. The extraCerts field can contain certificates that may be useful to the recipient. For example, this can be used by a CA or RA to present an end entity with certificates that it needs to verify its own new certificate (if, for example, the CA that issued the end entity's certificate is not a root CA for the end entity). Note that this field does not necessarily contain a certification path; the recipient may have to sort, select from, or otherwise process the extra certificates in order to use them.

PKI Message Header All PKI messages require some header information for addressing and transaction identification. Some of this information will also be present in a transport-specific envelope. However, if the PKI message is protected, then this information is also protected (i.e., we make no assumption about secure transport). The following data structure is used to contain this information: The usage of the protocol version number (pvno) is described in . The sender field contains the name of the sender of the PKIMessage. This name (in conjunction with senderKID, if supplied) should be sufficient to indicate the key to use to verify the protection on the message. If nothing about the sender is known to the sending entity (e.g., in the initial request message, where the end entity may not know its own Distinguished Name (DN), e-mail name, IP address, etc.), then the "sender" field MUST contain a "NULL-DN" value in the directoryName choice. A "NULL-DN" is a SEQUENCE OF relative distinguished names of zero length and is encoded as 0x3000. In such a case, the senderKID field MUST hold an identifier (i.e., a reference number) that indicates to the receiver the appropriate shared secret information to use to verify the message. The recipient field contains the name of the recipient of the PKIMessage. This name (in conjunction with recipKID, if supplied) should be usable to verify the protection on the message. The protectionAlg field specifies the algorithm used to protect the message. If no protection bits are supplied (note that PKIProtection is OPTIONAL) then this field MUST be omitted; if protection bits are supplied, then this field MUST be supplied. senderKID and recipKID are usable to indicate which keys have been used to protect the message (recipKID will normally only be required where protection of the message uses Diffie-Hellman (DH) or elliptic curve Diffie-Hellman (ECDH) keys). These fields MUST be used if required to uniquely identify a key (e.g., if more than one key is associated with a given sender name). The senderKID SHOULD be used in any case. Note: The recommendation of using senderKID was changed since , where it was recommended to be omitted if not needed to identify the protection key. The transactionID field within the message header is to be used to allow the recipient of a message to correlate this with an ongoing transaction. This is needed for all transactions that consist of more than just a single request/response pair. For transactions that consist of a single request/response pair, the rules are as follows. A client MUST populate the transactionID field if the message contains an infoValue of type KemCiphertextInfo, see . In all other cases the client MAY populate the transactionID field of the request. If a server receives such a request that has the transactionID field set, then it MUST set the transactionID field of the response to the same value. If a server receives such request with a missing transactionID field, then it MUST populate the transactionID field if the message contains a KemCiphertextInfo field. In all other cases the server MAY set transactionID field of the response. For transactions that consist of more than just a single request/response pair, the rules are as follows. If the message contains an infoValue of type KemCiphertextInfo, the client MUST generate a transactionID, otherwise the client SHOULD generate a transactionID for the first request. If a server receives such a request that has the transactionID field set, then it MUST set the transactionID field of the response to the same value. If a server receives such request with a missing transactionID field, then it MUST populate the transactionID field of the response with a server-generated ID. Subsequent requests and responses MUST all set the transactionID field to the thus established value. In all cases where a transactionID is being used, a given client MUST NOT have more than one transaction with the same transactionID in progress at any time (to a given server). Servers are free to require uniqueness of the transactionID or not, as long as they are able to correctly associate messages with the corresponding transaction. Typically, this means that a server will require the {client, transactionID} tuple to be unique, or even the transactionID alone to be unique, if it cannot distinguish clients based on transport-level information. A server receiving the first message of a transaction (which requires more than a single request/response pair) that contains a transactionID that does not allow it to meet the above constraints (typically because the transactionID is already in use) MUST send back an ErrorMsgContent with a PKIFailureInfo of transactionIdInUse. It is RECOMMENDED that the clients fill the transactionID field with 128 bits of (pseudo-) random data for the start of a transaction to reduce the probability of having the transactionID in use at the server. The senderNonce and recipNonce fields protect the PKIMessage against replay attacks. The senderNonce will typically be 128 bits of (pseudo-) random data generated by the sender, whereas the recipNonce is copied from the senderNonce field of the previous message in the transaction. The messageTime field contains the time at which the sender created the message. This may be useful to allow end entities to correct/check their local time for consistency with the time on a central system. The freeText field may be used to send a human-readable message to the recipient (in any number of languages). The first language used in this sequence indicates the desired language for replies. The generalInfo field may be used to send machine-processable additional data to the recipient. The following generalInfo extensions are defined and MAY be supported.

ImplicitConfirm This is used by the EE to inform the CA or RA that it does not wish to send a certificate confirmation for issued certificates. If the CA grants the request to the EE, it MUST put the same extension in the PKIHeader of the response. If the EE does not find the extension in the response, it MUST send the certificate confirmation.

ConfirmWaitTime This is used by the CA or RA to inform the EE how long it intends to wait for the certificate confirmation before revoking the certificate and deleting the transaction.

OrigPKIMessage An RA MAY include the original PKIMessage from the EE in the generalInfo field of the PKIHeader of a PKIMessage. This is used by the RA to inform the CA of the original PKIMessage that it received from the EE and modified in some way (e.g., added or modified particular field values or added new extensions) before forwarding the new PKIMessage. This accommodates, for example, cases in which the CA wishes to check the message origin, the POP, or other information on the original EE message. Note: If the changes made by the RA to the original PKIMessage break the POP of a certificate request, the RA can set the popo field of the new PKIMessage to raVerified, see . Unless the OrigPKIMessage infoValue is in the header of a nested message, it MUST contain exactly one PKIMessage. The contents of OrigPKIMessage infoValue in the header of a nested message MAY contain multiple PKIMessage structures, which MUST be in the same order as the PKIMessage structures in PKIBody.

CertProfile This is used by the EE to indicate specific certificate profiles, e.g., when requesting a new certificate or a certificate request template; see . When used in a p10cr message, the CertProfileValue sequence MUST NOT contain multiple certificate profile names. When used in an ir/cr/kur/genm message, the CertProfileValue sequence MUST NOT contain more certificate profile names than the number of CertReqMsg or GenMsgContent InfoTypeAndValue elements contained in the message body. The certificate profile names in the CertProfileValue sequence relate to the CertReqMsg or GenMsgContent InfoTypeAndValue elements in the given order. An empty string means no certificate profile name is associated with the respective CertReqMsg or GenMsgContent InfoTypeAndValue element. If the CertProfileValue sequence contains less certificate profile entries than CertReqMsg or GenMsgContent InfoTypeAndValue elements, the remaining CertReqMsg or GenMsgContent InfoTypeAndValue elements have no profile name associated with them.

KemCiphertextInfo A PKI entity MAY provide the KEM ciphertext for MAC-based message protection using KEM (see Section 5.1.3.4) in the generalInfo field of a request message to a PKI management entity if it knows that the PKI management entity uses a KEM key pair and has its public key. For more details of KEM-based message protection see . See for the definition of {id-it TBD1}.

PKI Message Body The specific types are described in below.

PKI Message Protection Some PKI messages will be protected for integrity. Note: If an asymmetric algorithm is used to protect a message and the relevant public component has been certified already, then the origin of the message can also be authenticated. On the other hand, if the public component is uncertified, then the message origin cannot be automatically authenticated, but may be authenticated via out-of-band means. When protection is applied, the following structure is used: The input to the calculation of PKIProtection is the DER encoding of the following data structure: There MAY be cases in which the PKIProtection BIT STRING is deliberately not used to protect a message (i.e., this OPTIONAL field is omitted) because other protection, external to PKIX, will be applied instead. Such a choice is explicitly allowed in this specification. Examples of such external protection include CMS and Security Multiparts encapsulation of the PKIMessage (or simply the PKIBody (omitting the CHOICE tag), if the relevant PKIHeader information is securely carried in the external mechanism). It is noted, however, that many such external mechanisms require that the end entity already possesses a public-key certificate, and/or a unique Distinguished Name, and/or other such infrastructure-related information. Thus, they may not be appropriate for initial registration, key-recovery, or any other process with "boot-strapping" characteristics. For those cases it may be necessary that the PKIProtection parameter be used. In the future, if/when external mechanisms are modified to accommodate boot-strapping scenarios, the use of PKIProtection may become rare or non-existent. Depending on the circumstances, the PKIProtection bits may contain a Message Authentication Code (MAC) or signature. Only the following cases can occur:

Shared Secret Information In this case, the sender and recipient share secret information with sufficient entropy (established via out-of-band means). PKIProtection will contain a MAC value and the protectionAlg MAY be one of the options described in CMP Algorithms Section 6.1 . The algorithm identifier id-PasswordBasedMac is defined in Section 4.4 of and updated by . It is mentioned in Section 6.1.1 of for backward compatibility. More modern alternatives are listed in Section 6.1 of . The following text gives a method of key expansion to be used when the MAC-algorithm requires an input length that is larger than the size of the one-way-function. Note: Section 4.4 of and do not mention this key expansion method and gives an example using HMAC algorithms where key expansion is not needed. It is recognized that this omission in can lead to confusion and possible incompatibility if key expansion is not used when needed. Therefore, when key expansion is required (when K > H) the key expansion defined in the following text MUST be used. In the above protectionAlg, the salt value is appended to the shared secret input. The OWF is then applied iterationCount times, where the salted secret is the input to the first iteration and, for each successive iteration, the input is set to be the output of the previous iteration. The output of the final iteration (called "BASEKEY" for ease of reference, with a size of "H") is what is used to form the symmetric key. If the MAC algorithm requires a K-bit key and K <= H, then the most significant K bits of BASEKEY are used. If K > H, then all of BASEKEY is used for the most significant H bits of the key, OWF("1" || BASEKEY) is used for the next most significant H bits of the key, OWF("2" || BASEKEY) is used for the next most significant H bits of the key, and so on, until all K bits have been derived. [Here "N" is the ASCII byte encoding the number N and "||" represents concatenation.] Note: It is RECOMMENDED that the fields of PBMParameter remain constant throughout the messages of a single transaction (e.g., ir/ip/certConf/pkiConf) to reduce the overhead associated with PasswordBasedMac computation.

DH Key Pairs Where the sender and receiver possess finite-field or elliptic-curve-based Diffie-Hellman certificates with compatible DH parameters, in order to protect the message the end entity must generate a symmetric key based on its private DH key value and the DH public key of the recipient of the PKI message. PKIProtection will contain a MAC value keyed with this derived symmetric key and the protectionAlg will be the following: In the above protectionAlg, OWF is applied to the result of the Diffie-Hellman computation. The OWF output (called "BASEKEY" for ease of reference, with a size of "H") is what is used to form the symmetric key. If the MAC algorithm requires a K-bit key and K <= H, then the most significant K bits of BASEKEY are used. If K > H, then all of BASEKEY is used for the most significant H bits of the key, OWF("1" || BASEKEY) is used for the next most significant H bits of the key, OWF("2" || BASEKEY) is used for the next most significant H bits of the key, and so on, until all K bits have been derived. [Here "N" is the ASCII byte encoding the number N and "||" represents concatenation.] Note: Hash algorithms that can be used as one-way functions are listed in CMP Algorithms Section 2.

Signature In this case, the sender possesses a signature key pair and simply signs the PKI message. PKIProtection will contain the signature value and the protectionAlg will be an AlgorithmIdentifier for a digital signature MAY be one of the options described in CMP Algorithms Section 3 .

Key Encapsulation In case the sender of a message has a Key Encapsulation Mechanism (KEM) key pair, it can be used to establish a shared secret key for MAC-based message protection. This can be used for message authentication. This approach uses the definition of Key Encapsulation Mechanism (KEM) algorithm functions in Section 1 of as follows: A KEM algorithm provides three functions:

KeyGen() -> (pk, sk):

Generate a public key (pk) and a private (secret) key (sk).

Encapsulate(pk) -> (ct, ss):

Given the public key (pk), produce a ciphertext (ct) and a shared secret (ss).

Decapsulate(sk, ct) -> (ss):

Given the private key (sk) and the ciphertext (ct), produce the shared secret (ss).

To support a particular KEM algorithm, the PKI entity that possesses a KEM key pair and wishes to use it for MAC-based message protection MUST support the KEM Decapsulate() function. The PKI entity that wishes to verify the MAC-based message protection MUST support the KEM Encapsulate() function. The respective public KEM key is usually carried in a certificate . Note: Both PKI entities send and receive messages in a PKI management operation. Both PKI entities may independently wish to protect messages using their KEM key pairs. For ease of explanation we use the term "Alice" to denote the PKI entity possessing the KEM key pair and who wishes to provide MAC-based message protection, and "Bob" to denote the PKI entity having Alice’s authentic public KEM key and who needs to verify the MAC-based protection provided by Alice. Assuming Bob has Alice's KEM public key, he generates the ciphertext using KEM encapsulation and transfers it to Alice in an InfoTypeAndValue structure. Alice then retrieves the KEM shared secret from the ciphertext using KEM decapsulation and the associated KEM private key. Using a key derivation function (KDF), she derives a shared secret key from the KEM shared secret and other data sent by Bob. PKIProtection will contain a MAC value calculated using that shared secret key, and the protectionAlg will be the following: Note: The OID for id-KemBasedMac was assigned on the private-use arc { iso(1) member-body(2) us(840) nortelnetworks(113533) entrust(7) }, and not assigned on an IANA-owned arc because the authors wished to placed it on the same branch as the existing OIDs for id-PasswordBasedMac and id-DHBasedMac. kdf is the algorithm identifier of the chosen KDF, and any associated parameters, used to derive the shared secret key. kemContext MAY be used to transfer additional algorithm specific context information, see also the definition of ukm in , Section 3. len is the output length of the KDF and MUST be the desired size of the key to be used for MAC-based message protection. mac is the algorithm identifier of the chosen MAC algorithm, and any associated parameters, used to calculate the MAC value. The KDF and MAC algorithms MAY be chosen from the options in CMP Algorithms . The InfoTypeAndValue transferring the KEM ciphertext uses OID id-it-KemCiphertextInfo. It contains a KemCiphertextInfo structure as defined in . Note: This InfoTypeAndValue can be carried in a genm/genp message body as specified in or in the generalInfo field of PKIHeader in messages of other types, see . In the following, a generic message flow for MAC-based protection using KEM is specified in more detail. It is assumed that Bob possesses the public KEM key of Alice. Alice can be the initiator of a PKI management operation or the responder. For more detailed figures see . Generic Message Flow:

Generic Message Flow when Alice has a KEM key pair message -> 3 perform key derivation, verify MAC-based protection ------------------- Alice authenticated by Bob -------------------- ]]>

Bob needs to possess the authentic public KEM key pk of Alice, for instance contained in a KEM certificate that was received and successfully validated by Bob beforehand. Bob generates a shared secret ss and the associated ciphertext ct using the KEM Encapsulate function with Alice's public KEM key pk. Bob MUST NOT reuse the ss and ct for other PKI management operations. From this data, Bob produces a KemCiphertextInfo structure including the KEM algorithm identifier and the ciphertext ct and sends it to Alice in an InfoTypeAndValue structure as defined in . (ct, ss) ]]>
Alice decapsulates the shared secret ss from the ciphertext ct using the KEM Decapsulate function and its private KEM key sk. (ss) ]]> If the decapsulation operation outputs an error, any failInfo field in an error response message SHALL contain the value badMessageCheck and the PKI management operation SHALL be terminated. Alice derives the shared secret key ssk using a KDF. The shared secret ss is used as input key material for the KDF, the value len is the desired output length of the KDF as required by the MAC algorithm to be used for message protection. KDF, len, and MAC will be transferred to Bob in the protectionAlg KemBMParameter. The DER-encoded KemOtherInfo structure, as defined below, is used as context for the KDF. (ssk) ]]> The shared secret key ssk is used for MAC-based protection by Alice.
Bob derives the same shared secret key ssk using the KDF. Also here the shared secret ss is used as input key material for the KDF, the value len is the desired output length for the KDF, and the DER-encoded KemOtherInfo structure constructed in the same way as on Alice's side is used as context for the KDF. (ssk) ]]> Bob uses the shared secret key ssk for verifying the MAC-based protection of the message received and in this way authenticates Alice.

This shared secret key ssk can be reused by Alice for MAC-based protection of further messages sent to Bob within the current PKI management operation. This approach employs the notation of KDF(IKM, L, info) as described in with the following changes:

IKM is the input key material. It is the symmetric secret called ss resulting from the key encapsulation mechanism.
L is dependent of the MAC algorithm that is used with the shared secret key for CMP message protection and is called len in this document.
info is an additional input to the KDF, is called context in this document, and contains the DER-encoded KemOtherInfo structure defined as: staticString MUST be "CMP-KEM". transactionID MUST be the value from the message containing the ciphertext ct in KemCiphertextInfo. Note: The transactionID is used to ensure domain separation of the derived shared secret key between different PKI management operations. For all PKI management operations with more than one exchange the transactionID MUST be set anyway, see . In case Bob provided a infoValue of type KemCiphertextInfo to Alice in the initial request message, see of , the transactionID MUST be set by Bob. kemContext MAY contain additional algorithm specific context information.
OKM is the output keying material of the KDF used for MAC-based message protection of length len and is called ssk in this document.

There are various ways how Alice can request, and Bob can provide the KEM ciphertext, see for details. The KemCiphertextInfo can be requested using PKI general messages as described in . Alternatively, the generalInfo field of the PKIHeader can be used to convey the same request and response InfoTypeAndValue structures as described in . The procedure works also without Alice explicitly requesting the KEM ciphertext in case Bob knows a KEM key of Alice beforehand and can expect that she is ready to use it. If both the initiator and responder in a PKI management operation have KEM key pairs, this procedure can be applied by both entities independently, establishing and using different shared secret keys for either direction.

Multiple Protection When receiving a protected PKI message, a PKI management entity, such as an RA, MAY forward that message adding its own protection. Additionally, multiple PKI messages MAY be aggregated. There are several use cases for such messages.

The RA confirms having validated and authorized a message and forwards the original message unchanged.
A PKI management entity collects several messages that are to be forwarded in the same direction and forwards them in a batch. Request messages can be transferred as batch upstream (towards the CA); response or announce messages can be transferred as batch downstream (towards an RA but not to the EE). For instance, this can be used when bridging an off-line connection between two PKI management entities.

These use cases are accomplished by nesting the messages within a new PKI message. The structure used is as follows: In case an RA needs to modify a request message, it MAY include the original PKIMessage in the generalInfo field of the modified message as described in .

Common Data Structures Before specifying the specific types that may be placed in a PKIBody, we define some data structures that are used in more than one case.

Requested Certificate Contents Various PKI management messages require that the originator of the message indicate some of the fields that are required to be present in a certificate. The CertTemplate structure allows an end entity or RA to specify as much as it wishes about the certificate it requires. CertTemplate is identical to a Certificate, but with all fields optional. Note: Even if the originator completely specifies the contents of a certificate it requires, a CA is free to modify fields within the certificate actually issued. If the modified certificate is unacceptable to the requester, the requester MUST send back a certConf message that either does not include this certificate (via a CertHash), or does include this certificate (via a CertHash) along with a status of "rejected". See for the definition and use of CertHash and the certConf message. Note: Before requesting a new certificate, an end entity can request a certTemplate structure as a kind of certificate request blueprint, in order to learn which data the CA expects to be present in the certificate request, see . See CRMF for CertTemplate syntax. If certTemplate is an empty SEQUENCE (i.e., all fields omitted), then the controls field in the CertRequest structure MAY contain the id-regCtrl-altCertTemplate control, specifying a template for a certificate other than an X.509v3 public-key certificate. Conversely, if certTemplate is not empty (i.e., at least one field is present), then controls MUST NOT contain id-regCtrl-altCertTemplate. The new control is defined as follows: See also for more details on how to manage certificates in alternative formats using CRMF syntax.

Encrypted Values When encrypted data like a private key, certificate, POP challenge, or revocation passphrase is sent in PKI messages it is RECOMMENDED to use the EnvelopedData structure. In some cases this is accomplished by using the EncryptedKey data structure instead of EncryptedValue. See Certificate Request Message Format (CRMF) for EncryptedKey and EncryptedValue syntax and Cryptographic Message Syntax (CMS) for EnvelopedData syntax. Using the EncryptedKey data structure offers the choice to either use EncryptedValue (for backward compatibility only) or EnvelopedData. The use of the EncryptedValue structure has been deprecated in favor of the EnvelopedData structure. Therefore, it is RECOMMENDED to use EnvelopedData. Note: The EncryptedKey structure defined in CRMF is used here, which makes the update backward compatible. Using the new syntax with the untagged default choice EncryptedValue is bits-on-the-wire compatible with the old syntax. To indicate support for EnvelopedData, the pvno cmp2021 has been introduced. Details on the usage of the protocol version number (pvno) are described in . The EnvelopedData structure is RECOMMENDED to use in CMP to transport a private key, certificate, POP challenge, or revocation passphrase in encrypted form as follows:

It contains only one RecipientInfo structure because the content is encrypted only for one recipient.
It may contain a private key in the AsymmetricKeyPackage structure (which is placed in the encryptedContentInfo field), as defined in , that is wrapped in a SignedData structure, as specified in Section 5 of and , signed by the Key Generation Authority or CA.
It may contain a certificate, POP challenge, or revocation passphrase directly in the encryptedContent field.

The content of the EnvelopedData structure, as specified in Section 6 of , MUST be encrypted using a newly generated symmetric content-encryption key. This content-encryption key MUST be securely provided to the recipient using one of four key management techniques. The choice of the key management technique to be used by the sender depends on the credential available at the recipient:

recipient's certificate with an algorithm identifier and a public key that supports key transport and where any given key usage extension allows keyEncipherment: The content-encryption key will be protected using the key transport key management technique, as specified in Section 6.2.1 of .
recipient's certificate with an algorithm identifier and a public key that supports key agreement and where any given key usage extension allows keyAgreement: The content-encryption key will be protected using the key agreement key management technique, as specified in Section 6.2.2 of .
a password or shared secret: The content-encryption key will be protected using the password-based key management technique, as specified in Section 6.2.4 of .
recipient's certificate with an algorithm identifier and a public key that supports key encapsulation mechanism and where any given key usage extension allows keyEncipherment: The content-encryption key will be protected using the key management technique for KEM keys, as specified in .

Note: There are cases where the algorithm identifier, the type of the public key, and the key usage extension will not be sufficient to decide on the key management technique to use, e.g., when rsaEncryption is the algorithm identifier. In such cases it is a matter of local policy to decide.

Status Codes and Failure Information for PKI Messages All response messages will include some status information. The following values are defined. Responders may use the following syntax to provide more information about failure cases.

Certificate Identification In order to identify particular certificates, the CertId data structure is used. See for CertId syntax.

Out-of-band root CA Public Key Each root CA must be able to publish its current public key via some "out-of-band" means or together with the respective link certificate using an online mechanism. While such mechanisms are beyond the scope of this document, we define data structures that can support such mechanisms. There are generally two methods available: Either the CA directly publishes its self-signed certificate, or this information is available via the directory (or equivalent) and the CA publishes a hash of this value to allow verification of its integrity before use. Note: As an alternative to out-of-band distribution of root CA public keys, the CA can provide the self-signed certificate together with link certificates, e.g., using RootCaKeyUpdateContent (). The fields within this certificate are restricted as follows:

The certificate MUST be self-signed (i.e., the signature must be verifiable using the SubjectPublicKeyInfo field);
The subject and issuer fields MUST be identical;
If the subject field contains a "NULL-DN", then both subjectAltNames and issuerAltNames extensions MUST be present and have exactly the same value;
The values of all other extensions must be suitable for a self-signed certificate (e.g., key identifiers for subject and issuer must be the same).

The intention of the hash value is that anyone who has securely received the hash value (via the out-of-band means) can verify a self-signed certificate for that CA.

Archive Options Requesters may indicate that they wish the PKI to archive a private key value using the PKIArchiveOptions structure. See for PKIArchiveOptions syntax.

Publication Information Requesters may indicate that they wish the PKI to publish a certificate using the PKIPublicationInfo structure. See for PKIPublicationInfo syntax.

Proof-of-Possession Structures The proof-of-possession structure used is indicated in the popo field of type ProofOfPossession in the CertReqMsg sequence, see Section 4 of .

raVerified An EE MUST NOT use raVerified. If an RA performs changes to a certification request breaking the provided proof-of-possession (POP), or if the RA requests a certificate on behalf of an EE and cannot provide the POP itself, the RA MUST use raVerified. Otherwise, it SHOULD NOT use raVerified. When introducing raVerified, the RA MUST check the existing POP, or it MUST ensure by other means that the EE is the holder of the private key. The RA MAY provide the original message containing the POP in the generalInfo field using the id-it-origPKIMessage, see , enabling the CA to verify it.

POPOSigningKey Structure If the certification request is for a key pair that supports signing (i.e., a request for a verification certificate), then the proof-of-possession of the private key is demonstrated through use of the POPOSigningKey structure, for details see Section 4.1 of . Note: For the purposes of this specification, the ASN.1 comment given in Appendix C of pertains not only to certTemplate, but also to the altCertTemplate control as defined in . If certTemplate (or the altCertTemplate control) contains the subject and publicKey values, then poposkInput MUST be omitted and the signature MUST be computed on the DER-encoded value of certReq field of the CertReqMsg (or the DER-encoded value of AltCertTemplate). If certTemplate/altCertTemplate does not contain both the subject and public key values (i.e., if it contains only one of these, or neither), then poposkInput MUST be present and the signature MUST be computed on the DER-encoded value of poposkInput (i.e., the "value" OCTETs of the POPOSigningKeyInput DER). In the special case that the CA/RA has a DH certificate that is known to the EE and the certification request is for a key agreement key pair, the EE can also use the POPOSigningKey structure (where the algorithmIdentifier field is DHBasedMAC and the signature field is the MAC) for demonstrating POP.

POPOPrivKey Structure If the certification request is for a key pair that does not support signing (i.e., a request for an encryption or key agreement certificate), then the proof-of-possession of the private key is demonstrated through use of the POPOPrivKey structure in one of the following three ways, for details see Section 4.2 and 4.3 of . When using agreeMAC or encryptedKey choices, the pvno cmp2021(3) MUST be used. Details on the usage of the protocol version number (pvno) are described in .

Inclusion of the Private Key This method mentioned previously in demonstrates proof-of-possession of the private key by including the encrypted private key in the CertRequest in the POPOPrivKey structure or in the PKIArchiveOptions control structure. This method SHALL only be used if archival of the private key is desired. For a certification request message indicating cmp2021(3) in the pvno field of the PKIHeader, the encrypted private key MUST be transferred in the encryptedKey choice of POPOPrivKey (or within the PKIArchiveOptions control) in a CMS EnvelopedData structure as defined in . Note: The thisMessage choice has been deprecated in favor of encryptedKey. When using cmp2000(2) in the certification request message header for backward compatibility, the thisMessage choice of POPOPrivKey is used containing the encrypted private key in an EncryptedValue structure wrapped in a BIT STRING. This allows the necessary conveyance and protection of the private key while maintaining bits-on-the-wire compatibility with .

Indirect Method - Encrypted Certificate The indirect method mentioned previously in demonstrates proof-of-possession of the private key by having the CA return the requested certificate in encrypted form, see . This method is indicated in the CertRequest by requesting the encrCert option in the subsequentMessage choice of POPOPrivKey. <--- rep (enc cert) ----- ---- conf (cert hash) ----> <--- ack ----- ]]> The end entity proves knowledge of the private key to the CA by providing the correct CertHash for this certificate in the certConf message. This demonstrates POP because the EE can only compute the correct CertHash if it is able to recover the encrypted certificate, and it can only recover the certificate if it is able to obtain the symmetric key using the required private key. Clearly, for this to work, the CA MUST NOT publish the certificate until the certConf message arrives (when certHash is to be used to demonstrate POP). See for further details and see for security considerations regarding use of Certificate Transparency logs. The recipient SHOULD maintain a context of the PKI management operation, e.g., using transactionID and certReqId, to identify the private key to use when decrypting the EnvelopedData containing the newly issued certificate. The recipient may be unable to use the RecipientInfo structure as it refers to the certificate that is still encrypted. The sender MUST populate the rid field as specified by CMS and the client MAY ignore it.

Direct Method - Challenge-Response Protocol The direct method mentioned previously in demonstrates proof-of-possession of the private key by having the end entity engage in a challenge-response protocol (using the messages popdecc of type POPODecKeyChall and popdecr of type POPODecKeyResp; see below) between CertReqMessages and CertRepMessage. This method is indicated in the CertRequest by requesting the challengeResp option in the subsequentMessage choice of POPOPrivKey. Note: This method would typically be used in an environment in which an RA verifies POP and then makes a certification request to the CA on behalf of the end entity. In such a scenario, the CA trusts the RA to have done POP correctly before the RA requests a certificate for the end entity. The complete protocol then looks as follows (note that req' does not necessarily encapsulate req as a nested message): <--- chall --- ---- resp ---> ---- req' ---> <--- rep ----- ---- conf ---> <--- ack ----- <--- rep ----- ---- conf ---> <--- ack ----- ]]> This protocol is obviously much longer than the exchange given in above, but allows a local Registration Authority to be involved and has the property that the certificate itself is not actually created until the proof-of-possession is complete. In some environments, a different order of the above messages may be required, such as the following (this may be determined by policy): <--- chall --- ---- resp ---> ---- req' ---> <--- rep ----- <--- rep ----- ---- conf ---> ---- conf ---> <--- ack ----- <--- ack ----- ]]> The challenge-response messages for proof-of-possession of a private key are specified as follows (for decryption keys see , p.404 for details). This challenge-response exchange is associated with the preceding certification request message (and subsequent certification response and confirmation messages) by the transactionID used in the PKIHeader and by the protection applied to the PKIMessage. More details on the fields in this syntax is available in . For a popdecc message indicating cmp2021(3) in the pvno field of the PKIHeader, the encryption of Rand MUST be transferred in the encryptedRand field in a CMS EnvelopedData structure as defined in . The challenge field MUST contain an empty OCTET STRING. The recipient SHOULD maintain a context of the PKI management operation, e.g., using transactionID and certReqId, to identify the private key to use when decrypting encryptedRand. The sender MUST populate the rid field in the EnvelopedData sequence using the issuerAndSerialNumber choice containing a NULL-DN as issuer and the certReqId as serialNumber. The client MAY ignore the rid field. Note: The challenge field has been deprecated in favor of encryptedRand. When using cmp2000(2) in the popdecc message header for backward compatibility, the challenge field MUST contain the encryption (involving the public key for which the certification request is being made) of Rand and encryptedRand MUST be omitted. Using challenge (omitting the optional encryptedRand field) is bit-compatible with . Note that the size of Rand, when used with challenge, needs to be appropriate for encryption, involving the public key of the requester. If, in some environment, names are so long that they cannot fit (e.g., very long DNs), then whatever portion will fit should be used (as long as it includes at least the common name, and as long as the receiver is able to deal meaningfully with the abbreviation). On receiving the popdecc message, the end entity decrypts all included challenges and responds with a popdecr message containing the decrypted integer values in the same order.

Summary of PoP Options The text in this section provides several options with respect to POP techniques. Using "SK" for "signing key", "EK" for "encryption key", "KAK" for "key agreement key", and "KEMK" for "key encapsulation mechanism key", the techniques may be summarized as follows: Given this array of options, it is natural to ask how an end entity can know what is supported by the CA/RA (i.e., which options it may use when requesting certificates). The following guidelines should clarify this situation for EE implementers. RAVerified: This is not an EE decision; the RA uses this if and only if it has verified POP before forwarding the request on to the CA, so it is not possible for the EE to choose this technique. SKPOP: If the EE has a signing key pair, this is the only POP method specified for use in the request for a corresponding certificate. EKPOPThisMessage (deprecated), KAKPOPThisMessage (deprecated), EKPOPEncryptedKey, KAKPOPEncryptedKey, KEMKPOPEncryptedKey: Whether or not to give up its private key to the CA/RA is an EE decision. If the EE decides to reveal its key, then these are the only POP methods available in this specification to achieve this (and the key pair type and protocol version used will determine which of these methods to use). The reason for deprecating EKPOPThisMessage and KAKPOPThisMessage options has been given in . KAKPOPThisMessageDHMAC: The EE can only use this method if (1) the CA/RA has a DH certificate available for this purpose, and (2) the EE already has a copy of this certificate. If both these conditions hold, then this technique is clearly supported and may be used by the EE, if desired. EKPOPEncryptedCert, KAKPOPEncryptedCert, KEMKPOPEncryptedCert, EKPOPChallengeResp, KAKPOPChallengeResp, and KEMKPOPChallengeResp: The EE picks one of these (in the subsequentMessage field) in the request message, depending upon preference and key pair type. The EE is not doing POP at this point; it is simply indicating which method it wants to use. Therefore, if the CA/RA replies with a "badPOP" error, the EE can re-request using the other POP method chosen in subsequentMessage. Note, however, that this specification encourages the use of the EncryptedCert choice and, furthermore, says that the challenge-response would typically be used when an RA is involved and doing POP verification. Thus, the EE should be able to make an intelligent decision regarding which of these POP methods to choose in the request message.

GeneralizedTime GeneralizedTime is a standard ASN.1 type and SHALL be used as specified in Section 4.1.2.5.2 of .

Operation-Specific Data Structures

Initialization Request An Initialization request message contains as the PKIBody a CertReqMessages data structure, which specifies the requested certificate(s). Typically, SubjectPublicKeyInfo, KeyId, and Validity are the template fields which may be supplied for each certificate requested (see the profiles defined in Section 4.1.1, and for further information). This message is intended to be used for entities when first initializing into the PKI. See and for CertReqMessages syntax.

Initialization Response An Initialization response message contains as the PKIBody a CertRepMessage data structure, which has for each certificate requested a PKIStatusInfo field, a subject certificate, and possibly a private key (normally encrypted using EnvelopedData, see Section 4.1.6 for further information). See for CertRepMessage syntax. Note that if the PKI Message Protection is "shared secret information" (see ), then any certificate transported in the caPubs field may be directly trusted as a root CA certificate by the initiator.

Certification Request A Certification request message contains as the PKIBody a CertReqMessages data structure, which specifies the requested certificates (see the profiles defined in Section 4.1.2 and for further information). This message is intended to be used for existing PKI entities who wish to obtain additional certificates. See and for CertReqMessages syntax. Alternatively, the PKIBody MAY be a CertificationRequest (this structure is fully specified by the ASN.1 structure CertificationRequest given in , see the profiles defined in Section 4.1.4 for further information). This structure may be required for certificate requests for signing key pairs when interoperation with legacy systems is desired, but its use is strongly discouraged whenever not absolutely necessary.

Certification Response A Certification response message contains as the PKIBody a CertRepMessage data structure, which has a status value for each certificate requested, and optionally has a CA public key, failure information, a subject certificate, and an encrypted private key. A p10cr message contains exactly one CertificationRequestInfo data structure, as specified in PKCS#10 , but no certReqId. Therefore, the certReqId in the corresponding Certification Response (cp) message MUST be set to -1. Only one of the failInfo (in PKIStatusInfo) and certificate (in CertifiedKeyPair) fields can be present in each CertResponse (depending on the status). For some status values (e.g., waiting), neither of the optional fields will be present. Given an EncryptedCert and the relevant decryption key, the certificate may be obtained. The purpose of this is to allow a CA to return the value of a certificate, but with the constraint that only the intended recipient can obtain the actual certificate. The benefit of this approach is that a CA may reply with a certificate even in the absence of a proof that the requester is the end entity that can use the relevant private key (note that the proof is not obtained until the certConf message is received by the CA). Thus, the CA will not have to revoke that certificate in the event that something goes wrong with the proof-of-possession (but MAY do so anyway, depending upon policy). The use of EncryptedKey is described in . Note: To indicate support for EnvelopedData, the pvno cmp2021 has been introduced. Details on the usage of different protocol version numbers (pvno) are described in .

Key Update Request Content For key update requests the CertReqMessages syntax is used. Typically, SubjectPublicKeyInfo, KeyId, and Validity are the template fields that may be supplied for each key to be updated (see the profiles defined in Section 4.1.3 and for further information). This message is intended to be used to request updates to existing (non-revoked and non-expired) certificates (therefore, it is sometimes referred to as a "Certificate Update" operation). An update is a replacement certificate containing either a new subject public key or the current subject public key (although the latter practice may not be appropriate for some environments). See and for CertReqMessages syntax.

Key Update Response Content For key update responses, the CertRepMessage syntax is used. The response is identical to the initialization response. See for CertRepMessage syntax.

Key Recovery Request Content For key recovery requests the syntax used is identical to the initialization request CertReqMessages. Typically, SubjectPublicKeyInfo and KeyId are the template fields that may be used to supply a signature public key for which a certificate is required. See and for CertReqMessages syntax. Note that if a key history is required, the requester must supply a Protocol Encryption Key control in the request message.

Key Recovery Response Content For key recovery responses, the following syntax is used. For some status values (e.g., waiting) none of the optional fields will be present.

Revocation Request Content When requesting revocation of a certificate (or several certificates), the following data structure is used (see the profiles defined in Section 4.2 for further information). The name of the requester is present in the PKIHeader structure.

Revocation Response Content The revocation response is the response to the above message. If produced, this is sent to the requester of the revocation. (A separate revocation announcement message MAY be sent to the subject of the certificate for which revocation was requested.)

Cross Certification Request Content Cross certification requests use the same syntax (CertReqMessages) as normal certification requests, with the restriction that the key pair MUST have been generated by the requesting CA and the private key MUST NOT be sent to the responding CA (see the profiles defined in for further information). This request MAY also be used by subordinate CAs to get their certificates signed by the parent CA. See and for CertReqMessages syntax.

Cross Certification Response Content Cross certification responses use the same syntax (CertRepMessage) as normal certification responses, with the restriction that no encrypted private key can be sent. See for CertRepMessage syntax.

CA Key Update Announcement Content When a CA updates its own key pair, the following data structure MAY be used to announce this event. When using RootCaKeyUpdateContent in the ckuann message, the pvno cmp2021 MUST be used. Details on the usage of the protocol version number (pvno) are described in Section 7. In contrast to CAKeyUpdAnnContent as supported with cmp2000, RootCaKeyUpdateContent offers omitting newWithOld and oldWithNew, depending on the needs of the EE.

Certificate Announcement This structure MAY be used to announce the existence of certificates. Note that this message is intended to be used for those cases (if any) where there is no pre-existing method for publication of certificates; it is not intended to be used where, for example, X.500 is the method for publication of certificates.

Revocation Announcement When a CA has revoked, or is about to revoke, a particular certificate, it MAY issue an announcement of this (possibly upcoming) event. A CA MAY use such an announcement to warn (or notify) a subject that its certificate is about to be (or has been) revoked. This would typically be used where the request for revocation did not come from the subject concerned. The willBeRevokedAt field contains the time at which a new entry will be added to the relevant CRLs.

CRL Announcement When a CA issues a new CRL (or set of CRLs) the following data structure MAY be used to announce this event.

PKI Confirmation Content This data structure is used in the protocol exchange as the final PKIMessage. Its content is the same in all cases -- actually there is no content since the PKIHeader carries all the required information. Use of this message for certificate confirmation is NOT RECOMMENDED; certConf SHOULD be used instead. Upon receiving a PKIConfirm for a certificate response, the recipient MAY treat it as a certConf with all certificates being accepted.

Certificate Confirmation Content This data structure is used by the client to send a confirmation to the CA/RA to accept or reject certificates. The hashAlg field SHOULD be used only in exceptional cases where the signatureAlgorithm of the certificate to be confirmed does not specify a hash algorithm in the OID or in the parameters or no hash algorithm is specified for hashing certificates signed using the signatureAlgorithm. Note that for EdDSA a hash algorithm is specified in Section 3.3 of , such that the hashAlg field is not needed for EdDSA. Otherwise, the certHash value SHALL be computed using the same hash algorithm as used to create and verify the certificate signature or as specified for hashing certificates signed using the signatureAlgorithm. If hashAlg is used, the CMP version indicated by the certConf message header must be cmp2021(3). For any particular CertStatus, omission of the statusInfo field indicates acceptance of the specified certificate. Alternatively, explicit status details (with respect to acceptance or rejection) MAY be provided in the statusInfo field, perhaps for auditing purposes at the CA/RA. Within CertConfirmContent, omission of a CertStatus structure corresponding to a certificate supplied in the previous response message indicates rejection of the certificate. Thus, an empty CertConfirmContent (a zero-length SEQUENCE) MAY be used to indicate rejection of all supplied certificates. See , for a discussion of the certHash field with respect to proof-of-possession.

PKI General Message Content

CA Protocol Encryption Certificate This MAY be used by the EE to get a certificate from the CA to use to protect sensitive information during the protocol. GenRep: {id-it 1}, Certificate | < absent > ]]> EEs MUST ensure that the correct certificate is used for this purpose.

Signing Key Pair Types This MAY be used by the EE to get the list of signature algorithm whose subject public key values the CA is willing to certify. GenRep: {id-it 2}, SEQUENCE SIZE (1..MAX) OF AlgorithmIdentifier ]]> Note: For the purposes of this exchange, rsaEncryption and sha256WithRSAEncryption, for example, are considered to be equivalent; the question being asked is, "Is the CA willing to certify an RSA public key?" Note: In case several elliptic curves are supported, several id-ecPublicKey elements as defined in need to be given, one per named curve.

Encryption/Key Agreement Key Pair Types This MAY be used by the client to get the list of encryption/key agreement algorithms whose subject public key values the CA is willing to certify. GenRep: {id-it 3}, SEQUENCE SIZE (1..MAX) OF AlgorithmIdentifier ]]> Note: In case several elliptic curves are supported, several id-ecPublicKey elements as defined in need to be given, one per named curve.

Preferred Symmetric Algorithm This MAY be used by the client to get the CA-preferred symmetric encryption algorithm for any confidential information that needs to be exchanged between the EE and the CA (for example, if the EE wants to send its private decryption key to the CA for archival purposes). GenRep: {id-it 4}, AlgorithmIdentifier ]]>

Updated CA Key Pair This MAY be used by the CA to announce a CA key update event. See for details of CA key update announcements.

CRL This MAY be used by the client to get a copy of the latest CRL. GenRep: {id-it 6}, CertificateList ]]>

Unsupported Object Identifiers This is used by the server to return a list of object identifiers that it does not recognize or support from the list submitted by the client.

Key Pair Parameters This MAY be used by the EE to request the domain parameters to use for generating the key pair for certain public-key algorithms. It can be used, for example, to request the appropriate P, Q, and G to generate the DH/DSA key, or to request a set of well-known elliptic curves. ]]> An absent infoValue in the GenRep indicates that the algorithm specified in GenMsg is not supported. EEs MUST ensure that the parameters are acceptable to it and that the GenRep message is authenticated (to avoid substitution attacks).

Revocation Passphrase This MAY be used by the EE to send a passphrase to a CA/RA for the purpose of authenticating a later revocation request (in the case that the appropriate signing private key is no longer available to authenticate the request). See for further details on the use of this mechanism. ]]> The use of EncryptedKey is described in .

ImplicitConfirm See for the definition and use of {id-it 13}.

ConfirmWaitTime See for the definition and use of {id-it 14}.

Original PKIMessage See for the definition and use of {id-it 15}.

Supported Language Tags This MAY be used to determine the appropriate language tag to use in subsequent messages. The sender sends its list of supported languages (in order, most preferred to least); the receiver returns the one it wishes to use. (Note: each UTF8String MUST include a language tag.) If none of the offered tags are supported, an error MUST be returned.

CA Certificates This MAY be used by the client to get CA certificates. GenRep: {id-it 17}, SEQUENCE SIZE (1..MAX) OF CMPCertificate | < absent > ]]>

Root CA Update This MAY be used by the client to get an update of a root CA certificate, which is provided in the body of the request message. In contrast to the ckuann message, this approach follows the request/response model. The EE SHOULD reference its current trust anchor in RootCaCertValue in the request body, giving the root CA certificate if available. GenRep: {id-it 18}, RootCaKeyUpdateValue | < absent > ]]> Note: In contrast to CAKeyUpdAnnContent (which was deprecated with pvno cmp2021), RootCaKeyUpdateContent offers omitting newWithOld and oldWithNew, depending on the needs of the EE.

Certificate Request Template This MAY be used by the client to get a template containing requirements for certificate request attributes and extensions. The controls id-regCtrl-algId and id-regCtrl-rsaKeyLen MAY contain details on the types of subject public keys the CA is willing to certify. The id-regCtrl-algId control MAY be used to identify a cryptographic algorithm (see Section 4.1.2.7 of ) other than rsaEncryption. The algorithm field SHALL identify a cryptographic algorithm. The contents of the optional parameters field will vary according to the algorithm identified. For example, when the algorithm is set to id-ecPublicKey, the parameters identify the elliptic curve to be used; see . Note: The client may specify a profile name in the certProfile field, see . The id-regCtrl-rsaKeyLen control SHALL be used for algorithm rsaEncryption and SHALL contain the intended modulus bit length of the RSA key. GenRep: {id-it 19}, CertReqTemplateContent | < absent > ]]> The CertReqTemplateValue contains the prefilled certTemplate to be used for a future certificate request. The publicKey field in the certTemplate MUST NOT be used. In case the PKI management entity wishes to specify supported public-key algorithms, the keySpec field MUST be used. One AttributeTypeAndValue per supported algorithm or RSA key length MUST be used. Note: The controls ASN.1 type is defined in Section 6 of CRMF

CRL Update Retrieval This MAY be used by the client to get new CRLs, specifying the source of the CRLs and the thisUpdate value of the latest CRL it already has, if available. A CRL source is given either by a DistributionPointName or the GeneralNames of the issuing CA. The DistributionPointName should be treated as an internal pointer to identify a CRL that the server already has and not as a way to ask the server to fetch CRLs from external locations. The server SHALL only provide those CRLs that are more recent than the ones indicated by the client. ]]>

KEM Ciphertext This MAY be used by a PKI entity to get the KEM ciphertext for MAC-based message protection using KEM (see ). The PKI entity which possesses a KEM key pair can request the ciphertext by sending an InfoTypeAndValue structure of type KemCiphertextInfo where the infoValue is absent. The ciphertext can be provided in the following genp message with an InfoTypeAndValue structure of the same type. GenRep: {id-it TBD1}, KemCiphertextInfo ]]> kem is the algorithm identifier of the KEM algorithm, and any associated parameters, used to generate the ciphertext ct. ct is the ciphertext output from the KEM Encapsulate function. NOTE: These InfoTypeAndValue structures can also be transferred in the generalInfo field of the PKIHeader in messages of other types (see ).

PKI General Response Content Examples of GenReps that MAY be supported include those listed in the subsections of .

Error Message Content This data structure MAY be used by EE, CA, or RA to convey error information and by a PKI management entity to initiate delayed delivery of responses. This message MAY be generated at any time during a PKI transaction. If the client sends this request, the server MUST respond with a PKIConfirm response, or another ErrorMsg if any part of the header is not valid. In case a PKI management entity sends an error message to the EE with the pKIStatusInfo field containing the status "waiting", the EE SHOULD initiate polling as described in . If the EE does not initiate polling, both sides MUST treat this message as the end of the transaction (if a transaction is in progress). If protection is desired on the message, the client MUST protect it using the same technique (i.e., signature or MAC) as the starting message of the transaction. The CA MUST always sign it with a signature key.

Polling Request and Response This pair of messages is intended to handle scenarios in which the client needs to poll the server to determine the status of an outstanding response (i.e., when the "waiting" PKIStatus has been received). Unless implicit confirmation has been requested and granted, in response to an ir, cr, p10cr, kur, krr, or ccr request message, polling is initiated with an ip, cp, kup, krp, or ccp response message containing status "waiting". For any type of request message, polling can be initiated with an error response messages with status "waiting". The following clauses describe how polling messages are used. It is assumed that multiple certConf messages can be sent during transactions. There will be one sent in response to each ip, cp, kup, krp, or ccp that contains a CertStatus for an issued certificate.

In response to an ip, cp, kup, krp, or ccp message, an EE will send a certConf for all issued certificates and expect a PKIconf for each certConf. An EE will send a pollReq message in response to each CertResponse element of an ip, cp, or kup message with status "waiting" and in response to an error message with status "waiting". Its certReqId MUST be either the index of a CertResponse data structure with status "waiting" or -1 referring to the complete response.
In response to a pollReq, a CA/RA will return an ip, cp, kup, krp, or ccp if one or more of still pending requested certificates are ready or the final response to some other type of request is available; otherwise, it will return a pollRep.
If the EE receives a pollRep, it will wait for at least the number of seconds given in the checkAfter field before sending another pollReq.

[RFC-Editor: Please fix the indentation. This note belongs to 3.] Note that the checkAfter value heavily depends on the certificate management environment. There are different reasons for a delayed delivery of response messages possible, e.g., high load on the server's backend, offline transfer of messages between two PKI management entities, or required RA operator approval. Therefore, the checkAfter time can vary greatly. This should also be considered by the transfer protocol.

[RFC-Editor: Please fix the enumeration and continue with '4'.] If the EE receives an ip, cp, kup, krp, or ccp, then it will be treated in the same way as the initial response; if it receives any other response, then this will be treated as the final response to the original request.

The following client-side state machine describes polling for individual CertResponse elements at the example of an ir request message. |<------------------+ | | | | | | (issued) v (waiting) | | Add to <----------- Check CertResponse ------> Add to | conf list for each certificate pending list | / \ | / \ | (conf list) / \ (empty conf list) | / \ ip | / \ +-----------------+ (empty pending list) / \ | pollRep END <---- Send certConf Send pollReq---------->Wait | ^ ^ | | | | | +-----------------+ +---------------+ (pending list) ]]> In the following exchange, the end entity is enrolling for two certificates in one request. ir -> 3 handle ir 4 manual intervention is required for both certs 5 <- ip <- 6 process ip 7 format pollReq 8 -> pollReq -> 9 check status of cert requests, certificates not ready 10 format pollRep 11 <- pollRep <- 12 wait 13 format pollReq 14 -> pollReq -> 15 check status of cert requests, one certificate is ready 16 format ip 17 <- ip <- 18 handle ip 19 format certConf 20 -> certConf -> 21 handle certConf 22 format ack 23 <- pkiConf <- 24 format pollReq 25 -> pollReq -> 26 check status of certificate, certificate is ready 27 format ip 28 <- ip <- 29 handle ip 30 format certConf 31 -> certConf -> 32 handle certConf 33 format ack 34 <- pkiConf <- ]]> The following client-side state machine describes polling for a complete response message. Polling | | | | | | Send pollReq | | | Receive response | | | | | pollRep | other response | +-----------+------------------->+<-------------------+ | v Handle response | v End ]]> In the following exchange, the end entity is sending a general message request, and the response is delayed by the server. genm -> 3 handle genm 4 delay in response is necessary 5 format error message "waiting" with certReqId set to -1 6 <- error <- 7 process error 8 format pollReq 9 -> pollReq -> 10 check status of original request, general message response not ready 11 format pollRep 12 <- pollRep <- 13 wait 14 format pollReq 15 -> pollReq -> 16 check status of original request, general message response is ready 17 format genp 18 <- genp <- 19 handle genp ]]>

Mandatory PKI Management Functions Some of the PKI management functions outlined in are described in this section. This section deals with functions that are "mandatory" in the sense that all end entity and CA/RA implementations MUST be able to provide the functionality described. This part is effectively the profile of the PKI management functionality that MUST be supported. Note, however, that the management functions described in this section do not need to be accomplished using the PKI messages defined in if alternate means are suitable for a given environment. See Section 7 and for profiles of the PKIMessage structures that MUST be supported for specific use cases.

Root CA Initialization [See for this document's definition of "root CA".] A newly created root CA must produce a "self-certificate", which is a Certificate structure with the profile defined for the "newWithNew" certificate issued following a root CA key update. In order to make the CA's self certificate useful to end entities that do not acquire the self certificate via "out-of-band" means, the CA must also produce a fingerprint for its certificate. End entities that acquire this fingerprint securely via some "out-of-band" means can then verify the CA's self-certificate and, hence, the other attributes contained therein. The data structure used to carry the fingerprint may be the OOBCertHash, see .

Root CA Key Update CA keys (as all other keys) have a finite lifetime and will have to be updated on a periodic basis. The certificates NewWithNew, NewWithOld, and OldWithNew (see ) MAY be issued by the CA to aid existing end entities who hold the current self-signed CA certificate (OldWithOld) to transition securely to the new self-signed CA certificate (NewWithNew), and to aid new end entities who will hold NewWithNew to acquire OldWithOld securely for verification of existing data.

Subordinate CA Initialization [See for this document's definition of "subordinate CA".] From the perspective of PKI management protocols, the initialization of a subordinate CA is the same as the initialization of an end entity. The only difference is that the subordinate CA must also produce an initial revocation list.

CRL production Before issuing any certificates, a newly established CA (which issues CRLs) must produce "empty" versions of each CRL which are to be periodically produced.

PKI Information Request When a PKI entity (CA, RA, or EE) wishes to acquire information about the current status of a CA, it MAY send that CA a request for such information. The CA MUST respond to the request by providing (at least) all of the information requested by the requester. If some of the information cannot be provided, then an error must be conveyed to the requester. If PKIMessages are used to request and supply this PKI information, then the request MUST be the GenMsg message, the response MUST be the GenRep message, and the error MUST be the Error message. These messages are protected using a MAC based on shared secret information (e.g., password-based MAC, see CMP Algorithms Section 6.1) or using any asymmetric authentication means such as a signature (if the end entity has an existing certificate).

Cross Certification The requester CA is the CA that will become the subject of the cross-certificate; the responder CA will become the issuer of the cross-certificate. The requester CA must be "up and running" before initiating the cross-certification operation.

One-Way Request-Response Scheme: The cross-certification scheme is essentially a one way operation; that is, when successful, this operation results in the creation of one new cross-certificate. If the requirement is that cross-certificates be created in "both directions", then each CA, in turn, must initiate a cross-certification operation (or use another scheme). This scheme is suitable where the two CAs in question can already verify each other's signatures (they have some common points of trust) or where there is an out-of-band verification of the origin of the certification request. Detailed Description: Cross certification is initiated at one CA known as the responder. The CA administrator for the responder identifies the CA it wants to cross certify and the responder CA equipment generates an authorization code. The responder CA administrator passes this authorization code by out-of-band means to the requester CA administrator. The requester CA administrator enters the authorization code at the requester CA in order to initiate the on-line exchange. The authorization code is used for authentication and integrity purposes. This is done by generating a symmetric key based on the authorization code and using the symmetric key for generating Message Authentication Codes (MACs) on all messages exchanged. (Authentication may alternatively be done using signatures instead of MACs, if the CAs are able to retrieve and validate the required public keys by some means, such as an out-of-band hash comparison.) The requester CA initiates the exchange by generating a cross-certification request (ccr) with a fresh random number (requester random number). The requester CA then sends the ccr message to the responder CA. The fields in this message are protected from modification with a MAC based on the authorization code. Upon receipt of the ccr message, the responder CA validates the message and the MAC, saves the requester random number, and generates its own random number (responder random number). It then generates (and archives, if desired) a new requester certificate that contains the requester CA public key and is signed with the responder CA signature private key. The responder CA responds with the cross certification response (ccp) message. The fields in this message are protected from modification with a MAC based on the authorization code. Upon receipt of the ccp message, the requester CA validates the message (including the received random numbers) and the MAC. The requester CA responds with the certConf message. The fields in this message are protected from modification with a MAC based on the authorization code. The requester CA MAY write the requester certificate to the Repository as an aid to later certificate path construction. Upon receipt of the certConf message, the responder CA validates the message and the MAC, and sends back an acknowledgement using the PKIConfirm message. It MAY also publish the requester certificate as an aid to later path construction. Notes:

The ccr message must contain a "complete" certification request; that is, all fields except the serial number (including, e.g., a BasicConstraints extension) must be specified by the requester CA.
The ccp message SHOULD contain the verification certificate of the responder CA; if present, the requester CA must then verify this certificate (for example, via the "out-of-band" mechanism).

(A simpler, non-interactive model of cross-certification may also be envisioned, in which the issuing CA acquires the subject CA's public key from some repository, verifies it via some out-of-band mechanism, and creates and publishes the cross-certificate without the subject CA's explicit involvement. This model may be perfectly legitimate for many environments, but since it does not require any protocol message exchanges, its detailed description is outside the scope of this specification.)

End Entity Initialization As with CAs, end entities must be initialized. Initialization of end entities requires at least two steps:

acquisition of PKI information
out-of-band verification of one root-CA public key

(other possible steps include the retrieval of trust condition information and/or out-of-band verification of other CA public keys).

Acquisition of PKI Information The information REQUIRED is:

the current root-CA public key
(if the certifying CA is not a root-CA) the certification path from the root CA to the certifying CA together with appropriate revocation lists
the algorithms and algorithm parameters that the certifying CA supports for each relevant usage

Additional information could be required (e.g., supported extensions or CA policy information) in order to produce a certification request that will be successful. However, for simplicity we do not mandate that the end entity acquires this information via the PKI messages. The end result is simply that some certification requests may fail (e.g., if the end entity wants to generate its own encryption key, but the CA doesn't allow that). The required information MAY be acquired as described in .

Out-of-Band Verification of Root-CA Key An end entity must securely possess the public key of its root CA. One method to achieve this is to provide the end entity with the CA's self-certificate fingerprint via some secure "out-of-band" means. The end entity can then securely use the CA's self-certificate. See for further details.

Certificate Request An initialized end entity MAY request an additional certificate at any time (for any purpose). This request will be made using the certification request (cr) message. If the end entity already possesses a signing key pair (with a corresponding verification certificate), then this cr message will typically be protected by the entity's digital signature. The CA returns the new certificate (if the request is successful) in a CertRepMessage.

Key Update When a key pair is due to expire, the relevant end entity MAY request a key update; that is, it MAY request that the CA issue a new certificate for a new key pair (or, in certain circumstances, a new certificate for the same key pair). The request is made using a key update request (kur) message (referred to, in some environments, as a "Certificate Update" operation). If the end entity already possesses a signing key pair (with a corresponding verification certificate), then this message will typically be protected by the entity's digital signature. The CA returns the new certificate (if the request is successful) in a key update response (kup) message, which is syntactically identical to a CertRepMessage.

Version Negotiation This section defines the version negotiation used to support older protocols between client and servers. If a client knows the protocol version(s) supported by the server (e.g., from a previous PKIMessage exchange or via some out-of-band means), then it MUST send a PKIMessage with the highest version supported by both it and the server. If a client does not know what version(s) the server supports, then it MUST send a PKIMessage using the highest version it supports with the following exception. Version cmp2021 SHOULD only be used if cmp2021 syntax is needed for the request being sent or for the expected response. Note: Using cmp2000 as the default pvno is done to avoid extra message exchanges for version negotiation and to foster compatibility with cmp2000 implementations. Version cmp2021 syntax is only needed if a message exchange uses EnvelopedData, hashAlg (in CertStatus), POPOPrivKey with agreeMAC, or ckuann with RootCaKeyUpdateContent. If a server receives a message with a version that it supports, then the version of the response message MUST be the same as the received version. If a server receives a message with a version higher or lower than it supports, then it MUST send back an ErrorMsg with the unsupportedVersion bit set (in the failureInfo field of the pKIStatusInfo). If the received version is higher than the highest supported version, then the version in the error message MUST be the highest version the server supports; if the received version is lower than the lowest supported version then the version in the error message MUST be the lowest version the server supports. If a client gets back an ErrorMsgContent with the unsupportedVersion bit set and a version it supports, then it MAY retry the request with that version.

Supporting RFC 2510 Implementations RFC 2510 did not specify the behavior of implementations receiving versions they did not understand since there was only one version in existence. With the introduction of the revision in , the following versioning behaviour is recommended.

Clients Talking to RFC 2510 Servers If, after sending a message with a protocol version number higher than cmp1999, a client receives an ErrorMsgContent with a version of cmp1999, then it MUST abort the current transaction. If a client receives a non-error PKIMessage with a version of cmp1999, then it MAY decide to continue the transaction (if the transaction hasn't finished) using RFC 2510 semantics. If it does not choose to do so and the transaction is not finished, then it MUST abort the transaction and send an ErrorMsgContent with a version of cmp1999.

Servers Receiving Version cmp1999 PKIMessages If a server receives a version cmp1999 message it MAY revert to RFC 2510 behaviour and respond with version cmp1999 messages. If it does not choose to do so, then it MUST send back an ErrorMsgContent as described above in .

Security Considerations

On the Necessity of Proof-Of-Possession It is well established that the role of a Certification Authority is to verify that the name and public key belong to the end entity prior to issuing a certificate. If an entity holding a private key obtains a certificate containing the corresponding public key issued for a different entity, it can authenticate as the entity named in the certificate. This facilitates masquerading. It is not entirely clear what security guarantees are lost if an end entity is able to obtain a certificate containing a public key that they do not possess the corresponding private key for. There are some scenarios, described as "forwarding attacks" in Appendix A of , in which this can lead to protocol attacks against a naively-implemented sign-then-encrypt protocol, but in general it merely results in the end entity obtaining a certificate that they can not use.

Proof-Of-Possession with a Decryption Key Some cryptographic considerations are worth explicitly spelling out. In the protocols specified above, when an end entity is required to prove possession of a decryption key, it is effectively challenged to decrypt something (its own certificate). This scheme (and many others!) could be vulnerable to an attack if the possessor of the decryption key in question could be fooled into decrypting an arbitrary challenge and returning the cleartext to an attacker. Although in this specification a number of other failures in security are required in order for this attack to succeed, it is conceivable that some future services (e.g., notary, trusted time) could potentially be vulnerable to such attacks. For this reason, we reiterate the general rule that implementations should be very careful about decrypting arbitrary "ciphertext" and revealing recovered "plaintext" since such a practice can lead to serious security vulnerabilities. The client MUST return the decrypted values only if they match the expected content type. In an Indirect Method, the decrypted value MUST be a valid certificate, and in the Direct Method, the decrypted value MUST be a Rand as defined in .

Proof-Of-Possession by Exposing the Private Key Note also that exposing a private key to the CA/RA as a proof-of-possession technique can carry some security risks (depending upon whether or not the CA/RA can be trusted to handle such material appropriately). Implementers are advised to:

Exercise caution in selecting and using this particular POP mechanism.
Only use this POP mechanism if archival of the private key is desired.
When appropriate, have the user of the application explicitly state that they are willing to trust the CA/RA to have a copy of their private key before proceeding to reveal the private key.

Attack Against Diffie-Hellman Key Exchange A small subgroup attack during a Diffie-Hellman key exchange may be carried out as follows. A malicious end entity may deliberately choose D-H parameters that enable it to derive (a significant number of bits of) the D-H private key of the CA during a key archival or key recovery operation. Armed with this knowledge, the EE would then be able to retrieve the decryption private key of another unsuspecting end entity, EE2, during EE2's legitimate key archival or key recovery operation with that CA. In order to avoid the possibility of such an attack, two courses of action are available. (1) The CA may generate a fresh D-H key pair to be used as a protocol encryption key pair for each EE with which it interacts. (2) The CA may enter into a key validation protocol (not specified in this document) with each requesting end entity to ensure that the EE's protocol encryption key pair will not facilitate this attack. Option (1) is clearly simpler (requiring no extra protocol exchanges from either party) and is therefore RECOMMENDED.

Perfect Forward Secrecy Long-term security typically requires perfect forward secrecy (pfs). When transferring encrypted long-term confidential values such as centrally generated private keys or revocation passphrases, pfs likely is important. Yet it is not needed for CMP message protection providing integrity and authenticity because transfer of PKI messages is usually completed in very limited time. For the same reason it typically is not required for the indirect method of providing a POP delivering the newly issued certificate in encrypted form. Encrypted values are transferred using CMS EnvelopedData , which does not offer pfs. In cases where long-term security is needed, CMP messages SHOULD be transferred over a mechanism that provides pfs, such as TLS with appropriate cipher suites selected.

Private Keys for Certificate Signing and CMP Message Protection A CA should not reuse its certificate signing key for other purposes, such as protecting CMP responses and TLS connections. This way, exposure to other parts of the system and the number of uses of this particularly critical key are reduced to a minimum.

Entropy of Random Numbers, Key Pairs, and Shared Secret Information Implementations must generate nonces and private keys from random input. The use of inadequate pseudorandom number generators (PRNGs) to generate cryptographic keys can result in little or no security. An attacker may find it much easier to reproduce the PRNG environment that produced the keys and to search the resulting small set of possibilities than brute-force searching the whole key space. As an example of predictable random numbers, see ; consequences of low-entropy random numbers are discussed in Mining Your Ps and Qs. The generation of quality random numbers is difficult. ISO/IEC 20543:2019, NIST SP 800-90A Rev.1, BSI AIS 31 V2.0, and other specifications offer valuable guidance in this area. If shared secret information is generated by a cryptographically secure random number generator (CSRNG), it is safe to assume that the entropy of the shared secret information equals its bit length. If no CSRNG is used, the entropy of shared secret information depends on the details of the generation process and cannot be measured securely after it has been generated. If user-generated passwords are used as shared secret information, their entropy cannot be measured and are typically insufficient for protected delivery of centrally generated keys or trust anchors. If the entropy of shared secret information protecting the delivery of a centrally generated key pair is known, it should not be less than the security strength of that key pair; if the shared secret information is reused for different key pairs, the security of the shared secret information should exceed the security strength of each individual key pair. For the case of a PKI management operation that delivers a new trust anchor (e.g., a root CA certificate) using caPubs or genp that is (a) not concluded in a timely manner or (b) where the shared secret information is reused for several key management operations, the entropy of the shared secret information, if known, should not be less than the security strength of the trust anchor being managed by the operation. The shared secret information should have an entropy that at least matches the security strength of the key material being managed by the operation. Certain use cases may require shared secret information that may be of a low security strength, e.g., a human-generated password. It is RECOMMENDED that such secret information be limited to a single PKI management operation. Importantly for this section further information about algorithm use profiles and their security strength is available in CMP Algorithms Section 7.

Recurring Usage of KEM Keys for Message Protection For each PKI management operation using MAC-based message protection involving KEM, see , the KEM Encapsulate() function, providing a fresh KEM ciphertext (ct) and shared secret (ss), MUST be invoked. It is assumed that the overall data size of the CMP messages in a PKI management operation protected by a single shared secret key is small enough not to introduce extra security risks. To be appropriate for use with this specification, the KEM algorithm MUST explicitly be designed to be secure when the public key is used many times. For example, a KEM algorithm with a single-use public key is not appropriate because the public key is expected to be carried in a long-lived certificate and used over and over. Thus, KEM algorithms that offer indistinguishability under adaptive chosen ciphertext attack (IND-CCA2) security are appropriate. A common design pattern for obtaining IND-CCA2 security with public key reuse is to apply the Fujisaki-Okamoto (FO) transform or a variant of the FO transform . Therefore, given a long-term public key using an IND-CCA2 secure KEM algorithm, there is no limit to the number of CMP messages that can be authenticated using KEM keys for MAC-based message protection.

Trust Anchor Provisioning Using CMP Messages A provider of trust anchors, which may be an RA involved in configuration management of its clients, MUST NOT include to-be-trusted CA certificates in a CMP message unless the specific deployment scenario can ensure that it is adequate that the receiving EE trusts these certificates, e.g., by loading them into its trust store. Whenever an EE receives in a CMP message a CA certificate to be used as a trust anchor (for example in the caPubs field of a certificate response or in a general response), it MUST properly authenticate the message sender with existing trust anchors without requiring new trust anchor information included in the message. Additionally, the EE MUST verify that the sender is an authorized source of trust anchors. This authorization is governed by local policy and typically indicated using shared secret information or with a signature-based message protection using a certificate issued by a PKI that is explicitly authorized for this purpose.

Authorizing Requests for Certificates with Specific EKUs When a CA issues a certificate containing extended key usage extensions as defined in , this expresses delegation of an authorization that originally is only with the CA certificate itself. Such delegation is a very sensitive action in a PKI and therefore special care must be taken when approving such certificate requests to ensure that only legitimate entities receive a certificate containing such an EKU.

Usage of Certificate Transparency Logs CAs that support indirect POP MUST NOT also publish final certificates to Certificate Transparency logs before having received the certConf message containing the certHash of that certificate to complete the POP. The risk is that a malicious actor could fetch the final certificate from the CT log and use that to spoof a response to the implicit POP challenge via a certConf response. This risk does not apply to CT precertificates, so those are ok to publish. If a certificate or its precertificate was published in a CT log it must be revoked, if a required certConf message could not be verified, especially when the implicit POP was used.

IANA Considerations This document updates the ASN.1 modules of CMP Updates Appendix A.2 . The OID TBD2 (id-mod-cmp2023-02) was registered in the "SMI Security for PKIX Module Identifier" registry to identify the updated ASN.1 module. In the SMI-numbers registry "SMI Security for PKIX CMP Information Types (1.3.6.1.5.5.7.4)" (see https://www.iana.org/assignments/smi-numbers/smi-numbers.xhtml#smi-numbers-1.3.6.1.5.5.7.4) as defined in one addition has been performed. One new entry has been added: Decimal: TBD1 Description: id-it-KemCiphertextInfo Reference: [RFCXXXX] The new OID 1.2.840.113533.7.66.16 was registered by Entrust for id-KemBasedMac in the arch 1.2.840.113533.7.66. Entrust registered also the OIDs for id-PasswordBasedMac and id-DHBasedMac there. All existing references to , , and at https://www.iana.org/assignments/smi-numbers/smi-numbers.xhtml except those in the "SMI Security for PKIX Module Identifier" registry should be replaced with references to this document.

Acknowledgements The authors of this document wish to thank Carlisle Adams, Stephen Farrell, Tomi Kause, and Tero Mononen, the original authors of , for their work. We also thank all reviewers of this document for their valuable feedback. Adding KEM support to this document was partly funded by the German Federal Ministry of Education and Research in the project Quoryptan through grant number 16KIS2033.

References Normative References PKCS #9: Selected Object Classes and Attribute Types Version 2.0 This memo represents a republication of PKCS #9 v2.0 from RSA Laboratories' Public-Key Cryptography Standards (PKCS) series, and change control is retained within the PKCS process. The body of this document, except for the security considerations section, is taken directly from that specification. This memo provides information for the Internet community. PKCS #10: Certification Request Syntax Specification Version 1.7 This memo represents a republication of PKCS #10 v1.7 from RSA Laboratories' Public-Key Cryptography Standards (PKCS) series, and change control is retained within the PKCS process. The body of this document, except for the security considerations section, is taken directly from the PKCS #9 v2.0 or the PKCS #10 v1.7 document. This memo provides information for the Internet community. UTF-8, a transformation format of ISO 10646 ISO/IEC 10646-1 defines a large character set called the Universal Character Set (UCS) which encompasses most of the world's writing systems. The originally proposed encodings of the UCS, however, were not compatible with many current applications and protocols, and this has led to the development of UTF-8, the object of this memo. UTF-8 has the characteristic of preserving the full US-ASCII range, providing compatibility with file systems, parsers and other software that rely on US-ASCII values but are transparent to other values. This memo obsoletes and replaces RFC 2279. Internet X.509 Public Key Infrastructure Certificate Request Message Format (CRMF) This document describes the Certificate Request Message Format (CRMF) syntax and semantics. This syntax is used to convey a request for a certificate to a Certification Authority (CA), possibly via a Registration Authority (RA), for the purposes of X.509 certificate production. The request will typically include a public key and the associated registration information. This document does not define a certificate request protocol. [STANDARDS-TRACK] Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile This memo profiles the X.509 v3 certificate and X.509 v2 certificate revocation list (CRL) for use in the Internet. An overview of this approach and model is provided as an introduction. The X.509 v3 certificate format is described in detail, with additional information regarding the format and semantics of Internet name forms. Standard certificate extensions are described and two Internet-specific extensions are defined. A set of required certificate extensions is specified. The X.509 v2 CRL format is described in detail along with standard and Internet-specific extensions. An algorithm for X.509 certification path validation is described. An ASN.1 module and examples are provided in the appendices. [STANDARDS-TRACK] Elliptic Curve Cryptography Subject Public Key Information This document specifies the syntax and semantics for the Subject Public Key Information field in certificates that support Elliptic Curve Cryptography. This document updates Sections 2.3.5 and 5, and the ASN.1 module of "Algorithms and Identifiers for the Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 3279. [STANDARDS-TRACK] Cryptographic Message Syntax (CMS) This document describes the Cryptographic Message Syntax (CMS). This syntax is used to digitally sign, digest, authenticate, or encrypt arbitrary message content. [STANDARDS-TRACK] Asymmetric Key Packages This document defines the syntax for private-key information and a content type for it. Private-key information includes a private key for a specified public-key algorithm and a set of attributes. The Cryptographic Message Syntax (CMS), as defined in RFC 5652, can be used to digitally sign, digest, authenticate, or encrypt the asymmetric key format content type. This document obsoletes RFC 5208. [STANDARDS-TRACK] Certificate Management over CMS (CMC) Updates This document contains a set of updates to the base syntax for CMC, a Certificate Management protocol using the Cryptographic Message Syntax (CMS). This document updates RFC 5272, RFC 5273, and RFC 5274. The new items in this document are: new controls for future work in doing server side key generation, definition of a Subject Information Access value to identify CMC servers, and the registration of a port number for TCP/IP for the CMC service to run on. [STANDARDS-TRACK] Update to the Cryptographic Message Syntax (CMS) for Algorithm Identifier Protection This document updates the Cryptographic Message Syntax (CMS) specified in RFC 5652 to ensure that algorithm identifiers in signed-data and authenticated-data content types are adequately protected. Algorithm Requirements Update to the Internet X.509 Public Key Infrastructure Certificate Request Message Format (CRMF) This document updates the cryptographic algorithm requirements for the Password-Based Message Authentication Code in the Internet X.509 Public Key Infrastructure Certificate Request Message Format (CRMF) specified in RFC 4211. Certificate Management Protocol (CMP) Algorithms This document describes the conventions for using several cryptographic algorithms with the Certificate Management Protocol (CMP). CMP is used to enroll and further manage the lifecycle of X.509 certificates. This document also updates the algorithm use profile from Appendix D.2 of RFC 4210. Using Key Encapsulation Mechanism (KEM) Algorithms in the Cryptographic Message Syntax (CMS) The Cryptographic Message Syntax (CMS) supports key transport and key agreement algorithms. In recent years, cryptographers have been specifying Key Encapsulation Mechanism (KEM) algorithms, including quantum-secure KEM algorithms. This document defines conventions for the use of KEM algorithms by the originator and recipients to encrypt and decrypt CMS content. This document updates RFC 5652. Handbook of Applied Cryptography Key words for use in RFCs to Indicate Requirement Levels In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Informative References Certificate Management Protocol (CMP) Updates This document contains a set of updates to the syntax of Certificate Management Protocol (CMP) version 2 and its HTTP transfer mechanism. This document updates RFCs 4210, 5912, and 6712. The aspects of CMP updated in this document are using EnvelopedData instead of EncryptedValue, clarifying the handling of p10cr messages, improving the crypto agility, as well as adding new general message types, extended key usages to identify certificates for use with CMP, and well-known URI path segments. CMP version 3 is introduced to enable signaling support of EnvelopedData instead of EncryptedValue and signal the use of an explicit hash AlgorithmIdentifier in certConf messages, as far as needed. Constrained Application Protocol (CoAP) Transfer for the Certificate Management Protocol This document specifies the use of the Constrained Application Protocol (CoAP) as a transfer mechanism for the Certificate Management Protocol (CMP). CMP defines the interaction between various PKI entities for the purpose of certificate creation and management. CoAP is an HTTP-like client-server protocol used by various constrained devices in the Internet of Things space. Lightweight Certificate Management Protocol (CMP) Profile This document aims at simple, interoperable, and automated PKI management operations covering typical use cases of industrial and Internet of Things (IoT) scenarios. This is achieved by profiling the Certificate Management Protocol (CMP), the related Certificate Request Message Format (CRMF), and transfer based on HTTP or Constrained Application Protocol (CoAP) in a succinct but sufficiently detailed and self-contained way. To make secure certificate management for simple scenarios and constrained devices as lightweight as possible, only the most crucial types of operations and options are specified as mandatory. More specialized or complex use cases are supported with optional features. Internet X.509 Public Key Infrastructure -- HTTP Transfer for the Certificate Management Protocol (CMP) Siemens Siemens Entrust Entrust This document describes how to layer the Certificate Management Protocol (CMP) over HTTP. It includes the updates on RFC 6712 specified in CMP Updates RFC 9480 Section 3 and obsoleted both documents. These updates introduce CMP URIs using a Well-known prefix. Security Multiparts for MIME: Multipart/Signed and Multipart/Encrypted This document defines a framework within which security services may be applied to MIME body parts. [STANDARDS-TRACK] This memo defines a new Simple Mail Transfer Protocol (SMTP) [1] reply code, 521, which one may use to indicate that an Internet host does not accept incoming mail. This memo defines an Experimental Protocol for the Internet community. This memo defines an extension to the SMTP service whereby an interrupted SMTP transaction can be restarted at a later time without having to repeat all of the commands and message content sent prior to the interruption. This memo defines an Experimental Protocol for the Internet community. Internet X.509 Public Key Infrastructure Certificate Management Protocols This document describes the Internet X.509 Public Key Infrastructure (PKI) Certificate Management Protocols. [STANDARDS-TRACK] Internet X.509 Public Key Infrastructure Operational Protocols: FTP and HTTP The protocol conventions described in this document satisfy some of the operational requirements of the Internet Public Key Infrastructure (PKI). This document specifies the conventions for using the File Transfer Protocol (FTP) and the Hypertext Transfer Protocol (HTTP) to obtain certificates and certificate revocation lists (CRLs) from PKI repositories. [STANDARDS-TRACK] Internet X.509 Public Key Infrastructure Certificate Management Protocol (CMP) This document describes the Internet X.509 Public Key Infrastructure (PKI) Certificate Management Protocol (CMP). Protocol messages are defined for X.509v3 certificate creation and management. CMP provides on-line interactions between PKI components, including an exchange between a Certification Authority (CA) and a client system. [STANDARDS-TRACK] Alternative Certificate Formats for the Public-Key Infrastructure Using X.509 (PKIX) Certificate Management Protocols The Public-Key Infrastructure using X.509 (PKIX) Working Group of the Internet Engineering Task Force (IETF) has defined a number of certificate management protocols. These protocols are primarily focused on X.509v3 public-key certificates. However, it is sometimes desirable to manage certificates in alternative formats as well. This document specifies how such certificates may be requested using the Certificate Request Message Format (CRMF) syntax that is used by several different protocols. It also explains how alternative certificate formats may be incorporated into such popular protocols as PKIX Certificate Management Protocol (PKIX-CMP) and Certificate Management Messages over CMS (CMC). This memo provides information for the Internet community. Lightweight Directory Access Protocol (LDAP): The Protocol This document describes the protocol elements, along with their semantics and encodings, of the Lightweight Directory Access Protocol (LDAP). LDAP provides access to distributed directory services that act in accordance with X.500 data and service models. These protocol elements are based on those described in the X.500 Directory Access Protocol (DAP). [STANDARDS-TRACK] New ASN.1 Modules for the Public Key Infrastructure Using X.509 (PKIX) The Public Key Infrastructure using X.509 (PKIX) certificate format, and many associated formats, are expressed using ASN.1. The current ASN.1 modules conform to the 1988 version of ASN.1. This document updates those ASN.1 modules to conform to the 2002 version of ASN.1. There are no bits-on-the-wire changes to any of the formats; this is simply a change to the syntax. This document is not an Internet Standards Track specification; it is published for informational purposes. Additional New ASN.1 Modules for the Cryptographic Message Syntax (CMS) and the Public Key Infrastructure Using X.509 (PKIX) The Cryptographic Message Syntax (CMS) format, and many associated formats, are expressed using ASN.1. The current ASN.1 modules conform to the 1988 version of ASN.1. This document updates some auxiliary ASN.1 modules to conform to the 2008 version of ASN.1; the 1988 ASN.1 modules remain the normative version. There are no bits- on-the-wire changes to any of the formats; this is simply a change to the syntax. This document is not an Internet Standards Track specification; it is published for informational purposes. Internet X.509 Public Key Infrastructure -- HTTP Transfer for the Certificate Management Protocol (CMP) This document describes how to layer the Certificate Management Protocol (CMP) over HTTP. It is the "CMPtrans" document referenced in RFC 4210; therefore, this document updates the reference given therein. [STANDARDS-TRACK] Internet Key Exchange Protocol Version 2 (IKEv2) This document describes version 2 of the Internet Key Exchange (IKE) protocol. IKE is a component of IPsec used for performing mutual authentication and establishing and maintaining Security Associations (SAs). This document obsoletes RFC 5996, and includes all of the errata for it. It advances IKEv2 to be an Internet Standard. Object Identifier Registry for the PKIX Working Group When the Public-Key Infrastructure using X.509 (PKIX) Working Group was chartered, an object identifier arc was allocated by IANA for use by that working group. This document describes the object identifiers that were assigned in that arc, returns control of that arc to IANA, and establishes IANA allocation policies for any future assignments within that arc. The Transport Layer Security (TLS) Protocol Version 1.3 This document specifies version 1.3 of the Transport Layer Security (TLS) protocol. TLS allows client/server applications to communicate over the Internet in a way that is designed to prevent eavesdropping, tampering, and message forgery. This document updates RFCs 5705 and 6066, and obsoletes RFCs 5077, 5246, and 6961. This document also specifies new requirements for TLS 1.2 implementations. Secure Zero Touch Provisioning (SZTP) This document presents a technique to securely provision a networking device when it is booting in a factory-default state. Variations in the solution enable it to be used on both public and private networks. The provisioning steps are able to update the boot image, commit an initial configuration, and execute arbitrary scripts to address auxiliary needs. The updated device is subsequently able to establish secure connections with other systems. For instance, a device may establish NETCONF (RFC 6241) and/or RESTCONF (RFC 8040) connections with deployment-specific network management systems. Hash Of Root Key Certificate Extension This document specifies the Hash Of Root Key certificate extension. This certificate extension is carried in the self-signed certificate for a trust anchor, which is often called a Root Certification Authority (CA) certificate. This certificate extension unambiguously identifies the next public key that will be used at some point in the future as the next Root CA certificate, eventually replacing the current one. Bootstrapping Remote Secure Key Infrastructure (BRSKI) This document specifies automated bootstrapping of an Autonomic Control Plane. To do this, a Secure Key Infrastructure is bootstrapped. This is done using manufacturer-installed X.509 certificates, in combination with a manufacturer's authorizing service, both online and offline. We call this process the Bootstrapping Remote Secure Key Infrastructure (BRSKI) protocol. Bootstrapping a new device can occur when using a routable address and a cloud service, only link-local connectivity, or limited/disconnected networks. Support for deployment models with less stringent security requirements is included. Bootstrapping is complete when the cryptographic identity of the new key infrastructure is successfully deployed to the device. The established secure connection can be used to deploy a locally issued certificate to the device as well. The Datagram Transport Layer Security (DTLS) Protocol Version 1.3 This document specifies version 1.3 of the Datagram Transport Layer Security (DTLS) protocol. DTLS 1.3 allows client/server applications to communicate over the Internet in a way that is designed to prevent eavesdropping, tampering, and message forgery. The DTLS 1.3 protocol is based on the Transport Layer Security (TLS) 1.3 protocol and provides equivalent security guarantees with the exception of order protection / non-replayability. Datagram semantics of the underlying transport are preserved by the DTLS protocol. This document obsoletes RFC 6347. Certificate Transparency Version 2.0 This document describes version 2.0 of the Certificate Transparency (CT) protocol for publicly logging the existence of Transport Layer Security (TLS) server certificates as they are issued or observed, in a manner that allows anyone to audit certification authority (CA) activity and notice the issuance of suspect certificates as well as to audit the certificate logs themselves. The intent is that eventually clients would refuse to honor certificates that do not appear in a log, effectively forcing CAs to add all issued certificates to the logs. This document obsoletes RFC 6962. It also specifies a new TLS extension that is used to send various CT log artifacts. Logs are network services that implement the protocol operations for submissions and queries that are defined in this document. BRSKI-AE: Alternative Enrollment Protocols in BRSKI Siemens AG Siemens AG Siemens AG This document defines enhancements to the Bootstrapping Remote Secure Key Infrastructure (BRSKI) protocol, known as BRSKI-AE (Alternative Enrollment). BRSKI-AE extends BRSKI to support certificate enrollment mechanisms instead of the originally specified use of EST. It supports certificate enrollment protocols, such as CMP, that use authenticated self-contained signed objects for certification messages, allowing for flexibility in network device onboarding scenarios. The enhancements address use cases where the existing enrollment mechanism may not be feasible or optimal, providing a framework for integrating suitable alternative enrollment protocols. This document also updates the BRSKI reference architecture to accommodate these alternative methods, ensuring secure and scalable deployment across a range of network environments. Internet X.509 Public Key Infrastructure - Algorithm Identifiers for the Module-Lattice-Based Key-Encapsulation Mechanism (ML-KEM) sn3rd AWS AWS Cloudflare The Module-Lattice-Based Key-Encapsulation Mechanism (ML-KEM) is a quantum-resistant key-encapsulation mechanism (KEM). This document describes the conventions for using the ML-KEM in X.509 Public Key Infrastructure. The conventions for the subject public keys and private keys are also described. Recommendation for Random Number Generation Using Deterministic Random Bit Generators Information Technology Laboratory Information Technology Laboratory National Institute of Standards and Technology

US Gaithersburg

IEEE Standard for Local and Metropolitan Area Networks - Secure Device Identity IEEE National Vulnerability Database - CVE-2008-0166 National Institute of Science and Technology (NIST) Mining Your Ps and Qs: Detection of Widespread Weak Keys in Network Devices Security'12: Proceedings of the 21st USENIX conference on Security symposium UC San Diego University of Michigan University of Michigan University of Michigan Information technology -- Security techniques -- Test and analysis methods for random bit generators within ISO/IEC 19790 and ISO/IEC 15408 International Organization for Standardization (ISO) A proposal for: Functionality classes for random number generators, version 2.0 Federal Office for Information Security (BSI) T-Systems GEI GmbH Federal Office for Information Security (BSI) Proof-of-possession for KEM certificates using verifiable generation Secure Integration of Asymmetric and Symmetric Encryption Schemes Springer Science and Business Media LLC A Modular Analysis of the Fujisaki-Okamoto Transformation Springer International Publishing Network Domain Security (NDS); Authentication Framework (AF) 3GPP ERTMS/ETCS On-line Key Management FFFIS UNISIG

Reasons for the Presence of RAs The reasons that justify the presence of an RA can be split into those that are due to technical factors and those which are organizational in nature. Technical reasons include the following.

If hardware tokens are in use, then not all end entities will have the equipment needed to initialize these; the RA equipment can include the necessary functionality (this may also be a matter of policy).
Some end entities may not have the capability to publish certificates; again, the RA may be suitably placed for this.
The RA will be able to issue signed revocation requests on behalf of end entities associated with it, whereas the end entity may not be able to do this (if the key pair is completely lost).

Some of the organizational reasons that argue for the presence of an RA are the following.

It may be more cost effective to concentrate functionality in the RA equipment than to supply functionality to all end entities (especially if special token initialization equipment is to be used).
Establishing RAs within an organization can reduce the number of CAs required, which is sometimes desirable.
RAs may be better placed to identify people with their "electronic" names, especially if the CA is physically remote from the end entity.
For many applications, there will already be in place some administrative structure so that candidates for the role of RA are easy to find (which may not be true of the CA).

Further reasons relevant for automated machine-to-machine certificate lifecycle management are available in the Lightweight CMP Profile .

The Use of Revocation Passphrase A revocation request must incorporate suitable security mechanisms, including proper authentication, in order to reduce the probability of successful denial-of-service attacks. A digital signature or DH/KEM-based message protection on the request -- REQUIRED to support within this specification depending on the key type used if revocation requests are supported -- can provide the authentication required, but there are circumstances under which an alternative mechanism may be desirable (e.g., when the private key is no longer accessible and the entity wishes to request a revocation prior to re-certification of another key pair). In order to accommodate such circumstances, a password-based MAC, see CMP Algorithms Section 6.1, on the request is also REQUIRED to support within this specification (subject to local security policy for a given environment) if revocation requests are supported and if shared secret information can be established between the requester and the responder prior to the need for revocation. A mechanism that has seen use in some environments is "revocation passphrase", in which a value of sufficient entropy (i.e., a relatively long passphrase rather than a short password) is shared between (only) the entity and the CA/RA at some point prior to revocation; this value is later used to authenticate the revocation request. In this specification, the following technique to establish shared secret information (i.e., a revocation passphrase) is OPTIONAL to support. Its precise use in CMP messages is as follows.

The OID and value specified in MAY be sent in a GenMsg message at any time or MAY be sent in the generalInfo field of the PKIHeader of any PKIMessage at any time. (In particular, the EncryptedKey structure as described in may be sent in the header of the certConf message that confirms acceptance of certificates requested in an initialization request or certificate request message.) This conveys a revocation passphrase chosen by the entity to the relevant CA/RA. When EnvelopedData is used, this is in the decrypted bytes of encryptedContent field. When EncryptedValue is used, this is in the decrypted bytes of the encValue field. Furthermore, the transfer is accomplished with appropriate confidentiality characteristics.
If a CA/RA receives the revocation passphrase (OID and value specified in ) in a GenMsg, it MUST construct and send a GenRep message that includes the OID (with absent value) specified in . If the CA/RA receives the revocation passphrase in the generalInfo field of a PKIHeader of any PKIMessage, it MUST include the OID (with absent value) in the generalInfo field of the PKIHeader of the corresponding response PKIMessage. If the CA/RA is unable to return the appropriate response message for any reason, it MUST send an error message with a status of "rejection" and, optionally, a failInfo reason set.
Either the localKeyId attribute of EnvelopedData as specified in or the valueHint field of EncryptedValue MAY contain a key identifier (chosen by the entity, along with the passphrase itself) to assist in later retrieval of the correct passphrase (e.g., when the revocation request is constructed by the end entity and received by the CA/RA).
The revocation request message is protected by a password-based MAC, see CMP Algorithms Section 6.1, with the revocation passphrase as the key. If appropriate, the senderKID field in the PKIHeader MAY contain the value previously transmitted in localKeyId or valueHint.

Note: For a message transferring a revocation passphrase indicating cmp2021(3) in the pvno field of the PKIHeader, the encrypted passphrase MUST be transferred in the envelopedData choice of EncryptedKey as defined in Section 5.2.2. When using cmp2000(2) in the message header for backward compatibility, the encryptedValue is used. This allows the necessary conveyance and protection of the passphrase while maintaining bits-on-the-wire compatibility with . The encryaptedValue choice has been deprecated in favor of encryptedData. Using the technique specified above, the revocation passphrase may be initially established and updated at any time without requiring extra messages or out-of-band exchanges. For example, the revocation request message itself (protected and authenticated through a MAC that uses the revocation passphrase as a key) may contain, in the PKIHeader, a new revocation passphrase to be used for authenticating future revocation requests for any of the entity's other certificates. In some environments this may be preferable to mechanisms that reveal the passphrase in the revocation request message, since this can allow a denial-of-service attack in which the revealed passphrase is used by an unauthorized third party to authenticate revocation requests on the entity's other certificates. However, because the passphrase is not revealed in the request message, there is no requirement that the passphrase must always be updated when a revocation request is made (that is, the same passphrase MAY be used by an entity to authenticate revocation requests for different certificates at different times). Furthermore, the above technique can provide strong cryptographic protection over the entire revocation request message even when a digital signature is not used. Techniques that do authentication of the revocation request by simply revealing the revocation passphrase typically do not provide cryptographic protection over the fields of the request message (so that a request for revocation of one certificate may be modified by an unauthorized third party to a request for revocation of another certificate for that entity).

PKI Management Message Profiles (REQUIRED) This appendix contains detailed profiles for those PKIMessages that MUST be supported by conforming implementations (see ). Note: and focus on PKI management operations managing certificates for human end entities. In contrast, the Lightweight CMP Profile focuses on typical use cases of industrial and IoT scenarios supporting highly automated certificate lifecycle management scenarios. Profiles for the PKIMessages used in the following PKI management operations are provided:

initial registration/certification
basic authenticated scheme
certificate request
key update

General Rules for Interpretation of These Profiles

Where OPTIONAL or DEFAULT fields are not mentioned in individual profiles, they SHOULD be absent from the relevant message (i.e., a receiver can validly reject a message containing such fields as being syntactically incorrect). Mandatory fields are not mentioned if they have an obvious value. The pvno MUST be set as specified in ).
Where structures occur in more than one message, they are separately profiled as appropriate.
The algorithmIdentifiers from PKIMessage structures are profiled separately.
A "special" X.500 DN is called the "NULL-DN"; this means a DN containing a zero-length SEQUENCE OF RelativeDistinguishedNames (its DER encoding is then '3000'H).
Where a GeneralName is required for a field, but no suitable value is available (e.g., an end entity produces a request before knowing its name), then the GeneralName is to be an X.500 NULL-DN (i.e., the Name field of the CHOICE is to contain a NULL-DN).
Where a profile omits to specify the value for a GeneralName, then the NULL-DN value is to be present in the relevant PKIMessage field. This occurs with the sender field of the PKIHeader for some messages.
Where any ambiguity arises due to naming of fields, the profile names these using a "dot" notation (e.g., "certTemplate.subject" means the subject field within a field called certTemplate).
Where a "SEQUENCE OF types" is part of a message, a zero-based array notation is used to describe fields within the SEQUENCE OF (e.g., crm[0].certReq.certTemplate.subject refers to a subfield of the first CertReqMsg contained in a request message).
All PKI message exchanges in to require a certConf message to be sent by the initiating entity and a PKIConfirm to be sent by the responding entity. The PKIConfirm is not included in some of the profiles given since its body is NULL and its header contents are clear from the context. Any authenticated means can be used for the protectionAlg (e.g., password-based MAC, if shared secret information is known, or signature).

Algorithm Use Profile For specifications of algorithm identifiers and respective conventions for conforming implementations, please refer to Section 7.1 of CMP Algorithms .

Proof-of-Possession Profile POP fields for use (in signature field of pop field of ProofOfPossession structure) when proving possession of a private signing key that corresponds to a public verification key for which a certificate has been requested. Note: For examples of MSG_SIG_ALG OIDs see CMP Algorithms Section 3 . Proof-of-possession of a private decryption key that corresponds to a public encryption key for which a certificate has been requested does not use this profile; the CertHash field of the certConf message is used instead. Not every CA/RA will do Proof-of-Possession (of signing key, decryption key, or key agreement key) in the PKIX-CMP in-band certification request protocol (how POP is done MAY ultimately be a policy issue that is made explicit for any given CA in its publicized Policy OID and Certification Practice Statement). However, this specification mandates that CA/RA entities MUST do POP (by some means) as part of the certification process. All end entities MUST be prepared to provide POP (i.e., these components of the PKIX-CMP protocol MUST be supported).

Initial Registration/Certification (Basic Authenticated Scheme) An (uninitialized) end entity requests a (first) certificate from a CA. When the CA responds with a message containing a certificate, the end entity replies with a certificate confirmation. The CA sends a PKIConfirm back, closing the transaction. All messages are authenticated. This scheme allows the end entity to request certification of a locally-generated public key (typically a signature key). The end entity MAY also choose to request the centralized generation and certification of another key pair (typically an encryption key pair). Certification may only be requested for one locally generated public key (for more, use separate PKIMessages). The end entity MUST support proof-of-possession of the private key associated with the locally-generated public key. Preconditions:

The end entity can authenticate the CA's signature based on out-of-band means
The end entity and the CA share a symmetric MACing key

Message flow: ir -> 3 handle ir 4 format ip 5 <- ip <- 6 handle ip 7 format certConf 8 -> certConf -> 9 handle certConf 10 format PKIConf 11 <- PKIConf <- 12 handle PKIConf ]]> For this profile, we mandate that the end entity MUST include all (i.e., one or two) CertReqMsg in a single PKIMessage, and that the PKI (CA) MUST produce a single response PKIMessage that contains the complete response (i.e., including the OPTIONAL second key pair, if it was requested and if centralized key generation is supported). For simplicity, we also mandate that this message MUST be the final one (i.e., no use of "waiting" status value). The end entity has an out-of-band interaction with the CA/RA. This transaction established the shared secret, the referenceNumber and OPTIONALLY the distinguished name used for both sender and subject name in the certificate template. See for security considerations on quality of shared secret information. Initialization Request -- ir Initialization Response -- ip Certificate confirm -- certConf Confirmation -- PKIConf

Certificate Request An (initialized) end entity requests a certificate from a CA (for any reason). When the CA responds with a message containing a certificate, the end entity replies with a certificate confirmation. The CA replies with a PKIConfirm, to close the transaction. All messages are authenticated. The profile for this exchange is identical to that given in , with the following exceptions:

sender name SHOULD be present
protectionAlg of MSG_SIG_ALG MUST be supported (MSG_MAC_ALG MAY also be supported) in request, response, certConfirm, and PKIConfirm messages;
senderKID and recipKID are only present if required for message verification;
body is cr or cp;
body may contain one or two CertReqMsg structures, but either CertReqMsg may be used to request certification of a locally-generated public key or a centrally-generated public key (i.e., the position-dependence requirement of is removed);
protection bits are calculated according to the protectionAlg field.

Key Update Request An (initialized) end entity requests a certificate from a CA (to update the key pair and/or corresponding certificate that it already possesses). When the CA responds with a message containing a certificate, the end entity replies with a certificate confirmation. The CA replies with a PKIConfirm, to close the transaction. All messages are authenticated. The profile for this exchange is identical to that given in , with the following exceptions:

sender name SHOULD be present
protectionAlg of MSG_SIG_ALG MUST be supported (MSG_MAC_ALG MAY also be supported) in request, response, certConfirm, and PKIConfirm messages;
senderKID and recipKID are only present if required for message verification;
body is kur or kup;
body may contain one or two CertReqMsg structures, but either CertReqMsg may be used to request certification of a locally-generated public key or a centrally-generated public key (i.e.,the position-dependence requirement of is removed);
protection bits are calculated according to the protectionAlg field;
regCtrl OldCertId SHOULD be used (unless it is clear to both sender and receiver -- by means not specified in this document -- that it is not needed).

PKI Management Message Profiles (OPTIONAL) This appendix contains detailed profiles for those PKIMessages that MAY be supported by implementations. Profiles for the PKIMessages used in the following PKI management operations are provided:

root CA key update
information request/response
cross-certification request/response (1-way)
in-band initialization using external identity certificate

Later versions of this document may extend the above to include profiles for the operations listed below (along with other operations, if desired).

revocation request
certificate publication
CRL publication

General Rules for Interpretation of These Profiles. Identical to .

Algorithm Use Profile Identical to .

Self-Signed Certificates Profile of how a Certificate structure may be "self-signed". These structures are used for distribution of CA public keys. This can occur in one of three ways (see above for a description of the use of these structures): Such certificates (including relevant extensions) must contain "sensible" values for all fields. For example, when present, subjectAltName MUST be identical to issuerAltName, and, when present, keyIdentifiers must contain appropriate values, et cetera.

Root CA Key Update A root CA updates its key pair. It then produces a CA key update announcement message that can be made available (via some transport mechanism) to the relevant end entities. A confirmation message is not required from the end entities. ckuann message:

PKI Information Request/Response The end entity sends a general message to the PKI requesting details that will be required for later PKI management operations. RA/CA responds with a general response. If an RA generates the response, then it will simply forward the equivalent message that it previously received from the CA, with the possible addition of certificates to the extraCerts fields of the PKIMessage. A confirmation message is not required from the end entity. Message Flows: genm -> 3 handle genm 4 produce genp 5 <- genp <- 6 handle genp ]]> genM: genP:

Cross Certification Request/Response (1-way) Creation of a single cross-certificate (i.e., not two at once). The requesting CA MAY choose who is responsible for publication of the cross-certificate created by the responding CA through use of the PKIPublicationInfo control. Preconditions:

Responding CA can verify the origin of the request (possibly requiring out-of-band means) before processing the request.
Requesting CA can authenticate the authenticity of the origin of the response (possibly requiring out-of-band means) before processing the response

The use of certificate confirmation and the corresponding server confirmation is determined by the generalInfo field in the PKIHeader (see ). The following profile does not mandate support for either confirmation. Message Flows: ccr -> 3 handle ccr 4 produce ccp 5 <- ccp <- 6 handle ccp ]]> ccr: ccp:

In-Band Initialization Using External Identity Certificate An (uninitialized) end entity wishes to initialize into the PKI with a CA, CA-1. It uses, for authentication purposes, a pre-existing identity certificate issued by another (external) CA, CA-X. A trust relationship must already have been established between CA-1 and CA-X so that CA-1 can validate the EE identity certificate signed by CA-X. Furthermore, some mechanism must already have been established within the Personal Security Environment (PSE) of the EE that would allow it to authenticate and verify PKIMessages signed by CA-1 (as one example, the PSE may contain a certificate issued for the public key of CA-1, signed by another CA that the EE trusts on the basis of out-of-band authentication techniques). The EE sends an initialization request to start the transaction. When CA-1 responds with a message containing the new certificate, the end entity replies with a certificate confirmation. CA-1 replies with a PKIConfirm to close the transaction. All messages are signed (the EE messages are signed using the private key that corresponds to the public key in its external identity certificate; the CA-1 messages are signed using the private key that corresponds to the public key in a certificate that can be chained to a trust anchor in the EE's PSE). The profile for this exchange is identical to that given in , with the following exceptions:

the EE and CA-1 do not share a symmetric MACing key (i.e., there is no out-of-band shared secret information between these entities);
sender name in ir MUST be present (and identical to the subject name present in the external identity certificate);
protectionAlg of MSG_SIG_ALG MUST be used in all messages;
external identity cert. MUST be carried in ir extraCerts field
senderKID and recipKID are not used;
body is ir or ip;
protection bits are calculated according to the protectionAlg field.

Variants of Using KEM Keys for PKI Message Protection As described in , any party in a PKI management operation may wish to use a KEM key pair for message protection. Below possible cases are described. For any PKI management operation started by a PKI entity with any type of request message, the following message flows describe the use of a KEM key. There are two cases to distinguish, namely whether the PKI entity or the PKI management entity owns a KEM key pair. If both sides own KEM key pairs, the flows need to be combined such that for each direction a shared secret key is established. In the following message flows Alice indicates the PKI entity that uses a KEM key pair for message authentication and Bob provides the KEM ciphertext using Alice's public KEM key, as described in . Message Flow when the PKI entity has a KEM key pair and certificate:

Message Flow when PKI entity has a KEM key pair genm -> 3 validate KEM certificate 4 perform KEM Encapsulate 5 format unprotected genp of type KemCiphertextInfo providing KEM ciphertext 6 <- genp <- 7 perform KEM Decapsulate 8 perform key derivation to get ssk 9 format request with MAC-based protection 10 -> request -> 11 perform key derivation to get ssk 12 verify MAC-based protection -------- PKI entity authenticated by PKI management entity -------- 13 format response with protection depending on available key material 14 <- response <- 15 verify protection provided by the PKI management entity 16 Further messages of this PKI management operation can be exchanged with MAC-based protection by the PKI entity using the established shared secret key (ssk) ]]> Message Flow when the PKI entity knows that the PKI management entity uses a KEM key pair and has the authentic public key:

Message Flow when the PKI entity knows that the PKI management entity uses a KEM key pair and has the authentic public key request -> 4 perform KEM Decapsulate 5 perform key derivation to get ssk 6 format response with MAC-based protection 7 <- response <- 8 perform key derivation to get ssk 9 verify MAC-based protection -------- PKI management entity authenticated by PKI entity -------- 10 Further messages of this PKI management operation can be exchanged with MAC-based protection by the PKI management entity using the established shared secret key (ssk) ]]> Note: describes the situation where KEM-based message protection may not require more that one message exchange. In this case, the transactionID MUST also be used by the PKI entity (Bob) to ensure domain separation between different PKI management operations. Message Flow when the PKI entity does not know that the PKI management entity uses a KEM key pair:

Message Flow when the PKI entity does not know that the PKI management entity uses a KEM key pair request -> 3 format unprotected error with status "rejection" and failInfo "wrongIntegrity" and KEM certificate in extraCerts 4 <- error <- 5 validate KEM certificate 6 proceed as shown in the Figure before ]]>

Compilable ASN.1 Definitions This section contains the updated 2002 ASN.1 module for as updated in . This module replaces the module in Section 9 of . The module contains those changes to the normative ASN.1 module from Appendix F of that were specified in , as well as changes made in this document.

History of Changes Note: This appendix will be deleted in the final version of the document. From version 14 -> 15:

Addressed SECDIR, OPSDIR, and TSVART review comments

From version 13 -> 14:

Implemented some editorial changes throughout the document, specifically in Sections 5.1.1, 5.1.1.3, 5.1.3.4, 5.2.2, 5.2.8.3, 5.3.18, 5.3.19.2, 5.2.22, 7, C.1, and C.4
Aligned formatting of message flow diagrams
Updated the page header to 'CMP'
Removed one instruction to RFC Editors
Fixed some nits in Section 5.2.2
Fixed one reference to RFC 9629 in the ASN.1 Module

From version 12 -> 13:

Updated the definition of "NULL-DN" in Section 5.1.1 and Appendix D.1 and added a specification of how the RA/CA shall generate the rid content to Section 5.2.8.3.3 to clarify direct POP (see thread "CMS RecipientInfo for EnvelopedData in CMC")
Added one minor clarification in Section 5.2.2
Updated reference from draft-ietf-lamps-cms-kemri to RFC 9629

From version 11 -> 12:

Adding a paragraph to Section 5.2.8.3.2 to clarify Indirect POP (see thread "Using cms-kemri this CMP Indirect POP")
Updated Appendix F addressing comments from Russ (see thread "WG Last Call for draft-ietf-lamps-rfc4210bis and draft-ietf-lamps-rfc6712bis")
Extended the Acknowledgments section.

From version 10 -> 11:

Updated Section 4.2.2 addressing the comment from Tomas Gustavsson and as presented during IETF 119 (see thread "draft-ietf-lamps-rfc4210bis-v10 Section 4.2.2 - removing normative language")

From version 09 -> 10:

Implemented some minor editorial changes modernizing the text in Section 3, 4, and 5.2.8 as proposed during IETF 119, without changing normative language.
Added to Section 4.2.2 two ToDos for further discussion, based on the comment from Tomas Gustavsson as presented during IETF 119.
Addressed erratum 7888

From version 08 -> 09:

Changed reference from ITU-T X.509 to RFC 5280 (see thread " CMP vs RFC5280").
Deprecated CAKeyUpdAnnContent in favor of RootCaKeyUpdateContent in CMP V3 as proposed by Tomas.
Updated Section 4.4 incorporating RootCaKeyUpdateContent as alternative to using a repository for providing root CA key updates.
Deleting an obsolete sentence in Section 8.8.
Added IANA considerations addressing IANA early review.

From version 07 -> 08:

Aligned with released RFC 9480 - RFC 9483
Updated Section 1.3
Added text on usage of transactionID with KEM-bases message protection to Section 5.1.1
Reverted a change to Section 5.1.3.1 from -02 and reinserting the deleted text and adding some text explaining when a key expansion is required.
Consolidated the definition and transferal of KemCiphertextInfo. Added a new Section 5.1.1.5 introducing KemCiphertextInfo in the generalInfo filed and moving text on how to request a KEM ciphertext using genm/genp from Section 5.1.3.4 to Section 5.3.19.18
Some editorial changes to Section 5.1.3.4 and Appendix E after discussion with David resolving #30 and discussing at IETF 117. Also introducing optional field kemContext to KemBasedMac and KemOtherInfo as CMP-specific alternative to ukm in cms-kemri.
Added ToDo for reviewing the reduced content of KemOtherInfo to Section 5.1.3.4
Added a cross-reference to Section 5.1.1.3 regarding use of OrigPKIMessage to Section 5.1.3.5
Added POP for KEM keys to Section 5.2.8. Restructured the section and fixed some references which broke from RFC2510 to RFC4210. Introduced a section on the usage of raVerified.
Fixed the issue in Section 5.3.19.15, resulting from a change made in draft-ietf-lamps-cmp-updates-14, that no plain public-key can be used in the request message in CMPCertificate.
Updated Appendix B regarding KEM-based message protection and usage of CMS EnvelopedData

From version 06 -> 07:

Updated section 5.1.1.4 addressing a question from Liao Lijun on how to interpret less profile names than certReqMsgs
Updated section 5.1.3.4 specifying establishing a shares secret key for one arbitrary side of the CMP communication only
Removed the note and the security consideration regarding combiner function for HPKE
Added security considerations 8.1 and 8.8
Updates IANA Considerations in section 9 to add new OID for the updates ASN.1 module and for id-it-KemCiphertextInfo
Added new appendix E showing different variants of using KEM keys for PKI message protection
Updates ASN.1 module in appendix F

From version 05 -> 06:

Updated section 5.1.3.4 exchanging HPKE with plain KEM+KDF as also used in draft-ietf-lamps-cms-kemri

From version 04 -> 05:

Updated sections 5.1.3.4, 5.2.2, and 8.9 addressing comments from Russ (see thread "I-D Action: draft-ietf-lamps-rfc4210bis-04.txt")

From version 03 -> 04:

Added Section 4.3.4 regarding POP for KEM keys
Added Section 5.1.3.4 on message protection using KEM keys and HPKE
Aligned Section 5.2.2 on guidance which CMS key management technique to use with encrypted values (see thread "CMS: selection of key management technique to use for EnvelopedData") also adding support for KEM keys
Added Section 8.9 and extended Section 3.1.2 regarding use of Certificate Transparency logs
Deleted former Appendix C as announced in the -03
Fixed some nits resulting from XML -> MD conversion

From version 02 -> 03:

Updated Section 4.4.1 clarifying the definition of "new with new" certificate validity period (see thread "RFC4210bis - notAfter time of newWithNew certificate")
Added ToDo to Section 4.3 and 5.2.8 on required alignment regarding POP for KEM keys.
Updated Sections 5.2.1, 5.2.8, and 5.2.8.1 incorporating text of former Appendix C (see thread "draft-ietf-lamps-rfc4210bis - ToDo on review of Appendix C")
Added a ToDo to Appendix B to indicate additional review need to try pushing the content to Sections 4 and Section 5

From version 01 -> 02:

Added Section 3.1.1.4 introducing the Key Generation Authority
Added Section 5.1.1.3 containing description of origPKIMessage content moved here from Section 5.1.3.4
Added ToDos on defining POP and message protection using KEM keys
Added a ToDo to Section 4.4.3
Added a ToDo to Appendix C to do a more detailed review
Removed concrete algorithms and referred to CMP Algorithms instead
Added references to Appendix D and E as well as the Lightweight CMP Profile for further information
Broaden the scope from human users also to devices and services
Addressed idnits feedback, specifically changing from historic LDAP V2 to LDAP V3 (RFC4511)
Did some further editorial alignment to the XML

From version 00 -> 01:

Performed all updates specified in CMP Updates Section 2 and Appendix A.2.
Did some editorial alignment to the XML

Version 00: This version consists of the text of RFC4210 with the following changes:

Introduced the authors of this document and thanked the authors of RFC4210 for their work.
Added a paragraph to the introduction explaining the background of this document.
Added the change history to this appendix.