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Security Automated Certificate Management Environment OpenID Federation ACME The Automatic Certificate Management Environment (ACME) protocol allows server operators to obtain TLS certificates for their websites, based on a demonstration of control over the website's domain via a fully-automated challenge/response protocol. OpenID Federation 1.0 defines how to build a trust infrastructure using a trusted third-party model. It uses a trust evaluation mechanism to attest the possession of public keys, protocol specific metadata and various administrative and technical information related to a specific entity. This document defines how X.509 certificates associated with a given OpenID Federation Entity can be issued by an X.509 Certification Authority through the ACME protocol to the organizations which are part of a federation built on top of OpenID Federation 1.0. About This Document Status information for this document may be found at . Discussion of this document takes place on the Automated Certificate Management Environment Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .

Introduction This document describes extensions to the ACME protocol that integrate with OpenID Federation 1.0, allowing an ACME server to issue X.509 Certificates associated with a given OpenID Federation Entity. X.509 Certificates can be provided to one or more organizations, without having pre-established any direct relationship or any stipulation of a contract. In a multilateral federation, composed by thousands of entities belonging to different organizations, all the participants adhere to the same regulation or trust framework. OpenID Federation 1.0 allows each participant to recognize other participants using a trust evaluation mechanism, with RESTful services and cryptographic materials. Considering that a requestor is an entity requesting the issuance of an X.509 Certificate to a server and the issuer is the ACME server that validates the entitlements of the requestor before issuing the X.509 Certificate, this specification defines how ACME and OpenID Federation 1.0 can be integrated to allow efficient issuance of X.509 Certificates to a requestor via the introduction of a new ACME challenge type. The new challenge type extends the ACME protocol in the following ways:

• It associates a cryptographic key with an OpenID Entity, rather than a domain, since the authentication and authorization of the requestor is asserted with OpenID Federation 1.0.
• It defines how to use and validate a basic OpenID Federation component, called Entity Configuration, that is a signed JWT published in a well-known resource (/.well-known/openid-federation).
• It defines how the OpenID Federation Subordinate Statements can be used for the publication of the X.509 Certificates, by a Superior Entity, that were previously issued with ACME.
• It extends the ACME newOrder resource, as defined in , defining a new payload identifier type called openid-federation.

Target Audience and Use Cases The audience of the document are the multilateral federations that require automatic issuance of X.509 Certificates using an infrastructure of trust based on OpenID Federation 1.0. This specification can be implemented by:

• Federation Entities that join to a federation staging area using HTTP only transport to attest themselves as trustworthy, and then retrieve X.509 Certificates for their official HTTPS Federation Entity ID.
• Federation Entities that want to ask for and obtain X.509 Certificate for use in other protocols.

Terminology The terms "Federation Entity", "Trust Anchor", "Entity Configuration", "Subordinate Statement", "Superior Entity", "Immediate Superior Entity", "Federation Entity Keys", "Trust Mark" and "Trust Chain" used in this document are defined in . The term "CSR" used in this document is defined in . The term Certification Authority used in this document is defined in . The terms "ACME Client" and "ACME Server" are defined in . The specification also defines the following terms:

Requestor:

A Federation Entity which wants to request X.509 certificates. It operates a web server for hosting its Entity Configuration. It also operates an ACME client, extended according to this document.

Certificate Issuer:

A Federation Entity which issues X.509 certificates. It operates a web server for hosting its Entity Configuration. It also operates an ACME server, extended according to this document.

Conventions and Definitions 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.

Certificates issued using OpenID Federation The Certificate Issuer establishes the authorization of a Federation Entity to obtain X.509 Certificates for the identifier configured in the Requestor's Entity Configuration. The Certificate Issuer establishes if a Federation Entity is eligible to obtain X.509 Certificates for the identifier configured in the Requestor's Entity Configuration. The cryptographic keys published within the Requestor's Entity Configuration are used to satisfy the Certificate Issuer's challenge. The protocol assumes the following discovery preconditions are met. The Issuer has the guarantee that:

1. The Requestor controls the private key related to the public part published in its Entity Configuration.
2. The Requestor controls its identifier, having published the Entity Configuration.

Protocol Flow This section presents the protocol flow. The protocol flow is subdivided in the following phases:

• Discovery, the Requestor obtains the available CAs within a federation, inspecting the ACME issuer entity types.
• Order request, the Requestor requests a X.509 Certificate to a Certificate Issuer using the ACME protocol.

Preconditions The protocol requires the following preconditions are met.

1. The Requestor and the Issuer MUST publish their Entity Configuration as defined in .
2. The Requestor and the Issuer MUST be able to establish a Trust Chain to each other, as defined in , from their respective Trust Anchors.
3. The Issuer MUST implement an ACME server, extended according to this document.
4. The Requestor MUST publish the entity type acme_requestor in its Entity Configuration, according to .
5. The Issuer MUST publish the entity type acme_issuer in its Entity Configuration, according to .

Discovery The Requestor's ACME client may either be configured to use a particular ACME server, or to automatically discover a Certificate Issuer through the federation. Requestors that use discovery MAY select any entity with an entity type of acme_issuer, or they may additionally require that such entities have a valid Trust Mark with a particular Trust Mark Identifier.

Overview

1. The Requestor checks if its superior Federation Entity supports the ACME protocol for OpenID Federation 1.0. If not, the Requestor starts the discovery process to find Issuers within the federation.
2. The Requestor requests and obtains a new nonce from the Certificate Issuer, by sending a HTTP HEAD request to the Issuer's newNonce resource, as described in .
3. The Requestor creates an ACME Account with the Issuer, as described in .
4. The Requestor begins the X.509 Certificate issuance process by sending a HTTP POST request to the Certificate Issuer's newOrder resource, as described in , and follows the remainder of the ACME protocol as specified in , using the new challenge defined in .
5. The Certificate Issuer evaluates the trust to the Requestor by checking if it is part of its federation. If not the CSR request MUST be rejected, with error type urn:ietf:params:acme:error:openIDFederationEntity, and an error code of invalid_trust_chain ().

There are two ways the Certificate Issuer is able to check if a Requestor is part of the federation, these are listed below:

• The Requestor adds the Trust Chain JWT header parameter related to itself, as described in . This option is RECOMMENDED since it reduces the effort of the Certificate Issuer in evaluating the trust to the Requestor.
• The Requestor doesn't add the Trust Chain in the request. The Certificate Issuer MUST start Federation Entity Discovery as described in to obtain the Trust Chain related to the Requestor.

The following diagram illustrates a successful interaction between Issuer and Requestor to retrieve an X.509 Certificate. The diagram assumes the Requestor has already discovered the Issuer, and the Requestor has already created an ACME account with the Issuer. | | | | | | |Authorization at | | | |/acme/authz/[authz-id] | | | |Finalize at | | | | /acme/order/[order-id]/finalize | | | |<- - - - - - - - - - - - - - - - - | | | | | | | POST /acme/authz/[authz-id] | | | |---------------------------------> | | | | | | | openid-federation-01 Challenge | | | | at /acme/chall/[chall-id] | | | |<- - - - - - - - - - - - - - - - - | | | | | | ----. | | | | | Sign challenge token | | | | | with private key | | | <---' | | | | | | | | POST /acme/chall/[chall-id] | | | | with signed | | | | token and entity ID | | | | set to Requestor's ID | | | |---------------------------------> | | | | | | | Challenge validation | | | | beginning | | | |<- - - - - - - - - - - - - - - - - | | | | | | GET /.well-known/openid-federation | | |<---------------------------------------------------- | | | | | | Requestor's Entity Configuration | | | - - - - - - - - - - - - - - - - - - - - - - - - - > | | | | | | | ______________________________________________________ | | ! OPT / If requestor didn't provide Trust Chain | ! | | !_____/ | | ! | | ! | Determine Trust Chain | ! | | ! | from Issuer's | ! | | ! | Trust Anchor to Requestor| ! | | ! | (Federation Discovery) | ! | | ! | <------------------------>| ! | | !~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~! | | | | | | |----. | | | | | Evaluate Trust Chain | | | |<---' | | | | | | | |----. | | | | | Check | | | | | Entity Configuration | | | | | sub matches | | | | | Entity identifier | | | |<---' in the order | | | | | | | | | | | |----. | | | | | Check challenge | | | | | sig is signed | | | | | with key in | | | |<---' Entity Configuration | | | | | | _________________________________________________________________________ | | ! LOOP / Poll until authz status | ! | | ! / is "valid" or "invalid" | ! | | !_____/ | | ! | | ! | POST-as-GET | ! | | ! | /acme/authz/[authz-id] | ! | | ! |---------------------------------> ! | | ! | | ! | | ! | Current authz status | ! | | ! |<--------------------------------- ! | | !~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~! | | | | | | | | | | ___________________________________________________________________________ | | ! OPT / If the authz status is "valid" | ! | | !_____/ | | ! | | ! | POST | ! | | ! | /acme/orders/[order-id]/finalize| ! | | ! | with CSR | ! | | ! |---------------------------------> ! | | ! | | ! | | ! | |----. ! | | ! | | | Check CSR ! | | ! | | | validity ! | | ! | | | according to ! | | ! | | | protocol ! | | ! | |<---' and CA policy ! | | ! | | ! | | ! | | ! | | ! | Order object with certificate | ! | | ! | at /acme/cert/[cert-id] | ! | | ! |<- - - - - - - - - - - - - - - - - ! | | ! | | ! | | ! | POST /acme/cert/[cert-id] | ! | | ! |---------------------------------> ! | | ! | | ! | | ! | Newly issued X.509 Certificate | ! | | ! |<- - - - - - - - - - - - - - - - - ! | | !~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~! | ,--------+--------. ,-----+-----. ,-----------+------------. ,-----------+-----------. |Requestor's | |Requestor's| |X.509 Certificate Issuer| |Federation Trust Anchor| |OpenID Federation| |ACME Client| `------------------------' `-----------------------' | Web Server | `-----------' `-----------------' ]]>

Entity Configuration Metadata This section describes the metadata a Requestor and Issuer MUST publish in their respective Entity Configurations.

Issuer Metadata The Issuer MUST publish its Entity Configuration including the acme_issuer Entity Type metadata within it. The acme_issuer metadata contains one parameter, directory_url, which is the URL of the ACME Directory, as defined in . Requestors MUST use the ACME Directory provided in the Issuer's Entity Configuration for client configuration of ACME endpoints. The following is a non-normative example of an Entity Configuration including the acme_issuer metadata:

Requestor Metadata The Requestor MUST publish in its Entity Configuration an acme_requestor metadata containing a JWK set, according to . The keys in the set are used to respond to ACME challenges. The following is a non-normative example of an Entity Configuration including the acme_requestor metadata and using the jwks metadata parameter. The Issuer MUST only use the Requestor's acme_requestor to validate an ACME challenge. Therefore, after completing the challenge, the Requestor MAY remove the acme_requestor metadata from its Entity Configuration.

OpenID Federation Identifier This document defines a new ACME identifier type for OpenID Federation entities, openid-federation, whose value is the sub parameter of the requestor's Entity Configuration, as defined in .

newOrder Request The Requestor begins certificate issuance by sending a HTTP POST request to the Issuer's newOrder resource, as specified in . A non-normative example of an ACME newOrder request:

OpenID Federation Challenge Type The OpenID Federation challenge type allows a Requestor to prove control of a Federation Entity using the trust evaluation mechanism provided by . The Requestor demonstrates control of a cryptographic public key published in its OpenID Federation Entity Configuration. The openid-federation-01 ACME challenge object has the following format: type (required, string): The string "openid-federation-01" token (required, string): A random value that uniquely identifies the challenge. This value MUST have at least 128 bits of entropy. It MUST NOT contain any characters outside the base64url alphabet as described in . Trailing '=' padding characters MUST be stripped. See for additional information on randomness requirements. trustAnchors (optional, array of string): An array of strings containing Entity Identifiers of the Issuer's Trust Anchors. When solving the challenge, the Requestor can construct a Trust Chain from itself to one of these Trust Anchors. It is RECOMMENDED that the Issuer includes this field to make it easier for the Requestor to construct a Trust Chain. A non-normative example of a challenge with trustAnchors specified: The openid-federation-01 challenge MUST NOT be used to issue X.509 Certificates for any identifiers except openid-federation identifiers. The openid-federation identifier MUST NOT be validated except by the openid-federation-01 challenge. The Requestor responds to the challenge with an object with the following format: sig (required, string): the compact JSON serialization (as described in ) of a JWS, signing the key authorization encoded in UTF-8. The key authorization is computed from the token in the challenge and the Requestor's ACME account key, as defined in . The signature must be made by one of the keys published in the Requestor's acme_requestor metadata in its Entity Configuration, as specified in . The JWS MUST include a kid header parameter corresponding to the key used to sign the key authorization and a typ header parameter set to "signed-acme-challenge+jwt". trustChain (optional, array of string): an array of strings containing signed JWTs, representing a Trust Chain from the Requestor to one of the Issuer's Trust Anchors (see ). The Entity Configuration of the Trust Chain subject MUST contain acme_requestor metadata that is valid under the Trust Chain's resolved metadata policy () and contains the key used to compute sig. It is RECOMMENDED that the Requestor includes this field. If the Requestor cannot construct a Trust Chain to one of the Trust Anchors indicated by the Issuer, or if no Trust Anchors were indicated, it MAY use some other Trust Anchor that it believes the Issuer trusts. If the Requestor cannot construct a Trust Chain to any Trust Anchor, it MAY omit the trustChain field from the challenge response. A non-normative example for an authorization with trustChain specified: On receiving a challenge response, the Certificate Issuer verifies that the Requestor is trusted. If the Requestor did not provide a trustChain, the Issuer MUST perform Federation Entity Discovery () to obtain a Trust Chain for the Requestor. Once it has obtained a Trust Chain, the Issuer verifies:

• That the Requestor's acme_requestor metadata is valid under the Trust Chain's resolved metadata policy ().
• That the requested openid-federation identifier value matches the sub parameter of the Requestor's Entity Configuration.
• That the sig field of the payload is the compact JSON serialization of a JWS signing the key authorization, signed with a key from the Requestor's acme_requestor metadata () whose kid matches the kid claim in the challenge response.

If all of the above verifications succeed, then the validation is successful. Otherwise, it has failed. In either case, the Certificate Issuer responds according to . A non-normative example for the challenge object post-validation:

CSR and Certificate Fields Depending on the Certificate Issuer's X.509 Certificate profile, the CSR and X.509 Certificate MAY associate the X.509 Certificate to the Federation Entity by including the Entity ID in the X.509 Certificate. To do so, the Issuer includes a Subject Alternative Name extension containing an otherName with a type-id of id-on-OpenIdFederationEntityId. The value of the name is an Octet String containing the UTF-8 encoding of the Entity ID (i.e., the URI in the corresponding openid-federation identifier from the newOrder request).

Publication of the Certificates within the Federation The X.509 Certificates issued by federation Immediate Superior Entities pertaining to one or more Federation Entity Keys in control of their Subordinates MAY publish this information by including the x5c member in each JWK contained within the matching Subordinate Statement. The contents of the published x5c member, including whether it contains a full or partial Trust Chain, and if so, to what Trust Anchor, are policy decisions out of scope for this document.

Certificate Lifecycle and Revocation The identity of the Requestor is verified through proof of possession of a private key corresponding to a public key attested within a Trust Chain. It is up to the Certificate Issuer to decide the expiration time of the X.509 Certificate. In some cases, and when required, it MAY be set to match the expiration of the Trust Chain. A Requestor SHOULD request the revocation of its X.509 Certificate when the related cryptographic material is revoked. The Requestor SHOULD publish the revoked or expired cryptographic keys in the Federation Historical Key Registry. The X.509 Certificate revocation request is defined in .

Errors This document defines one new error type URI to be used in problem documents , as described in . The error type urn:ietf:params:acme:error:openIDFederationEntity can be used to encapsulate any OAuth error code returned while resolving OpenID Federation Entities. The title of this error type is "OpenID Federation Error". The detail member of the problem document MAY include the description of the particular OAuth error code that caused the error. The problem document for this error type SHOULD include an extension member named error_code. The error_code member SHOULD be set to the OAuth error code, taken from the IANA "OAuth Extensions Error Registry" .

Security Considerations The openid-federation-01 challenge defined in defends against replay attacks by malicious ACME servers because the signature in challenge responses is over an ACME key authorization, which binds the ACME account key. The cryptographic keys in the acme_requestor metadata SHOULD NOT be reused for other purposes than signing responses to acme-federation-01 challenges. For example, the same keys SHOULD NOT be reused in the issued X.509 Certificate. If the keys are reused for other purposes, cross-protocol attacks MUST be considered. The cryptographic keys in the acme_requestor metadata SHOULD be rotated periodically.

IANA Considerations IANA is kindly asked to make the following updates to registries:

ACME Registry Group The following updates are all assignments in the "Automated Certificate Management Environment (ACME) Protocol" registry group .

ACME Identifier Types IANA is asked to add to the "ACME Identifier Types" registry, defined in , the entry below, as specified here in :

Label	Reference
openid-federation	this document

ACME Validation Methods IANA is also asked to add to the "ACME Validation Methods" registry, defined in , the entry below, as specified here in :

Label	Identifier Type	Reference
openid-federation-01	openid-federation	this document

ACME Error Types IANA is also asked to add to the "ACME Error Types" registry, defined in , the entry below, as specified here in :

Type	Description	Reference
openIDFederationEntity	An error occurred while resolving an OpenID Federation entity	this document

Assign X.509 PKIX Other Name IANA is asked to add to the "PKIX Other Name Forms" registry (1.3.6.1.5.5.7.8) the entry below, as specified here in

Decimal	Description	Reference
TBA	id-on-OpenIdFederationEntityId	this document

Normative References 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. 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] IANA n.d. IANA n.d. Public Key Infrastructure using X.509 (PKIX) certificates are used for a number of purposes, the most significant of which is the authentication of domain names. Thus, certification authorities (CAs) in the Web PKI are trusted to verify that an applicant for a certificate legitimately represents the domain name(s) in the certificate. As of this writing, this verification is done through a collection of ad hoc mechanisms. This document describes a protocol that a CA and an applicant can use to automate the process of verification and certificate issuance. The protocol also provides facilities for other certificate management functions, such as certificate revocation. 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. 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. This document describes the commonly used base 64, base 32, and base 16 encoding schemes. It also discusses the use of line-feeds in encoded data, use of padding in encoded data, use of non-alphabet characters in encoded data, use of different encoding alphabets, and canonical encodings. [STANDARDS-TRACK] Security systems are built on strong cryptographic algorithms that foil pattern analysis attempts. However, the security of these systems is dependent on generating secret quantities for passwords, cryptographic keys, and similar quantities. The use of pseudo-random processes to generate secret quantities can result in pseudo-security. A sophisticated attacker may find it easier to reproduce the environment that produced the secret quantities and to search the resulting small set of possibilities than to locate the quantities in the whole of the potential number space. Choosing random quantities to foil a resourceful and motivated adversary is surprisingly difficult. This document points out many pitfalls in using poor entropy sources or traditional pseudo-random number generation techniques for generating such quantities. It recommends the use of truly random hardware techniques and shows that the existing hardware on many systems can be used for this purpose. It provides suggestions to ameliorate the problem when a hardware solution is not available, and it gives examples of how large such quantities need to be for some applications. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. JSON Web Signature (JWS) represents content secured with digital signatures or Message Authentication Codes (MACs) using JSON-based data structures. Cryptographic algorithms and identifiers for use with this specification are described in the separate JSON Web Algorithms (JWA) specification and an IANA registry defined by that specification. Related encryption capabilities are described in the separate JSON Web Encryption (JWE) specification. This document defines a "problem detail" to carry machine-readable details of errors in HTTP response content to avoid the need to define new error response formats for HTTP APIs. This document obsoletes RFC 7807.

Contributors ISRG

divergentdave@gmail.com

ISRG

inahga@letsencrypt.org

ISRG

ietf@insufficient.coffee

ISRG

timgeog+ietf@gmail.com