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Security Limited Additional Mechanisms for PKIX and SMIME Internet-Draft Certification Authorities (CAs) issue certificates for public keys conveyed to the CA via a certificate management message or protocol. In some cases, a CA may wish to tailor certificate contents based on whether the corresponding private key is secured by hardware in non-exportable form. This document describes extensions that may be included in any of several widely used certificate management protocols to convey attestations about the private key to the CA to support this determination. About This Document Status information for this document may be found at . Discussion of this document takes place on the spasm Working Group mailing list (), which is archived at .

Introduction Many different certificate management protocols exist, including:

• PKCS #10
• Simple Certificate Enrolment Protocol (SCEP)
• Certificate Management over CMS (CMC)
• Certificate Management Protocol (CMP)
• Certificate Request Management Format (CRMF)
• Enrollment over Secure Transport (EST)
• Automatic Certificate Management Environment (ACME)

Each of these specifications defines extensibility mechanisms to customize requests sent to a Certification Authority (CA), Registration Authority (RA), or certificate management server. This document addresses the first six specifications in the above list, as all can be customized using attributes or extensions. is somewhat different and is addressed by . Many operating system and device vendors offer functionality enabling a device to generate a cryptographic attestation that can be used to establish the provenance of a key:

• Android Key Attestation
• Trusted Platform Module
• Apple Key Attestation
• Yubico PIV Attestation

defines an "API enabling the creation and use of strong, attested, scoped, public key-based credentials by web applications, for the purpose of strongly authenticating users." In support of this goal, it defines a model and corresponding formats to support attestation functionality. Section 6.5 of describes the general attestation structure and section 8 defines some specific attestation formats. Similar to , this specification uses the attestation object definition from as a means of supporting a variety of attestation formats, which are defined in the IANA registry that was established by ; see . This document defines a structure, KeyAttestation, that can be used to convey a attestation statement as an attribute or extension when using the protocols listed above.

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.

Key Attestation Attribute or Extension A key attestation attribute or extension MAY be included in certificate request messages to convey an attestation statement for the private key corresponding to the public key contained in the request. The attribute definition and the certificate extension definition are exactly the same, and they are identified by the same object identifier. The structure consists of two fields: hardwareSecured and attestationStatement. Where the hardwareSecured field is set to TRUE, the private key corresponding to the public key in the request MUST NOT be able to be exported from the cryptoprocessor. The attestationStatement field features an attestation statement as defined in encoded as an OCTET STRING. While the format of an attestation statement varies, all attestation statement formats conveyed via a keyAttestation extension MUST include the public key that is the subject of the corresponding certificate management request. Certificate request messages that contain a key attestation that does not include a public key or that contain a public key that does not match the public key in the certificate request SHOULD be rejected with no certificate issued, however, a CA MAY elect to issue a certificate as if the request did not contain a key attestation per local policy. Some attestation statement formats support the use of challenge password or nonce values. While the means of conveying challenge password value or a nonce value to certificate request clients is outside the scope of this document, each of SCEP , CMC , CMP and EST define means for conveying nonce values to certificate request clients. In some cases, challenge password or nonce values may be conveyed outside of a certificate management protocol. For example, SCEP payloads in Apple's Over-the-Air Profile Delivery and Configuration specification deliver challenge passwords in an XML-formatted set of instructions. Similarly, use and verification of a nonce value relative to an attestation statement is outside the scope of this document. Verification procedures for currently defined attestation statement formats can be found in Section 8 of . Certificate request messages that contain a key attestation that cannot be validated, including processing any nonce or challenge password values, SHOULD be rejected with no certificate issued, however, a CA MAY elect to issue a certificate as if the request did not contain a key attestation per local policy.

Usage in PKCS #10 requests The PKCS #10 structure may be used directly or in SCEP, CMC, CMP or EST contexts. Where PKCS #10 is used, the public key in the attestation statement MUST match the public key in the CertificationRequestInfo.subjectPKInfo field and the keyAttestation attribute MUST appear in the CertificationRequestInfo.attributes field.

Usage in CRMF requests The CRMF structure may be used in CMC, CMP or EST. Where CRMF is used, the public key in the attestation statement MUST match the public key in the CertTemplate.publicKey field and the keyAttestation extension MUST appear in the CertTemplate.extensions field.

Example extension The example extension below features a CBOR-encoded attestation statement of type "apple-appattest", which was genrated using the attestKey method. The contents of the attestationStatement field in the above example are shown below in JSON format. The attestation certificate, i.e., the first entry in the x5c array, contains an extension that features a nonce value generated by concatenating the authData value with a SHA256 hash of the challenge password (i.e., the string "Sample Nonce Value" in this case) then hashing the concatenation. The value from the extension is below. Note, at present there is no registration for the "apple-appattest" type in the https://www.iana.org/assignments/webauthn/webauthn.xhtml registry, which is WebAuthn-centric. New registrations will be required for common formats, like the one above.

Security Considerations See Section 13 of for additional security considerations related to attestation statement formats, including certificate revocation. CAs, RAs and certificate management servers will need a set of trust anchors to validate attestation statements that may originate from any number of sources. Where possible, a dedicated trust anchor and issuing CA should be used when verifying a given type of attestation statement. Where a trust anchor or issuing CA are shared for mulitple sources of attestation statements, including constraints in attestation signer certificates or attestation certificates is recommended. and define structures for conveying trust anchors that may be used for verifying attestations such that constraints are implied or are explicitly stated. Expression and validation of constraints imposed on trust anchors, CAs or attestation signers is beyond the scope of this specification. Key attestation statements may include a variety of information in addition to the public key being attested. While not described in this document, CAs, RAs and certificate management servers are free to use any policy when evaluating this information. This evaluation can result in rejection of a certificate request that features a verifiable key attestation for the public key contained in the request. For example, an attestation statement may indicate use of an unacceptable firmware version.

IANA Considerations

Key attestation extension object identifier An object identifier from the id-pe arc defined in should be assigned for id-pe-keyAttestation.

Key attestation extension ASN.1 module object identifier An object identifier from the id-mod arc defined in should be assigned for id-mod-keyAttestation.

Attestation statement formats describes registration of new attestation statement format types.

ASN.1 Module The following ASN.1 module makes use of the conventions from .

References 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 document specifies the Simple Certificate Enrolment Protocol (SCEP), a PKI protocol that leverages existing technology by using Cryptographic Message Syntax (CMS, formerly known as PKCS #7) and PKCS #10 over HTTP. SCEP is the evolution of the enrolment protocol sponsored by Cisco Systems, which enjoys wide support in both client and server implementations, as well as being relied upon by numerous other industry standards that work with certificates. This document defines the base syntax for CMC, a Certificate Management protocol using the Cryptographic Message Syntax (CMS). This protocol addresses two immediate needs within the Internet Public Key Infrastructure (PKI) community: 1. The need for an interface to public key certification products and services based on CMS and PKCS #10 (Public Key Cryptography Standard), and 2. The need for a PKI enrollment protocol for encryption only keys due to algorithm or hardware design. CMC also requires the use of the transport document and the requirements usage document along with this document for a full definition. [STANDARDS-TRACK] 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] 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] This document profiles certificate enrollment for clients using Certificate Management over CMS (CMC) messages over a secure transport. This profile, called Enrollment over Secure Transport (EST), describes a simple, yet functional, certificate management protocol targeting Public Key Infrastructure (PKI) clients that need to acquire client certificates and associated Certification Authority (CA) certificates. It also supports client-generated public/private key pairs as well as key pairs generated by the CA. 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. This specification defines IANA registries for W3C Web Authentication (WebAuthn) attestation statement format identifiers and extension identifiers. 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. Google Mozilla Microsoft Microsoft Yubico 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. Informative References 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. Google This document specifies new identifiers and a challenge for the Automated Certificate Management Environment (ACME) protocol which allows validating the identity of a device using attestation. IANA Red Hound Software Vigil Security, LLC Apple FIDO Alliance

Acknowledgments TODO acknowledge.