ACME Working Group B. Weeks Internet-Draft Google Intended status: Standards Track 24 July 2023 Expires: 25 January 2024 Automated Certificate Management Environment (ACME) Device Attestation Extension draft-acme-device-attest-01 Abstract 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. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on 25 January 2024. Copyright Notice Copyright (c) 2023 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/ license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License. Weeks Expires 25 January 2024 [Page 1] Internet-Draft ACME DA July 2023 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3 3. Permanent Identifier . . . . . . . . . . . . . . . . . . . . 3 4. Hardware Module . . . . . . . . . . . . . . . . . . . . . . . 4 5. Device Attestation Challenge . . . . . . . . . . . . . . . . 4 6. Security Considerations . . . . . . . . . . . . . . . . . . . 5 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 7.1. ACME Identifier Types . . . . . . . . . . . . . . . . . . 6 7.2. ACME Validation Method . . . . . . . . . . . . . . . . . 6 7.3. New Error Types . . . . . . . . . . . . . . . . . . . . . 6 7.4. Attestation statement formats . . . . . . . . . . . . . . 7 7.4.1. WebAuthn Attestation Statement Format Identifiers for Certificate Request Protocols . . . . . . . . . . . . 7 7.4.2. WebAuthn Extension Identifiers for Certificate Request Protocols . . . . . . . . . . . . . . . . . . . . . . 10 8. Normative References . . . . . . . . . . . . . . . . . . . . 12 Appendix A. Enterprise PKI . . . . . . . . . . . . . . . . . . . 12 A.1. External Account Binding . . . . . . . . . . . . . . . . 13 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 13 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 13 1. Introduction The Automatic Certificate Management Environment (ACME) [RFC8555] standard specifies methods for validating control over identifiers, such as domain names. It is also useful to be able to validate properties of the device requesting the certificate, such as the identity of the device /and whether the certificate key is protected by a secure cryptoprocessor. Many operating systems and device vendors offer functionality enabling a device to generate a cryptographic attestation of their identity, such as: * Android Key Attestation (https://source.android.com/security/keystore/attestation) * Chrome OS Verified Access (https://developers.google.com/chrome/ verified-access/overview) * Trusted Platform Module (https://trustedcomputinggroup.org/resource/trusted-platform- module-tpm-summary/) Weeks Expires 25 January 2024 [Page 2] Internet-Draft ACME DA July 2023 Using ACME and device attestation to issue client certificates for enterprise PKI is anticipated to be the most common use case. The following variances to the ACME specification are described in this document: * Addition of permanent-identifier [RFC4043] and hardware-module [RFC4108] identifier types. * Addition of the device-attest-01 challenge type to prove control of the permanent-identifier and hardware-module identifier types. * The challenge response payload contains a serialized WebAuthn attestation statement format instead of an empty JSON object ({}). * Accounts and external account binding being used as a mechanism to pre-authenticate requests to an enterprise CA. This document does not specify the attestation verification procedures. Section 13 of [WebAuthn] gives some guidance, however verification procedures are complex and may require changes to address future security issues. 2. 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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. 3. Permanent Identifier A new identifier type, "permanent-identifier" is introduced to represent the identity of a device assigned by the manufacturer, typically a serial number. The name of this identifier type was chosen to align with [RFC4043], it does not prescribe the lifetime of the identifier, which is at the discretion of the Assigner Authority. The identity along with the assigning organization can be included in the Subject Alternate Name Extension using the PermanentIdentifier form described in [RFC4043]. Clients MAY include this identifier in the certificate signing request (CSR). Alternatively if the server wishes to only issue privacy-preserving certificates, it MAY reject CSRs containing a PermanentIdentifier in the subjectAltName extension. Weeks Expires 25 January 2024 [Page 3] Internet-Draft ACME DA July 2023 4. Hardware Module A new identifier type, "hardware-module" is introduced to represent the identity of the secure cryptoprocessor that generated the certificate key. If the server includes HardwareModule in the subjectAltName extension the CA MUST verify that the certificate key was generated on the secure cryptoprocessor with the asserted identity and type. The key MUST NOT be able to be exported from the cryptoprocessor. If the server wishes to issue privacy-preserving certificates, it MAY omit HardwareModule from the subjectAltName extension. 5. Device Attestation Challenge The client can prove control over a permanent identifier of a device by providing an attestation statement containing the identifier of the device. The device-attest-01 ACME challenge object has the following format: type (required, string): The string "device-attest-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, including padding characters ("="). See [RFC4086] for additional information on randomness requirements. { "type": "device-attest-01", "url": "https://example.com/acme/chall/Rg5dV14Gh1Q", "status": "pending", "token": "evaGxfADs6pSRb2LAv9IZf17Dt3juxGJ-PCt92wr-oA" } A client fulfills this challenge by constructing a key authorization ([RFC4086] Section 8.1) from the "token" value provided in the challenge and the client's account key. The client then generates a WebAuthn attestation object using the key authorization as the challenge. This specification borrows the WebAuthn _attestation object_ representation as described in Section 6.5.4 of [WebAuthn] for encapsulating attestation formats, but with these modifications: Weeks Expires 25 January 2024 [Page 4] Internet-Draft ACME DA July 2023 * The key authorization is used to form _attToBeSigned_. This replaces the concatenation of _authenticatorData_ and _clientDataHash_. _attToBeSigned_ is hashed using an algorithm specified by the attestation format. * The _authData_ field is unused and SHOULD be omitted. A client responds with the response object containing the WebAuthn attestation object in the "attObj" field to acknowledge that the challenge can be validated by the server. On receiving a response, the server constructs and stores the key authorization from the challenge's "token" value and the current client account key. To validate a device attestation challenge, the server performs the following steps: 1. Perform the verification procedures described in Section 6 of [WebAuthn]. 2. Verify that key authorization conveyed by _attToBeSigned_ matches the key authorization stored by the server. POST /acme/chall/Rg5dV14Gh1Q Host: example.com Content-Type: application/jose+json { "protected": base64url({ "alg": "ES256", "kid": "https://example.com/acme/acct/evOfKhNU60wg", "nonce": "SS2sSl1PtspvFZ08kNtzKd", "url": "https://example.com/acme/chall/Rg5dV14Gh1Q" }), "payload": base64url({ "attObj": base64url(/* WebAuthn attestation object */), }), "signature": "Q1bURgJoEslbD1c5...3pYdSMLio57mQNN4" } 6. Security Considerations See Section 13 of [WebAuthn] for additional security considerations related to attestation statement formats, including certificate revocation. Weeks Expires 25 January 2024 [Page 5] Internet-Draft ACME DA July 2023 Key attestation statements may include a variety of information in addition to the public key being attested. While not described in this document, the server MAY 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. 7. IANA Considerations 7.1. ACME Identifier Types The "ACME Validation Methods" registry is to be updated to include the following entries: +======================+===========+ | Label | Reference | +======================+===========+ | permanent-identifier | RFC XXXX | +----------------------+-----------+ | hardware-module | RFC XXXX | +----------------------+-----------+ Table 1 7.2. ACME Validation Method The "ACME Validation Methods" registry is to be updated to include the following entry: +==================+======================+===========+ | Label | Identifier Type | Reference | +==================+======================+===========+ | device-attest-01 | permanent-identifier | RFC XXXX | +------------------+----------------------+-----------+ Table 2 7.3. New Error Types This document adds the following entries to the ACME Error Type registry: Weeks Expires 25 January 2024 [Page 6] Internet-Draft ACME DA July 2023 +=========================+===========================+===========+ | Type | Description | Reference | +=========================+===========================+===========+ | badAttestationStatement | The attestation statement | RFC XXXX | | | is unacceptable (e.g. not | | | | signed by an attestation | | | | authority trusted by the | | | | CA) | | +-------------------------+---------------------------+-----------+ Table 3 7.4. Attestation statement formats Section 2.1 of [RFC8809] describes registration of new attestation statement format types used when authenticating users via [WebAuthn]. This specification reuses the same format, but, because the context for use is different, a different registry is required. This section defines IANA registries for W3C Web Authentication (WebAuthn) attestation statement format identifiers and extension identifiers used in the context of a certificate request. This specification establishes two registries: * the "WebAuthn Attestation Statement Format Identifiers for Certificate Request Protocols" registry * the "WebAuthn Extension Identifiers for Certificate Request Protocols" registry Any additional processes established by the expert(s) after the publication of this document will be recorded on the registry web page at the discretion of the expert(s), who may differ from the experts associated with the registry established by [RFC8809]. 7.4.1. WebAuthn Attestation Statement Format Identifiers for Certificate Request Protocols WebAuthn attestation statement format identifiers are strings whose semantic, syntactic, and string-matching criteria are specified in the "Attestation Statement Format Identifiers" (https://www.w3.org/TR/2019/REC-webauthn-1-20190304/#sctn-attstn-fmt- ids) section of [WebAuthn], along with the concepts of attestation and attestation statement formats. Registered attestation statement format identifiers are those that have been added to the registry by following the procedure in Section 7.4.1.1. Weeks Expires 25 January 2024 [Page 7] Internet-Draft ACME DA July 2023 Each attestation statement format identifier added to this registry MUST be unique amongst the set of registered attestation statement format identifiers. Registered attestation statement format identifiers MUST be a maximum of 32 octets in length and MUST consist only of printable ASCII [RFC20] characters, excluding backslash and double quote, i.e., VCHAR as defined in [RFC5234] but without %x22 and %x5c. Attestation statement format identifiers are case sensitive and may not match other registered identifiers in a case-insensitive manner unless the designated experts determine that there is a compelling reason to allow an exception. 7.4.1.1. Registering Attestation Statement Format Identifiers WebAuthn attestation statement format identifiers are registered using the Specification Required policy (see Section 4.6 of [RFC8126]). The "WebAuthn Attestation Statement Format Identifiers for Certificate Request Protocols" registry is located at https://www.iana.org/assignments/webauthn_for_certreq (https://www.iana.org/assignments/webauthn_for_certreq). Registration requests can be made by following the instructions located there or by sending an email to the webauthn-for-certreq-reg- review@ietf.org mailing list. Registration requests consist of at least the following information: * WebAuthn Attestation Statement Format Identifier: - An identifier meeting the requirements given in Section 7.4.1. * Description: - A relatively short description of the attestation format. * Specification Document(s): - Reference to the document or documents that specify the attestation statement format. * Change Controller: - For Standards Track RFCs, list "IETF". For others, give the name of the responsible party. Other details (e.g., postal address, email address, home page URI) may also be included. Weeks Expires 25 January 2024 [Page 8] Internet-Draft ACME DA July 2023 * Notes: - [optional] Registrations MUST reference a freely available, stable specification, e.g., as described in Section 4.6 of [RFC8126]. This specification MUST include security and privacy considerations relevant to the attestation statement format. Note that WebAuthn attestation statement format identifiers can be registered by third parties (including the expert(s) themselves), if the expert(s) determines that an unregistered attestation statement format is widely deployed and not likely to be registered in a timely manner otherwise. Such registrations still are subject to the requirements defined, including the need to reference a specification. 7.4.1.2. Registration Request Processing As noted in Section 7.4.1.1, WebAuthn attestation statement format identifiers are registered using the Specification Required policy. The expert(s) will clearly identify any issues that cause a registration to be refused, such as an incompletely specified attestation format. When a request is approved, the expert(s) will inform IANA, and the registration will be processed. The IESG is the arbiter of any objection. 7.4.1.3. Initial Values in the WebAuthn Attestation Statement Format Identifiers for Certificate Request Protocols Registry The initial values for the "WebAuthn Attestation Statement Format Identifiers for Certificate Request Protocols" registry have been populated with the values listed in the "WebAuthn Attestation Statement Format Identifier Registrations" (https://www.w3.org/TR/2019/REC-webauthn-1-20190304/#sctn-att-fmt- reg) section of [WebAuthn]. Also, the Change Controller entry for each of those registrations is: * Change Controller: - W3C Web Authentication Working Group (public-webauthn@w3.org) Weeks Expires 25 January 2024 [Page 9] Internet-Draft ACME DA July 2023 7.4.2. WebAuthn Extension Identifiers for Certificate Request Protocols WebAuthn extension identifiers are strings whose semantic, syntactic, and string-matching criteria are specified in the "Extension Identifiers" (https://www.w3.org/TR/2019/REC-webauthn-1- 20190304/#sctn-extension-id) section of [WebAuthn]. Registered extension identifiers are those that have been added to the registry by following the procedure in Section 7.4.2.1. Each extension identifier added to this registry MUST be unique amongst the set of registered extension identifiers. Registered extension identifiers MUST be a maximum of 32 octets in length and MUST consist only of printable ASCII characters, excluding backslash and double quote, i.e., VCHAR as defined in [RFC5234] but without %x22 and %x5c. Extension identifiers are case sensitive and may not match other registered identifiers in a case-insensitive manner unless the designated experts determine that there is a compelling reason to allow an exception. 7.4.2.1. Registering Extension Identifiers WebAuthn extension identifiers are registered using the Specification Required policy (see Section 4.6 of [RFC8126]). The "WebAuthn Extension Identifiers" registry is located at https://www.iana.org/assignments/webauthn (https://www.iana.org/assignments/webauthn). Registration requests can be made by following the instructions located there or by sending an email to the webauthn-for-certreq-reg-review@ietf.org mailing list. Registration requests consist of at least the following information: * WebAuthn Extension Identifier: - An identifier meeting the requirements given in Section 7.4.2. * Description: - A relatively short description of the extension. * Specification Document(s): - Reference to the document or documents that specify the extension. Weeks Expires 25 January 2024 [Page 10] Internet-Draft ACME DA July 2023 * Change Controller: - For Standards Track RFCs, list "IETF". For others, give the name of the responsible party. Other details (e.g., postal address, email address, home page URI) may also be included. * Notes: - [optional] Registrations MUST reference a freely available, stable specification, e.g., as described in Section 4.6 of [RFC8126]. This specification MUST include security and privacy considerations relevant to the extension. Note that WebAuthn extensions can be registered by third parties (including the expert(s) themselves), if the expert(s) determines that an unregistered extension is widely deployed and not likely to be registered in a timely manner otherwise. Such registrations still are subject to the requirements defined, including the need to reference a specification. 7.4.2.2. Registration Request Processing As noted in Section 7.4.2.1, WebAuthn extension identifiers are registered using the Specification Required policy. The expert(s) will clearly identify any issues that cause a registration to be refused, such as an incompletely specified extension. When a request is approved, the expert(s) will inform IANA, and the registration will be processed. The IESG is the arbiter of any objection. 7.4.2.3. Initial Values in the WebAuthn Extension Identifiers Registry The initial values for the "WebAuthn Extension Identifiers" registry have been populated with the values listed in the "WebAuthn Extension Identifier Registrations" https://www.w3.org/TR/2019/REC-webauthn-1- 20190304/#sctn-extensions-reg (https://www.w3.org/TR/2019/REC- webauthn-1-20190304/#sctn-extensions-reg) section of [WebAuthn]. Also, the Change Controller entry for each of those registrations is: * Change Controller: - W3C Web Authentication Working Group (public-webauthn@w3.org) Weeks Expires 25 January 2024 [Page 11] Internet-Draft ACME DA July 2023 8. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC4043] Pinkas, D. and T. Gindin, "Internet X.509 Public Key Infrastructure Permanent Identifier", RFC 4043, DOI 10.17487/RFC4043, May 2005, . [RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker, "Randomness Requirements for Security", BCP 106, RFC 4086, DOI 10.17487/RFC4086, June 2005, . [RFC4108] Housley, R., "Using Cryptographic Message Syntax (CMS) to Protect Firmware Packages", RFC 4108, DOI 10.17487/RFC4108, August 2005, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [RFC8555] Barnes, R., Hoffman-Andrews, J., McCarney, D., and J. Kasten, "Automatic Certificate Management Environment (ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019, . [RFC8809] Hodges, J., Mandyam, G., and M. Jones, "Registries for Web Authentication (WebAuthn)", RFC 8809, DOI 10.17487/RFC8809, August 2020, . [WebAuthn] Hodges, J., Jones, J., Jones, M. B., Kumar, A., and E. Lundberg, "Web Authentication: An API for accessing Public Key Credentials Level 2", April 2021, . Appendix A. Enterprise PKI ACME was originally envisioned for issuing certificates in the Web PKI, however this extension will primarily be useful in enterprise PKI. The subsection below covers some operational considerations for an ACME-based enterprise CA. Weeks Expires 25 January 2024 [Page 12] Internet-Draft ACME DA July 2023 A.1. External Account Binding An enterprise CA likely only wants to receive requests from authorized devices. It is RECOMMENDED that the server require a value for the "externalAccountBinding" field to be present in "newAccount" requests. If an enterprise CA desires to limit the number of certificates that can be requested with a given account, including limiting an account to a single certificate. After the desired number of certificates have been issued to an account, the server MAY revoke the account as described in Section 7.1.2 of [RFC8555]. Acknowledgments TODO acknowledge. Author's Address Brandon Weeks Google Email: bweeks@google.com Weeks Expires 25 January 2024 [Page 13]