ACME Working Group B. Weeks Internet-Draft Google Intended status: Standards Track 17 May 2022 Expires: 18 November 2022 Automated Certificate Management Environment (ACME) Device Attestation Extension draft-bweeks-acme-device-attest-00 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 18 November 2022. Copyright Notice Copyright (c) 2022 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 18 November 2022 [Page 1] Internet-Draft ACME DA May 2022 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3 3. Permanent Identifier . . . . . . . . . . . . . . . . . . . . 3 4. Hardware Module . . . . . . . . . . . . . . . . . . . . . . . 3 5. Device Attestation Challenge . . . . . . . . . . . . . . . . 4 6. Security Considerations . . . . . . . . . . . . . . . . . . . 5 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5 7.1. ACME Identifier Types . . . . . . . . . . . . . . . . . . 5 7.2. ACME Validation Method . . . . . . . . . . . . . . . . . 6 8. Normative References . . . . . . . . . . . . . . . . . . . . 6 Appendix A. Enterprise PKI . . . . . . . . . . . . . . . . . . . 7 A.1. External Account Binding . . . . . . . . . . . . . . . . 7 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 7 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7 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 if 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/) 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 and hardware-module identifier types. Weeks Expires 18 November 2022 [Page 2] Internet-Draft ACME DA May 2022 * 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. 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]. The server MAY allow the client to 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. 4. Hardware Module A new identifier type, "hardware-module" is introduced to represent the identity of the secure cryptoprocessor, if any, that generated the certificate key. (TODO describe the certificate representation) 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. Weeks Expires 18 November 2022 [Page 3] Internet-Draft ACME DA May 2022 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 an 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 with these modification: * 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. Weeks Expires 18 November 2022 [Page 4] Internet-Draft ACME DA May 2022 On receiving a response, the server constructs and stores the key authorization from the challenge "token" value and the current client account key. To validate a device attestation challenge, the server performs the following steps: 1. Perform the verification proceedures 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 TODO Security 7. IANA Considerations 7.1. ACME Identifier Types The "ACME Validation Methods" registry is to be updated to include the following entry: Weeks Expires 18 November 2022 [Page 5] Internet-Draft ACME DA May 2022 +======================+===========+ | 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 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, . [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, . Weeks Expires 18 November 2022 [Page 6] Internet-Draft ACME DA May 2022 [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. 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 18 November 2022 [Page 7]