Automated Certificate Management Environment (ACME) Device Attestation Extension
draft-ietf-acme-device-attest-01

ACME Working Group                                              B. Weeks
Internet-Draft                                                          
Intended status: Standards Track                              G. Mallaya
Expires: 2 August 2026                                                  
                                                               S. Rajala
                                                         29 January 2026

 Automated Certificate Management Environment (ACME) Device Attestation
                               Extension
                    draft-ietf-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.

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   This Internet-Draft will expire on 2 August 2026.

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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.  Operational Considerations  . . . . . . . . . . . . . . . . .   6
     6.1.  Enterprise PKI  . . . . . . . . . . . . . . . . . . . . .   6
       6.1.1.  External Account Binding  . . . . . . . . . . . . . .   6
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
     8.1.  ACME Identifier Types . . . . . . . . . . . . . . . . . .   7
     8.2.  ACME Validation Method  . . . . . . . . . . . . . . . . .   7
     8.3.  New Error Types . . . . . . . . . . . . . . . . . . . . .   8
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

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/)

   *  Managed Device Attestation for Apple Devices
      (https://support.apple.com/en-om/guide/deployment/dep28afbde6a/
      web)

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   Using ACME and device attestation to issue client certificates for
   enterprise PKI is to be a 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.

   Efforts are underway within the Remote ATtestation ProcedureS (RATS)
   working group to define a set of standard formats and protocols for
   attestation.  An explict aim of this document is to support vendor
   specific formats and protocols that are widely deployed at
   publication time of this specification.

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].

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   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.

4.  Hardware Module

   A new identifier type, "hardware-module" is introduced to represent
   the identity of the secure cryptoprocessor that generated the
   certificate key.

   The hardware module identity can be included in the Subject Alternate
   Name Extension using the HardwareModuleName form described in
   [RFC4108].  The HardwareModuleName is encoded as an otherName with
   the OID id-on-hardwareModuleName (1.3.6.1.5.5.7.8.4) and consists of:

   *  hwType: An OBJECT IDENTIFIER that identifies the type of hardware
      module

   *  hwSerialNum: An OCTET STRING containing the hardware module serial
      number

   Clients MAY include this identifier in the certificate signing
   request (CSR).  When included in a CSR, it MUST appear in an
   extensionRequest attribute [RFC2985] requesting a subjectAltName
   extension.

   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.

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   {
     "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
   (Section 8.1 of [RFC8555]) 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:

   *  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.

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   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"
   }

   The webauthn payload MAY contain any identifiers registered in
   "WebAuthn Attestation Statement Format Identifiers" and any
   extensions registered in "WebAuthn Extension Identifiers"
   [IANA-Webauthn], [RFC8809].

6.  Operational Considerations

   Although this document focuses guidance on implementing new type and
   challenge for certificate issuance using ACME, it does not define a
   New Protocol, a Protocol Extension, or an architecture.

6.1.  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.

6.1.1.  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].

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7.  Security Considerations

   Please reference [RFC8555] for other security considerations.

   See Section 13 of [WebAuthn] for additional security considerations
   related to attestation statement formats, including certificate
   revocation.

   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.

   The "token" value MUST have at least 128 bits of entropy.  It MUST
   NOT contain any characters outside the base64url alphabet, including
   padding characters ("=").  See [I-D.ietf-tls-rfc8446bis],
   Appendix C.1 for additional information on randomness requirements.

8.  IANA Considerations

8.1.  ACME Identifier Types

   The "ACME Identifier Types" registry is to be updated to include the
   following entries:

                   +======================+===========+
                   | Label                | Reference |
                   +======================+===========+
                   | permanent-identifier | RFC XXXX  |
                   +----------------------+-----------+
                   | hardware-module      | RFC XXXX  |
                   +----------------------+-----------+

                                 Table 1

8.2.  ACME Validation Method

   The "ACME Validation Methods" registry is to be updated to include
   the following entry:

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      +==================+======================+======+===========+
      | Label            | Identifier Type      | ACME | Reference |
      +==================+======================+======+===========+
      | device-attest-01 | permanent-identifier | Y    | RFC XXXX  |
      +------------------+----------------------+------+-----------+

                                 Table 2

8.3.  New Error Types

   This document adds the following entries to the ACME Error Type
   registry:

    +=========================+===========================+===========+
    | 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

9.  References

9.1.  Normative References

   [I-D.ietf-tls-rfc8446bis]
              Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", Work in Progress, Internet-Draft, draft-
              ietf-tls-rfc8446bis-14, 13 September 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-tls-
              rfc8446bis-14>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/rfc/rfc2119>.

   [RFC2985]  Nystrom, M. and B. Kaliski, "PKCS #9: Selected Object
              Classes and Attribute Types Version 2.0", RFC 2985,
              DOI 10.17487/RFC2985, November 2000,
              <https://www.rfc-editor.org/rfc/rfc2985>.

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   [RFC4043]  Pinkas, D. and T. Gindin, "Internet X.509 Public Key
              Infrastructure Permanent Identifier", RFC 4043,
              DOI 10.17487/RFC4043, May 2005,
              <https://www.rfc-editor.org/rfc/rfc4043>.

   [RFC4108]  Housley, R., "Using Cryptographic Message Syntax (CMS) to
              Protect Firmware Packages", RFC 4108,
              DOI 10.17487/RFC4108, August 2005,
              <https://www.rfc-editor.org/rfc/rfc4108>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.

   [RFC8555]  Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
              Kasten, "Automatic Certificate Management Environment
              (ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019,
              <https://www.rfc-editor.org/rfc/rfc8555>.

   [RFC8809]  Hodges, J., Mandyam, G., and M. Jones, "Registries for Web
              Authentication (WebAuthn)", RFC 8809,
              DOI 10.17487/RFC8809, August 2020,
              <https://www.rfc-editor.org/rfc/rfc8809>.

   [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,
              <https://www.w3.org/TR/webauthn-2/>.

9.2.  Informative References

   [IANA-Webauthn]
              "IANA Registries for Web Authentication (WebAuthn)", n.d.,
              <https://www.iana.org/assignments/webauthn/
              webauthn.xhtml>.

Acknowledgments

   Thank you for all the reivews from the work group.

Authors' Addresses

   Brandon Weeks
   Email: me@brandonweeks.com

   Ganesh Mallaya
   Email: ganesh.mallaya@appviewx.com

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   Sven Rajala
   Email: sven.rajala@keyfactor.com

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