Automated Certificate Management Environment (ACME) Device Attestation Extension

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Table of Contents

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¶
• Chrome OS Verified Access¶
• Trusted Platform Module¶

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.¶

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:¶

• 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.¶

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

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.¶

Acknowledgments

TODO acknowledge.¶

Author's Address