Automated Certificate Management Environment (ACME) Device Attestation Extension
draft-bweeks-acme-device-attest-00
The information below is for an old version of the document.
| Document | Type |
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|---|---|---|---|
| Author | Brandon Weeks | ||
| Last updated | 2022-05-17 | ||
| Replaced by | draft-acme-device-attest | ||
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draft-bweeks-acme-device-attest-00
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.
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This Internet-Draft will expire on 18 November 2022.
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document authors. All rights reserved.
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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.
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* 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.
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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.
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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:
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+======================+===========+
| 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,
<https://www.rfc-editor.org/rfc/rfc2119>.
[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>.
[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC4086, June 2005,
<https://www.rfc-editor.org/rfc/rfc4086>.
[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>.
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[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/>.
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
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