Internet DRAFT - draft-bweeks-acme-device-attest
draft-bweeks-acme-device-attest
ACME Working Group B. Weeks
Internet-Draft Google
Intended status: Standards Track 7 August 2022
Expires: 8 February 2023
Automated Certificate Management Environment (ACME) Device Attestation
Extension
draft-bweeks-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.
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This Internet-Draft will expire on 8 February 2023.
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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. Security Considerations . . . . . . . . . . . . . . . . . . . 5
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
7.1. ACME Identifier Types . . . . . . . . . . . . . . . . . . 6
7.2. ACME Validation Method . . . . . . . . . . . . . . . . . 6
7.3. Attestation statement formats . . . . . . . . . . . . . . 6
7.3.1. WebAuthn Attestation Statement Format Identifiers for
Certificate Request Protocols . . . . . . . . . . . . 7
7.3.2. WebAuthn Extension Identifiers for Certificate Request
Protocols . . . . . . . . . . . . . . . . . . . . . . 9
8. Normative References . . . . . . . . . . . . . . . . . . . . 11
Appendix A. Enterprise PKI . . . . . . . . . . . . . . . . . . . 12
A.1. External Account Binding . . . . . . . . . . . . . . . . 12
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 12
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 12
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/)
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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.
* 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.
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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:
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* 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.
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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. 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:
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* 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].
7.3.1. 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 7.3.1.1.
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.
7.3.1.1. 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).
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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 7.3.1.
* 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.
* 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.
7.3.1.2. Registration Request Processing
As noted in Section 7.3.1.1, WebAuthn attestation statement format
identifiers are registered using the Specification Required policy.
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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.
7.3.1.3. 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)
7.3.2. 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 7.3.2.1.
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.
7.3.2.1. Registering Extension Identifiers
WebAuthn extension identifiers are registered using the Specification
Required policy (see Section 4.6 of [RFC8126]).
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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 7.3.2.
* Description:
- A relatively short description of the extension.
* Specification Document(s):
- Reference to the document or documents that specify the
extension.
* 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.
7.3.2.2. Registration Request Processing
As noted in Section 7.3.2.1, WebAuthn extension identifiers are
registered using the Specification Required policy.
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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.
7.3.2.3. 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)
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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[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/>.
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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