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Automated Certificate Management Environment (ACME) DNS Labeled With ACME Account ID Challenge
draft-ietf-acme-dns-account-label-00

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Document	Type	Active Internet-Draft (acme WG)
	Authors	Antonios Chariton , Amir Omidi , James Kasten , Fotis Loukos , Stanislaw A. Janikowski
	Last updated	2024-11-13
	Replaces	draft-ietf-acme-scoped-dns-challenges
	RFC stream	Internet Engineering Task Force (IETF)
	Intended RFC status	(None)
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	Additional resources	Mailing list discussion
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draft-ietf-acme-dns-account-label-00

Automated Certificate Management Environment A. A. Chariton
Internet-Draft Independent Contributor
Intended status: Standards Track A. A. Omidi
Expires: 17 May 2025 Spirl
J. Kasten
F. Loukos
S. A. Janikowski
Google
13 November 2024

Automated Certificate Management Environment (ACME) DNS Labeled With
ACME Account ID Challenge
draft-ietf-acme-dns-account-label-00

Abstract

This document outlines a new DNS-based challenge type for the ACME
protocol that enables multiple independent systems to authorize a
single domain name concurrently. By adding a unique label to the DNS
validation record name, the dns-account-01 challenge avoids CNAME
delegation conflicts inherent to the dns-01 challenge type. This is
particularly valuable for multi-region or multi-cloud deployments
that wish to rely upon DNS-based domain control validation and need
to independently obtain certificates for the same domain.

About This Document

This note is to be removed before publishing as an RFC.

The latest revision of this draft can be found at
https://github.com/aaomidi/draft-ietf-acme-dns-account-challenge.
Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-ietf-acme-dns-account-label/.

Discussion of this document takes place on the WG Working Group
mailing list (mailto:acme@ietf.org), which is archived at
https://datatracker.ietf.org/wg/acme/about/. Subscribe at
https://www.ietf.org/mailman/listinfo/acme/.

Source for this draft and an issue tracker can be found at
https://github.com/aaomidi/draft-ietf-acme-dns-account-challenge.

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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Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on 17 May 2025.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
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Please review these documents carefully, as they describe your rights
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provided without warranty as described in the Revised BSD License.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3
3. DNS-ACCOUNT-01 Challenge . . . . . . . . . . . . . . . . . . 4
3.1. Challenge Definition . . . . . . . . . . . . . . . . . . 4
3.2. Challenge Fulfillment . . . . . . . . . . . . . . . . . . 4
3.3. Server Validation . . . . . . . . . . . . . . . . . . . . 5
3.4. Errors . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.5. Implementation Considerations . . . . . . . . . . . . . . 6
4. Security Considerations . . . . . . . . . . . . . . . . . . . 6
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
5.1. ACME Validation Method . . . . . . . . . . . . . . . . . 7
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
6.1. Normative References . . . . . . . . . . . . . . . . . . 7
6.2. Informative References . . . . . . . . . . . . . . . . . 8
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8

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1. Introduction

The dns-01 challenge specified in section 8.4 of [RFC8555] uses a
single DNS authorization label (_acme-challenge) for domain
validation. This single-label approach creates a limitation in
domain validation: each domain can only delegate its validation to
one ACME client at a time. Since delegation requires the use of
CNAME records, of which only one can exist per DNS name, operators
are forced to choose a single ACME challenge solver for their domain
name.

This limitation becomes particularly problematic in modern deployment
architectures. In multi-region deployments, separate availability
zones serve the same content while avoiding cross-zone dependencies.
These zones need to independently obtain and manage certificates for
the same domain name. Similarly, during zero-downtime migrations,
two different infrastructure setups may coexist for extended periods,
with both requiring access to valid certificates. Other use cases
include multi-CDN deployments and the provision of backup
certificates for use when an active certificate must be quickly
revoked.

This document specifies a new challenge type: dns-account-01, which
addresses these operational needs. The dns-account-01 challenge
incorporates the ACME account URL into the DNS validation record
name, allowing multiple independent ACME clients to perform domain
validation concurrently. Since these authorization labels depend on
the ACME account KID ([RFC8555], Section 7.3), operators can generate
and configure the necessary DNS records in advance.

This RFC does not deprecate the dns-01 challenge specified in
[RFC8555]. The ability to complete the dns-account-01 challenge
requires ACME server operators to deploy new code, making adoption of
this challenge an opt-in process.

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.

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3. DNS-ACCOUNT-01 Challenge

The dns-account-01 challenge allows a client to prove control of a
domain name by provisioning a TXT resource record containing a
designated value for a specific validation domain name. It leverages
the ACME account URL to construct a unique but stable validation
domain name. The ACME server validates control of the domain name by
performing one or more DNS queries to this validation domain name,
following CNAME records, to arrive at one or more TXT resource
record. The ACME server verifies that the contents of one or more of
these TXT record(s) match the digest value of the key authorization
that is constructed from the token value provided in the challenge.

3.1. Challenge Definition

The challenge object contains the following fields:

* type (required, string): The string "dns-account-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 additional requirements for secure randomness.

Example challenge object:

{
"type": "dns-account-01",
"url": "https://example.com/acme/chall/i00MGYwLWIx",
"status": "pending",
"token": "ODE4OWY4NTktYjhmYS00YmY1LTk5MDgtZTFjYTZmNjZlYTUx"
}

3.2. Challenge Fulfillment

To fulfill this challenge, a client performs the following steps:

1. Construct Key Authorization - Construct a key authorization
[RFC8555], Section 8.1 from the token value provided in the
challenge and the client's account key - Compute the SHA-256
digest [FIPS180-4] of the key authorization

2. DNS Record Creation - Construct the validation domain name by
prepending the following two labels to the domain name being
validated:

"_" || base32(SHA-256(<ACCOUNT_URL>)[0:10]) || "._acme-challenge"

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* SHA-256 is the SHA hashing operation defined in [RFC6234]

* [0:10] is the operation that selects the first ten bytes
(bytes 0 through 9 inclusive) from the previous SHA-256
operation

* base32 is the operation defined in [RFC4648]

* ACCOUNT_URL is defined in [RFC8555], Section 7.3 as the value
in the Location header field

* The || operator indicates concatenation of strings

* Provision a DNS TXT record with the base64url digest value under
the constructed domain validation name

1. Challenge Response - Respond to the ACME server with an empty
object ({}) to acknowledge that the challenge can be validated by
the server

Example DNS record for domain example.org with account URL
https://example.com/acme/acct/ExampleAccount:

_ujmmovf2vn55tgye._acme-challenge.example.org 300 IN TXT "LoqXcYV8...jxAjEuX0.9jg46WB3...fm21mqTI"

Example response to server:

POST /acme/chall/Rg5dV14Gh1Q
Host: example.com
Content-Type: application/jose+json

{
"protected": base64url({
"alg": "ES256",
"kid": "https://example.com/acme/acct/ExampleAccount",
"nonce": "SS2sSl1PtspvFZ08kNtzKd",
"url": "https://example.com/acme/chall/Rg5dV14Gh1Q"
}),
"payload": base64url({}),
"signature": "Q1bURgJoEslbD1c5...3pYdSMLio57mQNN4"
}

3.3. Server Validation

Upon receiving the challenge response, the server:

1. Performs the typical JWS validation.

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2. Constructs and stores the key authorization

3. Computes the SHA-256 digest [FIPS180-4] of the stored key
authorization

4. Computes the validation domain name using the KID value from the
JWS message

5. Queries for TXT records at the validation domain name

6. Verifies that one TXT record matches the computed digest value

The validation succeeds only if all verifications pass. The server
MUST mark the challenge as invalid if any verification fails.

The client SHOULD de-provision the resource record(s) provisioned for
this challenge once the challenge is complete, i.e., once the
"status" field of the challenge has the value "valid" or "invalid".

3.4. Errors

The server SHOULD follow the guidelines set in [RFC8555], Section 6.7
for error conditions that occur during challenge validation.

If the server is unable to find a TXT record for the validation
domain name, it SHOULD include the account URL it used to construct
the validation domain name in the problem document. Clients MUST NOT
use or rely on the presence of this field to construct the validation
domain name.

3.5. Implementation Considerations

As this challenge creates strong dependency on the kid account
identifier, the server SHOULD ensure that the account identifier is
not changed during the lifetime of the account. This contains the
entire URI, including the ACME endpoint domain name, port, and full
HTTP path.

4. Security Considerations

The same security considerations apply for the integrity of
authorizations ([RFC8555], Section 10.2) and DNS security ([RFC8555],
Section 11.2) as in the original specification for dns-01.

To allow for seamless account key rollover without the label
changing, the dynamic part of the label depends on the ACME account
and not the account key. This allows for long-lived labels, without
the security considerations of keeping the account key static.

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In terms of the construction of the account label prepended to the
domain name, there is no need for a cryptographic hash. The goal is
to simply create a long-lived and statistically distinct label of
minimal size. SHA-256 was chosen due to its existing use in the
dns-01 challenge ([RFC8555], Section 8.1).

The first 10 bytes were picked as a tradeoff: the value needs to be
short enough to stay lower than the size limits for DNS ([RFC1035],
Section 2.3.4), long enough to provide sufficient probability of
collision avoidance across ACME accounts, and just the right size to
have Base32 require no padding. As the algorithm is used for a
uniform distribution of inputs, and not for integrity, we do not
consider the trimming a security issue.

5. IANA Considerations

5.1. ACME Validation Method

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

label: dns-account-01
identifier-type: dns
ACME: Y
Reference: This document

6. References

6.1. Normative References

[FIPS180-4]
National Institute of Standards and Technology, "Secure
Hash Standard (SHS)", August 2015,
<https://csrc.nist.gov/publications/detail/fips/180/4/
final>.

[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <https://www.rfc-editor.org/rfc/rfc1035>.

[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>.

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[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>.

[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/rfc/rfc4648>.

[RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
(SHA and SHA-based HMAC and HKDF)", RFC 6234,
DOI 10.17487/RFC6234, May 2011,
<https://www.rfc-editor.org/rfc/rfc6234>.

[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>.

6.2. Informative References

[I-D.draft-ietf-dnsop-domain-verification-techniques]
Sahib, S. K., Huque, S., Wouters, P., and E. Nygren,
"Domain Control Validation using DNS", Work in Progress,
Internet-Draft, draft-ietf-dnsop-domain-verification-
techniques-06, 21 October 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-dnsop-
domain-verification-techniques-06>.

Acknowledgments

Authors' Addresses

Antonios A. Chariton
Independent Contributor
Email: daknob@daknob.net

Amir A. Omidi
Spirl
Email: amir@aaomidi.com

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James Kasten
Google
Email: jdkasten@google.com

Fotis Loukos
Google
Email: fotisl@google.com

Stanislaw A. Janikowski
Google
Email: stanwise@google.com

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Automated Certificate Management Environment (ACME) DNS Labeled With ACME Account ID Challenge draft-ietf-acme-dns-account-label-00

Automated Certificate Management Environment (ACME) DNS Labeled With ACME Account ID Challenge
draft-ietf-acme-dns-account-label-00