Internet DRAFT - draft-ietf-acme-dns-account-challenge
draft-ietf-acme-dns-account-challenge
Automated Certificate Management Environment A. A. Chariton
Internet-Draft Independent Contributor
Intended status: Standards Track A. A. Omidi
Expires: 13 July 2024 Spirl
J. Kasten
F. Loukos
S. A. Janikowski
Google
10 January 2024
Automated Certificate Management Environment (ACME) DNS Labeled With
ACME Account ID Challenge
draft-ietf-acme-dns-account-challenge-01
Abstract
This document outlines a new challenge for the ACME protocol,
enabling an ACME client to answer a domain control validation
challenge from an ACME server using a DNS resource linked to the ACME
Account ID. This allows multiple systems or environments to handle
challenge-solving for a single 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://aaomidi.github.io/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-
challenge/.
Discussion of this document takes place on the WG Working Group
mailing list (mailto:acme@ietf.org), which is archived at
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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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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3
3. DNS-ACCOUNT-01 Challenge . . . . . . . . . . . . . . . . . . 3
3.1. Errors . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.2. Implementation Considerations . . . . . . . . . . . . . . 6
4. Security Considerations . . . . . . . . . . . . . . . . . . . 6
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
5.1. DNS Parameters . . . . . . . . . . . . . . . . . . . . . 7
5.2. ACME Validation Method . . . . . . . . . . . . . . . . . 7
6. Normative References . . . . . . . . . . . . . . . . . . . . 7
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
The dns-01 challenge specified in section 8.4 of [RFC8555] requires
that ACME clients validate the domain under the _acme-challenge label
for the TXT record. This unique label creates an impediment limiting
the number of other entities domain validation can be delegated to.
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In multi-region deployments, where separate availability zones serve
the same content, and dependencies across them are avoided, operators
need a way to obtain a separate certificate per zone, for the same
domain name. Similarly, in cases of zero-downtime migration, two
different setups of the infrastructure may coexist for a long period
of time, and both need access to valid certificates.
Due to the uniqueness of the _acme-challenge label, operators today
have to pick a single ACME challenge solver for their domain name,
and then add a CNAME record to this infrastructure. A domain name
can only have one CNAME in DNS.
This document specifies a new challenge type, dns-account-01. This
challenge leverages the ACME Account Resource URL to present an
account-unique stable challenge to an ACME server. This challenge
allows any domain name to delegate its domain validation to more than
one service through ACME account-unique DNS records.
As now multiple labels can be used to prove domain control, and they
depend on the ACME account, any number of them can be generated in
advance, and then all required CNAME records can be created
statically. The dynamic part of the label depends on the ACME
account and not the account key, to allow for seamless account key
rollover without the label changing. This ensures very long-lived
labels, without any security considerations.
This RFC does not intend to deprecate the dns-01 challenge specified
in [RFC8555]. Since this new challenge does not modify or build on
any pre-existing challenges, the ability to complete the dns-
account-01 challenge requires ACME server operators to deploy new
changes to their codebase. This makes adopting and using 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.
3. DNS-ACCOUNT-01 Challenge
When the identifier being validated is a domain name, the client can
prove control of that domain by provisioning a TXT resource record
containing a designated value for a specific validation domain name.
* type (required, string): The string "dns-account-01".
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* 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.
{
"type": "dns-account-01",
"url": "https://example.com/acme/chall/i00MGYwLWIx",
"status": "pending",
"token": "ODE4OWY4NTktYjhmYS00YmY1LTk5MDgtZTFjYTZmNjZlYTUx"
}
A client can fulfill this challenge by performing the following
steps:
* 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
* Construct the validation domain name by prepending the following
label to the domain name being validated:
"_acme-challenge_" || base32(SHA-256(Account Resource URL)[0:9])
- SHA-256 is the SHA hashing operation defined in [RFC6234]
- [0:9] is the operation that selects the first ten bytes (bytes
0 through 9 inclusive) from the previous SHA256 operation
- base32 is the operation defined in [RFC4648]
- Account Resource 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
For example, if the domain name being validated is "www.example.org",
and the account URL of "https://example.com/acme/acct/ExampleAccount"
then the client would provision the following DNS record:
_acme-challenge_ujmmovf2vn55tgye.www.example.org 300 IN TXT "LoqXcYV8...jxAjEuX0.9jg46WB3...fm21mqTI"
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(In the above, "..." indicates that the token and the JWK thumbprint
in the key authorization have been truncated to fit on the page.)
Respond to the ACME server with an empty object ({}) to acknowledge
that the challenge can be validated 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({}),
"signature": "Q1bURgJoEslbD1c5...3pYdSMLio57mQNN4"
}
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 the dns-account-01 challenge, the server performs the
following steps:
* Compute the SHA-256 digest [FIPS180-4] of the stored key
authorization
* Compute the validation domain name with the account URL of the
ACME account requesting validation
* Query for TXT records for the validation domain name
* Verify that the contents of one of the TXT records match the
digest value
If all the above verifications succeed, then the validation is
successful. If no DNS record is found, or DNS record and response
payload do not pass these checks, then the server MUST fail the
validation and mark the challenge as invalid.
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".
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3.1. 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.
3.2. 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.
If this change occurs, the existing long-term CNAME records created
by all account holders will no longer be valid. The clients will not
be able to issue certificates automatically moving forward.
4. Security Considerations
As this challenge that is introduced only differs in the left-most
label of the domain name from the existing dns-01 challenge, the same
security considerations apply.
In terms of the construction of the label prepended to the domain
name, there is no need for a cryptographic hash. The purpose of that
is to create a long-lived and statistically distinctive record of
minimal size.
SHA-256 was picked due to its broad adoption, hardware support, and
existing need in implementations that would likely support dns-
account-01.
The first 10 bytes were picked as a tradeoff: the value needs to be
short enough to not significantly impact DNS record and response
size, 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 uniform
distribution of inputs, and not for integrity, we do not consider the
trimming a security issue.
5. IANA Considerations
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5.1. DNS Parameters
The Underscored and Globally Scoped DNS Node Names is to be updated
to include the following entry:
RR Type: TXT
_NODE NAME: _acme-challenge_*
Reference: This document
Where _acme-challenge_* denotes all node names beginning with the
string _acme-challenge_. It does NOT refer to a DNS wildcard
specification.
5.2. ACME Validation Method
The "ACME Validation Methods" registry is to be updated to include
the following entry:
label: dns-account-01
identifier-type: dns
ACME: Y
Reference: This document
6. 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>.
[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>.
[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>.
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[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>.
Acknowledgments
Authors' Addresses
Antonios A. Chariton
Independent Contributor
Email: daknob@daknob.net
Amir A. Omidi
Spirl
Email: amir@aaomidi.com
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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