Automated Certificate Management Environment (ACME) DNS Labeled With ACME Account ID Challenge

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Table of Contents

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.¶

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".¶
• 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"

(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".¶

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

RR Type: TXT
_NODE NAME: _acme-challenge_*
Reference: This document

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

Acknowledgments

Authors' Addresses