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

Document Type Active Internet-Draft (acme WG)
Authors Antonios A. Chariton , Amir Omidi , James Kasten , Fotis Loukos , Stanislaw A. Janikowski
Last updated 2023-01-09
Replaces draft-todo-chariton-dns-account-01
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status (None)
Additional resources Mailing list discussion
Stream WG state WG Document
On agenda acme at IETF-116
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IESG IESG state I-D Exists
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Automated Certificate Management Environment              A. A. Chariton
Internet-Draft                                               A. A. Omidi
Intended status: Standards Track                               J. Kasten
Expires: 11 July 2023                                          F. Loukos
                                                        S. A. Janikowski
                                                          7 January 2023

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


   This document specifies a new challenge type for the Automated
   Certificate Management Environment (ACME) protocol which allows an
   ACME client to respond to a domain control validation challenge
   presented by an ACME server with a DNS resource that is keyed by the
   ACME account identification.

About This Document

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

   The latest revision of this draft can be found at  Status
   information for this document may be found at

   Discussion of this document takes place on the WG Working Group
   mailing list (, which is archived at  Subscribe at

   Source for this draft and an issue tracker can be found at

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 11 July 2023.

Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (
   license-info) in effect on the date of publication of this document.
   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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   3
   3.  DNS-ACCOUNT-01 Challenge  . . . . . . . . . . . . . . . . . .   3
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
     5.1.  DNS Parameters  . . . . . . . . . . . . . . . . . . . . .   6
     5.2.  ACME Validation Method  . . . . . . . . . . . . . . . . .   6
   6.  Normative References  . . . . . . . . . . . . . . . . . . . .   6
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

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.

   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.

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   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",
   "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": "",
       "status": "pending",
       "token": "ODE4OWY4NTktYjhmYS00YmY1LTk5MDgtZTFjYTZmNjZlYTUx"

   A client can fulfill this challenge by performing the following

   *  Construct a key authorization 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])

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      -  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 "",
   and the account URL of ""
   then the client would provision the following DNS record: 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
   Content-Type: application/jose+json

     "protected": base64url({
       "alg": "ES256",
       "kid": "",
       "nonce": "SS2sSl1PtspvFZ08kNtzKd",
       "url": ""
     "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:

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   *  Compute the SHA-256 digest [FIPS180-4] of the stored key

   *  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-

   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

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

              National Institute of Standards and Technology, "Secure
              Hash Standard (SHS)", August 2015,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC4086]  Eastlake 3rd, D., Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106, RFC 4086,
              DOI 10.17487/RFC4086, June 2005,

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,

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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,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [RFC8555]  Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
              Kasten, "Automatic Certificate Management Environment
              (ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019,


Authors' Addresses

   Antonios A. Chariton

   Amir A. Omidi

   James Kasten

   Fotis Loukos

   Stanislaw A. Janikowski

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