Automated Certificate Management Environment A. A. Chariton Internet-Draft Independent Contributor Intended status: Standards Track A. A. Omidi Expires: 20 February 2025 Spirl J. Kasten F. Loukos S. A. Janikowski Google 19 August 2024 Automated Certificate Management Environment (ACME) Scoped DNS Challenges draft-ietf-acme-scoped-dns-challenges-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-scoped-dns-challenges. Status information for this document may be found at https://datatracker.ietf.org/doc/draft-ietf-acme-scoped-dns- challenges/. 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-scoped-dns-challenges. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Chariton, et al. Expires 20 February 2025 [Page 1] Internet-Draft ACME-SCOPED-DNS-CHALLENGES August 2024 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 https://datatracker.ietf.org/drafts/current/. 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 20 February 2025. Copyright Notice Copyright (c) 2024 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 (https://trustee.ietf.org/ license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. DNS-02 . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.2. DNS-ACCOUNT-01 . . . . . . . . . . . . . . . . . . . . . 4 2. Conventions and Definitions . . . . . . . . . . . . . . . . . 4 3. DNS-02 Challenge . . . . . . . . . . . . . . . . . . . . . . 4 3.1. Errors . . . . . . . . . . . . . . . . . . . . . . . . . 6 4. DNS-ACCOUNT-01 Challenge . . . . . . . . . . . . . . . . . . 7 4.1. Errors . . . . . . . . . . . . . . . . . . . . . . . . . 9 4.2. Implementation Considerations . . . . . . . . . . . . . . 9 5. Security Considerations . . . . . . . . . . . . . . . . . . . 9 5.1. DNS-ACCOUNT-01 . . . . . . . . . . . . . . . . . . . . . 10 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 6.1. ACME Validation Method . . . . . . . . . . . . . . . . . 10 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 10 7.1. Normative References . . . . . . . . . . . . . . . . . . 10 7.2. Informative References . . . . . . . . . . . . . . . . . 11 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 12 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12 Chariton, et al. Expires 20 February 2025 [Page 2] Internet-Draft ACME-SCOPED-DNS-CHALLENGES August 2024 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 label creates several limitations in domain validation. First, the _acme-challenge label does not specify if the authorization is intended for a specific host, a wildcard domain, or a domain and all of its subdomains. Consequently, domain owners who may be delegating or provisioning authorization labels for a domain must concede control over the domain and all subdomains, violating the principle of least privilege. Furthermore, since each domain only has a single authorization label, it creates an impediment limiting the number of other entities domain validation can be delegated to. Delegating authorization to an entity requires the use of CNAME records, which can only used once per DNS name (or in this case, once per authorization label). This limitation requires operators to pick a single ACME challenge solver for their domain name. 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. This document specifies two new challenge types. dns-02 and dns- account-01, which aim to address these deficiencies. This work follows all recommendations set forth in "Domain Control Validation using DNS" [I-D.draft-ietf-dnsop-domain-verification-techniques]. This RFC does not intend to deprecate the dns-01 challenge specified in [RFC8555]. Since these new challenges do not modify any pre- existing challenges, the ability to complete the dns-02 or dns- account-01 challenge requires ACME server operators to deploy new changes to their codebase. This makes adopting and using these challenges an opt-in process. Chariton, et al. Expires 20 February 2025 [Page 3] Internet-Draft ACME-SCOPED-DNS-CHALLENGES August 2024 1.1. DNS-02 The dns-02 challenge adds onto dns-01 by introducing a scoping mechanism to the domain authorization label. This allows for the client to specify if the intended domain validation is for a specific host, a wildcard domain, or a domain and all of its subdomains. 1.2. DNS-ACCOUNT-01 The dns-account-01 challenge leverages the ACME account 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 unique per ACME account DNS records. With this new challenge, domain validation of the same DNS name can be done through different authorization labels. Since these authorization labels will depend on the ACME account KID ([RFC8555], Section 6.2), any number of them can be generated in advance. This allows all required CNAME records for domain validation delegation to be constructed statically. 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-02 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-02". * 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. Chariton, et al. Expires 20 February 2025 [Page 4] Internet-Draft ACME-SCOPED-DNS-CHALLENGES August 2024 { "type": "dns-02", "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 specifying the scope for the domain name being validated: "_acme-" || || "-challenge" - SCOPE is o "host" if the associated authorization applies only to the specific host name and no labels beneath it o "wildcard" if the associated authorization is for a wildcard domain and contains the wildcard field set to true ([RFC8555], Section 7.1.4) o "domain" if the authorization is for both the host and all subdomains by containing the subdomainAuthAllowed field set to true ([RFC9444], Section 4.1). - 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 the wildcard of *.example.org then the client would provision the following DNS record: _acme-wildcard-challenge.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.) Chariton, et al. Expires 20 February 2025 [Page 5] Internet-Draft ACME-SCOPED-DNS-CHALLENGES August 2024 * 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. To validate the dns-02 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 scope of the associated authorization similar to the client * 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". 3.1. Errors The server SHOULD follow the guidelines set in [RFC8555], Section 6.7 for error conditions that occur during challenge validation. Chariton, et al. Expires 20 February 2025 [Page 6] Internet-Draft ACME-SCOPED-DNS-CHALLENGES August 2024 4. 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 two labels to the domain name being validated: "_" || base32(SHA-256()[0:10]) || "._acme-" || || "-challenge" - 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 - SCOPE is o "host" if the associated authorization applies only to the specific host name and no labels beneath it Chariton, et al. Expires 20 February 2025 [Page 7] Internet-Draft ACME-SCOPED-DNS-CHALLENGES August 2024 o "wildcard" if the associated authorization is for a wildcard domain and contains the wildcard field set to true ([RFC8555], Section 7.1.4) o "domain" if the authorization is for both the host and all subdomains by containing the subdomainAuthAllowed field set to true ([RFC9444], Section 4.1). - 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 *.example.org, and the account URL of https://example.com/acme/acct/ ExampleAccount then the client would provision the following DNS record: _ujmmovf2vn55tgye._acme-wildcard-challenge.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/ExampleAccount", "nonce": "SS2sSl1PtspvFZ08kNtzKd", "url": "https://example.com/acme/chall/Rg5dV14Gh1Q" }), "payload": base64url({}), "signature": "Q1bURgJoEslbD1c5...3pYdSMLio57mQNN4" } On receiving this response, the server validates the message and 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: Chariton, et al. Expires 20 February 2025 [Page 8] Internet-Draft ACME-SCOPED-DNS-CHALLENGES August 2024 * Compute the SHA-256 digest [FIPS180-4] of the stored key authorization * Compute the validation domain name with the KID value in the JWS message * 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". 4.1. 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. 4.2. 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. This contains the entire URI, including the ACME endpoint domain name, port, and full HTTP path. 5. Security Considerations These challenges follow the recommendations set out in "Domain Control Validation using DNS" [I-D.draft-ietf-dnsop-domain-verification-techniques] for supporting multiple intermediates within the context of [RFC8555]. Chariton, et al. Expires 20 February 2025 [Page 9] Internet-Draft ACME-SCOPED-DNS-CHALLENGES August 2024 In both dns-account-01 and dns-02, 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. They both differ from dns-01 in that the challenges are scoped to the particular name being validated without relying upon CAA ([RFC8659]). 5.1. DNS-ACCOUNT-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. 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. 6. IANA Considerations 6.1. ACME Validation Method The "ACME Validation Methods" registry is to be updated to include the following entries: label: dns-02 identifier-type: dns ACME: Y Reference: This document label: dns-account-01 identifier-type: dns ACME: Y Reference: This document 7. References 7.1. Normative References Chariton, et al. Expires 20 February 2025 [Page 10] Internet-Draft ACME-SCOPED-DNS-CHALLENGES August 2024 [FIPS180-4] National Institute of Standards and Technology, "Secure Hash Standard (SHS)", August 2015, . [RFC1035] Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, November 1987, . [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, . [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, . [RFC8659] Hallam-Baker, P., Stradling, R., and J. Hoffman-Andrews, "DNS Certification Authority Authorization (CAA) Resource Record", RFC 8659, DOI 10.17487/RFC8659, November 2019, . 7.2. 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, Chariton, et al. Expires 20 February 2025 [Page 11] Internet-Draft ACME-SCOPED-DNS-CHALLENGES August 2024 Internet-Draft, draft-ietf-dnsop-domain-verification- techniques-05, 8 July 2024, . [RFC9444] Friel, O., Barnes, R., Hollebeek, T., and M. Richardson, "Automated Certificate Management Environment (ACME) for Subdomains", RFC 9444, DOI 10.17487/RFC9444, August 2023, . 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 Chariton, et al. Expires 20 February 2025 [Page 12]