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Security Automated Certificate Management Environment acme 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. 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 .

Introduction The dns-01 challenge specified in section 8.4 of 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 (), operators can generate and configure the necessary DNS records in advance. This document does not deprecate the dns-01 challenge specified in . 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.

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 when, and only when, they appear in all capitals, as shown here.

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 records. 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.

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 for information on additional requirements for secure randomness. Example challenge object:

Challenge Fulfillment To fulfill this challenge, a client performs the following steps: Construct Key Authorization Construct a key authorization from the token value provided in the challenge and the client's account key Compute the SHA-256 digest of the key authorization DNS Record Creation Construct the validation domain name by prepending the following two labels to the domain name being validated:

)[0:10]) || "._acme-challenge" ]]>

SHA-256 is the SHA hashing operation defined in [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 and its use throughout this document is case-insensitive ACCOUNT_URL is defined in as the value in the Location header field The || operator indicates concatenation of strings Provision a DNS TXT record under the constructed validation domain name, containing the base64url encoding (per Section 5 of , without padding) of the SHA-256 digest computed in step 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:

Example response to server:

Server Validation Upon receiving the challenge response, the server: Performs the typical JWS validation according to Constructs and stores the key authorization Computes the SHA-256 digest of the stored key authorization Computes the validation domain name using the KID value from the JWS message Queries for TXT records at the validation domain name Verifies that at least 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".

Errors The server SHOULD follow the guidelines set in 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.

Implementation Considerations As this challenge creates a 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, scheme, and full HTTP path. URL normalization MUST NOT be used differently by the server in the Location header and while validating challenges.

Security Considerations The same security considerations apply for the integrity of authorizations () and DNS security () 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. 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 (). 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 (), 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.

IANA Considerations

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

National Institute of Standards and Technology Public Key Infrastructure using X.509 (PKIX) certificates are used for a number of purposes, the most significant of which is the authentication of domain names. Thus, certification authorities (CAs) in the Web PKI are trusted to verify that an applicant for a certificate legitimately represents the domain name(s) in the certificate. As of this writing, this verification is done through a collection of ad hoc mechanisms. This document describes a protocol that a CA and an applicant can use to automate the process of verification and certificate issuance. The protocol also provides facilities for other certificate management functions, such as certificate revocation. In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Security systems are built on strong cryptographic algorithms that foil pattern analysis attempts. However, the security of these systems is dependent on generating secret quantities for passwords, cryptographic keys, and similar quantities. The use of pseudo-random processes to generate secret quantities can result in pseudo-security. A sophisticated attacker may find it easier to reproduce the environment that produced the secret quantities and to search the resulting small set of possibilities than to locate the quantities in the whole of the potential number space. Choosing random quantities to foil a resourceful and motivated adversary is surprisingly difficult. This document points out many pitfalls in using poor entropy sources or traditional pseudo-random number generation techniques for generating such quantities. It recommends the use of truly random hardware techniques and shows that the existing hardware on many systems can be used for this purpose. It provides suggestions to ameliorate the problem when a hardware solution is not available, and it gives examples of how large such quantities need to be for some applications. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. This document describes the commonly used base 64, base 32, and base 16 encoding schemes. It also discusses the use of line-feeds in encoded data, use of padding in encoded data, use of non-alphabet characters in encoded data, use of different encoding alphabets, and canonical encodings. [STANDARDS-TRACK] This RFC is the revised specification of the protocol and format used in the implementation of the Domain Name System. It obsoletes RFC-883. This memo documents the details of the domain name client - server communication. Brave Software Salesforce Aiven Akamai Technologies Cox Communications Many application services on the Internet need to verify ownership or control of a domain in the Domain Name System (DNS). The general term for this process is "Domain Control Validation", and can be done using a variety of methods such as email, HTTP/HTTPS, or the DNS itself. This document focuses only on DNS-based methods, which typically involve the Application Service Provider requesting a DNS record with a specific format and content to be visible in the domain to be verified. There is wide variation in the details of these methods today. This document provides some best practices to avoid known problems.

Acknowledgments