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Security ACME Working Group acme 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. 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 . Source for this draft and an issue tracker can be found at .

Introduction The dns-01 challenge specified in section 8.4 of 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. This RFC does not intend to deprecate the dns-01 challenge specified in . 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.

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 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 for additional information on additional requirements for secure randomness.

A client can fulfill this challenge by performing the following steps: 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 Construct the validation domain name by prepending the following label to the domain name being validated:

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 SHA256 operation base32 is the operation defined in Account Resource URL is defined in 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:

(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

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 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".

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.

IANA Considerations

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

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. Federal Information Processing Standard, FIPS 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]

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