title: "Automated Certificate Management Environment (ACME) Scoped DNS Challenges" abbrev: "ACME-SCOPED-DNS-CHALLENGES" category: std

docname: draft-ietf-acme-scoped-dns-challenges-00 v: 3 area: Security workgroup: Automated Certificate Management Environment keyword: acme ipr: full3978trust200902 venue: group: WG type: Working Group mail: acme@ietf.org arch: https://datatracker.ietf.org/wg/acme/about/ github: aaomidi/draft-ietf-acme-scoped-dns-challenges latest: https://aaomidi.github.io/draft-ietf-acme-scoped-dns-challenges

author:

fullname: Antonios A. Chariton
organization: Independent Contributor
email: daknob@daknob.net

fullname: Amir A. Omidi organization: Spirl email: amir@aaomidi.com
fullname: James Kasten organization: Google email: jdkasten@google.com
fullname: Fotis Loukos organization: Google email: fotisl@google.com
fullname: Stanislaw A. Janikowski organization: Google email: stanwise@google.com

normative: FIPS180-4: title: "Secure Hash Standard (SHS)" date: 2015-08 target: https://csrc.nist.gov/publications/detail/fips/180/4/final author: org: National Institute of Standards and Technology

informative: I-D.draft-ietf-dnsop-domain-verification-techniques:

--- 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.

--- middle

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.

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.

DNS-ACCOUNT-01

The dns-account-01 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 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.

Conventions and Definitions

{::boilerplate bcp14-tagged}

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.

{
    "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-" || <SCOPE> || "-challenge"
```
- SCOPE is
  - "host" if the associated authorization applies only to the specific host name and no labels beneath it
  - "wildcard" if the associated authorization is for a wildcard domain and contains the wildcard field set to true ({{?RFC8555, Section 7.1.4}})
  - "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.)

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 similiar 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".

Errors

The server SHOULD follow the guidelines set in {{!RFC8555, Section 6.7}} for error conditions that occur during challenge validation.

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(<ACCOUNT_RESOURCE_URL>)[0:10]) || "._acme-" || <SCOPE> || "-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_RESOURCE_URL is defined in {{!RFC8555, Section 7.3}} as the value in the Location header field
- SCOPE is
  - "host" if the associated authorization applies only to the specific host name and no labels beneath it
  - "wildcard" if the associated authorization is for a wildcard domain and contains the wildcard field set to true ({{?RFC8555, Section 7.1.4}})
  - "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/evOfKhNU60wg",
    "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:

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".

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.

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.

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}}.

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}}).

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.

IANA Considerations

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

--- back

Acknowledgments

{:numbered="false"}

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

draft-ietf-acme-scoped-dns-challenges.mkd

draft-ietf-acme-scoped-dns-challenges.mkd

author:

Introduction

DNS-02

DNS-ACCOUNT-01

Conventions and Definitions

DNS-02 Challenge

Errors

DNS-ACCOUNT-01 Challenge

Errors

Implementation Considerations

Security Considerations

DNS-ACCOUNT-01

IANA Considerations

ACME Validation Method

Acknowledgments

Files

draft-ietf-acme-scoped-dns-challenges.mkd

Latest commit

History

draft-ietf-acme-scoped-dns-challenges.mkd

File metadata and controls

author:

Introduction

DNS-02

DNS-ACCOUNT-01

Conventions and Definitions

DNS-02 Challenge

Errors

DNS-ACCOUNT-01 Challenge

Errors

Implementation Considerations

Security Considerations

DNS-ACCOUNT-01

IANA Considerations

ACME Validation Method

Acknowledgments