ACME Working Group                                             H. Landau
Internet-Draft                                          October 19, 2016
Intended status: Informational
Expires: April 22, 2017

                 CA Account URI Binding for CAA Records


   The CAA DNS record allows a domain to communicate issuance policy to
   CAs, but only allows a domain to define policy with CA-level
   granularity.  However, the CAA specification also provides facilities
   for extension to admit more granular, CA-specific policy.  This
   specification defines two such parameters, one allowing specific
   accounts of a CA to be identified by URI and one allowing specific
   methods of domain control validation as defined by the ACME protocol
   to be required.

Status of This Memo

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   This Internet-Draft will expire on April 22, 2017.

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   include Simplified BSD License text as described in Section 4.e of
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Extensions to the CAA Record: account-uri Parameter . . . . .   2
     3.1.  Use with ACME . . . . . . . . . . . . . . . . . . . . . .   3
     3.2.  Use without ACME  . . . . . . . . . . . . . . . . . . . .   3
   4.  Extensions to the CAA Record: acme-methods Parameter  . . . .   4
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
     5.1.  URI Ambiguity . . . . . . . . . . . . . . . . . . . . . .   4
     5.2.  Authorization Freshness . . . . . . . . . . . . . . . . .   5
     5.3.  DNSSEC  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     5.4.  Use without DNSSEC  . . . . . . . . . . . . . . . . . . .   6
     5.5.  Restrictions Supercedeable by DNS Delegation  . . . . . .   6
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   7.  Normative References  . . . . . . . . . . . . . . . . . . . .   6
   Appendix A.  Examples . . . . . . . . . . . . . . . . . . . . . .   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   This specification defines two parameters for the 'issue' and
   'issuewild' properties of the Certification Authority Authorization
   (CAA) DNS resource record [RFC6844].  The first, 'account-uri',
   allows authorization conferred by a CAA policy to be restricted to
   specific accounts of a CA, which are identified by URIs.  The second,
   'acme-methods', allows the set of validation [I-D.ietf-acme-acme]
   methods supported by an ACME-based CA to validate domain control to
   be limited to a subset of the full set of methods which it supports.

2.  Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   and "OPTIONAL" are to be interpreted as described in BCP 14, RFC 2119
   [RFC2119] and indicate requirement levels for compliant CAA-URI

3.  Extensions to the CAA Record: account-uri Parameter

   A CAA parameter "account-uri" is defined for the 'issue' and
   'issuewild' properties defined by [RFC6844].  The value of this
   parameter, if specified, MUST be a URI [RFC3986] identifying a
   specific CA account.

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   "CA account" means an object maintained by a specific CA representing
   a specific entity, or group of related entities, which may request
   the issuance of certificates.

   The presence of this parameter constrains the property to which it is
   attached.  A CA MUST only consider a property with an "account-uri"
   parameter to authorize issuance where the URI specified is an URI
   that the CA recognises as identifying the account making a
   certificate issuance request.

   If a certificate issuance request is made to a CA such that no
   account URI is available, because the request is made in the absence
   of any account or the account has no URI assigned to it, a CA MUST
   NOT consider any property having an "account-uri" parameter as
   authorizing issuance.

   If an CA finds multiple CAA records pertaining to it (i.e., having
   property 'issue' or 'issuewild' as applicable and a domain that the
   CA recognises as its own) with different "account-uri" parameters,
   the CA MUST NOT consider the CAA record set to authorize issuance
   unless at least one of the specified account URIs identifies the
   account of the CA by which issuance is requested.  A property without
   an "account-uri" parameter matches any account.  A property with an
   invalid or unrecognised "account-uri" parameter is unsatisfiable.

   The presence of an "account-uri" parameter does not replace or
   supercede the need to validate the domain name specified in an
   "issue" or "issuewild" record in the manner described in the CAA
   specification.  CAs MUST still perform such verification.  For
   example, a CAA property which specifies a domain name belonging to CA
   A and an account URI identifying an account at CA B is unsatisfiable.

3.1.  Use with ACME

   An ACME [I-D.ietf-acme-acme] registration object MAY be identified by
   setting the "account-uri" parameter to the URI of the ACME
   registration object.

   Implementations of this specification which also implement ACME MUST
   recognise such URIs.

3.2.  Use without ACME

   The "account-uri" specification provides a general mechanism to
   identify entities which may request certificate issuance via URIs.
   The use of specific kinds of URI may be specified in future RFCs, and
   CAs not implementing ACME MAY assign and recognise their own URIs

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4.  Extensions to the CAA Record: acme-methods Parameter

   A CAA parameter "acme-methods" is also defined for the 'issue' and
   'issuewild' properties.  The value of this parameter, if specified,
   MUST be a comma-separated string of ACME challenge method names.  The
   use of this parameter is specific to ACME and CAs implementing it.

   The presence of this parameter constrains the property to which it is
   attached.  A CA MUST only consider a property with the "acme-methods"
   parameter to authorize issuance where the name of the challenge
   method being used is one of the names listed in the comma separated

   The special method value 'non-acme' is defined.  Where a CA supports
   ACME, but also allows the issuance of certificates by other means, it
   MUST ensure that all of its other issuance channels recognise the
   'acme-methods' parameter.  For the purposes of validation, such non-
   ACME transactions shall be considered to have a method name of 'non-
   acme'.  Thus, domains implementing CAA which wish to nominate a CA
   which supports issuance via both ACME and non-ACME means can choose
   whether to allow one or both.

5.  Security Considerations

   This specification describes an extension to the CAA record
   specification increasing the granularity at which CAA policy can be
   expressed.  This allows the set of entities capable of successfully
   requesting issuance of certificates for a given domain to be
   restricted beyond that which would otherwise be possible, while still
   allowing issuance for specific accounts of a CA.  This improves the
   security of issuance for domains which choose to employ it, when
   combined with a CA which implements this specification.

5.1.  URI Ambiguity

   Suppose that CA A recognises "" as identifying itself,
   CA B is a subsidiary of CA A which recognises both ""
   and "" as identifying itself.

   Suppose that both CA A and CA B issue account URIs of the form


   If the CA domain name in a CAA record is specified as ""
   then this could be construed as identifying account number 1234 at CA
   A or at CA B.  These may be different accounts, creating ambiguity.

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   Thus, CAs MUST ensure that the URIs they recognise as pertaining to a
   specific account of that CA are unique within the scope of all domain
   names which they recognise as identifying that CA for the purpose of
   CAA record validation.

   It is RECOMMENDED that CAs satisfy this requirement by using URIs
   which include an authority:


5.2.  Authorization Freshness

   The CAA specification governs the act of issuance by a CA.  In some
   cases, a CA may establish authorization for an account to request
   certificate issuance for a specific domain separately to the act of
   issuance itself.  Such authorization may occur substantially prior to
   a certificate issuance request.  The CAA policy expressed by a domain
   may have changed in the meantime, creating the risk that a CA will
   issue certificates in a manner inconsistent with the presently
   published CAA policy.

   CAs SHOULD consider adopting practices to reduce the risk of such
   circumstances.  Possible countermeasures include issuing
   authorizations with very limited validity periods, such as an hour,
   or revalidating the CAA policy for a domain at certificate issuance

5.3.  DNSSEC

   Where a domain chooses to secure its nameservers using DNSSEC, the
   authenticity of its DNS data can be assured, providing that a CA
   makes all DNS resolutions via an appropriate, trusted DNSSEC-
   validating resolver.  A domain can use this property to protect
   itself from the threat posed by a global adversary capable of
   performing man-in-the-middle attacks, which is not ordinarily
   mitigated by the "domain validation" model.

   In order to facilitate this, a CA validation process must either rely
   solely on information obtained via DNSSEC, or meaningfully bind the
   other parts of the validation transaction using material obtained via

   The CAA parameters described in this specification can be used to
   ensure that only validation methods meeting these criteria are used.
   In particular, a domain secured via DNSSEC SHOULD either:

   1.  Use the "account-uri" parameter to ensure that only accounts
       which it controls are authorized to obtain certificates, or

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   2.  Exclusively use validation methods which rely solely on
       information obtained via DNSSEC, and use the "acme-methods"
       parameter to ensure that only such methods are used.

5.4.  Use without DNSSEC

   Where a domain does not secure its nameservers using DNSSEC, or one
   or more of the CAs it authorizes do not perform CAA validation
   lookups using a trusted DNSSEC-validating resolver, use of the
   "account-uri" parameter does not confer additional security against
   an attacker capable of performing a man-in-the-middle attack against
   all validation attempts made by a CA, as such an attacker could
   simply fabricate the responses to DNS lookups for CAA records.

   In this case, the "account-uri" mechanism still provides an effective
   means of administrative control over issuance, except where control
   over DNS is subdelegated (see below).

5.5.  Restrictions Supercedeable by DNS Delegation

   Because CAA records are located during validation by walking up the
   DNS hierarchy until one or more records are found, the use of the
   "account-uri" parameter, or any CAA policy, is not an effective way
   to restrict or control issuance for subdomains of a domain, where
   control over those subdomains is delegated to another party (such as
   via DNS delegation or providing limited access to manage subdomain
   DNS records).

6.  IANA Considerations

   None.  As per the CAA specification, the parameter namespace for the
   CAA 'issue' and 'issuewild' properties has CA-defined semantics.
   This document merely specifies a RECOMMENDED semantic for a parameter
   of the name "account-uri".

7.  Normative References

              Barnes, R., Hoffman-Andrews, J., and J. Kasten, "Automatic
              Certificate Management Environment (ACME)", draft-ietf-
              acme-acme-03 (work in progress), July 2016.

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

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   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66, RFC
              3986, DOI 10.17487/RFC3986, January 2005,

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

   [RFC6844]  Hallam-Baker, P. and R. Stradling, "DNS Certification
              Authority Authorization (CAA) Resource Record", RFC 6844,
              DOI 10.17487/RFC6844, January 2013,

   [RFC7517]  Jones, M., "JSON Web Key (JWK)", RFC 7517, DOI 10.17487/
              RFC7517, May 2015,

Appendix A.  Examples

   The following shows an example DNS zone file fragment which nominates
   two account URIs as authorized to issue certificates for the domain
   "".  Issuance is restricted to the CA "". IN CAA 0 issue "; \
     account-uri=" IN CAA 0 issue "; \

   The following shows a zone file fragment which restricts the ACME
   methods which can be used; only ACME methods "dns-01" and "xyz-01"
   can be used. IN CAA 0 issue "; \

   The following shows a zone file fragment in which one account can be
   used to issue with the "dns-01" method and one account can be used to
   issue with the "http-01" method. IN CAA 0 issue "; \
     account-uri=; \
     acme-methods=dns-01" IN CAA 0 issue "; \
     account-uri=; \

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   The following shows a zone file fragment in which only ACME method
   "dns-01" can be used, but non-ACME methods of issuance are also
   allowed. IN CAA 0 issue "; \

Author's Address

   Hugo Landau


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