ACME Account Key Binding via CAA Records
draft-landau-acme-caa-00

ACME Working Group                                             H. Landau
Internet-Draft                                            April 21, 2016
Intended status: Informational
Expires: October 23, 2016

                ACME Account Key Binding via CAA Records
                        draft-landau-acme-caa-00

Abstract

   The ACME protocol provides a means for hosts to automatically request
   and obtain X.509 certificates from certificate authorities.
   Certification authorities which implement ACME may also choose to
   implement the CAA DNS record, which allows a domain to communicate
   issuance policy to CAs.  The CAA specification alone 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 such a
   parameter.

Status of This Memo

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   This Internet-Draft will expire on October 23, 2016.

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   to this document.  Code Components extracted from this document must
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Extensions to the CAA Record  . . . . . . . . . . . . . . . .   2
     3.1.  acme-ak Parameter . . . . . . . . . . . . . . . . . . . .   2
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   3
     4.1.  DNSSEC  . . . . . . . . . . . . . . . . . . . . . . . . .   3
     4.2.  Authorization Freshness . . . . . . . . . . . . . . . . .   3
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   6.  Normative References  . . . . . . . . . . . . . . . . . . . .   4
   Appendix A.  Examples . . . . . . . . . . . . . . . . . . . . . .   4
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   4

1.  Introduction

   This specification defines a parameter for the 'issue' and
   'issuewild' properties of the Certification Authority Authorization
   (CAA) DNS resource record [RFC6844], allowing authorization conferred
   by a CAA policy to be restricted to specific ACME
   [I-D.ietf-acme-acme] accounts.  The accounts are identified by
   account key thumbprint.

2.  Terminology

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

3.  Extensions to the CAA Record

3.1.  acme-ak Parameter

   A CAA parameter "acme-ak" is defined for the 'issue' and 'issuewild'
   properties defined by [RFC6844].  The value of this parameter, if
   specified, MUST be the base64url [RFC4648] encoding (without padding)
   of the JWK thumbprint [RFC7517] of the ACME account key
   [I-D.ietf-acme-acme].

   If an ACME server finds multiple CAA records pertaining to it (i.e.,
   having property 'issue' or 'issuewild' as applicable and a domain

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   that the ACME server recognises as its own) with different "acme-ak"
   parameters, the ACME server MUST NOT consider the CAA record set to
   authorize issuance unless at least one of the specified account key
   thumbprints matches the requesting ACME account key.  A property
   without an "acme-ak" parameter matches any account key.  A property
   with an invalid "acme-ak" parameter (i.e. not 43 characters long or
   not a valid base64url string), or multiple "acme-ak" parameters is
   unsatisfiable.

4.  Security Considerations

   This specification describes an extension to the CAA record
   specification increasing the granularity at which CAA policy can be
   expressed for ACME-based CAs.  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 ACME account
   keys.  This improves the security of issuance for domains which
   choose to employ it, when combined with a CA which implements this
   specification.

4.1.  DNSSEC

   Where a domain chooses to secure its nameservers using DNSSEC, the
   authenticity of an ACME account key nomination placed in a CAA record
   can be assured, providing that a CA makes all DNS resolutions via an
   appropriate, trusted DNSSEC-validating resolver.  In this case and so
   long as control of nominated keys is retained, a domain is protected
   from the threat posed by a global adversary capable of performing
   man-in-the-middle attacks, which could otherwise forge DNS responses
   and successfully obtain ACME authorizations and certificates for the
   domain.

4.2.  Authorization Freshness

   The CAA specification governs the act of issuance by a CA.  The act
   of authorization as described by the ACME protocol occurs separately
   to issuance and may occur substantially prior to an 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 ACME
   authorizations with very limited validity periods, such as an hour,
   or revalidating the CAA policy for a domain at certificate issuance
   time.

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5.  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 "acme-ak" for ACME-based CAs.

6.  Normative References

   [I-D.ietf-acme-acme]
              Barnes, R., Hoffman-Andrews, J., and J. Kasten, "Automatic
              Certificate Management Environment (ACME)", draft-ietf-
              acme-acme-02 (work in progress), March 2016.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
              RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <http://www.rfc-editor.org/info/rfc4648>.

   [RFC6844]  Hallam-Baker, P. and R. Stradling, "DNS Certification
              Authority Authorization (CAA) Resource Record", RFC 6844,
              DOI 10.17487/RFC6844, January 2013,
              <http://www.rfc-editor.org/info/rfc6844>.

   [RFC7517]  Jones, M., "JSON Web Key (JWK)", RFC 7517, DOI 10.17487/
              RFC7517, May 2015,
              <http://www.rfc-editor.org/info/rfc7517>.

Appendix A.  Examples

   The following shows an example DNS configuration which nominates two
   account keys as authorized to issue certificates for the domain
   "example.com".  Issuance is restricted to the CA "example.net".

   example.com. IN CAA 0 issue "example.net; \
     acme-ak=UKNmi2whPhuAhDvAxGa_aOZgPzyJDhhsrt-8Bt2fWh0"
   example.com. IN CAA 0 issue "example.net; \
     acme-ak=rlp4OZPOR9MKejkOdZAKQ5Tfwce6llawmrDIh-BtNJ0"

Author's Address

   Hugo Landau

   Email: hlandau@devever.net

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