Tor design proposals

Filename: 343-rend-caa.txt
Title: CAA Extensions for the Tor Rendezvous Specification
Author: Q Misell <q@as207960.net>
Created: 2023-04-25
Status: Open
Ticket: https://gitlab.torproject.org/tpo/core/tor/-/merge_requests/716

Overview:
  The document defines extensions to the Tor Rendezvous Specification Hidden
  Service descriptor format to allow the attachment of DNS style CAA records to
  Tor hidden services to allow the same security benefits as CAA provides in the
  DNS.

Motivation:
  As part of the work on draft-misell-acme-onion [I-D.misell-acme-onion] at the
  IETF it was felt necessary to define a method to incorporate CAA records
  [RFC8659] into Tor hidden services.

  CAA records in the DNS provide an mechanism to indicate which Certificate
  Authorities are permitted to issue certificates for a given domain name, and
  restrict which validation methods are permitted for certificate validation.

  As Tor hidden service domains are not in the DNS another way to provide the
  same security benefits as CAA does in the DNS needed to be devised.

  It is important to note that a hidden service is not required to publish a CAA
  record to obtain a certificate, as is the case in the DNS.

  More information about this project in general can be found at
  https://acmeforonions.org.

Specification:
  To enable maximal code re-use in CA codebases the same CAA record format is
  used in Tor hidden services as in the DNS. To this end a new field is added to
  the second layer hidden service descriptor [tor-rend-spec-v3] § 2.5.2.2.
  with the following format:

    "caa" SP flags SP tag SP value NL
    [Any number of times]

  The contents of "flag", "tag", and "value" are as per [RFC8659] § 4.1.1.
  Multiple CAA records may be present, as is the case in the DNS.

  A hidden service's second layer descriptor using CAA may look
  something like the following:

    create2-formats 2
    single-onion-service
    caa 0 issue "example.com"
    caa 0 iodef "mailto:security@example.com"
    caa 128 validationmethods "onion-csr-01"
    introduction-point AwAGsAk5nSMpAhRqhMHbTFCTSlfhP8f5PqUhe6DatgMgk7kSL3KHCZ...

  As the CAA records are in the second layer descriptor and in the case of a
  hidden service requiring client authentication it is impossible to read them
  without the hidden service trusting a CA's public key, a method is required to
  signal that there are CAA records present (but not reveal their contents,
  which may disclose unwanted information about the hidden service operator to
  third parties). This is to allow a CA to know that it must attempt to check
  CAA records before issuance, and fail if it is unable to do so.

  To this end a new field is added to the first layer hidden service descriptor
  [tor-rend-spec-v3] § 2.5.1.2. with the following format:

    "caa-critical" NL
    [At most once]

Security Considerations:
  The second layer descriptor is signed, encrypted and MACed in a way that only
  a party with access to the secret key of the hidden service could manipulate
  what is published there. Therefore, Tor CAA records have at least the same
  security as those in the DNS secured by DNSSEC.

  The "caa-critical" flag is visible to anyone with knowledge of the hidden
  service's public key, however it reveals no information that could be used to
  de-anonymize the hidden service operator.

  The CAA flags in the second layer descriptor may reveal information about the
  hidden service operator if they choose to publish an "iodef", "contactemail",
  or "contactphone" tag. These however are not required for primary goal of CAA,
  that is to restrict which CAs may issue certificates for a given domain name.

  No more information is revealed by the "issue" nor "issuewild" tags than would
  be revealed by someone making a connection to the hidden service and noting
  which certificate is presented.

Compatibility:
  The hidden service spec [tor-rend-spec-v3] already requires that clients
  ignore unknown lines when decoding hidden service descriptors, so this change
  should not cause any compatibility issues. Additionally in testing no
  compatibility issues where found with existing Tor implementations.

  A hidden service with CAA records published in its descriptor is available at
  znkiu4wogurrktkqqid2efdg4nvztm7d2jydqenrzeclfgv3byevnbid.onion, to allow
  further compatibility testing.

References:
  [I-D.misell-acme-onion]
             Misell, Q., "Automated Certificate Management Environment (ACME)
             Extensions for ".onion" Domain Names", Internet-Draft
             draft-misell-acme-onion-02, April 2023,
             <https://datatracker.ietf.org/doc/html/draft-misell-acme-onion-02>.

  [RFC8659]  Hallam-Baker, P., Stradling, R., and J. Hoffman-Andrews,
             "DNS Certification Authority Authorization (CAA) Resource
             Record", RFC 8659, DOI 10.17487/RFC8659, November 2019,
             <https://www.rfc-editor.org/info/rfc8659>.

  [tor-rend-spec-v3]
             The Tor Project, "Tor Rendezvous Specification - Version 3",
             <https://spec.torproject.org/rend-spec-v3>.```