[Rats] using symmetric keys in an Endorsement/Verifier flow

[Michael Richardson <[e-mail]>]

In https://github.com/ietf-rats-wg/architecture/issues/162 it was argued that
we needed to be clear that:

  "Endorsements should have confidentiality protection when carrying
  symmetric verification keys. "

but, many of the Architecture Design Team didn't feel that the architecture
was intended to provide any kind of specification on Endorsements.
Within the dataflow diagram:
  https://www.ietf.org/archive/id/draft-ietf-rats-architecture-09.html#dataflow
  (attached below, for those who use a fixed-width font)
That only the "Evidence" and "Attestation Results", which are between
solid-lined entities was really in scope.  That the rest was for future
documents.

We felt that this requirement went into the Design Requirements which the
IETF usually tries to stay out of, sticking to Functional Requirements.
Further, we felt that the Functional Requirements in the RATS Architecture
should not even cover Endorsements.
At least, not in this edition of the document.

The recommendation is that those who need to do Endorsements with symmetric
verifification keys should write another document in which this topic would
get adequately dealt with.



  ************   *************    ************    *****************
  * Endorser *   * Reference *    * Verifier *    * Relying Party *
  ************   * Value     *    *  Owner   *    *  Owner        *
     |           * Provider  *    ************    *****************
     |           *************          |                 |
     |                  |               |                 |
     |Endorsements      |Reference      |Appraisal        |Appraisal
     |                  |Values         |Policy           |Policy for
     |                  |               |for              |Attestation
     .-----------.      |               |Evidence         |Results
                 |      |               |                 |
                 |      |               |                 |
                 v      v               v                 |
               .---------------------------.              |
        .----->|          Verifier         |------.       |
        |      '---------------------------'      |       |
        |                                         |       |
        |                              Attestation|       |
        |                              Results    |       |
        | Evidence                                |       |
        |                                         |       |
        |                                         v       v
  .----------.                                .---------------.
  | Attester |                                | Relying Party |
  '----------'                                '---------------'