Rename Feed to Subject

[SteveLasker]

PR addressing the conversation around Feed is redundant with Subject, as defined in CWT

• 3.1.2. sub (Subject) Claim

  The "sub" (subject) claim has the same meaning and processing rules
  as the "sub" claim defined in Section 4.1.2 of [RFC7519], except that
  the value is a StringOrURI, as defined in Section 2 of this
  specification. The Claim Key 2 is used to identify this claim.

• 4.1.2. "sub" (Subject) Claim

  The "sub" (subject) claim identifies the principal that is the
  subject of the JWT. The claims in a JWT are normally statements
  about the subject. The subject value MUST either be scoped to be
  locally unique in the context of the issuer or be globally unique.
  The processing of this claim is generally application specific. The
  "sub" value is a case-sensitive string containing a StringOrURI
  value. Use of this claim is OPTIONAL.

This PR intentionally contrasts #108

Choice for folks to vote upon:

• Rename Feed to Subject
• Should we also include CWT_Claims, which contains the subject Use CWT Claims in Headers #108

Fixes #11

[JAG-UK]

One small but important nit

[JAG-UK]

LGTM

[yogeshbdeshpande]

Does the subject (old feed) is mandatory?

[SteveLasker]

I didn't make that explicit change, but yes an issuer of a signed statement must have a subject.

[henkbirkholz]

Isn't 2 'crit`? Shouldn't 'sub' be nested in a 13 (see https://www.ietf.org/archive/id/draft-ietf-cose-cwt-claims-in-headers-06.html#name-representation)?

[SteveLasker]

Ahh, yes, I incorrectly pulled 2 from the CWT spec.
In this PR, we're not pulling in CWT. That is deferred to #108
This PR renames Feed to Subject, aligning with the definition of Subject, within CWT.
Can we agree with this incremental step? If so, what tag should we use? I can repurpose 392, but I think we agree cbor-tags should have a common tag for subject, which is outside the scope of this PR.

• Do we agree with the rename?
• What CBOR tag should we use?

[SteveLasker]

To unblock, I've set the label to 392 (previous temporary label), as a bytestring, similar to #111. This unblocks the rename, with the ability to relax to string if we decide, or possibly shift to a CWT (#108).

[SteveLasker]

Above reference is outdated. Please see updated line 658

392 => bstr ; Subject

[SteveLasker]

With #108 merged, removing this entry.

[yogeshbdeshpande]

LGTM!