CDDL schema and tests for COSE endorsements

[maxtropets]

@maxtropets we should have an endpoint that takes a historical service identity, and returns the chain of endorsements up to the current service, if found, and a clear message otherwise. This will be useful for users who have large amounts of receipts, and want to quickly check/confirm that a new service endorses them.

We also want a CDDL schema of the endorsements, with an automatic test, and Merkle root chaining as discussed.

Originally posted by @achamayou in #6500

Prerequisites

• [x] Define COSE endorsement schema
• [x] Do we need COSE signature schema too? Define if yes
• [x] CDDL schema and test for COSE signature
• [x] CDDL schema and test for COSE endorsement

[maxtropets]

COSE signature currently

uhdr: nil
phdr: {
  4 (int) -> key digest (tstr)
  395 (int) -> int: VDS
  "iat" (tstr) -> int: issued at, standard (coming in #6565)
  "ccf.txid" (tstr) -> signed transaction id
}
payload: root (bstr) (detached, therefore 'nil' de facto)

[achamayou]

To clarify:

• Yes, we should should have a CDDL of schema for every bit of CBOR (including COSE) we emit (and ideally every bit of COSE we accept too)
• Like our other schemas, this must be validated in automated tests to make sure it does not go out of date

[maxtropets]

COSE endorsement currently

uhdr: nil
phdr: {
  "iat" (tstr) -> int: issued at, standard (coming in #6565)
  "ccf.merkle.root" (tstr) -> root (bstr): last signed merkle root of previous epoch
  "ccf.epoch.begin" (tstr) -> tx id (tstr) : signed epoch start
  "ccf.epoch.end" (tstr) -> tx id (tstr) : signed epoch end
}
payload: root (bstr) (attached)

[maxtropets]

Proposed schema for cose signature

uhdr: nil
phdr: {
    1 (int) -> signing algorithm (int)
    4 (int) -> key digest (tstr)
    15 (int) -> CWT claims map {
        6 (int) -> IAT (int)
    }
    395 (int) -> int: VDS
    "ccf.v1" (tstr) -> CCF specific claims {
        tx.id (tstr) -> last TxID signed (tstr)
    }
}
payload: root (bstr) (detached, therefore 'nil' de facto)

[maxtropets]

Proposed schema for cose endorsement

uhdr: nil
phdr: {
    1 (int) -> signing algorithm (int)
    4 (int) -> key digest (tstr)
    15 (int) -> CWT claims map {
        6 (int) -> IAT (int)
    }
    "ccf.v1" (tstr) -> CCF specific claims map {
        "epoch.begin" (tstr) -> first TX in the endorsed epoch (tstr)
        [optional] "epoch.end" (tstr) -> first TX in the endorsed epoch (tstr)
        [optional] "last.root" (tstr) -> root (bstr)
    }
}
payload: root (bstr) (detached, therefore 'nil' de facto)

[achamayou]

Suggest:

"epoch.txid.first" "epoch.txid.last" "epoch.root.last"

or

"epoch.first.txid" "epoch.last.txid" "epoch.last.root"

not keen on begin/end, because that would typically exclude end.

[achamayou]

epoch.first.txid epoch.last.txid epoch.last.root.digest range.last.txid

Where range is the range of transactions that's being signed, and an epoch the full range signed by a given service identity followed by at least one DR.