Implement JcsTezosSignature2021

[clehner]

Linked data proof type for signing with Tezos wallets. Like TezosSignature2021 (#170) but not using RDF canonicalization. Instead, uses JCS.

• Initial implementation
• Allow proof with Tezos-style public key instead of publicKeyJwk
  • Support tz1, tz2, and tz3
• Add warning about this method being experimental
• Discuss public key and signature encoding: wrote paragraph below

Public key and signature encoding

While TezosSignature2021 includes the public key in the proof object in the publicKeyJwk property, here the public key is instead encoded in a publicKeyMultibase property. The purpose of this is to more closely follow Tezos's native format (e.g. edpk...), for easier inspection and verification. Tezos's native key and signature format use base58 with ASCII prefixes. There is a publicKeyBase58 format which used to be defined in DID Core but now has been deprecated in favor of publicKeyMultibase. Fortunately a base58 string can be converted to a multibase string simply by prepending the character "z". So that is the format used here, as seen in the added example VC. publicKeyMultibase is expected to use multicodec, according to Security Vocab, but that doesn't seem to be a hard requirement. A helper function to convert a JWK to a Tezos string is needed here and added in #267.
For the signature value (proofValue property), the format is the same as in TezosSignature2021: the edpk value. There doesn't seem to be an expected or required type for proofValue, so this seems okay.

Example

Signing request (Temple wallet):

Payload:

Tezos Signed Message: {\"@context\":[\"https://www.w3.org/2018/credentials/v1\"],\"credentialSubject\":{\"id\":\"did:pkh:tz:tz1TwZZZSShtM73oEr74aDtDcns3UmFqaca6\"},\"issuanceDate\":\"2021-07-27T19:37:15.715Z\",\"issuer\":\"did:pkh:tz:tz1TwZZZSShtM73oEr74aDtDcns3UmFqaca6\",\"proof\":{\"@context\":{\"TezosJcsSignature2021\":{\"@context\":{\"@protected\":true,\"@version\":1.1,\"challenge\":\"https://w3id.org/security#challenge\",\"created\":{\"@id\":\"http://purl.org/dc/terms/created\",\"@type\":\"http://www.w3.org/2001/XMLSchema#dateTime\"},\"domain\":\"https://w3id.org/security#domain\",\"expires\":{\"@id\":\"https://w3id.org/security#expiration\",\"@type\":\"http://www.w3.org/2001/XMLSchema#dateTime\"},\"id\":\"@id\",\"nonce\":\"https://w3id.org/security#nonce\",\"proofPurpose\":{\"@context\":{\"@protected\":true,\"@version\":1.1,\"assertionMethod\":{\"@container\":\"@set\",\"@id\":\"https://w3id.org/security#assertionMethod\",\"@type\":\"@id\"},\"authentication\":{\"@container\":\"@set\",\"@id\":\"https://w3id.org/security#authenticationMethod\",\"@type\":\"@id\"},\"id\":\"@id\",\"type\":\"@type\"},\"@id\":\"https://w3id.org/security#proofPurpose\",\"@type\":\"@vocab\"},\"proofValue\":\"https://w3id.org/security#proofValue\",\"publicKeyMultibase\":{\"@id\":\"https://w3id.org/security#publicKeyMultibase\",\"@type\":\"https://w3id.org/security#multibase\"},\"type\":\"@type\",\"verificationMethod\":{\"@id\":\"https://w3id.org/security#verificationMethod\",\"@type\":\"@id\"}},\"@id\":\"https://w3id.org/security#TezosJcsSignature2021\"}},\"created\":\"2021-07-27T19:37:15.716Z\",\"proofPurpose\":\"assertionMethod\",\"publicKeyMultibase\":\"zedpkuxZ5AQVCeEJ9inUG3w6VFhio5KBwC22ekPLBzcvub3QY2DvJ7n\",\"type\":\"TezosJcsSignature2021\",\"verificationMethod\":\"did:pkh:tz:tz1TwZZZSShtM73oEr74aDtDcns3UmFqaca6#TezosMethod2021\"},\"type\":[\"VerifiableCredential\"]}

Resulting VC: https://github.com/spruceid/ssi/blob/57ca1bfd1e451f75dc3c86e7f8cfa22307d2613e/did-pkh/tests/vc-tz1-jcs.jsonld

[sbihel]

I'm thinking the name of the signature should maybe be TezosSignedMessageJcsSignature2021 or something like that to reflect that it's still using the Tezos Signed Message representation (meaning it's not "pure" JCS)?

[bumblefudge]

Merging soon pending further warning message feature but future PRs will further refine based on ongoing discussions re: standard Tezos signing format.

[clehner]

• Warning added. Verification result now looks like this:

$ didkit vc-verify-credential < did-pkh/tests/vc-tz1-jcs.jsonld | jq .
{
  "checks": [
    "proof"
  ],
  "warnings": [
    "TezosJcsSignature2021 is experimental."
  ],
  "errors": []
}

• Some intermediate changes factored out into Convert JWK to Tezos key #267.
• Rebased and squashed other intermediate commits.
• Added description of the public key encoding to the PR message.
• CI is failing due to CI failure about crypto-mac #269