Support publicKeyBase58 for Ed25519Signature2018

[clehner]

Support the publicKeyBase58 property of verification material, for the Ed25519 Signature 2018 verification method.

DID Core allows either publicKeyJwk or publicKeyBase58. Although there is discussion of the spec replacing publicKeyBase58 with publicKeyMultibase, there is currently a test case in vc-http-api-test-suite that relies on it.

[sbihel]

LGTM.

I realise I use publicKeyBase58 for the did:tz resolution, should I change that to JWK as we currently only support ed25519? My thinking was that it was more "idiomatic" as Tezos tools always manipulate those keys as base 58.

PS: little typo in your PR message, a publicKeyBase58 needs to be publicKeyMultiBase

[clehner]

@sbihel I would suggest using publicKeyJwk where possible. Tezos uses base58 with prefixes e.g. edpk, but this is not consistent with other linked data proof types; for example the Ed25519Signature2018 spec uses just the 32-byte public key in publicKeyBase58. I think it could create confusion/noninteroperability if we allow both Tezos-prefixed keys and raw bytes in publicKeyBase58.

I have leaned away from publicKeyBase58 because it doesn't seem as clear how the public keys should be serialized. I see now that did-method-key has test vectors for EcdsaSecp256k1VerificationKey2019 using publicKeyBase58. But I am unable to find any examples or specification for EcdsaSecp256r1VerificationKey2019 (P-256), e.g. to know whether it should use compressed, uncompressed or untagged representation. publicKeyJwk is more well-specified, in IETF RFCs.

If the smart contract and/or other tools need to use Tezos-style base58 keys, we could probably convert them to JWK internally.