Make server ciphersuite-blind

[conradoplg]

Problem: the server needs to know the ciphersuite and only works with one. This is because the Args/Output structs have FROST structs in them, which are (de)serialized as needed. This requires specifying a ciphersuite.

Solution: the server doesn't need to do the serialization roundtrip. It can receive FROST structs serialized and just pass them around.

Coding approach suggestion:

• Change the FROST structs in server/src/types.go to use Vec<u8> instead, e.g. pub commitments: Vec<frost::round1::SigningCommitments> becomes pub commitments: Vec<Vec<u8>>.
  • By default serde encodes these Vec<u8> as integer arrays which is awkward. It's possible to encode them as hex strings (in human-readable formats like JSON) or directly binary. Check how we do that in SigningFormat::message field in frost-core for a reference on how to do that with serde(serialize_with).
  • For type safety it's possible to use newtypes for each struct, e.g. define a EncodedSigningCommitments(Vec<u8>) and use that instead.