feat: support for multisig sign

ggutoski commented 3 years ago

Continue implementing support for naive multisig. This PR implements sign.

Draft status: Quick-and-dirty implementation to establish API for tofnd asap. Lots of TODOs in the code.

Change pubkey type VerifyingKey -> ProjectivePoint and sig type SigningKey -> Scalar because SigningKey does not support signing of a pre-hashed message.
After lots of investigation I had to resort to the low-level hazmat::RecoverableSignPrimitive API to sign a pre-hashed message. This API also requires us to sample the ephemeral scalar ourselves, so I added a rng_seed_sign helper function for this purpose.
add Signature to k256_serde.
TODO: need to decide on final output for sign. Right now it's a vec of Signature.
TODO: support robustness, check number of valid sigs against threshold. Right now we succeed if and only if all sign participants succeed.

ggutoski commented 3 years ago

As discussed, keygen/sign results are now presented in a format that's convenient for tofnd to put into grpc.

Keygen: GroupPublicInfo now has a method all_verifying_keys_bytes that returns a Vec<(party_id, subshare_index, pubkey_bytes)>. Compare to the gg20 method pubkey_bytes.
Sign: protocol output is a Vec<(keygen_party_id, subshare_index, signature_bytes)>. Compare to the gg20 output, which is merely signature_bytes.

ggutoski commented 3 years ago

fixes #183 because we no longer use SigningKey

axelarnetwork / tofn