Module schnorrkel::musig

Implementation for Ristretto Schnorr signatures of "Simple Schnorr Multi-Signatures with Applications to Bitcoin" by Gregory Maxwell, Andrew Poelstra, Yannick Seurin, and Pieter Wuille https://eprint.iacr.org/2018/068

We observe the security arguments from the original 2-round version were found lacking in "On the Provable Security of Two-Round Multi-Signatures" by Manu Drijvers, Kasra Edalatnejad, Bryan Ford, and Gregory Neven https://eprint.iacr.org/2018/417 (slides) so we implement only the 3-round version.

Appendix A of the MuSig paper discusses Interactive Aggregate Signatures (IAS) in which cosigners' messages differ. Appendix A.3 gives a secure scheme that correctly binds signers to their messages. See https://github.com/w3f/schnorrkel/issues/5#issuecomment-477912319

Structs

AggregatePublicKeySlice	Aggregation helper for public keys kept in slices
CollectStage	Initial stage for cosignature collectors who do not themselves cosign.
CommitStage	Commitment stage for cosigner's R values
Commitment	Commitments to R_i values shared between cosigners during signing
CosignStage	Final cosigning stage colelction
Cosignature	Cosignatures shared between cosigners during signing
MuSig	Schnorr multi-signature (MuSig) container generic over its session types
Reveal	Revealed R_i values shared between cosigners during signing
RevealStage	Reveal stage for cosigner's R values

Traits

AggregatePublicKey	Any data structure used for aggregating public keys.
TranscriptStages	Initial cosigning stages during which transcript modification remains possible but not advisable.

Functions

aggregate_public_key_from_refs_slice	Aggregate public keys stored in a mutable slice
aggregate_public_key_from_slice	Aggregate public keys stored in a mutable slice
aggregate_public_key_from_sorted_slice	Aggregate public keys stored in a sorted slice
collect_cosignatures	Initialize a collector of cosignatures who does not themselves cosign.