BLS m-n threshold signature support

[franjoespejo]

Please see below the set of available operations for BLS at the moment:

BLS12-381 signature (sign, verify, aggregate)

The BLS scheme also supports threshold signatures. This is where a secret key is divided between n parties. For a predefined value of m≤n, if m of the parties sign a message then a single joint public key of all the parties can be used to verify the signature.

It would be very useful to have a new operation "recover" or "reconstruct" that builds a collective BLS signature. Supporting BLS threshold signature completes the BLS scheme. This seems to have been requested previously in #50

For reference:

BLS threshold is implemented:
https://github.com/herumi/bls#api-for-k-of-n-threshold-signature

Specifically the "recover" function can be found at: https://github.com/herumi/bls/blob/cf2b578f1c414fdcef74f1a01bc43991b4f5fd76/src/bls_c_impl.hpp#L602

[dot-asm]

Have you seen blsttc? Just in case, I haven't looked into it myself, so it's not like I'm saying that it will actually solve your problem.

[mratsim]

Additionally: https://github.com/status-im/nim-blscurve/blob/master/blscurve/blst/blst_recovery.nim

but unaudited.

[mcdee]

Looking to implement this feature in Go as it isn't supplied in blst itself, but the functions for field element creation and maths appear to be missing from the Go bindings. Are there any plans to either implement this natively in blst, and if not to expose the full set of blst functions in the Go bindings to allow others to do so?