Aggsig compatibility with BIP-schnorr

[jaspervdm]

Optionally add a public key to the aggsig challenge in both the creation and verification of the signature. Compatibility with BIP-schnorr (under "key prefixing")

Also fixes https://github.com/mimblewimble/secp256k1-zkp/pull/21 and fixes https://github.com/mimblewimble/secp256k1-zkp/pull/23

[ignopeverell]

Can't we just rely on the upstream musig code instead?

[jaspervdm]

@ignopeverell could you point me to where to find this code? I couldnt find any references to aggsig or musig in the ElementsProject fork or the original secp256k1 repository

[ignopeverell]

Mmmh looks like you're right. I was assuming the release of the BIP came with code but looks like I was wrong, sorry about that.

[jaspervdm]

Updated the tests to reflect the changes, however 1 is failing (L245, validating combined signature, even though separate signatures work). I don't understand why but will research it more

[garyyu]

One more comment:
change the title of this PR to change aggsig to bip-schnorr, something like that. and put this link in this PR: https://github.com/sipa/bips/blob/bip-schnorr/bip-schnorr.mediawiki

[yeastplume]

You guys have been all over this already and it all looks good to me. (Also very glad to have had more pairs of eyes on this code). Happy to merge when ready (it won't affect Grin builds until it's explicitly tagged)

[jaspervdm]

If there are no further comments, I think the PR is ready to be merged in

[jaspervdm]

Final small change on suggestion of @garyyu: write the full 33 bytes of the public key, as in the BIP-schnorr reference code

[yeastplume]

Great, merging! Thanks for all your work on this