Ability to parse / serialize nonces / partial signatures to do collaborative musig operations

[RubenWaterman]

As discussed in #696, this library currently does not provide the functionality to aggregate serialized nonces / parial signatures, which is useful if not all the signing happens within the same Swift app (i.e. a collaborative signing ceremony), such as in Boltz claims (or refunds).

I did an attempt of achieving this https://github.com/RubenWaterman/swift-secp256k1/pull/1/files but not sure it's the right way.

[csjones]

Hey @RubenWaterman 👋

Thanks for creating the thread and sharing your attempt. I've been reviewing the changes in your fork and have a question regarding your creation of secp256k1.Schnorr.Nonce(hexString: String) and secp256k1.Schnorr.Nonce(serializedNonce: Data). Is there a specific reason for choosing these initializations over the default one?

This package internally converts hexadecimal strings through the String extension and its bytes property which can be used to initialize a Data object directly. Could you clarify if this approach works for your use case?

Example:

let stringNonce = "2bd806c97f0e00af1a1fc3328fa763a9269723c8db8fac4f93af71db186d6e90"

let nonceBytes = try! stringNonce.bytes

let pubnonce = secp256k1.Schnorr.Nonce(pubnonce: Data(nonceBytes))

[RubenWaterman]

I wasn't able to work on this for a while but in the last two weeks or so I've made good progress and did a new attempt. I had tried to do what you suggested in the previous comment but then I ran into problems that in a different location, the Nonce was initialized with pubnonce in a different way (https://github.com/21-DOT-DEV/swift-secp256k1/blob/main/Sources/ZKP/Nonces.swift#L150).

I'm not a Swift developer at all (as I normally write Elixir) so all of this I've achieved with incredible help of Cursor/Claude. Feel free to make any changes to these proposed changes to make it better!

As of now, I can get to a working result (I've started a new PR based off the latest state of the package): RubenWaterman#2 and I've included an as detailed as possible example of how I use it: https://github.com/RubenWaterman/swift-secp256k1/tree/import-signatures-and-nonces?tab=readme-ov-file#musig2-with-external-nonces--signatures

The strangest part is that it works most of the times but in some cases it doesn't and the problem seems to be in aggregating the signatures. Would you be able to take a look at my changes and see what might cause this?