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))