feat: update signer impl for XOnlyPubkey

[chaitika]

Description
Updating signer implemention for XOnlyPublicKey key request

Fixes #19

All Submissions:

• I've signed all my commits
• I followed the conventional commit guidelines
• I ran cargo +nigthly fmt before committing

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

[ValuedMammal]

I suggest we do the following:

1. Use an if let ... to return the PrivateKey only if we found it in the current map entry.
2. Fix bug introduced by an earlier commit 94089eb that confused xkey.depth with the origin path length.

--- a/src/signer.rs
+++ b/src/signer.rs
@@ -1,6 +1,7 @@
 use alloc::string::ToString;
+use alloc::vec::Vec;
 use std::collections::BTreeMap;
 
 use bitcoin::{
     psbt::{GetKey, GetKeyError, KeyRequest},
     secp256k1::{self, Secp256k1},
@@ -25,11 +26,13 @@ impl GetKey for Signer {
         for entry in &self.0 {
             match entry {
                 (_, DescriptorSecretKey::Single(prv)) => {
                     let map: BTreeMap<_, _> =
                         core::iter::once((prv.key.public_key(secp), prv.key)).collect();
-                    return GetKey::get_key(&map, key_request, secp);
+                    if let Ok(Some(prv)) = GetKey::get_key(&map, key_request.clone(), secp) {
+                        return Ok(Some(prv));
+                    }
                 }
                 (_, desc_sk) => {
                     for desc_sk in desc_sk.clone().into_single_keys() {
                         if let KeyRequest::Bip32((fingerprint, derivation)) = &key_request {
                             if let DescriptorSecretKey::XPrv(k) = desc_sk {
@@ -42,11 +45,15 @@ impl GetKey for Signer {
                                 // The key origin is a strict prefix of the request derivation
                                 if let Some((fp, path)) = &k.origin {
                                     if fingerprint == fp
                                         && derivation.to_string().starts_with(&path.to_string())
                                     {
-                                        let to_derive = &derivation[k.xkey.depth as usize..];
+                                        let to_derive = derivation
+                                            .into_iter()
+                                            .copied()
+                                            .skip(path.len())
+                                            .collect::<Vec<_>>();
                                         let derived = k.xkey.derive_priv(secp, &to_derive)?;
                                         return Ok(Some(derived.to_priv()));
                                     }
                                 }
                             }

[ValuedMammal]

Here's another test

#[test]
fn get_key_xpriv_with_key_origin() -> anyhow::Result<()> {
    let secp = Secp256k1::new();
    let s = "wpkh([d34db33f/84h/1h/0h]tprv8ZgxMBicQKsPd3EupYiPRhaMooHKUHJxNsTfYuScep13go8QFfHdtkG9nRkFGb7busX4isf6X9dURGCoKgitaApQ6MupRhZMcELAxTBRJgS/*)";
    let (_, keymap) = Descriptor::parse_descriptor(&secp, s)?;

    let desc_sk = DescriptorSecretKey::from_str("[d34db33f/84h/1h/0h]tprv8ZgxMBicQKsPd3EupYiPRhaMooHKUHJxNsTfYuScep13go8QFfHdtkG9nRkFGb7busX4isf6X9dURGCoKgitaApQ6MupRhZMcELAxTBRJgS/*")?;
    let desc_xkey = match desc_sk {
        DescriptorSecretKey::XPrv(k) => k,
        _ => panic!(),
    };

    let (fp, _) = desc_xkey.origin.clone().unwrap();
    let path = DerivationPath::from_str("84h/1h/0h/7")?;
    let req = KeyRequest::Bip32((fp, path));

    let exp_prv = desc_xkey
        .xkey
        .derive_priv(&secp, &[ChildNumber::from(7)])?
        .to_priv();

    let res = Signer(keymap).get_key(req, &secp);

    assert!(matches!(
        res,
        Ok(Some(k)) if k == exp_prv,
    ));

    Ok(())
}

[ValuedMammal]

I tested using BDK test descriptors.

wpkh([e273fe42/84'/1'/0']tprv8g5otvsaxi79T7WyYCAucSgYsa9tcrM2mdZzC5NjcmsRJbgxZtmFsWBjysVerFe3DvKT5tLbJYUssERJUP5miQEtSwV7mcKC263Aikiw8mN/0/*)
wpkh([e273fe42/84'/1'/0']tprv8g5otvsaxi79T7WyYCAucSgYsa9tcrM2mdZzC5NjcmsRJbgxZtmFsWBjysVerFe3DvKT5tLbJYUssERJUP5miQEtSwV7mcKC263Aikiw8mN/1/*)

https://mempool.space/testnet4/tx/d93eb295818b70ca75569169fd188ef3c2ffdf386cdddf4652ae30626b9a0664

[ValuedMammal]

ACK fe83b69