Merge rust-bitcoin/rust-bitcoin#1583: Add conversions for `TweakedKeyPair` -> `TweakedPublicKey`

facaefc49c Add conversions for TweakedKeyPair -> TweakedPublicKey (Tobin C. Harding)
2407f241e4 Remove sep256k1 path from Parity (Tobin C. Harding)

Pull request description:

  It is trivially possible to get `TweakedPublicKey` from a `TweakedKeyPair`, add conversion methods for doing so.

  Patch 1 is preparatory cleanup. Please note `From` is not implemented because the conversion returns the `Parity` also.

  Fix: #1452

ACKs for top commit:
  apoelstra:
    ACK facaefc49c
  Kixunil:
    ACK facaefc49c

Tree-SHA512: 597026c481fe2622a625cbeb381cac345af6f49f4a115418b69817345fc3c2140bbdbc5208eae1149d7d171f94c776365d302ffe1f9c01d944e738807db28a89

bitcoin/src/crypto/schnorr.rs

@@ -11,7 +11,7 @@ use core::fmt;
 
 use bitcoin_internals::write_err;
 
-pub use secp256k1::{self, constants, Secp256k1, KeyPair, XOnlyPublicKey, Verification};
+pub use secp256k1::{self, constants, Secp256k1, KeyPair, XOnlyPublicKey, Verification, Parity};
 
 use crate::prelude::*;
 
@@ -44,6 +44,20 @@ impl fmt::Display for TweakedPublicKey {
 pub type UntweakedKeyPair = KeyPair;
 
 /// Tweaked BIP-340 key pair
+///
+/// # Examples
+/// ```
+/// # #[cfg(feature = "rand-std")] {
+/// # use bitcoin::schnorr::{TweakedKeyPair, TweakedPublicKey};
+/// # use bitcoin::secp256k1::{rand, Secp256k1};
+/// # let secp = Secp256k1::new();
+/// # let keypair = TweakedKeyPair::dangerous_assume_tweaked(KeyPair::new(&secp, &mut rand::thread_rng()));
+/// // There are various conversion methods available to get a tweaked pubkey from a tweaked keypair.
+/// let (_pk, _parity) = keypair.public_parts();
+/// let _pk  = TweakedPublicKey::from_keypair(keypair);
+/// let _pk = TweakedPublicKey::from(keypair);
+/// # }
+/// ```
 #[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash, Debug)]
 #[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
 #[cfg_attr(feature = "serde", serde(crate = "actual_serde"))]
@@ -79,7 +93,7 @@ pub trait TapTweak {
 }
 
 impl TapTweak for UntweakedPublicKey {
-    type TweakedAux = (TweakedPublicKey, secp256k1::Parity);
+    type TweakedAux = (TweakedPublicKey, Parity);
     type TweakedKey = TweakedPublicKey;
 
     /// Tweaks an untweaked public key with corresponding public key value and optional script tree
@@ -94,7 +108,7 @@ impl TapTweak for UntweakedPublicKey {
     ///
     /// # Returns
     /// The tweaked key and its parity.
-    fn tap_tweak<C: Verification>(self, secp: &Secp256k1<C>, merkle_root: Option<TapNodeHash>) -> (TweakedPublicKey, secp256k1::Parity) {
+    fn tap_tweak<C: Verification>(self, secp: &Secp256k1<C>, merkle_root: Option<TapNodeHash>) -> (TweakedPublicKey, Parity) {
         let tweak = TapTweakHash::from_key_and_tweak(self, merkle_root).to_scalar();
         let (output_key, parity) = self.add_tweak(secp, &tweak).expect("Tap tweak failed");
 
@@ -136,6 +150,13 @@ impl TapTweak for UntweakedKeyPair {
 }
 
 impl TweakedPublicKey {
+    /// Returns the [`TweakedPublicKey`] for `keypair`.
+    #[inline]
+    pub fn from_keypair(keypair: TweakedKeyPair) -> Self {
+        let (xonly, _parity) = keypair.0.x_only_public_key();
+        TweakedPublicKey(xonly)
+    }
+
     /// Creates a new [`TweakedPublicKey`] from a [`XOnlyPublicKey`]. No tweak is applied, consider
     /// calling `tap_tweak` on an [`UntweakedPublicKey`] instead of using this constructor.
     ///
@@ -176,6 +197,13 @@ impl TweakedKeyPair {
     pub fn to_inner(self) -> KeyPair {
         self.0
     }
+
+    /// Returns the [`TweakedPublicKey`] and its [`Parity`] for this [`TweakedKeyPair`].
+    #[inline]
+    pub fn public_parts(&self) -> (TweakedPublicKey, Parity) {
+        let (xonly, parity) = self.0.x_only_public_key();
+        (TweakedPublicKey(xonly), parity)
+    }
 }
 
 impl From<TweakedPublicKey> for XOnlyPublicKey {
@@ -192,6 +220,13 @@ impl From<TweakedKeyPair> for KeyPair {
     }
 }
 
+impl From<TweakedKeyPair> for TweakedPublicKey {
+    #[inline]
+    fn from(pair: TweakedKeyPair) -> Self {
+        TweakedPublicKey::from_keypair(pair)
+    }
+}
+
 /// A BIP340-341 serialized schnorr signature with the corresponding hash type.
 #[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
 #[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]