Add convenience methods for keys

We have a bunch of `from_<key>` methods for converting between key types.
To improve the API and make it more ergonomic to use we can add methods
that do the same but can be called on the initial key instead of on the
resulting key's type. E.g. once applied the following are equivalent:

- `let pk = PublicKey::from_keypair(kp)`
- `let pk = kp.public_key()`

Do this for `SecretKey`, `PublicKey`, `KeyPair`, and `XOnlyKeyPair`.

src/key.rs

@@ -292,6 +292,31 @@ impl SecretKey {
     pub fn sign_ecdsa(&self, msg: Message) -> ecdsa::Signature {
         SECP256K1.sign_ecdsa(&msg, self)
     }
+
+    /// Returns the [`KeyPair`] for this [`SecretKey`].
+    ///
+    /// This is equivalent to using [`KeyPair::from_secret_key`].
+    #[inline]
+    pub fn keypair<C: Signing>(&self, secp: &Secp256k1<C>) -> KeyPair {
+        KeyPair::from_secret_key(secp, self)
+    }
+
+    /// Returns the [`PublicKey`] for this [`SecretKey`].
+    ///
+    /// This is equivalent to using [`PublicKey::from_secret_key`].
+    #[inline]
+    pub fn public_key<C: Signing>(&self, secp: &Secp256k1<C>) -> PublicKey {
+        PublicKey::from_secret_key(secp, self)
+    }
+
+    /// Returns the [`XOnlyPublicKey`] (and it's [`Parity`]) for this [`SecretKey`].
+    ///
+    /// This is equivalent to `XOnlyPublicKey::from_keypair(self.keypair(secp))`.
+    #[inline]
+    pub fn x_only_public_key<C: Signing>(&self, secp: &Secp256k1<C>) -> (XOnlyPublicKey, Parity) {
+        let kp = self.keypair(secp);
+        XOnlyPublicKey::from_keypair(&kp)
+    }
 }
 
 #[cfg(feature = "serde")]
@@ -418,6 +443,20 @@ impl PublicKey {
         }
     }
 
+    /// Creates a [`PublicKey`] using the key material from `pk` combined with the `parity`.
+    pub fn from_x_only_public_key(pk: XOnlyPublicKey, parity: Parity) -> PublicKey {
+        let mut buf = [0u8; 33];
+
+        // First byte of a compressed key should be `0x02 AND parity`.
+        buf[0] = match parity {
+            Parity::Even => 0x02,
+            Parity::Odd => 0x03,
+        };
+        buf[1..].clone_from_slice(&pk.serialize());
+
+        PublicKey::from_slice(&buf).expect("we know the buffer is valid")
+    }
+
     #[inline]
     /// Serializes the key as a byte-encoded pair of values. In compressed form the y-coordinate is
     /// represented by only a single bit, as x determines it up to one bit.
@@ -589,6 +628,25 @@ impl PublicKey {
             }
         }
     }
+
+    /// Returns the [`XOnlyPublicKey`] (and it's [`Parity`]) for this [`PublicKey`].
+    #[inline]
+    pub fn x_only_public_key(&self) -> (XOnlyPublicKey, Parity) {
+        let mut pk_parity = 0;
+        unsafe {
+            let mut xonly_pk = ffi::XOnlyPublicKey::new();
+            let ret = ffi::secp256k1_xonly_pubkey_from_pubkey(
+                ffi::secp256k1_context_no_precomp,
+                &mut xonly_pk,
+                &mut pk_parity,
+                self.as_ptr(),
+            );
+            debug_assert_eq!(ret, 1);
+            let parity = Parity::from_i32(pk_parity).expect("should not panic, pk_parity is 0 or 1");
+
+            (XOnlyPublicKey(xonly_pk), parity)
+        }
+    }
 }
 
 impl CPtr for PublicKey {
@@ -866,9 +924,27 @@ impl KeyPair {
         }
     }
 
-    /// Gets the [XOnlyPublicKey] for this [KeyPair].
+    /// Returns the [`SecretKey`] for this [`KeyPair`].
+    ///
+    /// This is equivalent to using [`SecretKey::from_keypair`].
     #[inline]
-    pub fn public_key(&self) -> XOnlyPublicKey {
+    pub fn secret_key(&self) -> SecretKey {
+        SecretKey::from_keypair(self)
+    }
+
+    /// Returns the [`PublicKey`] for this [`KeyPair`].
+    ///
+    /// This is equivalent to using [`PublicKey::from_keypair`].
+    #[inline]
+    pub fn public_key(&self) -> PublicKey {
+        PublicKey::from_keypair(self)
+    }
+
+    /// Returns the [`XOnlyPublicKey`] (and it's [`Parity`]) for this [`KeyPair`].
+    ///
+    /// This is equivalent to using [`XOnlyPublicKey::from_keypair`].
+    #[inline]
+    pub fn x_only_public_key(&self) -> (XOnlyPublicKey, Parity) {
         XOnlyPublicKey::from_keypair(self)
     }
 
@@ -1014,9 +1090,9 @@ impl XOnlyPublicKey {
         &mut self.0
     }
 
-    /// Creates a new Schnorr public key from a Schnorr key pair.
+    /// Returns the [`XOnlyPublicKey`] (and it's [`Parity`]) for `keypair`.
     #[inline]
-    pub fn from_keypair(keypair: &KeyPair) -> XOnlyPublicKey {
+    pub fn from_keypair(keypair: &KeyPair) -> (XOnlyPublicKey, Parity) {
         let mut pk_parity = 0;
         unsafe {
             let mut xonly_pk = ffi::XOnlyPublicKey::new();
@@ -1027,7 +1103,9 @@ impl XOnlyPublicKey {
                 keypair.as_ptr(),
             );
             debug_assert_eq!(ret, 1);
-            XOnlyPublicKey(xonly_pk)
+            let parity = Parity::from_i32(pk_parity).expect("should not panic, pk_parity is 0 or 1");
+
+            (XOnlyPublicKey(xonly_pk), parity)
         }
     }
 
@@ -1098,7 +1176,7 @@ impl XOnlyPublicKey {
     /// thread_rng().fill_bytes(&mut tweak);
     ///
     /// let mut key_pair = KeyPair::new(&secp, &mut thread_rng());
-    /// let mut public_key = key_pair.public_key();
+    /// let (mut public_key, _parity) = key_pair.x_only_public_key();
     /// public_key.tweak_add_assign(&secp, &tweak).expect("Improbable to fail with a randomly generated tweak");
     /// # }
     /// ```
@@ -1163,7 +1241,7 @@ impl XOnlyPublicKey {
     /// thread_rng().fill_bytes(&mut tweak);
     ///
     /// let mut key_pair = KeyPair::new(&secp, &mut thread_rng());
-    /// let mut public_key = key_pair.public_key();
+    /// let (mut public_key, _) = key_pair.x_only_public_key();
     /// let original = public_key;
     /// let parity = public_key.tweak_add_assign(&secp, &tweak).expect("Improbable to fail with a randomly generated tweak");
     /// assert!(original.tweak_add_check(&secp, &public_key, parity, tweak));
@@ -1189,6 +1267,14 @@ impl XOnlyPublicKey {
             err == 1
         }
     }
+
+    /// Returns the [`PublicKey`] for this [`XOnlyPublicKey`].
+    ///
+    /// This is equivalent to using [`PublicKey::from_xonly_and_parity(self, parity)`].
+    #[inline]
+    pub fn public_key(&self, parity: Parity) -> PublicKey {
+        PublicKey::from_x_only_public_key(*self, parity)
+    }
 }
 
 /// Represents the parity passed between FFI function calls.
@@ -1978,12 +2064,15 @@ mod test {
             thread_rng().fill_bytes(&mut tweak);
 
             let mut kp = KeyPair::new(&s, &mut thread_rng());
-            let mut pk = kp.public_key();
+            let (mut pk, _parity) = kp.x_only_public_key();
 
             let orig_pk = pk;
             kp.tweak_add_assign(&s, &tweak).expect("Tweak error");
             let parity = pk.tweak_add_assign(&s, &tweak).expect("Tweak error");
-            assert_eq!(XOnlyPublicKey::from_keypair(&kp), pk);
+
+            let (back, _) = XOnlyPublicKey::from_keypair(&kp);
+
+            assert_eq!(back, pk);
             assert!(orig_pk.tweak_add_check(&s, &pk, parity, tweak));
         }
     }
@@ -2052,4 +2141,150 @@ mod test {
         assert_tokens(&sk.readable(), &[Token::Str(SK_STR)]);
         assert_tokens(&sk.readable(), &[Token::String(SK_STR)]);
     }
+
+    #[cfg(all(not(fuzzing), any(feature = "alloc", feature = "std")))]
+    fn keys() -> (SecretKey, PublicKey, KeyPair, XOnlyPublicKey) {
+        let secp = Secp256k1::new();
+
+        static SK_BYTES: [u8; 32] = [
+            0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01,
+            0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07,
+            0xff, 0xff, 0x00, 0x00, 0xff, 0xff, 0x00, 0x00,
+            0x63, 0x63, 0x63, 0x63, 0x63, 0x63, 0x63, 0x63,
+        ];
+
+        static PK_BYTES: [u8; 32] = [
+            0x18, 0x84, 0x57, 0x81, 0xf6, 0x31, 0xc4, 0x8f,
+            0x1c, 0x97, 0x09, 0xe2, 0x30, 0x92, 0x06, 0x7d,
+            0x06, 0x83, 0x7f, 0x30, 0xaa, 0x0c, 0xd0, 0x54,
+            0x4a, 0xc8, 0x87, 0xfe, 0x91, 0xdd, 0xd1, 0x66
+        ];
+
+        let mut pk_bytes = [0u8; 33];
+        pk_bytes[0] = 0x02;     // Use positive Y co-ordinate.
+        pk_bytes[1..].clone_from_slice(&PK_BYTES);
+
+        let sk = SecretKey::from_slice(&SK_BYTES).expect("failed to parse sk bytes");
+        let pk = PublicKey::from_slice(&pk_bytes).expect("failed to create pk from iterator");
+        let kp = KeyPair::from_secret_key(&secp, &sk);
+        let xonly = XOnlyPublicKey::from_slice(&PK_BYTES).expect("failed to get xonly from slice");
+
+        (sk, pk, kp, xonly)
+    }
+
+    #[test]
+    #[cfg(all(not(fuzzing), any(feature = "alloc", feature = "std")))]
+    fn convert_public_key_to_xonly_public_key() {
+        let (_sk, pk, _kp, want) = keys();
+        let (got, parity) = pk.x_only_public_key();
+
+        assert_eq!(parity, Parity::Even);
+        assert_eq!(got, want)
+    }
+
+    #[test]
+    #[cfg(all(not(fuzzing), any(feature = "alloc", feature = "std")))]
+    fn convert_secret_key_to_public_key() {
+        let secp = Secp256k1::new();
+
+        let (sk, want, _kp, _xonly) = keys();
+        let got = sk.public_key(&secp);
+
+        assert_eq!(got, want)
+    }
+
+    #[test]
+    #[cfg(all(not(fuzzing), any(feature = "alloc", feature = "std")))]
+    fn convert_secret_key_to_x_only_public_key() {
+        let secp = Secp256k1::new();
+
+        let (sk, _pk, _kp, want) = keys();
+        let (got, parity) = sk.x_only_public_key(&secp);
+
+        assert_eq!(parity, Parity::Even);
+        assert_eq!(got, want)
+    }
+
+    #[test]
+    #[cfg(all(not(fuzzing), any(feature = "alloc", feature = "std")))]
+    fn convert_keypair_to_public_key() {
+        let (_sk, want, kp, _xonly) = keys();
+        let got = kp.public_key();
+
+        assert_eq!(got, want)
+    }
+
+    #[test]
+    #[cfg(all(not(fuzzing), any(feature = "alloc", feature = "std")))]
+    fn convert_keypair_to_x_only_public_key() {
+        let (_sk, _pk, kp, want) = keys();
+        let (got, parity) = kp.x_only_public_key();
+
+        assert_eq!(parity, Parity::Even);
+        assert_eq!(got, want)
+    }
+
+    // SecretKey -> KeyPair -> SecretKey
+    #[test]
+    #[cfg(all(not(fuzzing), any(feature = "alloc", feature = "std")))]
+    fn roundtrip_secret_key_via_keypair() {
+        let secp = Secp256k1::new();
+        let (sk, _pk, _kp, _xonly) = keys();
+
+        let kp = sk.keypair(&secp);
+        let back = kp.secret_key();
+
+        assert_eq!(back, sk)
+    }
+
+    // KeyPair -> SecretKey -> KeyPair
+    #[test]
+    #[cfg(all(not(fuzzing), any(feature = "alloc", feature = "std")))]
+    fn roundtrip_keypair_via_secret_key() {
+        let secp = Secp256k1::new();
+        let (_sk, _pk, kp, _xonly) = keys();
+
+        let sk = kp.secret_key();
+        let back = sk.keypair(&secp);
+
+        assert_eq!(back, kp)
+    }
+
+    // XOnlyPublicKey -> PublicKey -> XOnlyPublicKey
+    #[test]
+    #[cfg(all(not(fuzzing), any(feature = "alloc", feature = "std")))]
+    fn roundtrip_x_only_public_key_via_public_key() {
+        let (_sk, _pk, _kp, xonly) = keys();
+
+        let pk = xonly.public_key(Parity::Even);
+        let (back, parity) = pk.x_only_public_key();
+
+        assert_eq!(parity, Parity::Even);
+        assert_eq!(back, xonly)
+    }
+
+    // PublicKey -> XOnlyPublicKey -> PublicKey
+    #[test]
+    #[cfg(all(not(fuzzing), any(feature = "alloc", feature = "std")))]
+    fn roundtrip_public_key_via_x_only_public_key() {
+        let (_sk, pk, _kp, _xonly) = keys();
+
+        let (xonly, parity) = pk.x_only_public_key();
+        let back = xonly.public_key(parity);
+
+        assert_eq!(back, pk)
+    }
+
+    #[test]
+    fn public_key_from_x_only_public_key_and_odd_parity() {
+        let s = "18845781f631c48f1c9709e23092067d06837f30aa0cd0544ac887fe91ddd166";
+        let mut want = String::from("03");
+        want.push_str(s);
+
+        let xonly = XOnlyPublicKey::from_str(s).expect("failed to parse xonly pubkey string");
+        let pk = xonly.public_key(Parity::Odd);
+        let got = format!("{}", pk);
+
+        assert_eq!(got, want)
+    }
 }

src/schnorr.rs

@@ -268,7 +268,7 @@ impl <C: Signing> Secp256k1<C> {
         rng: &mut R,
     ) -> (KeyPair, XOnlyPublicKey) {
         let sk = KeyPair::new(self, rng);
-        let pubkey = XOnlyPublicKey::from_keypair(&sk);
+        let (pubkey, _parity) = XOnlyPublicKey::from_keypair(&sk);
         (sk, pubkey)
     }
 }
@@ -344,7 +344,7 @@ mod tests {
 
         let mut rng = thread_rng();
         let kp = KeyPair::new(&secp, &mut rng);
-        let pk = kp.public_key();
+        let (pk, _parity) = kp.x_only_public_key();
 
         let mut msg = [0u8; 32];
 
@@ -414,7 +414,7 @@ mod tests {
     fn test_pubkey_serialize_roundtrip() {
         let secp = Secp256k1::new();
         let kp = KeyPair::new(&secp, &mut thread_rng());
-        let pk = kp.public_key();
+        let (pk, _parity) = kp.x_only_public_key();
 
         let ser = pk.serialize();
         let pubkey2 = XOnlyPublicKey::from_slice(&ser).unwrap();
@@ -431,7 +431,7 @@ mod tests {
         assert_eq!(SecretKey::from_str(sk_str).unwrap(), sk);
         let pk = ::key::PublicKey::from_keypair(&keypair);
         assert_eq!(::key::PublicKey::from_secret_key(&secp, &sk), pk);
-        let xpk = keypair.public_key();
+        let (xpk, _parity) = keypair.x_only_public_key();
         assert_eq!(XOnlyPublicKey::from(pk), xpk);
     }
 
@@ -478,7 +478,8 @@ mod tests {
 
             // In fuzzing mode secret->public key derivation is different, so
             // hard-code the expected result.
-            kp.public_key()
+            let (pk, _parity) = kp.x_only_public_key();
+            pk
         };
         #[cfg(fuzzing)]
         let pk = XOnlyPublicKey::from_slice(&[0x18, 0x84, 0x57, 0x81, 0xf6, 0x31, 0xc4, 0x8f, 0x1c, 0x97, 0x09, 0xe2, 0x30, 0x92, 0x06, 0x7d, 0x06, 0x83, 0x7f, 0x30, 0xaa, 0x0c, 0xd0, 0x54, 0x4a, 0xc8, 0x87, 0xfe, 0x91, 0xdd, 0xd1, 0x66]).expect("pk");
@@ -544,7 +545,7 @@ mod tests {
 
         let secp = Secp256k1::new();
         let kp = KeyPair::new(&secp, &mut DumbRng(0));
-        let pk = kp.public_key();
+        let (pk, _parity) = kp.x_only_public_key();
         assert_eq!(
             &pk.serialize()[..],
             &[