Merge #430: Add convenience methods for keys

f08276a Add convenience methods for keys (Tobin Harding)
b4c7fa0 Let the compiler work out int size (Tobin Harding)
c612130 Borrow secret key (Tobin Harding)

Pull request description:

  We have a bunch of `from_<key>` methods for converting between key types. To make the API more ergonomic to use we can add methods that do the same but called on a variable 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`.

  Fixes: #428

  ### Note to reviewers

  - `XOnlyPublicKey` -> `PublicKey` logic is made up by me, I could not work out how to get `libsecp256k1` to do this.
  - Please review the tests carefully, they include assumptions based on my current understanding of the cryptography :)

ACKs for top commit:
  sanket1729:
    ACK f08276a. Thanks for going through all the iterations.
  apoelstra:
    ACK f08276a

Tree-SHA512: 1503a6e570a3958110c6f24cd6d075fe5694b3b32b91a7a9d332c63aa0806198ff10bdd95e7f9de0cf73cbf4e3655c6826bd04e5044d1b019f551471b187c8ea

Author: apoelstra

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 {
@@ -674,7 +732,7 @@ impl Ord for PublicKey {
 ///
 /// let secp = Secp256k1::new();
 /// let (secret_key, public_key) = secp.generate_keypair(&mut rand::thread_rng());
-/// let key_pair = KeyPair::from_secret_key(&secp, secret_key);
+/// let key_pair = KeyPair::from_secret_key(&secp, &secret_key);
 /// # }
 /// ```
 /// [`Deserialize`]: serde::Deserialize
@@ -700,7 +758,7 @@ impl KeyPair {
     #[inline]
     pub fn from_secret_key<C: Signing>(
         secp: &Secp256k1<C>,
-        sk: SecretKey,
+        sk: &SecretKey,
     ) -> KeyPair {
         unsafe {
             let mut kp = ffi::KeyPair::new();
@@ -860,9 +918,27 @@ 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 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 public_key(&self) -> XOnlyPublicKey {
+    pub fn x_only_public_key(&self) -> (XOnlyPublicKey, Parity) {
         XOnlyPublicKey::from_keypair(self)
     }
 
@@ -1008,9 +1084,9 @@ 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();
@@ -1021,7 +1097,9 @@ 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)
         }
     }
 
@@ -1092,7 +1170,7 @@ 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");
     /// # }
     /// ```
@@ -1105,6 +1183,7 @@ impl XOnlyPublicKey {
             return Err(Error::InvalidTweak);
         }
 
+        let mut pk_parity = 0;
         unsafe {
             let mut pubkey = ffi::PublicKey::new();
             let mut err = ffi::secp256k1_xonly_pubkey_tweak_add(
@@ -1117,18 +1196,17 @@ impl XOnlyPublicKey {
                 return Err(Error::InvalidTweak);
             }
 
-            let mut parity: ::secp256k1_sys::types::c_int = 0;
             err = ffi::secp256k1_xonly_pubkey_from_pubkey(
                 secp.ctx,
                 &mut self.0,
-                &mut parity,
+                &mut pk_parity,
                 &pubkey,
             );
             if err == 0 {
                 return Err(Error::InvalidPublicKey);
             }
 
-            Parity::from_i32(parity).map_err(Into::into)
+            Parity::from_i32(pk_parity).map_err(Into::into)
         }
     }
 
@@ -1157,7 +1235,7 @@ 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));
@@ -1183,6 +1261,14 @@ 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.
@@ -1420,7 +1506,7 @@ pub mod serde_keypair {
 
         Ok(KeyPair::from_secret_key(
             &::SECP256K1,
-            secret_key,
+            &secret_key,
         ))
     }
 }
@@ -1972,12 +2058,15 @@ 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));
         }
     }
@@ -2046,4 +2135,150 @@ 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)
+    }
 }