Add negation support

secp256k1-sys/src/lib.rs

@@ -243,6 +243,14 @@ extern "C" {
 //TODO secp256k1_ec_privkey_export
 //TODO secp256k1_ec_privkey_import
 
+    #[cfg_attr(not(feature = "external-symbols"), link_name = "rustsecp256k1_v0_1_1_ec_privkey_negate")]
+    pub fn secp256k1_ec_privkey_negate(cx: *const Context,
+                                       sk: *mut c_uchar) -> c_int;
+
+    #[cfg_attr(not(feature = "external-symbols"), link_name = "rustsecp256k1_v0_1_1_ec_pubkey_negate")]
+    pub fn secp256k1_ec_pubkey_negate(cx: *const Context,
+                                      pk: *mut PublicKey) -> c_int;
+
     #[cfg_attr(not(feature = "external-symbols"), link_name = "rustsecp256k1_v0_1_1_ec_privkey_tweak_add")]
     pub fn secp256k1_ec_privkey_tweak_add(cx: *const Context,
                                           sk: *mut c_uchar,

src/key.rs

@@ -148,6 +148,19 @@ impl SecretKey {
         }
     }
 
+    #[inline]
+    /// Negates one secret key.
+    pub fn negate_assign(
+        &mut self
+    ) {
+        unsafe {
+            ffi::secp256k1_ec_privkey_negate(
+                ffi::secp256k1_context_no_precomp,
+                self.as_mut_c_ptr()
+            );
+        }
+    }
+
     #[inline]
     /// Adds one secret key to another, modulo the curve order. WIll
     /// return an error if the resulting key would be invalid or if
@@ -291,6 +304,22 @@ impl PublicKey {
         ret
     }
 
+    #[inline]
+    /// Negates the pk to the pk `self` in place
+    /// Will return an error if the pk would be invalid.
+    pub fn negate_assign<C: Verification>(
+        &mut self,
+        secp: &Secp256k1<C>
+    ) -> Result<(), Error> {
+        unsafe {
+            if ffi::secp256k1_ec_pubkey_negate(secp.ctx, &mut self.0 as *mut _) == 1 {
+                Ok(())
+            } else {
+                Err(Error::InvalidPublicKey)
+            }
+        }
+    }
+
     #[inline]
     /// Adds the pk corresponding to `other` to the pk `self` in place
     /// Will return an error if the resulting key would be invalid or
@@ -752,6 +781,27 @@ mod test {
         assert_eq!(PublicKey::from_secret_key(&s, &sk2), pk2);
     }
 
+    #[test]
+    fn test_negation() {
+        let s = Secp256k1::new();
+
+        let (mut sk, mut pk) = s.generate_keypair(&mut thread_rng());
+
+        let original_sk = sk;
+        let original_pk = pk;
+
+        assert_eq!(PublicKey::from_secret_key(&s, &sk), pk);
+        sk.negate_assign();
+        assert!(pk.negate_assign(&s).is_ok());
+        assert_ne!(original_sk, sk);
+        assert_ne!(original_pk, pk);
+        sk.negate_assign();
+        assert!(pk.negate_assign(&s).is_ok());
+        assert_eq!(original_sk, sk);
+        assert_eq!(original_pk, pk);
+        assert_eq!(PublicKey::from_secret_key(&s, &sk), pk);
+    }
+
     #[test]
     fn pubkey_hash() {
         use std::collections::hash_map::DefaultHasher;