Revert "remove PublicKey::combine"

This reverts commit 17450e91fd.

CHANGELOG.md

@@ -1,4 +1,8 @@
 
+# 0.11.1 - 2018-08-22
+
+* Put `PublicKey::combine` back because it is currently needed to implement Lightning BOLT 3
+
 # 0.11.0 - 2018-08-22
 
 * Update `rand` to 0.4 and `gcc` 0.3 to `cc` 1.0. (`rand` 0.5 exists but has a lot of breaking changes and no longer compiles with 1.14.0.)

src/key.rs

@@ -273,6 +273,21 @@ impl PublicKey {
             }
         }
     }
+
+    /// Adds a second key to this one, returning the sum. Returns an error if
+    /// the result would be the point at infinity, i.e. we are adding this point
+    /// to its own negation
+    pub fn combine<C>(&self, secp: &Secp256k1<C>, other: &PublicKey) -> Result<PublicKey, Error> {
+        unsafe {
+            let mut ret = mem::uninitialized();
+            let ptrs = [self.as_ptr(), other.as_ptr()];
+            if ffi::secp256k1_ec_pubkey_combine(secp.ctx, &mut ret, ptrs.as_ptr(), 2) == 1 {
+                Ok(PublicKey(ret))
+            } else {
+                Err(InvalidPublicKey)
+            }
+        }
+    }
 }
 
 /// Creates a new public key from a FFI public key
@@ -551,6 +566,30 @@ mod test {
         assert_eq!(count, COUNT);
     }
 
+    #[test]
+    fn pubkey_combine() {
+        let s = Secp256k1::without_caps();
+        let compressed1 = PublicKey::from_slice(
+            &s,
+            &hex!("0241cc121c419921942add6db6482fb36243faf83317c866d2a28d8c6d7089f7ba"),
+        ).unwrap();
+        let compressed2 = PublicKey::from_slice(
+            &s,
+            &hex!("02e6642fd69bd211f93f7f1f36ca51a26a5290eb2dd1b0d8279a87bb0d480c8443"),
+        ).unwrap();
+        let exp_sum = PublicKey::from_slice(
+            &s,
+            &hex!("0384526253c27c7aef56c7b71a5cd25bebb66dddda437826defc5b2568bde81f07"),
+        ).unwrap();
+
+        let sum1 = compressed1.combine(&s, &compressed2);
+        assert!(sum1.is_ok());
+        let sum2 = compressed2.combine(&s, &compressed1);
+        assert!(sum2.is_ok());
+        assert_eq!(sum1, sum2);
+        assert_eq!(sum1.unwrap(), exp_sum);
+    }
+
     #[test]
     fn pubkey_equal() {
         let s = Secp256k1::new();