expose 2-key version of PublicKey::combine

Cargo.toml

@@ -1,7 +1,7 @@
 [package]
 
 name = "secp256k1"
-version = "0.8.0"
+version = "0.8.1"
 authors = [ "Dawid Ciężarkiewicz <dpc@ucore.info>",
             "Andrew Poelstra <apoelstra@wpsoftware.net>" ]
 license = "CC0-1.0"

src/key.rs

@@ -20,6 +20,8 @@
 #[cfg(any(test, feature = "rustc-serialize"))] use serialize::{Decoder, Decodable, Encoder, Encodable};
 #[cfg(any(test, feature = "serde"))] use serde::{Serialize, Deserialize, Serializer, Deserializer};
 
+use std::mem;
+
 use super::{Secp256k1, ContextFlag};
 use super::Error::{self, IncapableContext, InvalidPublicKey, InvalidSecretKey};
 use constants;
@@ -252,6 +254,21 @@ 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(&self, secp: &Secp256k1, 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)
+            }
+        }
+    }
 }
 
 #[cfg(any(test, feature = "rustc-serialize"))]
@@ -377,8 +394,11 @@ mod test {
     use super::{PublicKey, SecretKey};
     use super::super::constants;
 
+    use serialize::hex::FromHex;
     use rand::{Rng, thread_rng};
 
+    macro_rules! hex (($hex:expr) => (FromHex::from_hex($hex).unwrap()));
+
     #[test]
     fn skey_from_slice() {
         let s = Secp256k1::new();
@@ -734,6 +754,30 @@ mod test {
         }).count();
         assert_eq!(count, COUNT);
     }
+
+    #[test]
+    fn pubkey_combine() {
+        let s = Secp256k1::with_caps(ContextFlag::None);
+        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);
+    }
 }