Add combine_keys function to PublicKey

Author: Tibo-lg

src/key.rs

@@ -65,6 +65,7 @@ pub const ONE_KEY: SecretKey = SecretKey([0, 0, 0, 0, 0, 0, 0, 0,
 
 /// A Secp256k1 public key, used for verification of signatures
 #[derive(Copy, Clone, PartialEq, Eq, Debug, Hash)]
+#[repr(transparent)]
 pub struct PublicKey(ffi::PublicKey);
 
 impl fmt::LowerHex for PublicKey {
@@ -364,14 +365,26 @@ impl PublicKey {
     /// the result would be the point at infinity, i.e. we are adding this point
     /// to its own negation
     pub fn combine(&self, other: &PublicKey) -> Result<PublicKey, Error> {
+        PublicKey::combine_keys(&[self, other])
+    }
+
+    /// Adds the keys in the provided slice together, returning the sum. Returns
+    /// an error if the result would be the point at infinity, i.e. we are adding
+    /// a point to its own negation
+    pub fn combine_keys(keys: &[&PublicKey]) -> Result<PublicKey, Error> {
+        use core::mem::transmute;
+        use core::i32::MAX;
+
+        debug_assert!(keys.len() < MAX as usize);
         unsafe {
             let mut ret = ffi::PublicKey::new();
-            let ptrs = [self.as_c_ptr(), other.as_c_ptr()];
+            let ptrs : &[*const ffi::PublicKey] =
+                transmute::<&[&PublicKey], &[*const ffi::PublicKey]>(keys);
             if ffi::secp256k1_ec_pubkey_combine(
                 ffi::secp256k1_context_no_precomp,
                 &mut ret,
                 ptrs.as_c_ptr(),
-                2
+                keys.len() as i32
             ) == 1
             {
                 Ok(PublicKey(ret))
@@ -794,6 +807,29 @@ mod test {
         assert_eq!(sum1.unwrap(), exp_sum);
     }
 
+    #[test]
+    fn pubkey_combine_keys() {
+        let compressed1 = PublicKey::from_slice(
+            &hex!("0241cc121c419921942add6db6482fb36243faf83317c866d2a28d8c6d7089f7ba"),
+        ).unwrap();
+        let compressed2 = PublicKey::from_slice(
+            &hex!("02e6642fd69bd211f93f7f1f36ca51a26a5290eb2dd1b0d8279a87bb0d480c8443"),
+        ).unwrap();
+        let compressed3 = PublicKey::from_slice(
+            &hex!("03e74897d8644eb3e5b391ca2ab257aec2080f4d1a95cad57e454e47f021168eb0")
+        ).unwrap();
+        let exp_sum = PublicKey::from_slice(
+            &hex!("0252d73a47f66cf341e5651542f0348f452b7c793af62a6d8bff75ade703a451ad"),
+        ).unwrap();
+
+        let sum1 = PublicKey::combine_keys(&[&compressed1, &compressed2, &compressed3]);
+        assert!(sum1.is_ok());
+        let sum2 = PublicKey::combine_keys(&[&compressed1, &compressed2, &compressed3]);
+        assert!(sum2.is_ok());
+        assert_eq!(sum1, sum2);
+        assert_eq!(sum1.unwrap(), exp_sum);
+    }
+
     #[test]
     fn pubkey_equal() {
         let pk1 = PublicKey::from_slice(