Merge #53: Fix tweak_private_key

ed38ec2 tests: add coverage for confidential::bare::tweak_*key (Leonardo Comandini)
7734755 Fix tweak_private_key (Leonardo Comandini)

Pull request description:

  to return a `SecretKey` instead of a `PublicKey`

ACKs for top commit:
  sanket1729:
    ACK ed38ec2

Tree-SHA512: 021ac5e6db9048bf0117a88a4972ae55794ef7c203cf7cf257333603b12ee9c943f79ddd3a23739b345791cbcacf039c268f61291ae68224d0380e49f7ed92a9

Author: sanket1729

src/confidential/bare.rs

@@ -59,26 +59,22 @@ where
 }
 
 /// Tweaks a bare key using the scriptPubKey of a descriptor
-pub fn tweak_private_key<'a, Pk, V>(
+pub fn tweak_private_key<V>(
     secp: &secp256k1_zkp::Secp256k1<V>,
     spk: &elements::Script,
-    pk: &Pk,
-) -> secp256k1_zkp::PublicKey
+    sk: &secp256k1_zkp::SecretKey,
+) -> secp256k1_zkp::SecretKey
 where
-    Pk: ToPublicKey + 'a,
-    V: secp256k1_zkp::Verification,
+    V: secp256k1_zkp::Signing,
 {
     let mut eng = TweakHash::engine();
-    pk.to_public_key()
+    bitcoin::PublicKey::new(sk.public_key(secp))
         .write_into(&mut eng)
         .expect("engines don't error");
     spk.consensus_encode(&mut eng).expect("engines don't error");
     let hash_bytes = TweakHash::from_engine(eng).to_byte_array();
     let hash_scalar = secp256k1_zkp::Scalar::from_be_bytes(hash_bytes).expect("bytes from hash");
-    pk.to_public_key()
-        .inner
-        .add_exp_tweak(secp, &hash_scalar)
-        .unwrap()
+    sk.add_tweak(&hash_scalar).unwrap()
 }
 
 #[cfg(test)]
@@ -119,4 +115,16 @@ mod tests {
             "e1e52419a2934d278c50e29608969d2f23c1bd1243a09bfc8026d4ed4b085e39",
         );
     }
+
+    #[test]
+    fn tweak() {
+        // Check that tweaking blinding keys produce consistent results
+        let secp = secp256k1_zkp::Secp256k1::new();
+        let sk = secp256k1_zkp::SecretKey::from_slice(&[1u8; 32]).unwrap();
+        let pk = sk.public_key(&secp);
+        let spk = elements::Script::default();
+        let tweaked_pk = tweak_key(&secp, &spk, &pk);
+        let tweaked_sk = tweak_private_key(&secp, &spk, &sk);
+        assert_eq!(tweaked_pk, tweaked_sk.public_key(&secp));
+    }
 }