add TweakCheck error

signer/src/error.rs

@@ -237,6 +237,11 @@ pub enum Error {
     #[error("invalid tweak? seriously? {0}")]
     InvalidPublicKeyTweak(#[source] secp256k1::Error),
 
+    /// This happens when a tweak produced by [`XOnlyPublicKey::add_tweak`] was computed incorrectly.
+    /// One if possible reasons is that you tweaked same key twice.
+    #[error("Tweak was computed incorrectly.")]
+    InvalidPublicKeyTweakCheck,
+
     /// This occurs when converting a byte slice to our internal public key
     /// type, which is a thin wrapper around the secp256k1::SecretKey.
     #[error("invalid private key: {0}")]

signer/src/keys.rs

@@ -453,7 +453,9 @@ impl SignerScriptPubKey for secp256k1::XOnlyPublicKey {
             .add_tweak(SECP256K1, &tweak)
             .map_err(Error::InvalidPublicKeyTweak)?;
 
-        debug_assert!(self.tweak_add_check(SECP256K1, &output_key, parity, tweak));
+        if !self.tweak_add_check(SECP256K1, &output_key, parity, tweak) {
+            return Err(Error::InvalidPublicKeyTweakCheck);
+        }
         let pk = secp256k1::PublicKey::from_x_only_public_key(output_key, parity);
         Ok(PublicKey(pk))
     }