Fix memory safety in schnorrSign function

Ensure pointer from useAsCStringLen is used within its scope to prevent potential use-after-free errors.

Author: prolic

src/Crypto/Secp256k1.hs

@@ -755,18 +755,25 @@ schnorrSign mGen KeyPair{..} bs
         keyPairPtr <- ContT (withForeignPtr keyPairFPtr)
         lift $ do
             sigBuf <- mallocBytes 64
-            let randomPtr = case mGen of
-                    Just gen -> castPtr . unsafePerformIO . BS.useAsCString (BS.pack $ Prelude.take 32 $ randoms gen) $ return
-                    Nothing -> nullPtr
-            ret <- Prim.schnorrsigSign ctx sigBuf msgHashPtr keyPairPtr randomPtr
+            ret <- case mGen of
+                Just gen -> do
+                    let randomBytes = BS.pack $ Prelude.take 32 $ randoms gen
+                    BS.useAsCStringLen randomBytes $ \(ptr, _) ->
+                        Prim.schnorrsigSign ctx sigBuf msgHashPtr keyPairPtr (castPtr ptr)
+                Nothing ->
+                    Prim.schnorrsigSign ctx sigBuf msgHashPtr keyPairPtr nullPtr
             if isSuccess ret
                 then Just . SchnorrSignature <$> newForeignPtr finalizerFree sigBuf
-                else free sigBuf $> Nothing
+                else do
+                    free sigBuf
+                    return Nothing
+
 
 -- | Compute a deterministic schnorr signature using a 'KeyPair'.
 schnorrSignDeterministic :: KeyPair -> ByteString -> Maybe SchnorrSignature
 schnorrSignDeterministic = schnorrSign Nothing
 
+
 -- | Compute a non-deterministic schnorr signature using a 'KeyPair'.
 schnorrSignNondeterministic :: KeyPair -> ByteString -> IO (Maybe SchnorrSignature)
 schnorrSignNondeterministic kp bs = newStdGen >>= \gen -> pure $ schnorrSign (Just gen) kp bs