Fix ivAdd overflow behaviour

Crypto/Cipher/Types/Block.hs

@@ -36,6 +36,7 @@ module Crypto.Cipher.Types.Block
     --, cfb8Decrypt
     ) where
 
+import           Control.Monad (unless)
 import           Data.Word
 import           Data.Monoid
 import           Crypto.Error
@@ -167,24 +168,16 @@ nullIV = toIV undefined
 ivAdd :: BlockCipher c => IV c -> Int -> IV c
 ivAdd (IV b) i = IV $ copy b
   where copy :: ByteArray bs => bs -> bs
-        copy bs = B.copyAndFreeze bs $ \p -> do
-            let until0 accu = do
-                  r <- loop accu (B.length bs - 1) p
-                  case r of
-                      0 -> return ()
-                      _ -> until0 r
-            until0 i
+        copy bs = B.copyAndFreeze bs $ loop i (B.length bs - 1)
 
-        loop :: Int -> Int -> Ptr Word8 -> IO Int
-        loop 0   _   _ = return 0
+        loop :: Int -> Int -> Ptr Word8 -> IO ()
+        loop 0   _   _ = return ()
         loop acc ofs p = do
             v <- peek (p `plusPtr` ofs) :: IO Word8
             let accv    = acc + fromIntegral v
                 (hi,lo) = accv `divMod` 256
             poke (p `plusPtr` ofs) (fromIntegral lo :: Word8)
-            if ofs == 0
-                then return hi
-                else loop hi (ofs - 1) p
+            unless (ofs == 0) $ loop hi (ofs - 1) p
 
 cbcEncryptGeneric :: (ByteArray ba, BlockCipher cipher) => cipher -> IV cipher -> ba -> ba
 cbcEncryptGeneric cipher ivini input = mconcat $ doEnc ivini $ chunk (blockSize cipher) input