add zero padding and its test.

Crypto/Data/Padding.hs

@@ -21,6 +21,7 @@ import qualified Data.ByteArray as B
 data Format =
       PKCS5     -- ^ PKCS5: PKCS7 with hardcoded size of 8
     | PKCS7 Int -- ^ PKCS7 with padding size between 1 and 255
+    | ZERO Int  -- ^ zero padding with block size
     deriving (Show, Eq)
 
 -- | Apply some pad to a bytearray
@@ -30,6 +31,15 @@ pad (PKCS7 sz) bin = bin `B.append` paddingString
   where
     paddingString = B.replicate paddingByte (fromIntegral paddingByte)
     paddingByte   = sz - (B.length bin `mod` sz)
+pad (ZERO sz)  bin = bin `B.append` paddingString
+  where
+    paddingString = B.replicate paddingSz 0
+    paddingSz
+      | len == 0   =  sz
+      | m == 0     =  0
+      | otherwise  =  sz - m
+    m = len `mod` sz
+    len = B.length bin
 
 -- | Try to remove some padding from a bytearray.
 unpad :: ByteArray byteArray => Format -> byteArray -> Maybe byteArray
@@ -46,3 +56,4 @@ unpad (PKCS7 sz) bin
     paddingSz   = fromIntegral paddingByte
     (content, padding) = B.splitAt (len - paddingSz) bin
     paddingWitness     = B.replicate paddingSz paddingByte :: Bytes
+unpad (ZERO sz)  bin = Nothing

tests/Padding.hs

@@ -13,6 +13,12 @@ cases =
     , ("xyze", 5, "xyze\x01")
     ]
 
+zeroCases =
+    [ ("", 4, "\NUL\NUL\NUL\NUL")
+    , ("abcdef", 8, "abcdef\NUL\NUL")
+    , ("0123456789abcdef", 16, "0123456789abcdef")
+    ]
+
 --instance Arbitrary where
 
 testPad :: Int -> (B.ByteString, Int, B.ByteString) -> TestTree
@@ -21,6 +27,11 @@ testPad n (inp, sz, padded) =
                                          , eqTest "unpadded" (Just inp) (unpad (PKCS7 sz) padded)
                                          ]
 
+testZeroPad :: Int -> (B.ByteString, Int, B.ByteString) -> TestTree
+testZeroPad n (inp, sz, padded) =
+    testCase (show n) $ propertyHoldCase [ eqTest "padded" padded (pad (ZERO sz) inp) ]
+
 tests = testGroup "Padding"
     [ testGroup "Cases" $ map (uncurry testPad) (zip [1..] cases)
+    , testGroup "ZeroCases" $ map (uncurry testZeroPad) (zip [1..] zeroCases)
     ]