diff --git a/Crypto/Cipher/Types/Base.hs b/Crypto/Cipher/Types/Base.hs
index 3843a6c..d35fb06 100644
--- a/Crypto/Cipher/Types/Base.hs
+++ b/Crypto/Cipher/Types/Base.hs
@@ -10,7 +10,6 @@
 {-# LANGUAGE ExistentialQuantification #-}
 module Crypto.Cipher.Types.Base
     ( KeySizeSpecifier(..)
-    , IV(..)
     , Cipher(..)
     , AuthTag(..)
     , AEADMode(..)
@@ -35,11 +34,6 @@ data KeySizeSpecifier =
 -- | Offset inside an XTS data unit, measured in block size.
 type DataUnitOffset = Word32
 
--- | an IV parametrized by the cipher
-data IV c = forall byteArray . ByteArray byteArray => IV byteArray
-
-instance ByteArray (IV c) where
-
 -- | Authentification Tag for AE cipher mode
 newtype AuthTag = AuthTag ByteString
     deriving (Show)
diff --git a/Crypto/Cipher/Types/Block.hs b/Crypto/Cipher/Types/Block.hs
index 20c9c39..0dbe414 100644
--- a/Crypto/Cipher/Types/Block.hs
+++ b/Crypto/Cipher/Types/Block.hs
@@ -15,7 +15,7 @@ module Crypto.Cipher.Types.Block
     -- * BlockCipher
       BlockCipher(..)
     -- * initialization vector (IV)
-    , IV
+    , IV(..)
     , makeIV
     , nullIV
     , ivAdd
@@ -45,6 +45,12 @@ import Crypto.Internal.ByteArray
 import Foreign.Ptr
 import Foreign.Storable
 
+-- | an IV parametrized by the cipher
+data IV c = forall byteArray . ByteArray byteArray => IV byteArray
+
+instance BlockCipher c => ByteArray (IV c) where
+
+
 type XTS cipher = (cipher, cipher)
                -> IV cipher        -- ^ Usually represent the Data Unit (e.g. disk sector)
                -> DataUnitOffset   -- ^ Offset in the data unit in number of blocks
@@ -161,7 +167,26 @@ nullIV = toIV undefined
 --
 -- Assume the IV is in Big Endian format.
 ivAdd :: BlockCipher c => IV c -> Int -> IV c
-ivAdd i _ = i
+ivAdd (IV b) i = IV $ copy b
+  where copy :: ByteArray bs => bs -> bs
+        copy bs = byteArrayCopyAndFreeze bs $ \p -> do
+            let until0 accu = do
+                  r <- loop accu (byteArrayLength bs - 1) p
+                  case r of
+                      0 -> return ()
+                      _ -> until0 r
+            until0 i
+
+        loop :: Int -> Int -> Ptr Word8 -> IO Int
+        loop 0   _   _ = return 0
+        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
 {-
 ivAdd (IV b) i = IV $ snd $ B.mapAccumR addCarry i b
   where addCarry :: Int -> Word8 -> (Int, Word8)
diff --git a/Crypto/Internal/ByteArray.hs b/Crypto/Internal/ByteArray.hs
index 3db24e7..85971bc 100644
--- a/Crypto/Internal/ByteArray.hs
+++ b/Crypto/Internal/ByteArray.hs
@@ -14,6 +14,7 @@ module Crypto.Internal.ByteArray
     ( ByteArray(..)
     , byteArrayAllocAndFreeze
     , empty
+    , byteArrayCopyAndFreeze
     , byteArraySplit
     , byteArrayXor
     , byteArrayConcat
@@ -99,3 +100,9 @@ byteArrayConcat allBs = byteArrayAllocAndFreeze total (loop allBs)
             let sz = byteArrayLength b
             withByteArray b $ \p -> bufCopy dst p sz
             loop bs (dst `plusPtr` sz)
+
+byteArrayCopyAndFreeze :: ByteArray bs => bs -> (Ptr p -> IO ()) -> bs
+byteArrayCopyAndFreeze bs f =
+    byteArrayAllocAndFreeze (byteArrayLength bs) $ \d -> do
+        withByteArray bs $ \s -> bufCopy d s (byteArrayLength bs)
+        f (castPtr d)