diff --git a/Crypto/Number/Serialize.hs b/Crypto/Number/Serialize.hs
index ea1798e..b1b096f 100644
--- a/Crypto/Number/Serialize.hs
+++ b/Crypto/Number/Serialize.hs
@@ -18,64 +18,32 @@ module Crypto.Number.Serialize
 import           Data.Bits
 import           Data.Word
 import           Foreign.Storable
-import           Foreign.Ptr
 import           Crypto.Number.Compat
 import           Crypto.Internal.Compat (unsafeDoIO)
+import qualified Crypto.Number.Serialize.Internal as Internal
 import qualified Crypto.Internal.ByteArray as B
-import           Data.Memory.PtrMethods
-
-divMod256 :: Integer -> (Integer, Word8)
-divMod256 n = (n `shiftR` 8, fromIntegral n)
 
 -- | os2ip converts a byte string into a positive integer
 os2ip :: B.ByteArrayAccess ba => ba -> Integer
-os2ip bs = unsafeDoIO $ B.withByteArray bs (loop 0 0)
-  where
-        len = B.length bs
-
-        loop :: Integer -> Int -> Ptr Word8 -> IO Integer
-        loop !acc i p
-            | i == len  = return acc
-            | otherwise = do
-                w <- peekByteOff p i :: IO Word8
-                loop ((acc `shiftL` 8) .|. fromIntegral w) (i+1) p
+os2ip bs = unsafeDoIO $ B.withByteArray bs $ \src -> Internal.os2ip src (B.length bs)
 
 -- | i2osp converts a positive integer into a byte string
 --
 -- first byte is MSB (most significant byte), last byte is the LSB (least significant byte)
 i2osp :: B.ByteArray ba => Integer -> ba
 i2osp 0 = B.allocAndFreeze 1 $ \p -> pokeByteOff p 0 (0 :: Word8)
-i2osp m = B.allocAndFreeze sz (\p -> fillPtr p >> return ())
+i2osp m = B.allocAndFreeze sz (\p -> Internal.i2osp m p sz >> return ())
   where
-        !sz = lengthBytes m
-
-        fillPtr p = gmpExportInteger m p `onGmpUnsupported` export p (sz-1) m
-        export p ofs i
-            | ofs == 0  = pokeByteOff p ofs (fromIntegral i :: Word8)
-            | otherwise = do
-                let (i', b) = divMod256 i
-                pokeByteOff p ofs b
-                export p (ofs-1) i'
+    !sz = lengthBytes m
 
 -- | just like i2osp, but take an extra parameter for size.
 -- if the number is too big to fit in @len bytes, nothing is returned
 -- otherwise the number is padded with 0 to fit the @len required.
 i2ospOf :: B.ByteArray ba => Int -> Integer -> Maybe ba
-i2ospOf 0   _ = error "cannot create integer serialization in 0 bytes"
-i2ospOf len 0 = Just $ B.allocAndFreeze len $ \p -> memSet p 0 len
 i2ospOf len m
-    | sz > len  = Nothing
-    | otherwise = Just $ B.allocAndFreeze len $ \p -> memSet p 0 len >> fillPtr (p `plusPtr` (len - sz))
-  where
-        !sz = lengthBytes m
-
-        fillPtr p = gmpExportInteger m p `onGmpUnsupported` export p (sz-1) m
-        export p ofs i
-            | ofs == 0  = pokeByteOff p ofs (fromIntegral i :: Word8)
-            | otherwise = do
-                let (i', b) = divMod256 i
-                pokeByteOff p ofs b
-                export p (ofs-1) i'
+    | len <= 0  = Nothing
+    | m < 0     = Nothing
+    | otherwise = Just $ B.unsafeCreate len $ \p -> Internal.i2ospOf m p len >> return ()
 
 --
 -- | just like i2ospOf except that it doesn't expect a failure: i.e.
diff --git a/Crypto/Number/Serialize/Internal.hs b/Crypto/Number/Serialize/Internal.hs
new file mode 100644
index 0000000..b729891
--- /dev/null
+++ b/Crypto/Number/Serialize/Internal.hs
@@ -0,0 +1,79 @@
+-- |
+-- Module      : Crypto.Number.Serialize.Internal
+-- License     : BSD-style
+-- Maintainer  : Vincent Hanquez <vincent@snarc.org>
+-- Stability   : experimental
+-- Portability : Good
+--
+-- fast serialization primitives for integer using raw pointers
+{-# LANGUAGE BangPatterns #-}
+module Crypto.Number.Serialize.Internal
+    ( i2osp
+    , i2ospOf
+    , os2ip
+    ) where
+
+import           Crypto.Number.Compat
+import           Crypto.Number.Basic
+import           Data.Bits
+import           Data.Memory.PtrMethods
+import           Data.Word (Word8)
+import           Foreign.Ptr
+import           Foreign.Storable
+
+-- | fill a pointer with the big endian binary representation of an integer
+--
+-- if the room available @ptrSz is less than the number of bytes needed,
+-- 0 is returned. Likewise if a parameter is invalid, 0 is returned.
+--
+-- returns the number of bytes written
+i2osp :: Integer -> Ptr Word8 -> Int -> IO Int
+i2osp m ptr ptrSz
+    | ptrSz <= 0 = return 0
+    | m < 0      = return 0
+    | m == 0     = pokeByteOff ptr 0 (0 :: Word8) >> return 1
+    | ptrSz < sz = return 0
+    | otherwise  = fillPtr >> return sz
+  where
+    !sz = numBytes m
+
+    fillPtr = gmpExportInteger m ptr `onGmpUnsupported` export ptr (sz-1) m
+    export p ofs i
+        | ofs == 0  = pokeByteOff p ofs (fromIntegral i :: Word8)
+        | otherwise = do
+            let (i', b) = i `divMod` 256
+            pokeByteOff p ofs (fromIntegral b :: Word8)
+            export p (ofs-1) i'
+
+-- | Similar to 'i2osp', except it will pad any remaining space with zero.
+i2ospOf :: Integer -> Ptr Word8 -> Int -> IO Int
+i2ospOf m ptr ptrSz
+    | ptrSz <= 0 = return 0
+    | m < 0      = return 0
+    | ptrSz < sz = return 0
+    | otherwise  = (if padSz > 0 then memSet ptr 0 padSz else return ()) >> fillPtr (ptr `plusPtr` padSz) >> return ptrSz
+  where
+    !sz    = numBytes m
+    !padSz = ptrSz - sz
+
+    fillPtr p = gmpExportInteger m p `onGmpUnsupported` export p (sz-1) m
+    export p ofs i
+        | ofs == 0  = pokeByteOff p ofs (fromIntegral i :: Word8)
+        | otherwise = do
+            let (i', b) = i `divMod` 256
+            pokeByteOff p ofs (fromIntegral b :: Word8)
+            export p (ofs-1) i'
+
+-- | transform a big endian binary integer representation pointed by a pointer and a size
+-- into an integer
+os2ip :: Ptr Word8 -> Int -> IO Integer
+os2ip ptr ptrSz
+    | ptrSz <= 0 = return 0
+    | otherwise  = {-gmpImportInteger ptrSz ptr `onGmpUnsupported` -} loop 0 0 ptr 
+  where
+    loop :: Integer -> Int -> Ptr Word8 -> IO Integer
+    loop !acc i p
+        | i == ptrSz = return acc
+        | otherwise  = do
+            w <- peekByteOff p i :: IO Word8
+            loop ((acc `shiftL` 8) .|. fromIntegral w) (i+1) p
diff --git a/cryptonite.cabal b/cryptonite.cabal
index 62160ea..8f0e72d 100644
--- a/cryptonite.cabal
+++ b/cryptonite.cabal
@@ -86,6 +86,7 @@ Library
                      Crypto.Number.ModArithmetic
                      Crypto.Number.Prime
                      Crypto.Number.Serialize
+                     Crypto.Number.Serialize.Internal
                      Crypto.KDF.PBKDF2
                      Crypto.KDF.Scrypt
                      Crypto.Hash