Many thanks for the help on the original space leak which is now fixed -see the function pad below and test runs in small constant space. However, that has merely revealed the next space leak. The problem appears to be blockWord8sIn512 :: [Word8] -> [[Word8]] blockWord8sIn512 = unfoldr g where g [] = Nothing g xs = Just (splitAt 64 xs) or its alternative bws :: [Word8] -> [[Word8]] bws [] = [] bws xs = ys:(bws zs) where (ys,zs) = splitAt 64 xs But I can't see why a big expression is being built up. Shouldn't these produce elements of the list when they are required and they are then consumed by foldl'? Dominic. module Main(main) where import Data.Word import Data.Bits import Data.List ss = [0x67452301, 0xefcdab89, 0x98badcfe, 0x10325476, 0xc3d2e1f0] pad = pad' 0 where pad' l [] = [0x80] ++ ps where pl = (64-(l+9)) `mod` 64 ps = replicate pl 0x00 pad' l (x:xs) = x : (pad' $! l+1) xs -- otherwise (l+1) it will be deferred until replicate blockWord8sIn512 :: [Word8] -> [[Word8]] blockWord8sIn512 = unfoldr g where g [] = Nothing g xs = Just (splitAt 64 xs) bws :: [Word8] -> [[Word8]] bws [] = [] bws xs = ys:(bws zs) where (ys,zs) = splitAt 64 xs test :: Int -> Word8 test n = foldl' xor 0x55 (pad (replicate n 0x55)) test1 :: Int -> [Word8] test1 n = foldl' (zipWith xor) [0x01..0x40] (blockWord8sIn512 (pad (replicate n 0x55))) test2 :: Int -> [Word8] test2 n = foldl' (zipWith xor) [0x01..0x40] (bws (pad (replicate n 0x55))) main = do putStrLn (show (test 1000000)) putStrLn (show (test1 1000000)) putStrLn (show (test2 1000000))