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diff --git a/Data/OpenPGP/Util/Decrypt.hs b/Data/OpenPGP/Util/Decrypt.hs
new file mode 100644
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+++ b/Data/OpenPGP/Util/Decrypt.hs
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+{-# LANGUAGE RankNTypes, CPP #-}
+module Data.OpenPGP.Util.Decrypt where
+
+import qualified Data.ByteString.Lazy as LZ
+import Crypto.Error
+import qualified Crypto.Cipher.AES as Vincent
+import qualified Crypto.Cipher.Blowfish as Vincent
+import qualified Crypto.Cipher.Types as Vincent
+import Crypto.Cipher.Cast5 (CAST5_128)
+import Crypto.Cipher.ThomasToVincent
+import qualified Data.OpenPGP as OpenPGP
+import Data.OpenPGP.Util.Base
+
+
+-- decryption codec for withS2K
+simpleUnCFB :: (Vincent.BlockCipher k) => k -> Vincent.IV k -> LZ.ByteString -> LZ.ByteString
+simpleUnCFB k iv = padThenUnpad k (toLazyBS . Vincent.cfbDecrypt k iv . toStrictBS)
+
+withS2K' :: OpenPGP.SymmetricAlgorithm -> Maybe OpenPGP.S2K -> LZ.ByteString
+            -> (forall b. Vincent.BlockCipher b => b -> x) -> x
+withS2K' OpenPGP.AES128   s2k s f = f (string2key s2k s :: Vincent.AES128)
+withS2K' OpenPGP.AES192   s2k s f = f (string2key s2k s :: Vincent.AES192)
+withS2K' OpenPGP.AES256   s2k s f = f (string2key s2k s :: Vincent.AES256)
+withS2K' OpenPGP.Blowfish s2k s f = f (string2key s2k s :: Vincent.Blowfish128)
+withS2K' OpenPGP.CAST5    s2k s f = f (string2key s2k s :: ThomasToVincent CAST5_128)
+
+string2key :: (Vincent.BlockCipher k) => Maybe OpenPGP.S2K -> LZ.ByteString -> k
+string2key ms2k s = cipher
+    where
+#if defined(VERSION_cryptonite)
+    CryptoPassed cipher = Vincent.cipherInit k
+    k = toStrictBS $ LZ.take ksize $ maybe s (\s2k -> OpenPGP.string2key hashBySymbol s2k s) ms2k
+#else
+    cipher = Vincent.cipherInit k
+    Right k = Vincent.makeKey $ toStrictBS $
+        LZ.take ksize $ maybe s (\s2k -> OpenPGP.string2key hashBySymbol s2k s) ms2k
+#endif
+    ksize = case Vincent.cipherKeySize cipher of
+                Vincent.KeySizeFixed n -> fromIntegral n
+                Vincent.KeySizeEnum xs -> error $ "Unknown key size in string2key"
+                Vincent.KeySizeRange min max -> error $ "Unknown key size range in string2key"
+
+-- Apply a function f to a zero-padded bytestring s to a multiple
+-- of the blocksize for cyper k.
+-- Then drop the same number of bytes from the result of f.
+padThenUnpad :: (Vincent.BlockCipher k) => k -> (LZ.ByteString -> LZ.ByteString) -> LZ.ByteString -> LZ.ByteString
+padThenUnpad k f s = dropPadEnd (f padded)
+    where
+        dropPadEnd s = LZ.take (LZ.length s - padAmount) s
+        padded = s `LZ.append` LZ.replicate padAmount 0
+        padAmount = blksize - (LZ.length s `mod` blksize)
+        blksize = fromIntegral $ Vincent.blockSize k
+