From e295e4669be452c3aa855065cf6816c68d42df7a Mon Sep 17 00:00:00 2001
From: Clint Adams <clint@debian.org>
Date: Sat, 31 Mar 2012 18:53:45 -0400
Subject: Implement CRC24 for ASCII armor.

---
 Data/Digest/CRC24.hs | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)
 create mode 100644 Data/Digest/CRC24.hs

diff --git a/Data/Digest/CRC24.hs b/Data/Digest/CRC24.hs
new file mode 100644
index 0000000..5c8c27c
--- /dev/null
+++ b/Data/Digest/CRC24.hs
@@ -0,0 +1,25 @@
+-- CRC24.hs: OpenPGP (RFC4880) CRC24 implementation
+-- Copyright Ⓒ 2012  Clint Adams
+-- This software is released under the terms of the Expat (MIT) license.
+-- (See the LICENSE file).
+
+module Data.Digest.CRC24 (
+  crc24
+) where
+
+import Data.Bits (shiftL, (.&.), xor)
+import Data.ByteString (ByteString)
+import qualified Data.ByteString as B
+import Data.Word (Word8, Word32)
+
+crc24_init :: Word32
+crc24_init = 0xB704CE
+
+crc24_poly :: Word32
+crc24_poly = 0x1864CFB
+
+crc24_update :: Word32 -> Word8 -> Word32
+crc24_update c b = (last . take 9 $ iterate (\x -> if (shiftL x 1) .&. 0x1000000 == 0x1000000 then shiftL x 1 `xor` crc24_poly else shiftL x 1) (c `xor` shiftL (fromIntegral b) 16)) .&. 0xFFFFFF
+
+crc24 :: ByteString -> Word32
+crc24 bs = B.foldl crc24_update crc24_init bs
-- 
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