Added SKEIN256 and SKEIN512 backends

SHA3 is still waiting for final standardization.
Although this is looking less likely given
https://www.cdt.org/blogs/joseph-lorenzo-hall/2409-nist-sha-3

In the meantime, cryptohash implements skein, and it's used by some of the
haskell ecosystem (for yesod sessions, IIRC), so this implementation is
likely to continue working. Also, I've talked with the cryprohash author
and he's a reasonable guy.

It makes sense to have an alternate high security hash, in case some
horrible attack is found against SHA2 tomorrow, or in case SHA3 comes out
and worst fears are realized.

I'd also like to support using skein for HMAC. But no hurry there and
a new version of cryptohash has much nicer HMAC code, so I will probably
wait until I can use that version.

Backend.hs

@@ -27,12 +27,12 @@ import qualified Types.Backend as B
 import Config
 
 -- When adding a new backend, import it here and add it to the list.
-import qualified Backend.SHA
+import qualified Backend.Hash
 import qualified Backend.WORM
 import qualified Backend.URL
 
 list :: [Backend]
-list = Backend.SHA.backends ++ Backend.WORM.backends ++ Backend.URL.backends
+list = Backend.Hash.backends ++ Backend.WORM.backends ++ Backend.URL.backends
 
 {- List of backends in the order to try them when storing a new key. -}
 orderedList :: Annex [Backend]

Backend/Hash.hs

@@ -1,11 +1,11 @@
-{- git-annex SHA backends
+{- git-annex hashing backends
  -
  - Copyright 2011-2013 Joey Hess <joey@kitenet.net>
  -
  - Licensed under the GNU GPL version 3 or higher.
  -}
 
-module Backend.SHA (backends) where
+module Backend.Hash (backends) where
 
 import Common.Annex
 import qualified Annex
@@ -19,87 +19,64 @@ import qualified Build.SysConfig as SysConfig
 import qualified Data.ByteString.Lazy as L
 import Data.Char
 
-type SHASize = Int
+data Hash = SHAHash HashSize | SkeinHash HashSize
+type HashSize = Int
 
 {- Order is slightly significant; want SHA256 first, and more general
  - sizes earlier. -}
-sizes :: [Int]
-sizes = [256, 1, 512, 224, 384]
+hashes :: [Hash]
+hashes = concat 
+	[ map SHAHash [256, 1, 512, 224, 384]
+	, map SkeinHash [256, 512]
+	]
 
-{- The SHA256E backend is the default. -}
+{- The SHA256E backend is the default, so genBackendE comes first. -}
 backends :: [Backend]
-backends = catMaybes $ map genBackendE sizes ++ map genBackend sizes
+backends = catMaybes $ map genBackendE hashes ++ map genBackend hashes
 
-genBackend :: SHASize -> Maybe Backend
-genBackend size = Just Backend
-	{ name = shaName size
-	, getKey = keyValue size
-	, fsckKey = Just $ checkKeyChecksum size
+genBackend :: Hash -> Maybe Backend
+genBackend hash = Just Backend
+	{ name = hashName hash
+	, getKey = keyValue hash
+	, fsckKey = Just $ checkKeyChecksum hash
 	, canUpgradeKey = Just needsUpgrade
 	}
 
-genBackendE :: SHASize -> Maybe Backend
-genBackendE size = do
-	b <- genBackend size
+genBackendE :: Hash -> Maybe Backend
+genBackendE hash = do
+	b <- genBackend hash
 	return $ b 
-		{ name = shaNameE size
-		, getKey = keyValueE size
+		{ name = hashNameE hash
+		, getKey = keyValueE hash
 		}
 
-shaName :: SHASize -> String
-shaName size = "SHA" ++ show size
+hashName :: Hash -> String
+hashName (SHAHash size) = "SHA" ++ show size
+hashName (SkeinHash size) = "SKEIN" ++ show size
 
-shaNameE :: SHASize -> String
-shaNameE size = shaName size ++ "E"
+hashNameE :: Hash -> String
+hashNameE hash = hashName hash ++ "E"
 
-shaN :: SHASize -> FilePath -> Integer -> Annex String
-shaN shasize file filesize = do
-	showAction "checksum"
-	liftIO $ case shaCommand shasize filesize of
-		Left sha -> sha <$> L.readFile file
-		Right command ->
-			either error return 
-				=<< externalSHA command shasize file
-
-shaCommand :: SHASize -> Integer -> Either (L.ByteString -> String) String
-shaCommand shasize filesize
-	| shasize == 1 = use SysConfig.sha1 sha1
-	| shasize == 256 = use SysConfig.sha256 sha256
-	| shasize == 224 = use SysConfig.sha224 sha224
-	| shasize == 384 = use SysConfig.sha384 sha384
-	| shasize == 512 = use SysConfig.sha512 sha512
-	| otherwise = error $ "bad sha size " ++ show shasize
-  where
-	use Nothing hasher = Left $ show . hasher
-	use (Just c) hasher
-		{- Use builtin, but slightly slower hashing for
-		 - smallish files. Cryptohash benchmarks 90 to 101%
-		 - faster than external hashers, depending on the hash
-		 - and system. So there is no point forking an external
-		 - process unless the file is large. -}
-		| filesize < 1048576 = use Nothing hasher
-		| otherwise = Right c
-
-{- A key is a checksum of its contents. -}
-keyValue :: SHASize -> KeySource -> Annex (Maybe Key)
-keyValue shasize source = do
+{- A key is a hash of its contents. -}
+keyValue :: Hash -> KeySource -> Annex (Maybe Key)
+keyValue hash source = do
 	let file = contentLocation source
 	stat <- liftIO $ getFileStatus file
 	let filesize = fromIntegral $ fileSize stat
-	s <- shaN shasize file filesize
+	s <- hashFile hash file filesize
 	return $ Just $ stubKey
 		{ keyName = s
-		, keyBackendName = shaName shasize
+		, keyBackendName = hashName hash
 		, keySize = Just filesize
 		}
 
 {- Extension preserving keys. -}
-keyValueE :: SHASize -> KeySource -> Annex (Maybe Key)
-keyValueE size source = keyValue size source >>= maybe (return Nothing) addE
+keyValueE :: Hash -> KeySource -> Annex (Maybe Key)
+keyValueE hash source = keyValue hash source >>= maybe (return Nothing) addE
   where
 	addE k = return $ Just $ k
 		{ keyName = keyName k ++ selectExtension (keyFilename source)
-		, keyBackendName = shaNameE size
+		, keyBackendName = hashNameE hash
 		}
 
 selectExtension :: FilePath -> String
@@ -113,27 +90,27 @@ selectExtension f
 	shortenough e = length e <= 4 -- long enough for "jpeg"
 
 {- A key's checksum is checked during fsck. -}
-checkKeyChecksum :: SHASize -> Key -> FilePath -> Annex Bool
-checkKeyChecksum size key file = do
+checkKeyChecksum :: Hash -> Key -> FilePath -> Annex Bool
+checkKeyChecksum hash key file = do
 	fast <- Annex.getState Annex.fast
 	mstat <- liftIO $ catchMaybeIO $ getFileStatus file
 	case (mstat, fast) of
 		(Just stat, False) -> do
 			let filesize = fromIntegral $ fileSize stat
-			check <$> shaN size file filesize
+			check <$> hashFile hash file filesize
 		_ -> return True
   where
-	sha = keySha key
+	expected = keyHash key
 	check s
-		| s == sha = True
+		| s == expected = True
 		{- A bug caused checksums to be prefixed with \ in some
 		 - cases; still accept these as legal now that the bug has been
 		 - fixed. -}
-		| '\\' : s == sha = True
+		| '\\' : s == expected = True
 		| otherwise = False
 
-keySha :: Key -> String
-keySha key = dropExtensions (keyName key)
+keyHash :: Key -> String
+keyHash key = dropExtensions (keyName key)
 
 validExtension :: Char -> Bool
 validExtension c
@@ -144,5 +121,42 @@ validExtension c
 {- Upgrade keys that have the \ prefix on their sha due to a bug, or
  - that contain non-alphanumeric characters in their extension. -}
 needsUpgrade :: Key -> Bool
-needsUpgrade key = "\\" `isPrefixOf` keySha key ||
+needsUpgrade key = "\\" `isPrefixOf` keyHash key ||
 	any (not . validExtension) (takeExtensions $ keyName key)
+
+hashFile :: Hash -> FilePath -> Integer -> Annex String
+hashFile hash file filesize = do
+	showAction "checksum"
+	liftIO $ go hash
+  where
+  	go (SHAHash hashsize) = case shaCommand hashsize filesize of
+		Left sha -> sha <$> L.readFile file
+		Right command ->
+			either error return 
+				=<< externalSHA command hashsize file
+	go (SkeinHash hashsize) = skeinHasher hashsize <$> L.readFile file
+
+skeinHasher :: HashSize -> (L.ByteString -> String)
+skeinHasher hashsize 
+	| hashsize == 256 = show . skein256
+	| hashsize == 512 = show . skein512
+	| otherwise = error $ "bad skein size " ++ show hashsize
+
+shaCommand :: HashSize -> Integer -> Either (L.ByteString -> String) String
+shaCommand hashsize filesize
+	| hashsize == 1 = use SysConfig.sha1 sha1
+	| hashsize == 256 = use SysConfig.sha256 sha256
+	| hashsize == 224 = use SysConfig.sha224 sha224
+	| hashsize == 384 = use SysConfig.sha384 sha384
+	| hashsize == 512 = use SysConfig.sha512 sha512
+	| otherwise = error $ "bad sha size " ++ show hashsize
+  where
+	use Nothing hasher = Left $ show . hasher
+	use (Just c) hasher
+		{- Use builtin, but slightly slower hashing for
+		 - smallish files. Cryptohash benchmarks 90 to 101%
+		 - faster than external hashers, depending on the hash
+		 - and system. So there is no point forking an external
+		 - process unless the file is large. -}
+		| filesize < 1048576 = use Nothing hasher
+		| otherwise = Right c

Utility/Hash.hs

@@ -26,4 +26,8 @@ sha512 = hashlazy
 --sha3 :: L.ByteString -> Digest SHA3
 --sha3 = hashlazy
 
+skein256 :: L.ByteString -> Digest Skein256_256
+skein256 = hashlazy
 
+skein512 :: L.ByteString -> Digest Skein512_512
+skein512 = hashlazy

debian/changelog

@@ -12,6 +12,7 @@ git-annex (4.20130921) UNRELEASED; urgency=low
   * Use cryptohash rather than SHA for hashing when no external hash program
     is available. This is a significant speedup for SHA256 on OSX, for
     example.
+  * Added SKEIN256 and SKEIN512 backends.
   * Android build redone from scratch, many dependencies updated,
     and entire build can now be done using provided scripts.
   * assistant: Clear the list of failed transfers when doing a full transfer

doc/backends.mdwn

@@ -21,6 +21,8 @@ can use different ones for different files.
    but are not concerned about security.
 * `SHA384`, `SHA384E`, `SHA224`, `SHA224E` -- Hashes for people who like
   unusual sizes.
+* `SKEIN512`, `SKEIN256` -- [Skein hash](http://en.wikipedia.org/wiki/Skein_hash),
+  a well-regarded SHA3 hash competition finalist.
 
 The `annex.backends` git-config setting can be used to list the backends
 git-annex should use. The first one listed will be used by default when