add KeyVariety type

Where before the "name" of a key and a backend was a string, this makes
it a concrete data type.

This is groundwork for allowing some varieties of keys to be disabled
in file2key, so git-annex won't use them at all.

Benchmarks ran in my big repo:

old git-annex info:

real	0m3.338s
user	0m3.124s
sys	0m0.244s

new git-annex info:

real	0m3.216s
user	0m3.024s
sys	0m0.220s

new git-annex find:

real	0m7.138s
user	0m6.924s
sys	0m0.252s

old git-annex find:

real	0m7.433s
user	0m7.240s
sys	0m0.232s

Surprising result; I'd have expected it to be slower since it now parses
all the key varieties. But, the parser is very simple and perhaps
sharing KeyVarieties uses less memory or something like that.

This commit was supported by the NSF-funded DataLad project.

22 files changed, 202 insertions, 99 deletions

diff --git a/Annex/Content.hs b/Annex/Content.hs
index e879e4eeb..8e225548f 100644
--- a/Annex/Content.hs
+++ b/Annex/Content.hs
@@ -341,7 +341,7 @@ verifyKeyContent v UnVerified k f = ifM (shouldVerify v)
 		Just size -> do
 			size' <- liftIO $ catchDefaultIO 0 $ getFileSize f
 			return (size' == size)
-	verifycontent = case Types.Backend.verifyKeyContent =<< Backend.maybeLookupBackendName (keyBackendName k) of
+	verifycontent = case Types.Backend.verifyKeyContent =<< Backend.maybeLookupBackendVariety (keyVariety k) of
 		Nothing -> return True
 		Just verifier -> verifier k f
 
diff --git a/Annex/Locations.hs b/Annex/Locations.hs
index 3138b2322..6bc24c4a8 100644
--- a/Annex/Locations.hs
+++ b/Annex/Locations.hs
@@ -78,6 +78,7 @@ import Data.Default
 
 import Common
 import Key
+import Types.Key
 import Types.UUID
 import Types.GitConfig
 import Types.Difference
@@ -478,7 +479,7 @@ prop_isomorphic_fileKey s
 	| null s = True -- it's not legal for a key to have no keyName
 	| otherwise= Just k == fileKey (keyFile k)
   where
-	k = stubKey { keyName = s, keyBackendName = "test" }
+	k = stubKey { keyName = s, keyVariety = OtherKey "test" }
 
 {- A location to store a key on a special remote that uses a filesystem.
  - A directory hash is used, to protect against filesystems that dislike
diff --git a/Assistant/Threads/SanityChecker.hs b/Assistant/Threads/SanityChecker.hs
index 62ba8f0d0..0c79ef605 100644
--- a/Assistant/Threads/SanityChecker.hs
+++ b/Assistant/Threads/SanityChecker.hs
@@ -39,6 +39,7 @@ import Git.Index
 import Assistant.Unused
 import Logs.Unused
 import Types.Transfer
+import Types.Key
 import Annex.Path
 import qualified Annex
 #ifdef WITH_WEBAPP
@@ -308,7 +309,7 @@ cleanReallyOldTmp = do
 	cleanjunk check f = case fileKey (takeFileName f) of
 		Nothing -> cleanOld check f
 		Just k
-			| "GPGHMAC" `isPrefixOf` keyBackendName k ->
+			| "GPGHMAC" `isPrefixOf` formatKeyVariety (keyVariety k) ->
 				cleanOld check f
 			| otherwise -> noop
 
diff --git a/Assistant/Upgrade.hs b/Assistant/Upgrade.hs
index afbb61924..f91fde06c 100644
--- a/Assistant/Upgrade.hs
+++ b/Assistant/Upgrade.hs
@@ -115,7 +115,7 @@ distributionDownloadComplete d dest cleanup t
 	| otherwise = cleanup
   where
 	k = distributionKey d
-	fsckit f = case Backend.maybeLookupBackendName (Types.Key.keyBackendName k) of
+	fsckit f = case Backend.maybeLookupBackendVariety (Types.Key.keyVariety k) of
 		Nothing -> return $ Just f
 		Just b -> case Types.Backend.verifyKeyContent b of
 			Nothing -> return $ Just f
diff --git a/Backend.hs b/Backend.hs
index f7bbed6b5..40b618355 100644
--- a/Backend.hs
+++ b/Backend.hs
@@ -1,6 +1,6 @@
 {- git-annex key/value backends
  -
- - Copyright 2010-2014 Joey Hess <id@joeyh.name>
+ - Copyright 2010-2017 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU GPL version 3 or higher.
  -}
@@ -11,14 +11,15 @@ module Backend (
 	genKey,
 	getBackend,
 	chooseBackend,
-	lookupBackendName,
-	maybeLookupBackendName,
+	lookupBackendVariety,
+	maybeLookupBackendVariety,
 	isStableKey,
 ) where
 
 import Annex.Common
 import qualified Annex
 import Annex.CheckAttr
+import Types.Key
 import Types.KeySource
 import qualified Types.Backend as B
 
@@ -42,14 +43,15 @@ orderedList = do
 			f <- Annex.getState Annex.forcebackend
 			case f of
 				Just name | not (null name) ->
-					return [lookupBackendName name]
+					return [lookupname name]
 				_ -> do
 					l' <- gen . annexBackends <$> Annex.getGitConfig
 					Annex.changeState $ \s -> s { Annex.backends = l' }
 					return l'
   where
 	gen [] = list
-	gen l = map lookupBackendName l
+	gen ns = map lookupname ns
+	lookupname = lookupBackendVariety . parseKeyVariety
 
 {- Generates a key for a file, trying each backend in turn until one
  - accepts it. -}
@@ -73,33 +75,33 @@ genKey' (b:bs) source = do
 		| otherwise = c
 
 getBackend :: FilePath -> Key -> Annex (Maybe Backend)
-getBackend file k = let bname = keyBackendName k in
-	case maybeLookupBackendName bname of
-		Just backend -> return $ Just backend
-		Nothing -> do
-			warning $ "skipping " ++ file ++ " (unknown backend " ++ bname ++ ")"
-			return Nothing
+getBackend file k = case maybeLookupBackendVariety (keyVariety k) of
+	Just backend -> return $ Just backend
+	Nothing -> do
+		warning $ "skipping " ++ file ++ " (unknown backend " ++ formatKeyVariety (keyVariety k) ++ ")"
+		return Nothing
 
 {- Looks up the backend that should be used for a file.
  - That can be configured on a per-file basis in the gitattributes file. -}
 chooseBackend :: FilePath -> Annex (Maybe Backend)
 chooseBackend f = Annex.getState Annex.forcebackend >>= go
   where
-	go Nothing =  maybeLookupBackendName <$> checkAttr "annex.backend" f
+	go Nothing =  maybeLookupBackendVariety . parseKeyVariety
+		<$> checkAttr "annex.backend" f
 	go (Just _) = Just . Prelude.head <$> orderedList
 
-{- Looks up a backend by name. May fail if unknown. -}
-lookupBackendName :: String -> Backend
-lookupBackendName s = fromMaybe unknown $ maybeLookupBackendName s
+{- Looks up a backend by variety. May fail if unsupported or disabled. -}
+lookupBackendVariety :: KeyVariety -> Backend
+lookupBackendVariety v = fromMaybe unknown $ maybeLookupBackendVariety v
   where
-	unknown = error $ "unknown backend " ++ s
+	unknown = error $ "unknown backend " ++ formatKeyVariety v
 
-maybeLookupBackendName :: String -> Maybe Backend
-maybeLookupBackendName s = M.lookup s nameMap
+maybeLookupBackendVariety :: KeyVariety -> Maybe Backend
+maybeLookupBackendVariety v = M.lookup v varietyMap
 
-nameMap :: M.Map String Backend
-nameMap = M.fromList $ zip (map B.name list) list
+varietyMap :: M.Map KeyVariety Backend
+varietyMap = M.fromList $ zip (map B.backendVariety list) list
 
 isStableKey :: Key -> Bool
 isStableKey k = maybe False (`B.isStableKey` k) 
-	(maybeLookupBackendName (keyBackendName k))
+	(maybeLookupBackendVariety (keyVariety k))
diff --git a/Backend/Hash.hs b/Backend/Hash.hs
index c85047d51..a1640435c 100644
--- a/Backend/Hash.hs
+++ b/Backend/Hash.hs
@@ -1,6 +1,6 @@
 {- git-annex hashing backends
  -
- - Copyright 2011-2015 Joey Hess <id@joeyh.name>
+ - Copyright 2011-2017 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU GPL version 3 or higher.
  -}
@@ -14,6 +14,7 @@ module Backend.Hash (
 
 import Annex.Common
 import qualified Annex
+import Types.Key
 import Types.Backend
 import Types.KeySource
 import Utility.Hash
@@ -29,17 +30,16 @@ data Hash
 	| SHA2Hash HashSize
 	| SHA3Hash HashSize
 	| SkeinHash HashSize
-type HashSize = Int
 
 {- Order is slightly significant; want SHA256 first, and more general
  - sizes earlier. -}
 hashes :: [Hash]
 hashes = concat 
-	[ map SHA2Hash [256, 512, 224, 384]
+	[ map (SHA2Hash . HashSize) [256, 512, 224, 384]
 #ifdef WITH_CRYPTONITE
-	, map SHA3Hash [256, 512, 224, 384]
+	, map (SHA3Hash . HashSize) [256, 512, 224, 384]
 #endif
-	, map SkeinHash [256, 512]
+	, map (SkeinHash . HashSize) [256, 512]
 	, [SHA1Hash]
 	, [MD5Hash]
 	]
@@ -50,7 +50,7 @@ backends = concatMap (\h -> [genBackendE h, genBackend h]) hashes
 
 genBackend :: Hash -> Backend
 genBackend hash = Backend
-	{ name = hashName hash
+	{ backendVariety = hashKeyVariety hash (HasExt False)
 	, getKey = keyValue hash
 	, verifyKeyContent = Just $ checkKeyChecksum hash
 	, canUpgradeKey = Just needsUpgrade
@@ -60,19 +60,16 @@ genBackend hash = Backend
 
 genBackendE :: Hash -> Backend
 genBackendE hash = (genBackend hash)
-	{ name = hashNameE hash
+	{ backendVariety = hashKeyVariety hash (HasExt True)
 	, getKey = keyValueE hash
 	}
 
-hashName :: Hash -> String
-hashName MD5Hash = "MD5"
-hashName SHA1Hash = "SHA1"
-hashName (SHA2Hash size) = "SHA" ++ show size
-hashName (SHA3Hash size) = "SHA3_" ++ show size
-hashName (SkeinHash size) = "SKEIN" ++ show size
-
-hashNameE :: Hash -> String
-hashNameE hash = hashName hash ++ "E"
+hashKeyVariety :: Hash -> HasExt -> KeyVariety
+hashKeyVariety MD5Hash = MD5Key
+hashKeyVariety SHA1Hash = SHA1Key
+hashKeyVariety (SHA2Hash size) = SHA2Key size
+hashKeyVariety (SHA3Hash size) = SHA3Key size
+hashKeyVariety (SkeinHash size) = SKEINKey size
 
 {- A key is a hash of its contents. -}
 keyValue :: Hash -> KeySource -> Annex (Maybe Key)
@@ -82,7 +79,7 @@ keyValue hash source = do
 	s <- hashFile hash file filesize
 	return $ Just $ stubKey
 		{ keyName = s
-		, keyBackendName = hashName hash
+		, keyVariety = hashKeyVariety hash (HasExt False)
 		, keySize = Just filesize
 		}
 
@@ -92,7 +89,7 @@ keyValueE hash source = keyValue hash source >>= maybe (return Nothing) addE
   where
 	addE k = return $ Just $ k
 		{ keyName = keyName k ++ selectExtension (keyFilename source)
-		, keyBackendName = hashNameE hash
+		, keyVariety = hashKeyVariety hash (HasExt True)
 		}
 
 selectExtension :: FilePath -> String
@@ -149,24 +146,29 @@ needsUpgrade key = "\\" `isPrefixOf` keyHash key ||
 trivialMigrate :: Key -> Backend -> AssociatedFile -> Maybe Key
 trivialMigrate oldkey newbackend afile
 	{- Fast migration from hashE to hash backend. -}
-	| keyBackendName oldkey == name newbackend ++ "E" = Just $ oldkey
+	| migratable && hasExt newvariety = Just $ oldkey
 		{ keyName = keyHash oldkey
-		, keyBackendName = name newbackend
+		, keyVariety = newvariety
 		}
 	{- Fast migration from hash to hashE backend. -}
-	| keyBackendName oldkey ++"E" == name newbackend = case afile of
+	| migratable && hasExt oldvariety = case afile of
 		Nothing -> Nothing
 		Just file -> Just $ oldkey
 			{ keyName = keyHash oldkey ++ selectExtension file
-			, keyBackendName = name newbackend
+			, keyVariety = newvariety
 			}
 	| otherwise = Nothing
+  where
+	migratable = oldvariety /= newvariety 
+		&& sameExceptExt oldvariety newvariety
+	oldvariety = keyVariety oldkey
+	newvariety = backendVariety newbackend
 
 hashFile :: Hash -> FilePath -> Integer -> Annex String
 hashFile hash file filesize = go hash
   where
 	go MD5Hash = use md5Hasher
-	go SHA1Hash = usehasher 1
+	go SHA1Hash = usehasher (HashSize 1)
 	go (SHA2Hash hashsize) = usehasher hashsize
 	go (SHA3Hash hashsize) = use (sha3Hasher hashsize)
 	go (SkeinHash hashsize) = use (skeinHasher hashsize)
@@ -176,10 +178,10 @@ hashFile hash file filesize = go hash
 		-- Force full evaluation so file is read and closed.
 		return (length h `seq` h)
 	
-	usehasher hashsize = case shaHasher hashsize filesize of
+	usehasher hashsize@(HashSize sz) = case shaHasher hashsize filesize of
 		Left sha -> use sha
 		Right (external, internal) -> do
-			v <- liftIO $ externalSHA external hashsize file
+			v <- liftIO $ externalSHA external sz file
 			case v of
 				Right r -> return r
 				Left e -> do
@@ -189,7 +191,7 @@ hashFile hash file filesize = go hash
 					use internal
 
 shaHasher :: HashSize -> Integer -> Either (L.ByteString -> String) (String, L.ByteString -> String)
-shaHasher hashsize filesize
+shaHasher (HashSize hashsize) filesize
 	| hashsize == 1 = use SysConfig.sha1 sha1
 	| hashsize == 256 = use SysConfig.sha256 sha2_256
 	| hashsize == 224 = use SysConfig.sha224 sha2_224
@@ -209,7 +211,7 @@ shaHasher hashsize filesize
 	usehasher hasher = show . hasher
 
 sha3Hasher :: HashSize -> (L.ByteString -> String)
-sha3Hasher hashsize
+sha3Hasher (HashSize hashsize)
 #ifdef WITH_CRYPTONITE
 	| hashsize == 256 = show . sha3_256
 	| hashsize == 224 = show . sha3_224
@@ -219,7 +221,7 @@ sha3Hasher hashsize
 	| otherwise = error $ "unsupported SHA3 size " ++ show hashsize
 
 skeinHasher :: HashSize -> (L.ByteString -> String)
-skeinHasher hashsize 
+skeinHasher (HashSize hashsize)
 	| hashsize == 256 = show . skein256
 	| hashsize == 512 = show . skein512
 	| otherwise = error $ "unsupported SKEIN size " ++ show hashsize
@@ -236,7 +238,7 @@ md5Hasher = show . md5
  -}
 testKeyBackend :: Backend
 testKeyBackend = 
-	let b = genBackendE (SHA2Hash 256)
+	let b = genBackendE (SHA2Hash (HashSize 256))
 	in b { getKey = (fmap addE) <$$> getKey b } 
   where
 	addE k = k { keyName = keyName k ++ longext }
diff --git a/Backend/URL.hs b/Backend/URL.hs
index 92b7a4482..b9d8264d6 100644
--- a/Backend/URL.hs
+++ b/Backend/URL.hs
@@ -11,6 +11,7 @@ module Backend.URL (
 ) where
 
 import Annex.Common
+import Types.Key
 import Types.Backend
 import Backend.Utilities
 
@@ -19,7 +20,7 @@ backends = [backend]
 
 backend :: Backend
 backend = Backend
-	{ name = "URL"
+	{ backendVariety = URLKey
 	, getKey = const $ return Nothing
 	, verifyKeyContent = Nothing
 	, canUpgradeKey = Nothing
@@ -33,6 +34,6 @@ backend = Backend
 fromUrl :: String -> Maybe Integer -> Key
 fromUrl url size = stubKey
 	{ keyName = genKeyName url
-	, keyBackendName = "URL"
+	, keyVariety = URLKey
 	, keySize = size
 	}
diff --git a/Backend/WORM.hs b/Backend/WORM.hs
index 99a853f47..d7220a431 100644
--- a/Backend/WORM.hs
+++ b/Backend/WORM.hs
@@ -8,6 +8,7 @@
 module Backend.WORM (backends) where
 
 import Annex.Common
+import Types.Key
 import Types.Backend
 import Types.KeySource
 import Backend.Utilities
@@ -18,7 +19,7 @@ backends = [backend]
 
 backend :: Backend
 backend = Backend
-	{ name = "WORM"
+	{ backendVariety = WORMKey
 	, getKey = keyValue
 	, verifyKeyContent = Nothing
 	, canUpgradeKey = Nothing
@@ -37,7 +38,7 @@ keyValue source = do
 	relf <- getTopFilePath <$> inRepo (toTopFilePath $ keyFilename source)
 	return $ Just $ stubKey
 		{ keyName = genKeyName relf
-		, keyBackendName = name backend
+		, keyVariety = WORMKey
 		, keySize = Just sz
 		, keyMtime = Just $ modificationTime stat
 		}
diff --git a/CmdLine/GitAnnex/Options.hs b/CmdLine/GitAnnex/Options.hs
index f0d00981b..f7e0dcf07 100644
--- a/CmdLine/GitAnnex/Options.hs
+++ b/CmdLine/GitAnnex/Options.hs
@@ -15,6 +15,7 @@ import Annex.Common
 import qualified Git.Config
 import qualified Git.Construct
 import Git.Types
+import Types.Key
 import Types.TrustLevel
 import Types.NumCopies
 import Types.Messages
@@ -346,4 +347,5 @@ completeRemotes = completer $ mkCompleter $ \input -> do
 		
 		
 completeBackends :: HasCompleter f => Mod f a
-completeBackends = completeWith (map Backend.name Backend.list)
+completeBackends = completeWith $
+	map (formatKeyVariety . Backend.backendVariety) Backend.list
diff --git a/Command/Find.hs b/Command/Find.hs
index 553ddc419..d3571c6f8 100644
--- a/Command/Find.hs
+++ b/Command/Find.hs
@@ -13,6 +13,7 @@ import qualified Data.Map as M
 import Command
 import Annex.Content
 import Limit
+import Types.Key
 import qualified Utility.Format
 import Utility.DataUnits
 
@@ -76,7 +77,7 @@ showFormatted format unformatted vars =
 keyVars :: Key -> [(String, String)]
 keyVars key =
 	[ ("key", key2file key)
-	, ("backend", keyBackendName key)
+	, ("backend", formatKeyVariety $ keyVariety key)
 	, ("bytesize", size show)
 	, ("humansize", size $ roughSize storageUnits True)
 	, ("keyname", keyName key)
diff --git a/Command/Fsck.hs b/Command/Fsck.hs
index 96ffd35da..f1b0b78a6 100644
--- a/Command/Fsck.hs
+++ b/Command/Fsck.hs
@@ -175,7 +175,7 @@ performRemote key afile backend numcopies remote =
 
 startKey :: Maybe Remote -> Incremental -> Key -> ActionItem -> NumCopies -> CommandStart
 startKey from inc key ai numcopies =
-	case Backend.maybeLookupBackendName (keyBackendName key) of
+	case Backend.maybeLookupBackendVariety (keyVariety key) of
 		Nothing -> stop
 		Just backend -> runFsck inc ai key $
 			case from of
diff --git a/Command/Info.hs b/Command/Info.hs
index 9def38838..835a8498d 100644
--- a/Command/Info.hs
+++ b/Command/Info.hs
@@ -36,6 +36,7 @@ import qualified Git.LsTree as LsTree
 import Utility.Percentage
 import Types.Transfer
 import Logs.Transfer
+import Types.Key
 import Types.TrustLevel
 import Types.FileMatcher
 import qualified Limit
@@ -51,7 +52,7 @@ data KeyData = KeyData
 	{ countKeys :: Integer
 	, sizeKeys :: Integer
 	, unknownSizeKeys :: Integer
-	, backendsKeys :: M.Map String Integer
+	, backendsKeys :: M.Map KeyVariety Integer
 	}
 
 data NumCopiesStats = NumCopiesStats
@@ -451,7 +452,8 @@ disk_size = simpleStat "available local disk space" $
 
 backend_usage :: Stat
 backend_usage = stat "backend usage" $ json fmt $
-	ObjectMap . backendsKeys <$> cachedReferencedData
+	ObjectMap . (M.mapKeys formatKeyVariety) . backendsKeys
+		<$> cachedReferencedData
   where
 	fmt = multiLine . map (\(b, n) -> b ++ ": " ++ show n) . sort . M.toList . fromObjectMap
 
@@ -598,7 +600,7 @@ addKey key (KeyData count size unknownsize backends) =
 	{- All calculations strict to avoid thunks when repeatedly
 	 - applied to many keys. -}
 	!count' = count + 1
-	!backends' = M.insertWith (+) (keyBackendName key) 1 backends
+	!backends' = M.insertWith (+) (keyVariety key) 1 backends
 	!size' = maybe size (+ size) ks
 	!unknownsize' = maybe (unknownsize + 1) (const unknownsize) ks
 	ks = keySize key
diff --git a/Command/Smudge.hs b/Command/Smudge.hs
index cf5272f82..1644ee257 100644
--- a/Command/Smudge.hs
+++ b/Command/Smudge.hs
@@ -84,7 +84,7 @@ clean file = do
 				-- for this file before, so that when
 				-- git re-cleans a file its backend does
 				-- not change.
-				currbackend <- maybe Nothing (maybeLookupBackendName . keyBackendName)
+				currbackend <- maybe Nothing (maybeLookupBackendVariety . keyVariety)
 					<$> catKeyFile file
 				liftIO . emitPointer
 					=<< go
diff --git a/Command/TestRemote.hs b/Command/TestRemote.hs
index 4c0ff9e3c..78921b856 100644
--- a/Command/TestRemote.hs
+++ b/Command/TestRemote.hs
@@ -149,7 +149,7 @@ test st r k =
 		Annex.eval st (Annex.setOutput QuietOutput >> a) @? "failed"
 	present b = check ("present " ++ show b) $
 		(== Right b) <$> Remote.hasKey r k
-	fsck = case maybeLookupBackendName (keyBackendName k) of
+	fsck = case maybeLookupBackendVariety (keyVariety k) of
 		Nothing -> return True
 		Just b -> case Backend.verifyKeyContent b of
 			Nothing -> return True
diff --git a/Command/Version.hs b/Command/Version.hs
index e15f1fb91..ece5fbb05 100644
--- a/Command/Version.hs
+++ b/Command/Version.hs
@@ -11,6 +11,7 @@ import Command
 import qualified Build.SysConfig as SysConfig
 import Annex.Version
 import BuildFlags
+import Types.Key
 import qualified Types.Backend as B
 import qualified Types.Remote as R
 import qualified Remote
@@ -62,7 +63,8 @@ showPackageVersion :: IO ()
 showPackageVersion = do
 	vinfo "git-annex version" SysConfig.packageversion
 	vinfo "build flags" $ unwords buildFlags
-	vinfo "key/value backends" $ unwords $ map B.name Backend.list
+	vinfo "key/value backends" $ unwords $
+		map (formatKeyVariety . B.backendVariety) Backend.list
 	vinfo "remote types" $ unwords $ map R.typename Remote.remoteTypes
 
 showRawVersion :: IO ()
diff --git a/Crypto.hs b/Crypto.hs
index dc1d2e6d2..a37528619 100644
--- a/Crypto.hs
+++ b/Crypto.hs
@@ -42,6 +42,7 @@ import Annex.Common
 import qualified Utility.Gpg as Gpg
 import Types.Crypto
 import Types.Remote
+import Types.Key
 
 {- The beginning of a Cipher is used for MAC'ing; the remainder is used
  - as the GPG symmetric encryption passphrase when using the hybrid
@@ -159,14 +160,16 @@ type EncKey = Key -> Key
 encryptKey :: Mac -> Cipher -> EncKey
 encryptKey mac c k = stubKey
 	{ keyName = macWithCipher mac c (key2file k)
-	, keyBackendName = encryptedBackendNamePrefix ++ showMac mac
+	, keyVariety = OtherKey (encryptedBackendNamePrefix ++ showMac mac)
 	}
 
 encryptedBackendNamePrefix :: String
 encryptedBackendNamePrefix = "GPG"
 
 isEncKey :: Key -> Bool
-isEncKey k = encryptedBackendNamePrefix `isPrefixOf` keyBackendName k
+isEncKey k = case keyVariety k of
+	OtherKey s ->  encryptedBackendNamePrefix `isPrefixOf` s
+	_ -> False
 
 type Feeder = Handle -> IO ()
 type Reader m a = Handle -> m a
diff --git a/Key.hs b/Key.hs
index 99aa756e2..5eaf3d56b 100644
--- a/Key.hs
+++ b/Key.hs
@@ -35,7 +35,7 @@ import qualified Utility.SimpleProtocol as Proto
 stubKey :: Key
 stubKey = Key
 	{ keyName = ""
-	, keyBackendName = ""
+	, keyVariety = OtherKey ""
 	, keySize = Nothing
 	, keyMtime = Nothing
 	, keyChunkSize = Nothing
@@ -69,8 +69,8 @@ fieldSep = '-'
  - The name field is always shown last, separated by doubled fieldSeps,
  - and is the only field allowed to contain the fieldSep. -}
 key2file :: Key -> FilePath
-key2file Key { keyBackendName = b, keySize = s, keyMtime = m, keyChunkSize = cs, keyChunkNum = cn, keyName = n } =
-	b +++ ('s' ?: s) +++ ('m' ?: m) +++ ('S' ?: cs) +++ ('C' ?: cn) +++ (fieldSep : n)
+key2file Key { keyVariety = kv, keySize = s, keyMtime = m, keyChunkSize = cs, keyChunkNum = cn, keyName = n } =
+	formatKeyVariety kv +++ ('s' ?: s) +++ ('m' ?: m) +++ ('S' ?: cs) +++ ('C' ?: cn) +++ (fieldSep : n)
   where
 	"" +++ y = y
 	x +++ "" = x
@@ -80,12 +80,12 @@ key2file Key { keyBackendName = b, keySize = s, keyMtime = m, keyChunkSize = cs,
 
 file2key :: FilePath -> Maybe Key
 file2key s
-	| key == Just stubKey || (keyName <$> key) == Just "" || (keyBackendName <$> key) == Just "" = Nothing
+	| key == Just stubKey || (keyName <$> key) == Just "" || (keyVariety <$> key) == Just (OtherKey "") = Nothing
 	| otherwise = key
   where
 	key = startbackend stubKey s
 
-	startbackend k v = sepfield k v addbackend
+	startbackend k v = sepfield k v addvariety
 		
 	sepfield k v a = case span (/= fieldSep) v of
 		(v', _:r) -> findfields r $ a k v'
@@ -96,7 +96,7 @@ file2key s
 		| otherwise = sepfield k v $ addfield c
 	findfields _ v = v
 
-	addbackend k v = Just k { keyBackendName = v }
+	addvariety k v = Just k { keyVariety = parseKeyVariety v }
 
 	-- This is a strict parser for security reasons; a key
 	-- can contain only 4 fields, which all consist only of numbers.
@@ -126,31 +126,27 @@ file2key s
 		| validKeyName k v = Just $ k { keyName = v }
 		| otherwise = Nothing
 
-{- A key with a backend ending in "E" is an extension preserving key,
- - using some hash.
+{- When a key HasExt, the length of the extension is limited in order to
+ - mitigate against SHA1 collision attacks (specifically, chosen-prefix
+ - attacks).
  -
- - The length of the extension is limited in order to mitigate against
- - SHA1 collision attacks (specifically, chosen-prefix attacks).
  - In such an attack, the extension of the key could be made to contain
  - the collision generation data, with the result that a signed git commit
  - including such keys would not be secure. 
  -
  - The maximum extension length ever generated for such a key was 8
  - characters; 20 is used here to give a little future wiggle-room. 
- - The SHA1 common-prefix attack used 128 bytes of data.
- -
- - This code is here, and not in Backend.Hash (where it really belongs)
- - so that file2key can check it whenever a Key is constructed.
+ - The SHA1 common-prefix attack needs 128 bytes of data.
  -}
 validKeyName :: Key -> String -> Bool
-validKeyName k v
-	| end (keyBackendName k) == "E" = length (takeExtensions v) <= 20
+validKeyName k name
+	| hasExt (keyVariety k) = length (takeExtensions name) <= 20
 	| otherwise = True
 
 instance Arbitrary Key where
 	arbitrary = Key
 		<$> (listOf1 $ elements $ ['A'..'Z'] ++ ['a'..'z'] ++ ['0'..'9'] ++ "-_\r\n \t")
-		<*> (listOf1 $ elements ['A'..'Z']) -- BACKEND
+		<*> (parseKeyVariety <$> (listOf1 $ elements ['A'..'Z'])) -- BACKEND
 		<*> ((abs <$>) <$> arbitrary) -- size cannot be negative
 		<*> arbitrary
 		<*> ((abs <$>) <$> arbitrary) -- chunksize cannot be negative
diff --git a/Limit.hs b/Limit.hs
index 7b26f9e58..1485b4bce 100644
--- a/Limit.hs
+++ b/Limit.hs
@@ -19,6 +19,7 @@ import Annex.Action
 import Annex.UUID
 import Logs.Trust
 import Annex.NumCopies
+import Types.Key
 import Types.TrustLevel
 import Types.Group
 import Types.FileMatcher
@@ -251,7 +252,8 @@ addInBackend = addLimit . limitInBackend
 limitInBackend :: MkLimit Annex
 limitInBackend name = Right $ const $ checkKey check
   where
-	check key = pure $ keyBackendName key == name
+	check key = pure $ keyVariety key == variety
+	variety = parseKeyVariety name
 
 {- Adds a limit to skip files that are too large or too small -}
 addLargerThan :: String -> Annex ()
diff --git a/Test.hs b/Test.hs
index 1b724b5af..7ef0cb5f0 100644
--- a/Test.hs
+++ b/Test.hs
@@ -64,6 +64,7 @@ import qualified Logs.PreferredContent
 import qualified Types.MetaData
 import qualified Remote
 import qualified Key
+import qualified Types.Key
 import qualified Types.Messages
 import qualified Config
 import qualified Config.Cost
@@ -2152,7 +2153,7 @@ backendWORM :: Types.Backend
 backendWORM = backend_ "WORM"
 
 backend_ :: String -> Types.Backend
-backend_ = Backend.lookupBackendName
+backend_ = Backend.lookupBackendVariety . Types.Key.parseKeyVariety
 
 getKey :: Types.Backend -> FilePath -> IO Types.Key
 getKey b f = fromJust <$> annexeval go
diff --git a/Types/Backend.hs b/Types/Backend.hs
index 9a1c44cc8..f1d8919a4 100644
--- a/Types/Backend.hs
+++ b/Types/Backend.hs
@@ -2,7 +2,7 @@
  -
  - Most things should not need this, using Types instead
  -
- - Copyright 2010,2012 Joey Hess <id@joeyh.name>
+ - Copyright 2010-2017 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU GPL version 3 or higher.
  -}
@@ -13,7 +13,7 @@ import Types.Key
 import Types.KeySource
 
 data BackendA a = Backend
-	{ name :: String
+	{ backendVariety :: KeyVariety
 	, getKey :: KeySource -> a (Maybe Key) 
 	-- Verifies the content of a key.
 	, verifyKeyContent :: Maybe (Key -> FilePath -> a Bool)
@@ -28,7 +28,7 @@ data BackendA a = Backend
 	}
 
 instance Show (BackendA a) where
-	show backend = "Backend { name =\"" ++ name backend ++ "\" }"
+	show backend = "Backend { name =\"" ++ formatKeyVariety (backendVariety backend) ++ "\" }"
 
 instance Eq (BackendA a) where
-	a == b = name a == name b
+	a == b = backendVariety a == backendVariety b
diff --git a/Types/Key.hs b/Types/Key.hs
index 0615adfe4..27d56dfd9 100644
--- a/Types/Key.hs
+++ b/Types/Key.hs
@@ -7,13 +7,15 @@
 
 module Types.Key where
 
+import Utility.PartialPrelude
+
 import System.Posix.Types
 
 {- A Key has a unique name, which is derived from a particular backend,
  - and may contain other optional metadata. -}
 data Key = Key
 	{ keyName :: String
-	, keyBackendName :: String
+	, keyVariety :: KeyVariety
 	, keySize :: Maybe Integer
 	, keyMtime :: Maybe EpochTime
 	, keyChunkSize :: Maybe Integer
@@ -22,3 +24,85 @@ data Key = Key
 
 {- A filename may be associated with a Key. -}
 type AssociatedFile = Maybe FilePath
+
+{- There are several different varieties of keys. -}
+data KeyVariety
+	= SHA2Key HashSize HasExt
+	| SHA3Key HashSize HasExt
+	| SKEINKey HashSize HasExt
+	| SHA1Key HasExt
+	| MD5Key HasExt
+	| WORMKey
+	| URLKey
+ 	-- Some repositories may contain keys of other varieties,
+	-- which can still be processed to some extent.
+	| OtherKey String
+	deriving (Eq, Ord, Read, Show)
+
+{- Some varieties of keys may contain an extension at the end of the
+ - keyName -}
+newtype HasExt = HasExt Bool
+	deriving (Eq, Ord, Read, Show)
+
+newtype HashSize = HashSize Int
+	deriving (Eq, Ord, Read, Show)
+
+hasExt :: KeyVariety -> Bool
+hasExt (SHA2Key _ (HasExt b)) = b
+hasExt (SHA3Key _ (HasExt b)) = b
+hasExt (SKEINKey _ (HasExt b)) = b
+hasExt (SHA1Key (HasExt b)) = b
+hasExt (MD5Key (HasExt b)) = b
+hasExt WORMKey = False
+hasExt URLKey = False
+hasExt (OtherKey s) =  end s == "E"
+
+sameExceptExt :: KeyVariety -> KeyVariety -> Bool
+sameExceptExt (SHA2Key sz1 _) (SHA2Key sz2 _) = sz1 == sz2
+sameExceptExt (SHA3Key sz1 _) (SHA3Key sz2 _) = sz1 == sz2
+sameExceptExt (SKEINKey sz1 _) (SKEINKey sz2 _) = sz1 == sz2
+sameExceptExt (SHA1Key _) (SHA1Key _) = True
+sameExceptExt (MD5Key _) (MD5Key _) = True
+sameExceptExt _ _ = False
+
+formatKeyVariety :: KeyVariety -> String
+formatKeyVariety v = case v of
+	SHA2Key sz e -> adde e (addsz sz "SHA")
+	SHA3Key sz e -> adde e (addsz sz "SHA3_")
+	SKEINKey sz e -> adde e (addsz sz "SKEIN")
+	SHA1Key e -> adde e "SHA1"
+	MD5Key e -> adde e "MD5"
+	WORMKey -> "WORM"
+	URLKey -> "URL"
+	OtherKey s -> s
+  where
+	adde (HasExt False) s = s
+	adde (HasExt True) s = s ++ "E"
+	addsz (HashSize n) s =  s ++ show n
+
+parseKeyVariety :: String -> KeyVariety
+parseKeyVariety "SHA256"    =  SHA2Key (HashSize 256) (HasExt False)
+parseKeyVariety "SHA256E"   = SHA2Key (HashSize 256) (HasExt True)
+parseKeyVariety "SHA512"    = SHA2Key (HashSize 512) (HasExt False)
+parseKeyVariety "SHA512E"   = SHA2Key (HashSize 512) (HasExt True)
+parseKeyVariety "SHA224"    = SHA2Key (HashSize 224) (HasExt False)
+parseKeyVariety "SHA224E"   = SHA2Key (HashSize 224) (HasExt True)
+parseKeyVariety "SHA384"    = SHA2Key (HashSize 384) (HasExt False)
+parseKeyVariety "SHA384E"   = SHA2Key (HashSize 384) (HasExt True)
+parseKeyVariety "SHA3_512"  = SHA3Key (HashSize 512) (HasExt False)
+parseKeyVariety "SHA3_512E" = SHA3Key (HashSize 512) (HasExt True)
+parseKeyVariety "SHA3_384"  = SHA3Key (HashSize 384) (HasExt False)
+parseKeyVariety "SHA3_384E" = SHA3Key (HashSize 384) (HasExt True)
+parseKeyVariety "SHA3_256"  = SHA3Key (HashSize 256) (HasExt False)
+parseKeyVariety "SHA3_256E" = SHA3Key (HashSize 256) (HasExt True)
+parseKeyVariety "SHA3_224"  = SHA3Key (HashSize 224) (HasExt False)
+parseKeyVariety "SHA3_224E" = SHA3Key (HashSize 224) (HasExt True)
+parseKeyVariety "SKEIN512"  = SKEINKey (HashSize 512) (HasExt False)
+parseKeyVariety "SKEIN512E" = SKEINKey (HashSize 512) (HasExt True)
+parseKeyVariety "SKEIN256"  = SKEINKey (HashSize 256) (HasExt False)
+parseKeyVariety "SKEIN256E" = SKEINKey (HashSize 256) (HasExt True)
+parseKeyVariety "SHA1"      = SHA1Key (HasExt False)
+parseKeyVariety "MD5"       = MD5Key (HasExt False)
+parseKeyVariety "WORM"      = WORMKey
+parseKeyVariety "URL"       = URLKey
+parseKeyVariety s           = OtherKey s
diff --git a/Upgrade/V1.hs b/Upgrade/V1.hs
index 725bb4089..d0f9e51d3 100644
--- a/Upgrade/V1.hs
+++ b/Upgrade/V1.hs
@@ -14,6 +14,7 @@ import Data.Default
 import Annex.Common
 import Annex.Content
 import Annex.Link
+import Types.Key
 import Logs.Presence
 import qualified Annex.Queue
 import qualified Git
@@ -130,7 +131,7 @@ oldlog2key l
   where
 	len = length l - 4
 	k = readKey1 (take len l)
-	sane = (not . null $ keyName k) && (not . null $ keyBackendName k)
+	sane = (not . null $ keyName k) && (not . null $ formatKeyVariety $ keyVariety k)
 
 -- WORM backend keys: "WORM:mtime:size:filename"
 -- all the rest: "backend:key"
@@ -143,7 +144,7 @@ readKey1 v
 	| mixup = fromJust $ file2key $ intercalate ":" $ Prelude.tail bits
 	| otherwise = stubKey
 		{ keyName = n
-		, keyBackendName = b
+		, keyVariety = parseKeyVariety b
 		, keySize = s
 		, keyMtime = t
 		}
@@ -161,11 +162,12 @@ readKey1 v
 	mixup = wormy && isUpper (Prelude.head $ bits !! 1)
 
 showKey1 :: Key -> String
-showKey1 Key { keyName = n , keyBackendName = b, keySize = s, keyMtime = t } =
+showKey1 Key { keyName = n , keyVariety = v, keySize = s, keyMtime = t } =
 	intercalate ":" $ filter (not . null) [b, showifhere t, showifhere s, n]
   where
 	showifhere Nothing = ""
-	showifhere (Just v) = show v
+	showifhere (Just x) = show x
+	b = formatKeyVariety v
 
 keyFile1 :: Key -> FilePath
 keyFile1 key = replace "/" "%" $ replace "%" "&s" $ replace "&" "&a"  $ showKey1 key
@@ -189,7 +191,7 @@ lookupFile1 file = do
 		Right l -> makekey l
   where
 	getsymlink = takeFileName <$> readSymbolicLink file
-	makekey l = case maybeLookupBackendName bname of
+	makekey l = case maybeLookupBackendVariety (keyVariety k) of
 		Nothing -> do
 			unless (null kname || null bname ||
 			        not (isLinkToAnnex l)) $
@@ -198,7 +200,7 @@ lookupFile1 file = do
 		Just backend -> return $ Just (k, backend)
 	  where
 		k = fileKey1 l
-		bname = keyBackendName k
+		bname = formatKeyVariety (keyVariety k)
 		kname = keyName k
 		skip = "skipping " ++ file ++ 
 			" (unknown backend " ++ bname ++ ")"