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diff --git a/Backend/WORM.hs b/Backend/WORM.hs
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+{- git-annex "WORM" backend -- Write Once, Read Many
+ - -}
+
+module Backend.WORM (backend) where
+
+import Control.Monad.State
+import qualified Backend.File
+import BackendTypes
+import Utility
+import System.FilePath
+import System.Posix.Files
+import Data.Digest.Pure.SHA -- slow, but we only checksum filenames
+import qualified Data.ByteString.Lazy.Char8 as B
+
+backend = Backend.File.backend {
+	name = "WORM",
+	getKey = keyValue
+}
+
+-- A SHA1 of the basename of the filename, plus the file size and
+-- modification time, is used as the unique part of the key. That
+-- allows multiple files with the same names to have different keys,
+-- while also allowing a file to be moved around while retaining the
+-- same key.
+--
+-- The basename of the filename is also included in the key, so it's clear
+-- what the original filename was when a user sees the value.
+keyValue :: FilePath -> Annex (Maybe Key)
+keyValue file = do
+	stat <- liftIO $ getFileStatus file
+	return $ Just $ Key ((name backend), key stat)
+	where
+ 		key stat = (checksum $ uniqueid stat) ++ sep ++ base
+		checksum s = show $ sha1 $ B.pack s
+		uniqueid stat = (show $ fileSize stat) ++ sep ++
+			(show $ modificationTime stat)
+		base = takeFileName file
+		sep = ":"