broke out Verifiable to a utility library, and added a quickcheck test

Utility/Verifiable.hs

@@ -0,0 +1,37 @@
+{- values verified using a shared secret
+ -
+ - Copyright 2012 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Utility.Verifiable where
+
+import Data.Digest.Pure.SHA
+import Data.ByteString.Lazy.UTF8 (fromString)
+import qualified Data.ByteString.Lazy as L
+
+type Secret = L.ByteString
+type HMACDigest = String
+
+{- A value, verifiable using a HMAC digest and a secret. -}
+data Verifiable a = Verifiable
+	{ val :: a
+	, digest :: HMACDigest
+	}
+	deriving (Eq, Read, Show)
+
+mkVerifiable :: Show a => a -> Secret -> Verifiable a
+mkVerifiable a secret = Verifiable a (calcDigest (show a) secret)
+
+verified :: (Eq a, Show a) => Verifiable a -> Secret -> Bool
+verified v secret = v == mkVerifiable (val v) secret
+
+calcDigest :: String -> Secret -> HMACDigest
+calcDigest v secret = showDigest $ hmacSha1 secret $ fromString v
+
+{- for quickcheck -}
+prop_verifiable_sane :: String -> String -> Bool
+prop_verifiable_sane a s = verified (mkVerifiable a secret) secret
+	where
+		secret = fromString s