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

Assistant/Pairing.hs

@@ -8,13 +8,26 @@
 module Assistant.Pairing where
 
 import Assistant.Common
+import Utility.Verifiable
 
 import Network.Socket (HostName)
 
-type SshPubKey = String
-type HMACDigest = String
-type UserName = String
-type Secret = String
+{- Messages sent in pairing are all verifiable using a secret that
+ - should be shared between the systems being paired. -}
+type PairMsg = Verifiable (PairStage, HostInfo, SshPubKey)
+
+mkPairMsg :: Secret -> PairStage -> HostInfo -> SshPubKey -> PairMsg
+mkPairMsg secret pairstage hostinfo sshkey = mkVerifiable
+	(pairstage, hostinfo, sshkey) secret
+
+data PairStage
+	{- "I'd like to pair with somebody who knows a secret.
+	 - Here's my ssh key, and hostinfo." -}
+	= PairRequest
+	{- "I've checked your PairRequest, and like it; I set up
+	 - your ssh key already. Here's mine." -}
+	| PairAck
+	deriving (Eq, Read, Show)
 
 data HostInfo = HostInfo
 	{ hostName :: HostName
@@ -22,35 +35,5 @@ data HostInfo = HostInfo
 	}
 	deriving (Eq, Read, Show)
 
-data PairStage
-	{- "I'd like to pair with somebody who knows a secret.
-	 - Here's my ssh key, and hostinfo, both verifiable with
-	 - our shared secret." -}
-	= PairRequest
-	{- "I've checked your PairRequest, and like it; I set up
-	 - your ssh key already. Here's mine, also verified, please set it
-	 - up too, and start syncing!" -}
-	| PairAck
-	deriving (Eq, Read, Show)
-
-type PairMsg = Verifiable (PairStage, HostInfo, SshPubKey)
-
-mkPairMsg :: Secret -> PairStage -> HostInfo -> SshPubKey -> PairMsg
-mkPairMsg secret pairstage hostinfo sshkey = mkVerifiable
-	(pairstage, hostinfo, sshkey) secret
-
-{- A value, verifiable using a HMAC digest to encrypt using a shared 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 = undefined -- TODO
+type SshPubKey = String
+type UserName = String