use hmac in balanced preferred content

This deals with the possible security problem that someone could make an
unusually low UUID and generate keys that are all constructed to hash to
a number that, mod the number of repositories in the group, == 0.
So balanced preferred content would always put those keys in the
repository with the low UUID as long as the group contains the
number of repositories that the attacker anticipated.
Presumably the attacker than holds the data for ransom? Dunno.

Anyway, the partial solution is to use HMAC (sha256) with all the UUIDs
combined together as the "secret", and the key as the "message". Now any
change in the set of UUIDs in a group will invalidate the attacker's
constructed keys from hashing to anything in particular.

Given that there are plenty of other things someone can do if they can
write to the repository -- including modifying preferred content so only
their repository wants files, and numcopies so other repositories drom
them -- this seems like safeguard enough.

Note that, in balancedPicker, combineduuids is memoized.

Utility/Hash.hs

@@ -6,6 +6,7 @@
  -}
 
 {-# LANGUAGE BangPatterns, PackageImports #-}
+{-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE CPP #-}
 
 module Utility.Hash (
@@ -16,7 +17,6 @@ module Utility.Hash (
 	sha2_224_context,
 	sha2_256,
 	sha2_256_context,
-	sha2_256s,
 	sha2_384,
 	sha2_384_context,
 	sha2_512,
@@ -107,9 +107,6 @@ sha2_256 = hashlazy
 sha2_256_context :: Context SHA256
 sha2_256_context = hashInit
 
-sha2_256s :: S.ByteString -> Digest SHA256
-sha2_256s = hash
-
 sha2_384 :: L.ByteString -> Digest SHA384
 sha2_384 = hashlazy
 
@@ -267,25 +264,26 @@ data Mac = HmacSha1 | HmacSha224 | HmacSha256 | HmacSha384 | HmacSha512
 	deriving (Eq)
 
 calcMac
-	:: Mac          -- ^ MAC
+	:: (forall a. Digest a -> t)    -- ^ applied to MAC'ed message
+	-> Mac          -- ^ MAC
 	-> S.ByteString -- ^ secret key
 	-> S.ByteString -- ^ message
-	-> String       -- ^ MAC'ed message, in hexadecimal
-calcMac mac = case mac of
+	-> t
+calcMac f mac = case mac of
 	HmacSha1   -> use SHA1
 	HmacSha224 -> use SHA224
 	HmacSha256 -> use SHA256
 	HmacSha384 -> use SHA384
 	HmacSha512 -> use SHA512
   where
-	use alg k m = show (hmacGetDigest (hmacWitnessAlg alg k m))
+	use alg k m = f (hmacGetDigest (hmacWitnessAlg alg k m))
 
 	hmacWitnessAlg :: HashAlgorithm a => a -> S.ByteString -> S.ByteString -> HMAC a
 	hmacWitnessAlg _ = hmac
 
 -- Check that all the MACs continue to produce the same.
 props_macs_stable :: [(String, Bool)]
-props_macs_stable = map (\(desc, mac, result) -> (desc ++ " stable", calcMac mac key msg == result))
+props_macs_stable = map (\(desc, mac, result) -> (desc ++ " stable", calcMac show mac key msg == result))
 	[ ("HmacSha1", HmacSha1, "46b4ec586117154dacd49d664e5d63fdc88efb51")
 	, ("HmacSha224", HmacSha224, "4c1f774863acb63b7f6e9daa9b5c543fa0d5eccf61e3ffc3698eacdd")
 	, ("HmacSha256", HmacSha256, "f9320baf0249169e73850cd6156ded0106e2bb6ad8cab01b7bbbebe6d1065317")