diff --git a/hjugement-protocol.cabal b/hjugement-protocol.cabal
--- a/hjugement-protocol.cabal
+++ b/hjugement-protocol.cabal
@@ -2,7 +2,7 @@
 -- PVP:  +-+------- breaking API changes
 --       | | +----- non-breaking API additions
 --       | | | +--- code changes with no API change
-version: 0.0.0.20190519
+version: 0.0.1.20190623
 category: Politic
 synopsis: A cryptographic protocol for the Majority Judgment.
 description:
diff --git a/src/Voting/Protocol/Credential.hs b/src/Voting/Protocol/Credential.hs
--- a/src/Voting/Protocol/Credential.hs
+++ b/src/Voting/Protocol/Credential.hs
@@ -144,20 +144,17 @@
 -- using 'Crypto.fastPBKDF2_SHA256'.
 credentialSecretKey :: Reifies c FFC => UUID -> Credential -> (SecretKey c)
 credentialSecretKey (UUID uuid) (Credential cred) =
-	fromNatural $ BS.foldl'
-	 (\acc b -> acc`shiftL`3 + fromIntegral b)
-	 (0::Natural)
-	 (ByteArray.convert deriv)
-	where
-	deriv :: BS.ByteString
-	deriv =
-		Crypto.fastPBKDF2_SHA256
-		 Crypto.Parameters
-		 { Crypto.iterCounts   = 1000
-		 , Crypto.outputLength = 256 `div` 8
-		 }
-		 (Text.encodeUtf8 cred)
-		 (Text.encodeUtf8 uuid)
+	fromNatural $
+	BS.foldl' -- NOTE: interpret the SHA256 as a big-endian number.
+	 (\acc b -> acc`shiftL`8 + fromIntegral b)
+	 (0::Natural) $
+	Crypto.fastPBKDF2_SHA256
+	 Crypto.Parameters
+	 { Crypto.iterCounts   = 1000
+	 , Crypto.outputLength = 32 -- bytes, ie. 256 bits
+	 }
+	 (Text.encodeUtf8 cred)
+	 (Text.encodeUtf8 uuid)
 
 -- ** Type 'PublicKey'
 type PublicKey = G
diff --git a/src/Voting/Protocol/FFC.hs b/src/Voting/Protocol/FFC.hs
--- a/src/Voting/Protocol/FFC.hs
+++ b/src/Voting/Protocol/FFC.hs
@@ -24,6 +24,7 @@
 import Data.Aeson (ToJSON(..),FromJSON(..),(.:),(.:?),(.=))
 import Data.Bits
 import Data.Bool
+import Data.Either (Either(..))
 import Data.Eq (Eq(..))
 import Data.Foldable (Foldable, foldl')
 import Data.Function (($), (.), id)
@@ -40,7 +41,7 @@
 import GHC.Natural (minusNaturalMaybe)
 import Numeric.Natural (Natural)
 import Prelude (Integer, Integral(..), fromIntegral, Enum(..))
-import Text.Read (readMaybe)
+import Text.Read (readMaybe, readEither)
 import Text.Show (Show(..))
 import qualified Control.Monad.Trans.State.Strict as S
 import qualified Crypto.Hash as Crypto
@@ -86,15 +87,38 @@
      -- WARNING: 'ffc_groupOrder' MUST be a prime number dividing @('ffc_fieldCharac'-1)@
      -- to ensure that ElGamal is secure in terms of the DDH assumption.
  } deriving (Eq,Show,Generic,NFData)
-deriving instance ToJSON FFC
+instance ToJSON FFC where
+	toJSON FFC{..} =
+		JSON.object
+		 [ "name" .= ffc_name
+		 , "p"    .= show ffc_fieldCharac
+		 , "g"    .= show ffc_groupGen
+		 , "q"    .= show ffc_groupOrder
+		 ]
+	toEncoding FFC{..} =
+		JSON.pairs
+		 (  "name" .= ffc_name
+		 <> "p"    .= show ffc_fieldCharac
+		 <> "g"    .= show ffc_groupGen
+		 <> "q"    .= show ffc_groupOrder
+		 )
 instance FromJSON FFC where
 	parseJSON = JSON.withObject "FFC" $ \o -> do
 		ffc_name <- fromMaybe "" <$> (o .:? "name")
-		ffc_fieldCharac <- o .: "p"
-		ffc_groupGen    <- o .: "g"
-		ffc_groupOrder  <- o .: "q"
+		p <- o .: "p"
+		g <- o .: "g"
+		q <- o .: "q"
 		-- TODO: check p is probable prime
 		-- TODO: check q is probable prime
+		ffc_fieldCharac <- case readEither (Text.unpack p) of
+		 Left err -> JSON.typeMismatch ("FFC: fieldCharac: "<>err) (JSON.String p)
+		 Right a -> return a
+		ffc_groupGen <- case readEither (Text.unpack g) of
+		 Left err -> JSON.typeMismatch ("FFC: groupGen: "<>err) (JSON.String g)
+		 Right a -> return a
+		ffc_groupOrder <- case readEither (Text.unpack q) of
+		 Left err -> JSON.typeMismatch ("FFC: groupOrder: "<>err) (JSON.String q)
+		 Right a -> return a
 		unless (nat ffc_groupGen < ffc_fieldCharac) $
 			JSON.typeMismatch "FFC: groupGen is not lower than fieldCharac" (JSON.Object o)
 		unless (ffc_groupOrder < ffc_fieldCharac) $
@@ -306,8 +330,12 @@
 hash :: Reifies c FFC => BS.ByteString -> [G c] -> E c
 hash bs gs = do
 	let s = bs <> BS.intercalate (fromString ",") (bytesNat <$> gs)
-	let h = ByteArray.convert (Crypto.hashWith Crypto.SHA256 s)
-	fromNatural (BS.foldl' (\acc b -> acc`shiftL`3 + fromIntegral b) (0::Natural) h)
+	let h = Crypto.hashWith Crypto.SHA256 s
+	fromNatural $
+		BS.foldl' -- NOTE: interpret the SHA256 as a big-endian number.
+		 (\acc b -> acc`shiftL`8 + fromIntegral b)
+		 (0::Natural)
+		 (ByteArray.convert h)
 
 -- * Type 'E'
 -- | An exponent of a (necessarily cyclic) subgroup of a Finite Prime Field.
diff --git a/tests/HUnit/Credential.hs b/tests/HUnit/Credential.hs
--- a/tests/HUnit/Credential.hs
+++ b/tests/HUnit/Credential.hs
@@ -1,7 +1,6 @@
 {-# LANGUAGE OverloadedStrings #-}
 module HUnit.Credential where
 
-import Control.Applicative (Applicative(..))
 import Test.Tasty.HUnit
 import qualified Control.Monad.Trans.State.Strict as S
 import qualified System.Random as Random
@@ -36,18 +35,15 @@
 	 , "xLcs7ev6Jy6FHHE"  ==> Right (Credential "xLcs7ev6Jy6FHHE")
 	 ]
  , testGroup "credentialSecretKey" $
-	 [ testSecretKey weakFFC 0 122
-	 , testSecretKey weakFFC 1 35
-	 , testSecretKey beleniosFFC 0 2317630607062989137269685509390
-	 , testSecretKey beleniosFFC 1 1968146140481358915910346867611
+	 [ testSecretKey beleniosFFC
+		 (UUID "xLcs7ev6Jy6FHH")
+		 (Credential "xLcs7ev6Jy6FHHE")
+		 24202898752499029126606335829564687069186982035759723128887013101942425902424
 	 ]
  ]
 
-testSecretKey :: FFC -> Int -> Natural -> TestTree
-testSecretKey ffc seed exp =
+testSecretKey :: FFC -> UUID -> Credential -> Natural -> TestTree
+testSecretKey ffc uuid cred exp =
 	reify ffc $ \(Proxy::Proxy c) ->
-	let (uuid@(UUID u), cred@(Credential c)) =
-		(`S.evalState` Random.mkStdGen seed) $
-			(,) <$> randomUUID <*> randomCredential in
-	testCase (show (u,c)) $
+	testCase (show (uuid,cred)) $
 		credentialSecretKey @c uuid cred @?= E exp
diff --git a/tests/HUnit/FFC.hs b/tests/HUnit/FFC.hs
--- a/tests/HUnit/FFC.hs
+++ b/tests/HUnit/FFC.hs
@@ -28,19 +28,19 @@
 		reify weakFFC $ \(Proxy::Proxy c) ->
 		 [ testCase "[groupGen]" $
 			hash "start" [groupGen @c] @?=
-				fromNatural 100
+				fromNatural 62
 		 , testCase "[groupGen, groupGen]" $
 			hash "start" [groupGen @c, groupGen] @?=
-				fromNatural 16
+				fromNatural 31
 		 ]
 	 , testGroup "BeleniosParams" $
 		reify beleniosFFC $ \(Proxy::Proxy c) ->
 		 [ testCase "[groupGen]" $
 			hash "start" [groupGen @c] @?=
-				fromNatural 1832875488615060263192702367259
+				fromNatural 75778590284190557660612328423573274641033882642784670156837892421285248292707
 		 , testCase "[groupGen, groupGen]" $
 			hash "start" [groupGen @c, groupGen] @?=
-				fromNatural 2495277906542783643199702546512
+				fromNatural 28798937720387703653439047952832768487958170248947132321730024269734141660223
 		 ]
 	 ]
  ]