diff --git a/.gitignore b/.gitignore
index df05efc..cb6a2b1 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,4 +1,5 @@
 .DS_Store
 _bck*
 json/
-.ghc.*
\ No newline at end of file
+.ghc.*
+*.json
\ No newline at end of file
diff --git a/Challenge.hs b/Challenge.hs
new file mode 100644
index 0000000..05bf586
--- /dev/null
+++ b/Challenge.hs
@@ -0,0 +1,148 @@
+
+-- | Fiat-Shamir challenges
+
+{-# LANGUAGE StrictData, GeneralizedNewtypeDeriving #-}
+module Challenge where
+
+--------------------------------------------------------------------------------
+
+import Data.Array
+
+import Control.Monad
+import qualified Control.Monad.State.Strict as S
+import Control.Monad.IO.Class
+
+import Goldilocks
+import GoldilocksExt
+import Poseidon
+import Digest
+import Types
+
+--------------------------------------------------------------------------------
+
+-- | Plonky2 uses the duplex sponge construction with overwrite mode
+data DuplexState 
+  = Absorbing { duplexOld :: State, duplexInp :: [F] } 
+  | Squeezing { duplexOld :: State, duplexOut :: [F] }
+  deriving (Eq,Show)
+
+duplexInitialState :: State -> DuplexState
+duplexInitialState state = Absorbing state [] 
+
+overwrite :: [F] -> State -> State
+overwrite new old = listToState $ new ++ drop (length new) (elems old)
+
+duplex :: [F] -> State -> State
+duplex inp old = permutation (overwrite inp old)
+
+extract :: State -> [F]
+extract state = reverse $ take rate (elems state) where 
+  rate = 8
+
+freshSqueezing :: State -> DuplexState
+freshSqueezing new = Squeezing new (extract new)
+
+--------------------------------------------------------------------------------
+
+absorbFelt :: Goldilocks -> DuplexState -> DuplexState
+absorbFelt x mode = 
+  case mode of
+    Squeezing old _   -> absorbFelt x (Absorbing old [])
+    Absorbing old inp -> if length inp < rate
+      then                Absorbing             old (inp ++ [x]) 
+      else absorbFelt x $ Absorbing (duplex inp old) [] 
+  where
+    rate = 8
+
+squeezeFelt :: DuplexState -> (Goldilocks, DuplexState)
+squeezeFelt mode =
+  case mode of
+    Squeezing old out -> case out of
+      []     -> let new = permutation old
+                in  squeezeFelt $ freshSqueezing new 
+      (y:ys) -> (y, Squeezing old ys)
+    Absorbing old inp -> case inp of
+      []     -> squeezeFelt $ freshSqueezing (permutation old) 
+      (x:xs) -> squeezeFelt $ freshSqueezing (duplex  inp old) 
+
+--------------------------------------------------------------------------------
+
+class Absorb a where
+  absorb :: a -> DuplexState -> DuplexState
+
+instance Absorb F where
+  absorb = absorbFelt
+
+instance Absorb FExt where absorb (MkExt a b) = absorb [a,b]
+
+instance Absorb a => Absorb [a] where
+  absorb []     = id
+  absorb (x:xs) = absorb xs . absorb x
+
+instance Absorb Digest where absorb h = absorb (digestToList h)
+
+instance Absorb MerkleCap where
+  absorb (MkMerkleCap digests) = absorb digests
+
+--------------------------------------------------------------------------------
+
+class Squeeze a where
+  squeeze :: DuplexState -> (a, DuplexState)
+
+squeezeN :: Squeeze a => Int -> DuplexState -> ([a], DuplexState)
+squeezeN 0 state0 = ([],state0)
+squeezeN n state0 = let (x   , state1) = squeeze        state0 
+                        (xs  , state2) = squeezeN (n-1) state1
+                    in  (x:xs, state2)
+
+instance Squeeze Goldilocks where squeeze = squeezeFelt
+
+instance Squeeze GoldilocksExt where 
+  squeeze state0 = 
+    let (x, state1) = squeeze state0
+        (y, state2) = squeeze state1
+    in  (MkExt x y, state2)
+
+--------------------------------------------------------------------------------
+-- * Monadic interface
+
+newtype DuplexT m a 
+  = DuplexT (S.StateT DuplexState m a)
+  deriving (Functor,Applicative,Monad)
+
+type DuplexIO a = DuplexT IO a
+
+instance MonadIO (DuplexT IO) where 
+  liftIO action = DuplexT (liftIO action)
+
+runDuplexT :: Monad m => DuplexT m a -> State -> m a
+runDuplexT (DuplexT action) ini = S.evalStateT action (duplexInitialState ini)
+
+doPrint :: Show a => a -> DuplexIO ()
+doPrint x = DuplexT (liftIO $ print x)
+
+doDebug :: DuplexIO ()
+doDebug = DuplexT $ do
+  state <- S.get
+  liftIO (print state)
+  return ()
+
+doAbsorb :: Absorb a => a -> DuplexIO ()
+doAbsorb x = DuplexT $ S.modify (absorb x)
+
+doSqueeze :: Squeeze a => DuplexIO a
+doSqueeze = DuplexT $ S.state squeeze
+
+doSqueezeN :: Squeeze a => Int -> DuplexIO [a]
+doSqueezeN n = DuplexT $ S.state (squeezeN n)
+
+--------------------------------------------------------------------------------
+
+duplexTest :: Int -> IO ()
+duplexTest m = flip runDuplexT zeroState $ do
+  forM_ [0..19] $ \k -> do
+    doAbsorb (map intToF [1..k])
+    ys <- doSqueezeN k :: DuplexIO [F]
+    doPrint ys
+
+--------------------------------------------------------------------------------
diff --git a/Goldilocks.hs b/Goldilocks.hs
index 1705e92..1649808 100644
--- a/Goldilocks.hs
+++ b/Goldilocks.hs
@@ -31,9 +31,17 @@ fromF (Goldilocks x) = fromInteger x
 toF :: Word64 -> F
 toF = mkGoldilocks . fromIntegral
 
+intToF :: Int -> F
+intToF = mkGoldilocks . fromIntegral
+
+integerToF :: Integer -> F
+integerToF = mkGoldilocks 
+
 rndF :: IO F
 rndF = Goldilocks <$> randomRIO ( 0 , 0xffff_ffff_0000_0000 )
 
+--------------------------------------------------------------------------------
+
 -- | The generator of the multiplicative subgroup of F used by Plonky2
 multGen :: F
 multGen = 0xc65c18b67785d900