diff --git a/Crypto/ECC.hs b/Crypto/ECC.hs
index 52890df..990105d 100644
--- a/Crypto/ECC.hs
+++ b/Crypto/ECC.hs
@@ -9,6 +9,7 @@
 --
 {-# LANGUAGE GeneralizedNewtypeDeriving #-}
 {-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE ScopedTypeVariables #-}
 module Crypto.ECC
     ( Curve_P256R1(..)
     , Curve_P384R1(..)
@@ -26,13 +27,12 @@ import qualified Crypto.PubKey.ECC.Types as H
 import qualified Crypto.PubKey.ECC.Prim as H
 import           Crypto.Random
 import           Crypto.Internal.Imports
-import           Crypto.Internal.ByteArray (ByteArrayAccess, ScrubbedBytes)
+import           Crypto.Internal.ByteArray (ByteArray, ByteArrayAccess, ScrubbedBytes)
+import qualified Crypto.Internal.ByteArray as B
 import           Crypto.Number.Serialize (i2ospOf_, os2ip)
 import qualified Crypto.PubKey.Curve25519 as X25519
 import           Data.Function (on)
 import           Data.ByteArray (convert)
-import           Data.ByteString (ByteString)
-import qualified Data.ByteString as B
 
 -- | An elliptic curve key pair composed of the private part (a scalar), and
 -- the associated point.
@@ -70,8 +70,8 @@ class EllipticCurve curve where
     -- | Generate a new random keypair
     curveGenerateKeyPair :: MonadRandom randomly => randomly (KeyPair curve)
 
-    encodePoint :: Point curve -> ByteString
-    decodePoint :: ByteString -> Point curve
+    encodePoint :: ByteArray bs => Point curve -> bs
+    decodePoint :: ByteArray bs => bs -> Point curve
 
 instance {-# OVERLAPPABLE #-} Show (Point a) where
     show _ = undefined
@@ -219,18 +219,22 @@ instance EllipticCurveDH Curve_X25519 where
       where
         secret = X25519.dh p s
 
-encodeECPoint :: Integer -> Integer -> Int -> ByteString
+encodeECPoint :: forall bs. ByteArray bs => Integer -> Integer -> Int -> bs
 encodeECPoint x y siz = B.concat [uncompressed,xb,yb]
   where
+    uncompressed, xb, yb :: bs
     uncompressed = B.singleton 4
     xb = i2ospOf_ siz x
     yb = i2ospOf_ siz y
 
-decodeECPoint :: ByteString -> (Integer,Integer)
-decodeECPoint mxy = (x,y)
-  where
-    xy = B.drop 1 mxy -- dropping 4 (uncompressed)
-    siz = B.length xy `div` 2
-    (xb,yb) = B.splitAt siz xy
-    x = os2ip xb
-    y = os2ip yb
+decodeECPoint :: ByteArray bs => bs -> (Integer,Integer)
+decodeECPoint mxy = case B.uncons mxy of
+  Nothing     -> error "decodeECPoint"
+  Just (m,xy)
+    -- uncompressed
+    | m == 4  -> let siz = B.length xy `div` 2
+                     (xb,yb) = B.splitAt siz xy
+                     x = os2ip xb
+                     y = os2ip yb
+                 in (x,y)
+   | otherwise -> error $ "decodeECPoint: unknown " ++ show m