diff --git a/Crypto/PubKey/ECC/ECDSA.hs b/Crypto/PubKey/ECC/ECDSA.hs
index 4cfc4b8..175f75c 100644
--- a/Crypto/PubKey/ECC/ECDSA.hs
+++ b/Crypto/PubKey/ECC/ECDSA.hs
@@ -11,8 +11,11 @@ module Crypto.PubKey.ECC.ECDSA
     , toPublicKey
     , toPrivateKey
     , signWith
+    , signDigestWith
     , sign
+    , signDigest
     , verify
+    , verifyDigest
     ) where
 
 import Control.Monad
@@ -24,7 +27,7 @@ import Crypto.Number.ModArithmetic (inverse)
 import Crypto.Number.Generate
 import Crypto.PubKey.ECC.Types
 import Crypto.PubKey.ECC.Prim
-import Crypto.PubKey.Internal (dsaTruncHash)
+import Crypto.PubKey.Internal (dsaTruncHashDigest)
 import Crypto.Random.Types
 
 -- | Represent a ECDSA signature namely R and S.
@@ -57,17 +60,16 @@ toPublicKey (KeyPair curve pub _) = PublicKey curve pub
 toPrivateKey :: KeyPair -> PrivateKey
 toPrivateKey (KeyPair curve _ priv) = PrivateKey curve priv
 
--- | Sign message using the private key and an explicit k number.
+-- | Sign digest using the private key and an explicit k number.
 --
 -- /WARNING:/ Vulnerable to timing attacks.
-signWith :: (ByteArrayAccess msg, HashAlgorithm hash)
-         => Integer    -- ^ k random number
-         -> PrivateKey -- ^ private key
-         -> hash       -- ^ hash function
-         -> msg        -- ^ message to sign
-         -> Maybe Signature
-signWith k (PrivateKey curve d) hashAlg msg = do
-    let z = dsaTruncHash hashAlg msg n
+signDigestWith :: HashAlgorithm hash
+               => Integer     -- ^ k random number
+               -> PrivateKey  -- ^ private key
+               -> Digest hash -- ^ digest to sign
+               -> Maybe Signature
+signDigestWith k (PrivateKey curve d) digest = do
+    let z = dsaTruncHashDigest digest n
         CurveCommon _ _ g n _ = common_curve curve
     let point = pointMul curve k g
     r <- case point of
@@ -78,26 +80,44 @@ signWith k (PrivateKey curve d) hashAlg msg = do
     when (r == 0 || s == 0) Nothing
     return $ Signature r s
 
+-- | Sign message using the private key and an explicit k number.
+--
+-- /WARNING:/ Vulnerable to timing attacks.
+signWith :: (ByteArrayAccess msg, HashAlgorithm hash)
+         => Integer    -- ^ k random number
+         -> PrivateKey -- ^ private key
+         -> hash       -- ^ hash function
+         -> msg        -- ^ message to sign
+         -> Maybe Signature
+signWith k pk hashAlg msg = signDigestWith k pk (hashWith hashAlg msg)
+
+-- | Sign digest using the private key.
+--
+-- /WARNING:/ Vulnerable to timing attacks.
+signDigest :: (HashAlgorithm hash, MonadRandom m)
+           => PrivateKey -> Digest hash -> m Signature
+signDigest pk digest = do
+    k <- generateBetween 1 (n - 1)
+    case signDigestWith k pk digest of
+         Nothing  -> signDigest pk digest
+         Just sig -> return sig
+  where n = ecc_n . common_curve $ private_curve pk
+
 -- | Sign message using the private key.
 --
 -- /WARNING:/ Vulnerable to timing attacks.
 sign :: (ByteArrayAccess msg, HashAlgorithm hash, MonadRandom m)
      => PrivateKey -> hash -> msg -> m Signature
-sign pk hashAlg msg = do
-    k <- generateBetween 1 (n - 1)
-    case signWith k pk hashAlg msg of
-         Nothing  -> sign pk hashAlg msg
-         Just sig -> return sig
-  where n = ecc_n . common_curve $ private_curve pk
+sign pk hashAlg msg = signDigest pk (hashWith hashAlg msg)
 
--- | Verify a bytestring using the public key.
-verify :: (ByteArrayAccess msg, HashAlgorithm hash) => hash -> PublicKey -> Signature -> msg -> Bool
-verify _       (PublicKey _ PointO) _ _ = False
-verify hashAlg pk@(PublicKey curve q) (Signature r s) msg
+-- | Verify a digest using the public key.
+verifyDigest :: HashAlgorithm hash => PublicKey -> Signature -> Digest hash -> Bool
+verifyDigest (PublicKey _ PointO) _ _ = False
+verifyDigest pk@(PublicKey curve q) (Signature r s) digest
     | r < 1 || r >= n || s < 1 || s >= n = False
     | otherwise = maybe False (r ==) $ do
         w <- inverse s n
-        let z  = dsaTruncHash hashAlg msg n
+        let z  = dsaTruncHashDigest digest n
             u1 = z * w `mod` n
             u2 = r * w `mod` n
             x  = pointAddTwoMuls curve u1 g u2 q
@@ -107,3 +127,7 @@ verify hashAlg pk@(PublicKey curve q) (Signature r s) msg
   where n = ecc_n cc
         g = ecc_g cc
         cc = common_curve $ public_curve pk
+
+-- | Verify a bytestring using the public key.
+verify :: (ByteArrayAccess msg, HashAlgorithm hash) => hash -> PublicKey -> Signature -> msg -> Bool
+verify hashAlg pk sig msg = verifyDigest pk sig (hashWith hashAlg msg)
diff --git a/Crypto/PubKey/Internal.hs b/Crypto/PubKey/Internal.hs
index b1631cc..3951d06 100644
--- a/Crypto/PubKey/Internal.hs
+++ b/Crypto/PubKey/Internal.hs
@@ -9,6 +9,7 @@ module Crypto.PubKey.Internal
     ( and'
     , (&&!)
     , dsaTruncHash
+    , dsaTruncHashDigest
     ) where
 
 import Data.Bits (shiftR)
@@ -32,8 +33,15 @@ False &&! False = False
 
 -- | Truncate and hash for DSA and ECDSA.
 dsaTruncHash :: (ByteArrayAccess msg, HashAlgorithm hash) => hash -> msg -> Integer -> Integer
-dsaTruncHash hashAlg m n
+dsaTruncHash hashAlg = dsaTruncHashDigest . hashWith hashAlg
+
+-- | Truncate a digest for DSA and ECDSA.
+dsaTruncHashDigest :: HashAlgorithm hash => Digest hash -> Integer -> Integer
+dsaTruncHashDigest digest n
     | d > 0 = shiftR e d
     | otherwise = e
-  where e = os2ip $ hashWith hashAlg m
-        d = hashDigestSize hashAlg * 8 - numBits n
+  where e = os2ip digest
+        d = hashDigestSize (getHashAlg digest) * 8 - numBits n
+
+getHashAlg :: Digest hash -> hash
+getHashAlg _ = undefined