Merge pull request #62 from clinty/dsa-truncate

Do DSS truncation on verify

Crypto/PubKey/DSA.hs

@@ -29,14 +29,17 @@ module Crypto.PubKey.DSA
     ) where
 
 import           Crypto.Random.Types
+import           Data.Bits (testBit)
 import           Data.Data
 import           Data.Maybe
+import           Crypto.Number.Basic (numBits)
 import           Crypto.Number.ModArithmetic (expFast, expSafe, inverse)
 import           Crypto.Number.Serialize
 import           Crypto.Number.Generate
-import           Crypto.Internal.ByteArray (ByteArrayAccess)
+import           Crypto.Internal.ByteArray (ByteArrayAccess(length), convert, index, dropView, takeView)
 import           Crypto.Internal.Imports
 import           Crypto.Hash
+import           Prelude hiding (length)
 
 -- | DSA Public Number, usually embedded in DSA Public Key
 type PublicNumber = Integer
@@ -145,9 +148,11 @@ verify hashAlg pk (Signature r s) m
     | otherwise                            = v == r
     where (Params p g q) = public_params pk
           y       = public_y pk
-          hm      = os2ip $ hashWith hashAlg m
+          hm      = os2ip . truncateHash $ hashWith hashAlg m
 
           w       = fromJust $ inverse s q
           u1      = (hm*w) `mod` q
           u2      = (r*w) `mod` q
           v       = ((expFast g u1 p) * (expFast y u2 p)) `mod` p `mod` q
+          -- if the hash is larger than the size of q, truncate it; FIXME: deal with the case of a q not evenly divisible by 8
+          truncateHash h = if numBits (os2ip h) > numBits q then takeView h (numBits q `div` 8) else dropView h 0