[hash] replace scrub by reset

reset make more sense as it leave the mutable context in a state where
it can be re-used by further update, whereas scrub doesn't.

Crypto/Hash/IO.hs

@@ -16,7 +16,7 @@ module Crypto.Hash.IO
     , hashMutableInitWith
     , hashMutableUpdate
     , hashMutableFinalize
-    , hashMutableScrub
+    , hashMutableReset
     ) where
 
 import           Crypto.Hash.Types
@@ -56,6 +56,9 @@ hashMutableFinalize mc = doFinalize undefined (B.withByteArray mc) B.alloc
                         hashInternalFinalize ctx dig
             return $ Digest b
 
--- FIXME not implemented just yet.
-hashMutableScrub :: HashAlgorithm a => MutableContext a -> IO ()
-hashMutableScrub (MutableContext _) = return ()
+-- | Reset the mutable context to the initial state of the hash
+hashMutableReset :: HashAlgorithm a => MutableContext a -> IO ()
+hashMutableReset mc = doReset mc (B.withByteArray mc)
+  where
+    doReset :: HashAlgorithm a => MutableContext a -> ((Ptr (Context a) -> IO ()) -> IO ()) -> IO ()
+    doReset _ withCtx = withCtx hashInternalInit

Crypto/MAC/HMAC.hs

@@ -69,7 +69,7 @@ initialize secret = unsafeDoIO (doHashAlg undefined)
                                 ctx <- hashMutableInitWith alg
                                 hashMutableUpdate ctx secret
                                 digest <- hashMutableFinalize ctx
-                                hashMutableScrub ctx
+                                hashMutableReset ctx
                                 -- pad it if necessary
                                 if digestSize < blockSize
                                     then do