diff --git a/automotive-cse.cabal b/automotive-cse.cabal
--- a/automotive-cse.cabal
+++ b/automotive-cse.cabal
@@ -1,6 +1,6 @@
 
 name:                automotive-cse
-version:             0.1.7.0
+version:             0.1.8.0
 synopsis:            Automotive CSE emulation
 description:         This package includes Cryptography Security Engine (CSE)
                      codec emulation for automotive things.
diff --git a/src/Codec/Automotive/CSE/Internal.hs b/src/Codec/Automotive/CSE/Internal.hs
--- a/src/Codec/Automotive/CSE/Internal.hs
+++ b/src/Codec/Automotive/CSE/Internal.hs
@@ -4,7 +4,7 @@
   M1, unM1, makeM1, extractM1, refineM1,
   M2, unM2, makeM2, extractM2, refineM2,
   M3, unM3, makeM3,
-  M4, unM4, makeM4, extractM4, refineM4, makeM4', extractM4',
+  M4, unM4, makeM4, extractM4, refineM4, indexM4, makeM4', extractM4',
   M5, unM5, makeM5,
 
   K1, K1', makeK1,
@@ -321,6 +321,13 @@
       ps@((uid, kid, akid), counter) = extractM4 k3 m4
   guard $ makeM4 k3 uid kid akid counter == m4
   return (m4, ps)
+
+indexM4 :: MonadPlus m
+        => ByteString
+        -> m (UID, Word8, Word8)
+indexM4 s = do
+  guard $ BS.length s == 32
+  snd `liftM` refineM1 (BS.take 16 s)
 
 newtype M5 = M5 ByteString deriving Eq