diff --git a/Network/Wai/Session.hs b/Network/Wai/Session.hs
--- a/Network/Wai/Session.hs
+++ b/Network/Wai/Session.hs
@@ -1,9 +1,7 @@
 {-# LANGUAGE CPP #-}
 module Network.Wai.Session (Session, SessionStore, withSession, genSessionId) where
 
-import Data.Unique (newUnique, hashUnique)
-import Data.Ratio (numerator, denominator)
-import Data.Time.Clock.POSIX (getPOSIXTime)
+import Data.Monoid (mconcat)
 import Data.String (fromString)
 import Control.Monad.IO.Class (liftIO)
 import Network.HTTP.Types (ResponseHeaders)
@@ -22,9 +20,13 @@
 import Data.Vault (Key)
 import qualified Data.Vault as Vault
 #endif
-import Data.ByteString (ByteString)
+import Data.ByteString (ByteString, foldr')
+import Data.ByteString.Lazy (toStrict)
+import Data.ByteString.Builder (word8Hex, toLazyByteString)
 import qualified Blaze.ByteString.Builder as Builder
 
+import System.Entropy (getEntropy)
+
 -- | Type representing a single session (a lookup, insert pair)
 type Session m k v = ((k -> m (Maybe v)), (k -> v -> m ()))
 
@@ -67,14 +69,17 @@
 	setCookie = fromString "Set-Cookie"
 	ciCookie = fromString "Cookie"
 
--- | Simple session ID generator based on time and 'Data.Unique'
+-- | Simple session ID generator using cryptographically strong random IDs
 --
 -- Useful for session stores that use session IDs.
 genSessionId :: IO ByteString
 genSessionId = do
-	u <- fmap (toInteger . hashUnique) newUnique
-	time <- fmap toRational getPOSIXTime
-	return $ fromString $ show (numerator time * denominator time * u)
+	randBytes <- getEntropy 32
+	return $ prettyPrint randBytes
+	where
+	prettyPrint :: ByteString -> ByteString
+	prettyPrint = toStrict . toLazyByteString . mconcat . Data.ByteString.foldr'
+		( \ byte acc -> word8Hex byte:acc ) []
 
 -- | Run a function over the headers in a 'Response'
 mapHeader :: (ResponseHeaders -> ResponseHeaders) -> Response -> Response
@@ -82,7 +87,7 @@
 mapHeader f (ResponseBuilder s h b) = ResponseBuilder s (f h) b
 #if MIN_VERSION_wai(3,0,0)
 mapHeader f (ResponseStream s h b) = ResponseStream s (f h) b
-mapHeader _ (ResponseRaw _ _) = error "Cannot mapHeader of Wai.Interal.ResponseRaw when trying to add session cookie header"
+mapHeader f (ResponseRaw io resp) = ResponseRaw io (mapHeader f resp)
 #else
 mapHeader f (ResponseSource s h b) = ResponseSource s (f h) b
 #endif
diff --git a/Network/Wai/Session/Map.hs b/Network/Wai/Session/Map.hs
--- a/Network/Wai/Session/Map.hs
+++ b/Network/Wai/Session/Map.hs
@@ -30,7 +30,7 @@
 		case Map.lookup k m of
 			Just sv -> return (sessionFromMapStateVar sv, return k)
 			-- Could not find key, so it's as if we were not sent one
-			Nothing -> mapStore' (return k) ssv Nothing
+			Nothing -> mapStore' gen ssv Nothing
 	mapStore' genNewKey ssv Nothing = do
 		newKey <- genNewKey
 		sv <- newThreadSafeStateVar Map.empty
diff --git a/README b/README
--- a/README
+++ b/README
@@ -1,10 +1,10 @@
-Provides a generic, cookie-based middleware for sessions that is
-parameterised over the session store, the cookie name, and the
-cookie parameters (such as path, expiry, etc).  Passes a pair of
-functions (lookup key, set key) for the current session through the
-'Vault' in the 'Request'.
-
-Also provides a simple example session store based on threadsafe
-'IORef's and 'Data.Map'.
-
-See example/Main.hs in git for example usage.
+        Provides a generic, cookie-based middleware for sessions that is
+        parameterised over the session store, the cookie name, and the
+        cookie parameters (such as path, expiry, etc).  Passes a pair of
+        functions (lookup key, set key) for the current session through the
+        'Vault' in the 'Request'.
+        .
+        Also provides a simple example session store based on threadsafe
+        'IORef's and 'Data.Map'.
+        .
+        See example/Main.hs in git for example usage.
diff --git a/wai-session.cabal b/wai-session.cabal
--- a/wai-session.cabal
+++ b/wai-session.cabal
@@ -1,5 +1,5 @@
 name:            wai-session
-version:         0.3.2
+version:         0.3.3
 cabal-version:   >= 1.8
 license:         OtherLicense
 license-file:    COPYING
@@ -37,6 +37,8 @@
                 base == 4.*,
                 containers,
                 bytestring,
+                bytestring-builder,
+                entropy,
                 transformers,
                 time,
                 StateVar,