diff --git a/clientsession.cabal b/clientsession.cabal
--- a/clientsession.cabal
+++ b/clientsession.cabal
@@ -1,5 +1,5 @@
 name:            clientsession
-version:         0.9.0.3
+version:         0.9.0.4
 license:         BSD3
 license-file:    LICENSE
 author:          Michael Snoyman <michael@snoyman.com>, Felipe Lessa <felipe.lessa@gmail.com>
diff --git a/src/Web/ClientSession.hs b/src/Web/ClientSession.hs
--- a/src/Web/ClientSession.hs
+++ b/src/Web/ClientSession.hs
@@ -101,9 +101,9 @@
 
 -- | The keys used to store the cookies.  We have an AES key used
 -- to encrypt the cookie and a Skein-MAC-512-256 key used verify
--- the authencity and integrity of the cookie.  The AES key needs
--- to have exactly 32 bytes (256 bits) while Skein-MAC-512-256
--- should have 64 bytes (512 bits).
+-- the authencity and integrity of the cookie.  The AES key must
+-- have exactly 32 bytes (256 bits) while Skein-MAC-512-256 must
+-- have 64 bytes (512 bits).
 --
 -- See also 'getDefaultKey' and 'initKey'.
 data Key = Key { aesKey ::
@@ -131,7 +131,7 @@
 instance Show Key where
     show _ = "<Web.ClientSession.Key>"
 
--- | The initialization vector used by AES.  Should be exactly 16
+-- | The initialization vector used by AES.  Must be exactly 16
 -- bytes long.
 newtype IV = IV S.ByteString
 
@@ -166,7 +166,7 @@
         | otherwise         = Nothing
 
 -- | Randomly construct a fresh initialization vector.  You
--- /should not/ reuse initialization vectors.
+-- /MUST NOT/ reuse initialization vectors.
 randomIV :: IO IV
 randomIV = aesRNG
 
@@ -208,6 +208,18 @@
 -- | Initializes a 'Key' from a random 'S.ByteString'.  Fails if
 -- there isn't exactly 96 bytes (256 bits for AES and 512 bits
 -- for Skein-MAC-512-512).
+--
+-- Note that the input string is assumed be uniformly chosen from
+-- the set of all 96-byte strings.  In order words, each byte
+-- should be chosen from the set of all byte values (0-255) with
+-- the same probability.
+--
+-- In particular, this function does not do any kind of key
+-- stretching.  You should never feed it a password, for example.
+--
+-- It's /highly/ recommended to feed @initKey@ only with values
+-- generated by 'randomKey', unless you really know what you're
+-- doing.
 initKey :: S.ByteString -> Either String Key
 initKey bs | S.length bs /= 96 = Left $ "Web.ClientSession.initKey: length of " ++
                                          show (S.length bs) ++ " /= 96."