diff --git a/Data/CipherSaber2.hs b/Data/CipherSaber2.hs
--- a/Data/CipherSaber2.hs
+++ b/Data/CipherSaber2.hs
@@ -8,7 +8,7 @@
 -- software for license terms.
 module Data.CipherSaber2 (
   ByteString, rc4, encrypt, decrypt,
-  toByteString, fromByteString )
+  toByteString, fromByteString, ivLength )
   where
 
 import Control.Monad
diff --git a/README.md b/README.md
--- a/README.md
+++ b/README.md
@@ -66,7 +66,7 @@
 >     *keystream* ← zero-based array of *n* bytes  
 >     *j* ← 0  
 >     **for** *i* **in** 0..n-1  
->         i' ← *i* **mod** 256  
+>         i' ← *(i + 1)* **mod** 256  
 >         *j* ← (*j* + *S*[i']) **mod** 256  
 > &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;*S*[i']&nbsp;<->&nbsp;*S*[*j*]  
 > &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;*keystream*[*i*]&nbsp;&#8592;&nbsp;*S*[(*S*[i']&nbsp;+&nbsp;*S*[*j*])&nbsp;**mod**&nbsp;256]  
diff --git a/ciphersaber2.cabal b/ciphersaber2.cabal
--- a/ciphersaber2.cabal
+++ b/ciphersaber2.cabal
@@ -14,7 +14,7 @@
 -- PVP summary:      +-+------- breaking API changes
 --                   | | +----- non-breaking API additions
 --                   | | | +--- code changes with no API change
-version:             0.1.0.0
+version:             0.1.1.0
 
 -- A short (one-line) description of the package.
 synopsis:            Implementation of CipherSaber2 RC4 cryptography.
@@ -61,7 +61,7 @@
 source-repository this
   type:     git
   location: http://github.com/BartMassey/ciphersaber2
-  tag:      v0.1.0.0
+  tag:      v0.1.1.0
 
 library
   -- Modules exported by the library.
diff --git a/cs2.hs b/cs2.hs
--- a/cs2.hs
+++ b/cs2.hs
@@ -6,12 +6,15 @@
 -- CipherSaber driver for UNIX-like systems with "/dev/random".
 
 import Control.Monad
-import qualified Data.ByteString.Char8 as BS
+import qualified Data.ByteString as BS
+import qualified Data.ByteString.Char8 as BSC
 import Data.CipherSaber2
+import Data.Char
+import Data.Word
 import System.Console.ParseArgs
 import System.IO
 
-data ArgInd = ArgEncrypt | ArgDecrypt | ArgKey | ArgReps
+data ArgInd = ArgEncrypt | ArgDecrypt | ArgKey | ArgReps | ArgIV
      deriving (Ord, Eq, Show)
 
 argd :: [ Arg ArgInd ]
@@ -39,6 +42,14 @@
                "(use 1 for CipherSaber-1, default 20)."
   },
   Arg {
+     argIndex = ArgIV,
+     argName = Just "iv",
+     argAbbr = Just 'i',
+     argData = argDataOptional "hex-string" ArgtypeString,
+     argDesc = "IV as " ++ show (ivLength * 2) ++ " hex digits with" ++
+               " leading 0x, or " ++ show ivLength ++ " chars."
+  },
+  Arg {
      argIndex = ArgKey,
      argName = Nothing,
      argAbbr = Nothing,
@@ -66,7 +77,26 @@
     usageError argv "Exactly one of -e or -d is required."
   case e of
     True -> do
-      iv <- makeIV
+      iv <- case getArg argv ArgIV of
+              Nothing -> makeIV
+              Just ivString -> return (makeBS argv ivString)
       BS.interact (encrypt r k iv)
     False -> do
       BS.interact (decrypt r k)
+  where
+    makeBS argv desc
+        | take 2 desc == "0x" && length desc == 2 * ivLength + 2 =
+              BS.pack $ reassembleIV $ drop 2 desc
+        | length desc == ivLength =
+            BSC.pack desc
+        | otherwise =
+            usageError argv $ "IV length must be " ++
+                              show ivLength ++ " bytes."
+        where
+          reassembleIV :: String -> [Word8]
+          reassembleIV "" = []
+          reassembleIV (c1 : c2 : cs)
+              | isHexDigit c1 && isHexDigit c2 =
+                  let aByte = 16 * digitToInt c1 + digitToInt c2 in
+                  fromIntegral aByte : reassembleIV cs
+          reassembleIV _ = usageError argv "Bad IV string."