Copyright © 2015 Bart Massey
This package provides a Haskell library and driver program implementing CipherSaber-2(CS2) stream encryption based on the RC4 stream encryption algorithm. This implementation has been tested against and is compatible with existing CipherSaber implementations.
The documentation for CS2 is a bit out-of-date and scattered.
CS2 is based on the RC4 stream cipher. Wikipedia has a nice history of RC4 as well as current reports on its cryptanalysis.
In 1999, Arnold Reinhold suggested using RC4 as the basis for citizens to learn to build their own encryption software, along the lines of Jedi Light Sabers. Reinhold proposed a stream protocol for RC4 ciphertext that he called CipherSaber (Note that the CipherSaber website is mostly abandoned and in some state of disrepair.)
In 2003, after cryptographic attacks were found against RC4 as used in CipherSaber, Reinhold modified the CipherSaber protocol to produce a new parameterized family of protocols known as CS2: the original CipherSaber is a special case of CS2, and is often referred to as CipherSaber-1.
Pseudocode for CS2 is available from a variety of places. The pseudocode given here attempts to be clear and normative.
CS2 encryption and decryption both require an RC4 implementation that has been modified to iterate the key schedule a given number of times.
-- Produce an RC4 keystream of length n with
-- r rounds of key scheduling given key k
rc4(n, r, k):
l ← length k
-- Initialize the array.
S ← zero-based array of 256 bytes
for i in 0..255
S[i] ← i
-- Do key scheduling.
j ← 0
repeat r times
for i in 0..255
j ← (j + S[i] + k[i mod l]) mod 256
S[i] ↔ S[j]
-- Finally, produce the stream.
keystream ← zero-based array of n bytes
j ← 0
for i in 0..n-1
i' ← (i + 1) mod 256
j ← (j + S[i']) mod 256
S[i'] ↔ S[j]
keystream[i] ← S[(S[i'] + S[j]) mod 256]
return keystream
CS2 encryption requires a plaintext message (treated as a bytestream), a key, and an "initial value" (IV) of 10 bytes. The key should be no more than 53 bytes, to ensure good mixing during key scheduling.
The IV is a nonce that must be different for each message sent: it should be chosen randomly if possible, but may be chosen pseudo-randomly or even just counted if necessary. CS2 appends the IV to the CS2 key to produce an RC4 key. RC4 uses only the first 256 RC4 key bytes. Thus, the CS2 key must be no more than 246 bytes: a longer CS2 key would cause some or all of the IV to not be used by RC4.
-- Ciphersaber-2 encrypt message m with key k and
-- r rounds of key scheduling
encrypt(m, r, k):
n ← length m
iv ← appropriately-chosen 10-byte IV
k' ← prepend k to iv
keystream ← rc4(n, r, k')
ciphertext ← zero-based array of n + 10 bytes
for i in 0..9
ciphertext[i] ← iv[i]
for i in 0..n
ciphertext[i + 10] ← m[i] xor keystream[i]
return ciphertext
CS2 decryption requires ciphertext and the encryption key used to produce the ciphertext.
-- Ciphersaber-2 decrypt ciphertext m with key k and
-- r rounds of key scheduling
decrypt(m, r, k):
n ← length m
iv ← m[0..9]
delete the first 10 characters of m
k' ← prepend k to iv
keystream ← rc4(n - 10, r, k')
plaintext ← zero-based array of n - 10 bytes
for i in 0..n-10
plaintext[i] ← m[i] xor keystream[i]
return plaintext
The CipherSaber2
library provides a relatively straightforward
ByteString
interface. See the haddock
documentation
for details.
The program cs2
uses the CipherSaber2
library to encrypt
or decrypt stdin
to stdout
.
cs2
is written in Haskell, so you will need a Haskell
installation to run it. It depends on the package
[parseargs](http://hackage.haskell.org/package/parseargs)
from Hackage, as well as the
bytestring
package that should probably have come with
your distribution but may need to be installed. Say "cabal install parseargs
" and "cabal install bytestring
" to get
things set up. (This in turn may require a cabal-install
package from your Linux distribution or thereabouts.)
Say "runghc cs2.hs --help
" for usage information.
Say "runghc cs2.hs -e whee <f.txt >g.cs2
" to encrypt the
file f.txt
with key whee
. An IV will be automatically
chosen.
Say "runghc cs2.hs -d whee <g.cs2 >ff.txt
" to decrypt the
file g.cs2
with key whee
.
The test
directory include a bunch of plaintext/ciphertext
pairs taken from the original CS2 materials. Please see
README.md
in that directory for more information.
This work is licensed under the "MIT License". Please see the file LICENSE in the source distribution of this software for license terms.