# ciphersaber2

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.

## CS2

The documentation for CS2 is a bit out-of-date and
scattered.

### History

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.

### Algorithm

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

## Library

The `CipherSaber2`

library provides a relatively straightforward
`ByteString`

interface. See the `haddock`

documentation
for details.

## Driver

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`

.

## Test Instances

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.

## License

This work is licensed under the "MIT License". Please
see the file LICENSE in the source distribution of this
software for license terms.