Node.js RC4 encryption/decryption

rc4.coffee

###
 Copyright (c) 2012 rhoot <https://github.com/rhoot>

 This software is provided 'as-is', without any express or implied
 warranty. In no event will the authors be held liable for any damages
 arising from the use of this software.

 Permission is granted to anyone to use this software for any purpose,
 including commercial applications, and to alter it and redistribute it
 freely, subject to the following restrictions:

    1. The origin of this software must not be misrepresented; you must not
    claim that you wrote the original software. If you use this software
    in a product, an acknowledgment in the product documentation would be
    appreciated but is not required.

    2. Altered source versions must be plainly marked as such, and must not be
    misrepresented as being the original software.

    3. This notice may not be removed or altered from any source
    distribution.
###

###

 Sample usage:

 rc4 = require 'rc4'
 engine = new rc4.Engine()

 # Encrypt 'Plaintext' with key 'Key'.
 # 'encrypted' contains a node.js Buffer with the results

 engine.init 'Key'
 encrypted = engine.encrypt 'Plaintext'

 # Encryption also works on byte arrays and node.js Buffer objects
 # This would give the same results as the above

 engine.init [ 0x4b, 0x65, 0x79 ]
 encrypted = engine.encrypt new Buffer([ 0x50, 0x6c, 0x61, 0x69, 0x6e, 0x74, 0x65, 0x78, 0x74 ])

 # Decrypting is just as easy 'Attack at dawn' with key 'Secret'

 engine.init 'Secret'
 message   = [ 0x45, 0xA0, 0x1F, 0x64, 0x5F, 0xC3, 0x5B, 0x38, 0x35, 0x52, 0x54, 0x4B, 0x9B, 0xF5 ]
 decrypted = engine.decrypt(message).toString()

 # Some encrypted messages require dropping the first x bytes of the keystream.

 engine.init 'Key'
 engine.drop 3072
 decrypted = engine.decrypt [ 0x36, 0x49, 0xbe, 0xa0, 0xdf, 0xb1, 0xd3, 0xcd, 0x3f ]
    
###

isArrayIsh = (obj) ->
  Array.isArray(obj) or
    Buffer.isBuffer(obj) or
    (obj? and typeof obj is 'object' and typeof obj.length is 'number')

stringToArray = (str) ->
  str.charCodeAt i for i in [0...str.length]

class Rc4Engine

  # @public
  constructor: (key) ->
    @state = new Array 256
    @i1 = 0
    @i2 = 0
    @init key if key?

  # @private
  swap = (state, i1, i2) ->
    temp = state[i1]
    state[i1] = state[i2]
    state[i2] = temp

  # @private
  step = (e) ->
    e.i1 = (e.i1 + 1) & 255
    e.i2 = (e.i2 + e.state[e.i1]) & 255
    swap e.state, e.i1, e.i2

  # @public
  init: (key) ->
    key = stringToArray key if typeof key is 'string'
    throw new TypeError 'Invalid key type' if not isArrayIsh key
    
    @state = [0...256]
    @i1 = 0
    @i2 = 0
    j = 0        

    for i in [0...256]
      j = (j + @state[i] + key[i % key.length]) & 255
      swap @state, i, j

  # @public
  drop: (num) ->
    throw new TypeError 'Invalid num argument.' unless typeof num is 'number'
    step this for i in [0...num]

  # @public
  encrypt: (bytes) ->
    bytes = stringToArray bytes if typeof bytes is 'string'
    throw new TypeError 'Invalid data type.' if not isArrayIsh bytes
        
    encrypted = new Buffer bytes.length
    for i in [0...bytes.length]
      step this
      key = @state[(@state[@i1] + @state[@i2]) & 0xff]
      encrypted[i] = bytes[i] ^ key
    encrypted

  # @public
  decrypt: (bytes) ->
    # RC4 encryption and decryption algorithms are exactly the same
    @encrypt bytes
  
module.exports =
  Engine: Rc4Engine