class Arcfour

Implementation of the “ARCFOUR” algorithm (“alleged RC4 (tm)”). Implemented as described at: www.mozilla.org/projects/security/pki/nss/draft-kaukonen-cipher-arcfour-03.txt

“RC4” is a trademark of RSA Data Security, Inc.

This is free software. Please see the LICENSE and COPYING files for details.

Public Class Methods

new(key) click to toggle source

# File lib/prawn/arcfour.rb, line 12
def initialize(key)
  # Convert string key to Array of integers
  key = key.unpack('c*') if key.is_a?(String)

  # 1. Allocate an 256 element array of 8 bit bytes to be used as an S-box
  # 2. Initialize the S-box.  Fill each entry first with it's index
  @sbox = (0..255).to_a
  
  # 3. Fill another array of the same size (256) with the key, repeating
  #    bytes as necessary.
  s2 = []
  while s2.length < 256
    s2 += key
  end
  s2 = s2[0, 256]

  # 4. Set j to zero and initialize the S-box
  j = 0
  (0..255).each do |i|
    j = (j + @sbox[i] + s2[i]) % 256
    @sbox[i], @sbox[j] = @sbox[j], @sbox[i]
  end

  @i = @j = 0
end

Public Instance Methods

encrypt(string) click to toggle source

# File lib/prawn/arcfour.rb, line 38
def encrypt(string)
  string.unpack('c*').map{|byte| byte ^ key_byte}.pack('c*')
end

Private Instance Methods

key_byte() click to toggle source

Produces the next byte of key material in the stream (3.2 Stream Generation)

# File lib/prawn/arcfour.rb, line 45
def key_byte
  @i = (@i + 1) % 256
  @j = (@j + @sbox[@i]) % 256
  @sbox[@i], @sbox[@j] = @sbox[@j], @sbox[@i]
  @sbox[(@sbox[@i] + @sbox[@j]) % 256]
end