#!/usr/bin/env ruby
# -*- coding: binary -*-

require 'rex/encoding/xor/exceptions'
require 'rex/text'

module Rex
module Encoding
module Xor

class Generic

  def Generic.keysize
    # special case:
    # 0 means we encode based on the length of the key
    # we don't enforce any perticular key length
    return 0
  end

  #
  # Now for some internal check methods
  #

  # hook stylies!
  # return index of offending byte or nil
  def Generic._check(data, key, badchars)
    return _check_key(key, badchars) || _check_encode(data, key, badchars)
  end
  def Generic._check_key(key, badchars)
    return Rex::Text.badchar_index(key, badchars)
  end
  def Generic._check_encode(data, key, badchars)
    return Rex::Text.badchar_index(encode(data, key), badchars)
  end

  def Generic.find_key(data, badchars)
    return _find_good_key(data, _find_bad_keys(data, badchars), badchars)
  end

  # !!! xxx MAKE THESE BITCHE PRIVATE

  #
  # Find a list of bytes that can't be valid xor keys, from the data and badchars.
  # This returns a Array of hashes, length keysize
  #
  def Generic._find_bad_keys(data, badchars)

    ksize = keysize

    # array of hashes for the bad characters based
    # on their position in the data
    badkeys = [ ]
    ksize.times { badkeys << { } }

    badchars.each_byte { |badchar|
      pos = 0
      data.each_byte { |char|
        badkeys[pos % ksize][char ^ badchar] = true
        pos += 1
      }
    }

    return badkeys
  end

  #
  # (Hopefully) find a good key, from badkeys and badchars
  #
  def Generic._find_good_key(data, badkeys, badchars)

    ksize = keysize
    strip = 0
    key   = ""

    while strip < keysize

      kbyte = rand(256)

      catch(:found_kbyte) do
        256.times {

          if !badkeys[strip][kbyte] && !badchars[kbyte.chr]
            throw :found_kbyte
          end

          kbyte = (kbyte + 1) & 0xff
        }

        raise KeySearchError, "Exhausted byte space for strip #{strip}!", caller
      end

      key << kbyte
      strip += 1
    end

    # ok, we should have a good key now, lets double check...
    if _check(data, key, badchars)
      raise KeySearchError, "Key found, but bad character check failed!", caller
    end

    return key
  end

  def Generic.encode(buf, key)

    if !key.kind_of?(String)
      raise ::ArgumentError, "Key must be a string!", caller
    end

    len = key.length

    if len == 0
      raise ::ArgumentError, "Zero key length!", caller
    end

    if keysize != 0 && keysize != len
      raise ::ArgumentError, "Key length #{len}, expected #{keysize}", caller
    end

    encoded = ""
    pos     = 0

    while pos < buf.length
      encoded += (buf[pos,1].unpack("C*")[0] ^ key[pos % len, 1].unpack("C*")[0]).chr
      key = _encode_mutate_key(buf, key, pos, len)
      pos += 1
    end

    return [ encoded, key ]

  end

  # kind of ghetto, but very convenient for mutating keys
  # by default, do no key mutations
  def Generic._encode_mutate_key(buf, key, pos, len)
    return key
  end

  # maybe a bit a smaller of method name?
  def Generic.find_key_and_encode(data, badchars)
    key        = find_key(data, badchars)
    enc, fkey  = encode(data, key)
    return [ enc, key, fkey ]
  end


end end end end # Generic/Xor/Encoding/Rex