Method: Rex::Encoding::Xor::DwordAdditive._find_good_key

Defined in:
lib/rex/encoding/xor/dword_additive.rb

._find_good_key(data, badkeys, badchars) ⇒ Object

I realize this algorithm is broken. We invalidate some keys in _find_bad_keys that could actually be perfectly fine. However, it seems to work ok for now, and this is all just a lame adhoc method. Maybe someday we can revisit this and make it a bit less ghetto…



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# File 'lib/rex/encoding/xor/dword_additive.rb', line 54

def DwordAdditive._find_good_key(data, badkeys, badchars)

  ksize  = keysize
  kstart = ""
  ksize.times { kstart << rand(256) } # random key starting place

  key = kstart.dup

  #
  # now for the ghettoness of an algorithm:
  #  try the random key we picked
  #  if the key failed, figure out which key byte corresponds
  #  increment that key byte
  #  if we wrapped a byte all the way around, fail :(
  #

  loop do
    # ok, try to encode it, any bad chars present?
    pos = _check(data, key, badchars)

    # yay, no problems, we found a key!
    break if !pos

    strip = pos % ksize

    # increment the offending key byte
    key[strip] = key[strip] + 1 & 0xff

    # We wrapped around!
    if key[strip] == kstart[strip]
      raise KeySearchError, "Key space exhausted on strip #{strip}!", caller
    end
  end

  return key
end