metasploit-framework/modules/encoders/x86/nonalpha.rb

##
# This module requires Metasploit: https://metasploit.com/download
# Current source: https://github.com/rapid7/metasploit-framework
##

require 'rex/encoder/nonalpha'

class MetasploitModule < Msf::Encoder::NonAlpha
  Rank = LowRanking

  def initialize
    super(
      'Name'             => "Non-Alpha Encoder",
      'Description'      => %q{
          Encodes payloads as non-alpha based bytes. This allows
        payloads to bypass both toupper() and tolower() calls,
        but will fail isalpha(). Table based design from
        Russel Sanford.
      },
      'Author'           => [ 'pusscat'],
      'Arch'             => ARCH_X86,
      'License'          => BSD_LICENSE,
      'EncoderType'      => Msf::Encoder::Type::NonAlpha,
      'Decoder'          =>
        {
          'BlockSize' => 1,
        })
  end

  #
  # Returns the decoder stub that is adjusted for the size of the buffer
  # being encoded.
  #
  def decoder_stub(state)
    state.key                   = ""
    state.decoder_key_size      = 0
    Rex::Encoder::NonAlpha::gen_decoder()
  end

  #
  # Encodes a one byte block with the current index of the length of the
  # payload.
  #
  def encode_block(state, block)
    newchar, state.key, state.decoder_key_size = Rex::Encoder::NonAlpha::encode_byte(block.unpack('C')[0], state.key, state.decoder_key_size)
    return newchar
  end

  #
  # Fix stuff, and add the table :)
  #
  def encode_end(state)
    state.encoded.gsub!(/A/, state.decoder_key_size.chr)
    state.encoded.gsub!(/B/, (state.decoder_key_size+5).chr)
    state.encoded[0x24, 0] = state.key
  end
end