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##
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# This module requires Metasploit: https://metasploit.com/download
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# Current source: https://github.com/rapid7/metasploit-framework
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##
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class MetasploitModule < Msf::Encoder::Xor
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  def initialize
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    super(
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      'Name' => 'XOR POLY Encoder',
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      'Description' => 'An x86 Simple POLY Xor encoding method. using polymorphism Register swapping, and instructions modification',
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      'Author' => [ 'Arthur RAOUT' ],
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      'Arch' => ARCH_X86,
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      'License' => MSF_LICENSE,
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      'Decoder' => {
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        'KeySize' => 4,
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        'BlockSize' => 4,
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        'KeyPack' => 'V'
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      }
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      )
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  end
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  # Indicate that this module can preserve the registers used
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  def can_preserve_registers?
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    true
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  end
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  # select a permutation from table
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  def choose_permutation(state, table)
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    table = table.shuffle
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    for i in 0..table.length - 1
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      if table[i].count(state.badchars).zero?
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        return table[i]
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      end
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    end
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    raise 'No permutation found for the badchar set :' + state.badchars.inspect
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  end
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  # generate instruction for a push
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  def register_preservation_generate(flag, regs)
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    ret = ''
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    pop = 0b0101_1000
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    push = 0b0101_0000
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    if flag == 0
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      for r in regs
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        ret += [push | r].pack('C')
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      end
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    end
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    if flag == 1
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      for r in regs.reverse
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        ret += [pop | r].pack('C')
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      end
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    end
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    return ret
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  end
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  def decoder_stub(state)
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    state.decoder_key_size = 4
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    state.decoder_key_pack = 'V'
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    # calculate the (negative) and positive block count.
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    block_count = [-(((state.buf.length - 1) / state.decoder_key_size) + 1)].pack('V')
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    block_count_positive = [(((state.buf.length - 1) / state.decoder_key_size) + 1)].pack('V')
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    regs = [0b0000, 0b0001, 0b0010, 0b0011, 0b0110, 0b0111]
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    pop = 0b0101_1000
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    push = 0b0101_0000
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    mov = 0b1011_1000
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    reg1 = regs[rand(6)]
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    regs.delete(reg1)
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    reg2 = regs[rand(5)]
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    regs.delete(reg2)
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    reg3 = regs[rand(4)]
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    regs.delete(reg3)
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    reg4 = regs[rand(3)] # reg4 is useless and used for nopLike operations
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    regs.delete(reg4)
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    # NOPS
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    nop_nop_nop_nop = "\x90\x90\x90\x90" # 4 bytes
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    push_pop12 = [push | reg1, push | reg2, pop | reg2, pop | reg1].pack('CCCC') # 4 bytes
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    push_pop34 = [push | reg3, push | reg4, pop | reg4, pop | reg3].pack('CCCC') # 4 bytes
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    push_pop56 = [push | reg4, push | reg1, pop | reg1, pop | reg4].pack('CCCC') # 4 bytes
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    sub_reg_0 = [0x83, (0xE8 | rand(6)), 0x00].pack('CCC') # 3 bytes
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    add_reg_0 = [0x83, (0xc0 | rand(6)), 0x00].pack('CCC') # 3 bytes
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    add_reg4_1 = [0x83, (0xc0 | reg4), 0x01].pack('CCC') # 3 bytes
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    add_reg4_33 = [0x83, (0xc0 | reg4), 0x33].pack('CCC') # 3 bytes
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    add_reg4_f1 = [0x83, (0xc0 | reg4), 0xf1].pack('CCC') # 3 bytes
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    nop_nop_nop = "\x90\x90\x90" # 3 bytes
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    push_pop1 = [push | reg1, pop | reg1].pack('CC') # 2 bytes
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    push_pop2 = [push | reg2, pop | reg2].pack('CC') # 2 bytes
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    push_pop3 = [push | reg3, pop | reg3].pack('CC') # 2 bytes
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    push_pop4 = [push | reg4, pop | reg4].pack('CC') # 2 bytes
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    inc_reg1_dec_reg1 = [0x40 | reg1, 0x48 | reg1].pack('CC') # 2 bytes
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    inc_reg2_dec_reg2 = [0x40 | reg2, 0x48 | reg2].pack('CC') # 2 bytes
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    inc_reg3_dec_reg3 = [0x40 | reg3, 0x48 | reg3].pack('CC') # 2 bytes
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    inc_reg4_dec_reg4 = [0x40 | reg4, 0x48 | reg4].pack('CC') # 2 bytes
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    # nops tables by size
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    nops_2_bytes = [push_pop1, push_pop2, push_pop3, push_pop4, "\x90\x90", inc_reg1_dec_reg1, inc_reg2_dec_reg2, inc_reg3_dec_reg3, inc_reg4_dec_reg4]
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    nops_3_bytes = [nop_nop_nop, push_pop1 + "\x90", push_pop2 + "\x90", push_pop3 + "\x90", push_pop4 + "\x90", sub_reg_0, add_reg_0, choose_permutation(state, nops_2_bytes) + "\x90", add_reg4_1, add_reg4_33, add_reg4_f1]
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    nops_4_bytes = [nop_nop_nop_nop, push_pop12, push_pop34, push_pop56, choose_permutation(state, nops_2_bytes) + choose_permutation(state, nops_2_bytes), choose_permutation(state, nops_3_bytes) + "\x90"]
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    # THE DECODER CODE
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    pop_reg1 = [pop | reg1].pack('C')
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    # sub 5 from reg1 on 5 byte
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    sub_reg1_5 = [0x83, (0xE8 | reg1), 0x05].pack('CCC') + choose_permutation(state, nops_2_bytes) # 5 bytes
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    add_reg1_neg5 = [0x83, (0xc0 | reg1), 0xfb].pack('CCC') + choose_permutation(state, nops_2_bytes) # 5 bytes
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    dec_reg1_5 = [0x48 | reg1, 0x48 | reg1, 0x48 | reg1, 0x48 | reg1, 0x48 | reg1].pack('CCCCC') # 5 bytes
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    # set reg2 to 0, on 6 bytes
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    xor_reg2_reg2 = [0x31, (0xC0 | (reg2 << 3) | reg2)].pack('CC') + choose_permutation(state, nops_4_bytes) # 6 bytes
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    and_reg2_0 = [0x83, (0xE0 | reg2), 0x00].pack('CCC') + choose_permutation(state, nops_3_bytes) # 6 bytes
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    lea_reg2_0 = [0x8D, (0x05 | (reg2 << 3)), 0x00, 0x00, 0x00, 0x00].pack('CCCCCC')
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    imul_reg2_reg2_0 = [0x6b, (0xC0 | (reg2 << 3) | reg2), 0x00].pack('CCC') + choose_permutation(state, nops_3_bytes) # 6 bytes
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    sub_reg2_reg2 = [0x29, (0xC0 | (reg2 << 3) | reg2)].pack('CC') + choose_permutation(state, nops_4_bytes) # 6 bytes
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    push0_popreg2 = [0x6A, 0x00, (0x58 | reg2)].pack('CCC') + choose_permutation(state, nops_3_bytes) # 6 bytes
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    # SET REG2 TO BLOCK_COUNT
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    sub_reg2_bc = [0x81, (0xe8 | reg2)].pack('CC') + block_count
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    add_reg2_bc = [0x81, (0xc0 | reg2)].pack('CC') + block_count_positive
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    mov_reg3 = [mov | reg3].pack('C')
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    xor_rel_reg1_reg3 = [0x31, (0x40 | (reg3 << 3 | reg1))].pack('cc')
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    # ADD 4 TO REG1
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    add_reg1_4 = [0x83, (0xC0 | reg1), 0x04].pack('CCC') + choose_permutation(state, nops_3_bytes) # 6 bytes
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    sub_reg1_neg4 = [0x83, (0xE8 | reg1), 0xFC].pack('CCC') + choose_permutation(state, nops_3_bytes) # 6 bytes
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    inc_reg1_4 = [0x40 | reg1, 0x40 | reg1, 0x40 | reg1, 0x40 | reg1].pack('CCCC') + choose_permutation(state, nops_2_bytes) # 6 bytes
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    # sub 1 from reg2 on 6 bytes
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    dec_r2 = [0xFF, (0xC8 | reg2)].pack('CC')
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    sub_reg2_1 = [0x83, (0xE8 | reg2), 0x01].pack('CCC')
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    add_reg2_neg1 = [0x83, (0xC0 | reg2), 0xFF].pack('CCC')
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    set_reg2_0 = [xor_reg2_reg2, and_reg2_0, lea_reg2_0, imul_reg2_reg2_0, sub_reg2_reg2, push0_popreg2]
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    sub_reg1_0x5 = [sub_reg1_5, add_reg1_neg5, dec_reg1_5]
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    set_reg2_bc = [sub_reg2_bc, add_reg2_bc]
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    # GET EIP TO REG1
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    call_pop = [0xE8, 0x00, 0x00, 0x00, 0x00].pack('CCCCC') + pop_reg1 + choose_permutation(state, sub_reg1_0x5)
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    fpu_inst = ["\xD9\xE0", "\xDF\xE9", "\xDB\xC9", "\xDA\xD9", "\xDA\xC1", "\xDA\xD1", "\xDB\xD9"] # 2 bytes
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    fnstenv_pop = choose_permutation(state, fpu_inst) + "\xD9\x74\x24\xF4" + pop_reg1
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    add_reg1_0x4 = [add_reg1_4, sub_reg1_neg4, inc_reg1_4]
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    dec_reg2 = [dec_r2, sub_reg2_1, add_reg2_neg1]
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    get_eip = [call_pop, fnstenv_pop]
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    small_junk = [choose_permutation(state, nops_2_bytes), choose_permutation(state, nops_3_bytes), choose_permutation(state, nops_4_bytes)]
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    reg_for_preservation = [reg1, reg2, reg3, reg4].shuffle
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    reg_push = register_preservation_generate(0, reg_for_preservation)
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    reg_pop = register_preservation_generate(1, reg_for_preservation)
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    geip = choose_permutation(state, get_eip)
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    junk = choose_permutation(state, small_junk)
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    reg2_0 = choose_permutation(state, set_reg2_0)
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    block_count_set = choose_permutation(state, set_reg2_bc)
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    reg1_add4 = choose_permutation(state, add_reg1_0x4)
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    decrement_reg2 = choose_permutation(state, dec_reg2)
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    decoder = reg_push +
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              geip +                                      # get EIP into REG1
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              junk +                                      # small junk
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              reg2_0 +                                    # set REG2 to 0
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              block_count_set + # sub reg2, block_count
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              mov_reg3 + 'XXXX' +                         # mov reg3, 0xKEY_KEY_KEY_KEY
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              xor_rel_reg1_reg3 + 'LL' +                  # xor [reg1+DECODER_LEN], reg3
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              reg1_add4 + # add reg1, 4
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              decrement_reg2 + # dec reg2
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              "\x75" + 'SS' + # jnz to xor
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              reg_pop
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    decoder_len = decoder.size
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    jmp = decoder.index(xor_rel_reg1_reg3) - decoder.index('SS')
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    decoder.sub! 'SS', [jmp].pack('C')
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    decoder.sub! 'LL', [decoder_len - 6].pack('C')
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    # example of decoder generated
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    # e800000000     call loc._start.continue
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    # 58             pop eax
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    # 83e805         sub eax, 5
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    # 31c9           xor ecx, ecx
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    # 81e9bbbbbbbb   sub ecx, 0xbbbbbbbb
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    # bbaaaaaaaa     mov ebx, 0xaaaaaaaa
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    # 31581f         xor dword [eax + 0x1f], ebx
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    # 83e8f4         sub eax, 0xfffffff4
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    # e2f8           loop loc._start.check
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    state.decoder_key_offset = decoder.index('XXXX')
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    return decoder
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  end
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end
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