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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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#
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# NOTE: this encoder currently has only be tested using bit 5 set to on.
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#
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# The decoder has been tested with all possible values, but the decoder stub
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# is was not designed to bypass restrictions other than "bit 5 must be on"..
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#
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class MetasploitModule < Msf::Encoder
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  # This encoder has a manual ranking because it should only be used in cases
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  # where information has been explicitly supplied, specifically
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  # BitNumber and BitValue.
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  Rank = ManualRanking
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  def initialize
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    super(
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      'Name'             => 'Single Static Bit',
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      'Description'      => 'Static value for specific bit',
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      'Author'           => 'jduck',
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      'Arch'             => ARCH_X86,
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      'License'          => MSF_LICENSE,
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      'EncoderType'      => Msf::Encoder::Type::SingleStaticBit
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      )
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    # this shouldn't be present in the decoder stub.
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    @key_marker = 0x1010
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  end
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  #
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  # Returns the decoder stub that is adjusted for the size of
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  # the buffer being encoded
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  #
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  def decoder_stub(state)
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    bit_num = (datastore['BitNumber'] || 5).to_i
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    bit_val = (datastore['BitValue'] || true)
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    # variables:
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    # bit to ignore                                  (global - hardcoded)
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    # buf len (can be deduced with a jmp/call/pop)   (global - ebx)
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    # current source byte ptr                        (global - esi)
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    # current dest byte ptr                          (global - edi) ?
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    # current dest byte                              (global - ah)  ?
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    # number of bits accumulated                     (global - ebp) ?
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    # current source byte                            (outer  - al)
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    # bit index (for this byte)                      (inner  - cl)  ?
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    pre_init = ""
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    pre_init << "\x31\xed"        # xor ebp, ebp               - no bits accumulated
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    pre_init << "\x83\xe1\x01"    # and ecx, $0x1              - init inner loop counter (set to 0/1)
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    pre_init << "\x83\xe3\x01"    # and ebx, $0x1              - init buffer length
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    pre_init << "\x66\xbb" + [@key_marker].pack('v')         # - load encrypted buffer length
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    pre_init << "\x66\x81\xf3" + [@key_marker].pack('v')     # - xor decrypt buffer length
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    # we stored an entire byte, move to the next one
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    next_byte = ""
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    next_byte << "\x83\xef\xff"  # sub edi, 0xffffffff         - increment dst pointer
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    next_byte << "\x31\xed"      # xor ebp, ebp                - no bits accumulated
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    # inside the loop, we need to extract a bit, as
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    # specified by:
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    #
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    # ecx-1  - bit number to extract
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    # al     - byte to extract it from
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    get_a_bit = ""
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    get_a_bit << "\x60"          # pusha                       - save all registers
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    get_a_bit << "\x83\xe9\x01"  # sub ecx, 1                  - account for 1-based counting
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    get_a_bit << "\x74\x06"      # jz +6                       - skip dividing if bit zero
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    get_a_bit << "\xb3\x02"      # mov bl, 2                   - set divisor to 2
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    # divide_it:
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    get_a_bit << "\xf6\xf3"      # div bl                      - do the division
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    get_a_bit << "\xe2" + [-1 * (2+2)].pack('C')             # - divide again..
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    # store_bit:
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    get_a_bit << "\x83\xe0\x01"  # and eax, 0x01               - we only want the lowest bit
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    get_a_bit << "\x6b\x2f\x02"  # imul ebp, 2, [edi]          - load [edi], shifted left by 1, to ebp
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    get_a_bit << "\x09\xe8"      # or ebp, eax                 - set bit 0
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    get_a_bit << "\xaa"          # stosb al, [edi]             - store byte back
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    get_a_bit << "\x61"          # popa                        - restore previous ebx/eax
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    get_a_bit << "\x83\xed\xff"  # sub ebp, 0xffffffff         - increment bits stored
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    inner_init = ""
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    inner_init << "\xb1\x08"      # mov cl, $0x8               - init loop counter
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    inner_loop = ""
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    # process_bits:
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    inner_loop << "\x80\xf9"     # cmp cl, <ignore_bit + 1>   - is this the one to ignore?
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    inner_loop << [(bit_num+1)].pack('C')
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    len = get_a_bit.length + 3 + 2 + next_byte.length
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    inner_loop << "\x74" + [len].pack('C')                   # - je next_bit
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    inner_loop << get_a_bit
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    inner_loop << "\x83\xfd\x08"  # cmp ebp, $0x8              - got 8 bits now?
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    inner_loop << "\x75" + [next_byte.length].pack('C')      # - jne to next_bit
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    # next_dst_byte:
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    inner_loop << next_byte
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    # next_bit:
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    # I really wish this silly padding wasn't necessary, however removing the bad characters in the
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    # jump/call displacements has proven difficult otherwise.
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    inner_loop << "\x90" * 0x1a   # nops                       - for padding (so relative jumps don't have badchars)
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    len = -1 * (inner_loop.length+2)
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    inner_loop << "\xe2" + [len].pack('C')                   # - loop process_bits
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    # prefixed by:                # jmp data_beg_call
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    outer_init = ""
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    # get_data_beg:
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    outer_init << "\x5e"          # pop esi                    - ptr to beginning of data
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    outer_init << pre_init
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    outer_init << "\x89\xf7"      # mov edi, esi               - decode in place, init dst ptr
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    outer_loop = ""
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    #outer_loop << "\x90" * (0xd+6)
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    outer_loop << "\x83\xe0\x7f"  # and eax, 0x7f              - we only want the low byte
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    outer_loop << "\xac"          # lods   al, [esi]           - load src byte
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    outer_loop << inner_init << inner_loop
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    outer_loop << "\x83\xeb\x01"  # sub ebx, 1                 - 1 byte down!
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    outer_loop << "\x74\x07"      # jz +(2+5)                  - jump to data!
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    len = -1 * (outer_loop.length+2)
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    # next_byte:
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    outer_loop << "\xeb" + [len].pack('C')                   # - jmp process_byte
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    # data_beg_call:
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    decoder = outer_init + outer_loop
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    jmp = "\xeb" + [decoder.length].pack('C')
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    call = "\xe8" + [-1 * (decoder.length+5)].pack('V')
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    decoder = jmp + decoder + call
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    # encoded sled
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    state.context = ''
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    return decoder
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  end
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  def encode_block(state, block)
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    bit_num = (datastore['BitNumber'] || 5).to_i
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    bit_num = (7-bit_num)
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    bit_val = (datastore['BitValue'] || true)
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    encoded = ''
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    new_byte = 0
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    nbits = 0
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    block.unpack('C*').each do |ch|
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      7.step(0,-1) do |x|
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        # is this the special bit?
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        if (nbits == bit_num)
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          new_byte <<= 1 if nbits > 0
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          new_byte |= 1 if bit_val
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          nbits += 1
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          # do we have a full byte?
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          if nbits == 8
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            encoded << new_byte.chr
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            new_byte = 0
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            nbits = 0
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          end
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        end
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        # we have space, add it in
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        new_byte <<= 1 if nbits > 0
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        new_byte += 1 if (((ch >> x) & 1) > 0)
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        nbits += 1
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        # do we have a full byte?
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        if nbits == 8
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          encoded << new_byte.chr
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          new_byte = 0
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          nbits = 0
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        end
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      end
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    end
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    # if we have bits left, pad out to a whole byte
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    if nbits > 0
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      while nbits < 8
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        new_byte <<= 1
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        new_byte |= 1 if (nbits == bit_num) and bit_val
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        nbits += 1
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      end
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      encoded << new_byte.chr
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    end
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    return encoded
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  end
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  #
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  # Appends the encoded context portion.
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  #
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  def encode_end(state)
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    state.encoded += state.context
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    xor_key = 0
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    xor_key_str = ''
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    enc_len_str = ''
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    loop do
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      xor_key = rand(0x10000)
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      xor_key_str = [xor_key].pack('v')
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      enc_len_str = [state.encoded.length ^ xor_key].pack('v')
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      next if has_badchars?(xor_key_str, state.badchars)
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      next if has_badchars?(enc_len_str, state.badchars)
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      break
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    end
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    marker_str = [@key_marker].pack('v')
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    state.encoded.sub!(marker_str, enc_len_str)
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    state.encoded.sub!(marker_str, xor_key_str)
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  end
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end
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