tarjeihsch · February 27, 2025 23:52
diff --git a/gistfile1.txt b/gistfile1.txt
 %define CHUNK_SIZE  4096
 %define KEY_SIZE    64
 %define PATH_SIZE   256

 section .bss
    input_buffer   resb CHUNK_SIZE
    output_buffer  resb CHUNK_SIZE
    
    argument_key   resb KEY_SIZE
    argument_path  resb PATH_SIZE
    
    file_handle    resq 1
    bytes_read     resq 1

 section .text
    global _start

 _start:
    ; Write all bytes in buffer to 0
    mov rdi, argument_key
    xor rax, rax
    mov rcx, KEY_SIZE
    rep stosb
    
    ; Write all bytes in buffer to 0
    mov rdi, argument_path
    xor rax, rax
    mov rcx, PATH_SIZE
    rep stosb

    ; Load argument 1 (Argv[1] = 8 * 2)
    mov rsi, [rsp + 16]
    mov rdi, argument_key
    mov rcx, KEY_SIZE
    rep movsb

    ; Load argument 2 (Argv[2] = 8 * 3)
    mov rsi, [rsp + 24]
    mov rdi, argument_path
    mov rcx, PATH_SIZE
    rep movsb

    ; Load file
    mov rax, 2
    mov rdi, argument_path
    mov rsi, 0x002 ; O_RDWR
    mov rdx, 0666 ; rw-rw-rw-
    syscall

    ; Exit on failure
    cmp rax, 0
    jl exit

    ; Store file handle
    mov [file_handle], rax

 ; Attempt to read a chunk of bytes into the input buffer
 .xor_read:
    ; Read chunk into input buffer
    mov rax, 0
    mov rdi, [file_handle]
    mov rsi, input_buffer
    mov rdx, CHUNK_SIZE
    syscall
    cmp rax, 0
    je stop
    jl stop

    ; Store file size
    mov [bytes_read], rax
    mov rcx, rax
    mov rbx, rax

 ; Perform XOR cipher operation on each byte (Output = Input XOR Key)
 .xor_translate:
    mov dl, [input_buffer + rcx - 1]
    mov r8, rcx
    and r8, KEY_SIZE - 1
    xor dl, [argument_key + r8]
    mov [output_buffer + rcx - 1], dl  
    loop .xor_translate

 ; Write the modified bytes back to the file at it's respective file offset
 .xor_write:
    ; Move file pointer
    mov rax, 8
    mov rdi, [file_handle]
    mov rsi, rbx
    neg rsi
    mov rdx, 1
    syscall
    cmp rax, 0
    jl stop
    
    ; Write output buffer to file
    mov rax, 1
    mov rdi, [file_handle]
    mov rsi, output_buffer
    mov rdx, rbx
    syscall
    cmp rax, rbx
    jl stop

    ; Prepare a new chunk of bytes and repeat the process
    jmp .xor_read

 stop:
    ; Dispose file handle
    mov rax, 3
    mov rdi, [file_handle]
    syscall

 exit:
    ; Exit application
    mov rax, 60
    xor rdi, rdi
    syscall
	%define CHUNK_SIZE 4096
	%define KEY_SIZE 64
	%define PATH_SIZE 256

	section .bss
	input_buffer resb CHUNK_SIZE
	output_buffer resb CHUNK_SIZE

	argument_key resb KEY_SIZE
	argument_path resb PATH_SIZE

	file_handle resq 1
	bytes_read resq 1

	section .text
	global _start

	_start:
	; Write all bytes in buffer to 0
	mov rdi, argument_key
	xor rax, rax
	mov rcx, KEY_SIZE
	rep stosb

	; Write all bytes in buffer to 0
	mov rdi, argument_path
	xor rax, rax
	mov rcx, PATH_SIZE
	rep stosb

	; Load argument 1 (Argv[1] = 8 * 2)
	mov rsi, [rsp + 16]
	mov rdi, argument_key
	mov rcx, KEY_SIZE
	rep movsb

	; Load argument 2 (Argv[2] = 8 * 3)
	mov rsi, [rsp + 24]
	mov rdi, argument_path
	mov rcx, PATH_SIZE
	rep movsb

	; Load file
	mov rax, 2
	mov rdi, argument_path
	mov rsi, 0x002 ; O_RDWR
	mov rdx, 0666 ; rw-rw-rw-
	syscall

	; Exit on failure
	cmp rax, 0
	jl exit

	; Store file handle
	mov [file_handle], rax

	; Attempt to read a chunk of bytes into the input buffer
	.xor_read:
	; Read chunk into input buffer
	mov rax, 0
	mov rdi, [file_handle]
	mov rsi, input_buffer
	mov rdx, CHUNK_SIZE
	syscall
	cmp rax, 0
	je stop
	jl stop

	; Store file size
	mov [bytes_read], rax
	mov rcx, rax
	mov rbx, rax

	; Perform XOR cipher operation on each byte (Output = Input XOR Key)
	.xor_translate:
	mov dl, [input_buffer + rcx - 1]
	mov r8, rcx
	and r8, KEY_SIZE - 1
	xor dl, [argument_key + r8]
	mov [output_buffer + rcx - 1], dl
	loop .xor_translate

	; Write the modified bytes back to the file at it's respective file offset
	.xor_write:
	; Move file pointer
	mov rax, 8
	mov rdi, [file_handle]
	mov rsi, rbx
	neg rsi
	mov rdx, 1
	syscall
	cmp rax, 0
	jl stop

	; Write output buffer to file
	mov rax, 1
	mov rdi, [file_handle]
	mov rsi, output_buffer
	mov rdx, rbx
	syscall
	cmp rax, rbx
	jl stop

	; Prepare a new chunk of bytes and repeat the process
	jmp .xor_read

	stop:
	; Dispose file handle
	mov rax, 3
	mov rdi, [file_handle]
	syscall

	exit:
	; Exit application
	mov rax, 60
	xor rdi, rdi
	syscall