%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