section .bss
input_string resb 255 ; Buffer for user input
output_forward resb 255 ; Buffer for forward encrypted output
output_backward resb 255 ; Buffer for backward encrypted output
section .text
global _start
_start:
; Ask the user for input
mov rdi, 0 ; File descriptor 0 is STDIN
mov rsi, input_string ; Pointer to the input buffer
mov rdx, 255 ; Maximum number of bytes to read
call read_input
; Encrypt the input string forward
mov rdi, input_string ; Pointer to the input string
mov rsi, output_forward ; Pointer to the forward encrypted string buffer
call encrypt_forward
; Print "Encoded Message:"
mov rdi, 1 ; File descriptor 1 is STDOUT
mov rax, 1 ; System call number for sys_write
lea rsi, [encoded_message] ; Pointer to the encoded message string
mov rdx, 16 ; Number of bytes to write (length of the encoded message)
syscall
; Print the forward encrypted string
mov rdi, 1 ; File descriptor 1 is STDOUT
mov rax, 1 ; System call number for sys_write
lea rsi, [output_forward] ; Pointer to the forward encrypted string
mov rdx, 255 ; Maximum number of bytes to write
syscall
; Print a newline
mov rdi, 1 ; File descriptor 1 is STDOUT
mov rax, 1 ; System call number for sys_write
lea rsi, [newline] ; Pointer to the newline string
mov rdx, 1 ; Number of bytes to write (newline character)
syscall
; Encrypt the input string backward
mov rdi, input_string ; Pointer to the input string
mov rsi, output_backward ; Pointer to the backward encrypted string buffer
call encrypt_backward
; Print "Decoded Message:"
mov rdi, 1 ; File descriptor 1 is STDOUT
mov rax, 1 ; System call number for sys_write
lea rsi, [decoded_message] ; Pointer to the decoded message string
mov rdx, 16 ; Number of bytes to write (length of the decoded message)
syscall
; Print the backward encrypted string
mov rdi, 1 ; File descriptor 1 is STDOUT
mov rax, 1 ; System call number for sys_write
lea rsi, [output_backward] ; Pointer to the backward encrypted string
mov rdx, 255 ; Maximum number of bytes to write
syscall
; Exit the program
mov rax, 60 ; System call number for sys_exit
xor rdi, rdi ; Exit code 0
syscall
read_input:
; Input: rdi - File descriptor
; rsi - Pointer to the input buffer
; rdx - Maximum number of bytes to read
mov rax, 0 ; System call number for sys_read
syscall
ret
encrypt_forward:
; Input: rdi - Pointer to the input string
; rsi - Pointer to the forward encrypted string buffer
xor rdx, rdx ; Clear rdx (counter for string length)
xor rcx, rcx ; Clear rcx (counter for loop)
; Loop through the input string
forward_char:
movzx rax, byte [rdi + rcx] ; Load the next character from the input string
cmp rax, 0 ; Check if it is the null terminator (end of string)
je end_forward ; If yes, end the forward encryption
add rax, 1 ; Caesar cipher shift forward
mov [rsi + rcx], al ; Store the forward encrypted character in the output string
inc rcx ; Move to the next character
inc rdx ; Increment the string length counter
jmp forward_char ; Repeat the loop
end_forward:
mov byte [rsi + rdx], 0 ; Add null terminator to the forward encrypted string
ret
encrypt_backward:
; Input: rdi - Pointer to the input string
; rsi - Pointer to the backward encrypted string buffer
xor rdx, rdx ; Clear rdx (counter for string length)
xor rcx, rcx ; Clear rcx (counter for loop)
; Loop through the input string
backward_char:
movzx rax, byte [rdi + rcx] ; Load the next character from the input string
cmp rax, 0 ; Check if it is the null terminator (end of string)
je end_backward ; If yes, end the backward encryption
sub rax, 1 ; Caesar cipher shift backward
mov [rsi + rcx], al ; Store the backward encrypted character in the output string
inc rcx ; Move to the next character
inc rdx ; Increment the string length counter
jmp backward_char ; Repeat the loop
end_backward:
mov byte [rsi + rdx], 0 ; Add null terminator to the backward encrypted string
ret
section .data
newline db 10 ; Newline character
encoded_message db "Encoded Message: ", 0
decoded_message db "Decoded Message: ", 0 Write, Run & Share Assembly code online using OneCompiler's Assembly online compiler for free. It's one of the robust, feature-rich online compilers for Assembly language. Getting started with the OneCompiler's Assembly compiler is simple and pretty fast. The editor shows sample boilerplate code when you choose language as Assembly and start coding.
Assembly language(asm) is a low-level programming language, where the language instructions will be more similar to machine code instructions.
Every assembler may have it's own assembly language designed for a specific computers or an operating system.
Assembly language requires less execution time and memory. It is more helful for direct hardware manipulation, real-time critical applications. It is used in device drivers, low-level embedded systems etc.
Assembly language usually consists of three sections,
Data section
To initialize variables and constants, buffer size these values doesn't change at runtime.
bss section
To declare variables
text section
_start specifies the starting of this section where the actually code is written.
There are various define directives to allocate space for variables for both initialized and uninitialized data.
variable-name define-directive initial-value
| Define Directive | Description | Allocated Space |
|---|---|---|
| DB | Define Byte | 1 byte |
| DW | Define Word | 2 bytes |
| DD | Define Doubleword | 4 bytes |
| DQ | Define Quadword | 8 bytes |
| DT | Define Ten Bytes | 10 bytes |
| Define Directive | Description |
|---|---|
| RESB | Reserve a Byte |
| RESW | Reserve a Word |
| RESD | Reserve a Doubleword |
| RESQ | Reserve a Quadword |
| REST | Reserve a Ten Bytes |
Constants can be defined using
CONSTANT_NAME EQU regular-exp or value
%assign constant_name value
%define constant_name value
Loops are used to iterate a set of statements for a specific number of times.
mov ECX,n
L1:
;<loop body>
loop L1
where n specifies the no of times loops should iterate.
Procedure is a sub-routine which contains set of statements. Usually procedures are written when multiple calls are required to same set of statements which increases re-usuability and modularity.
procedure_name:
;procedure body
ret