 ; Encryption Program (Encrypt.asm)
INCLUDE Irvine32.
KEY = 239 ; any value between 1-255
BUFMAX = 128 ; maximum buffer size
.data
sPrompt BYTE "Enter the plain text:",0
sEncrypt BYTE "Cipher text: ",0
sDecrypt BYTE "Decrypted: ",0
buffer BYTE BUFMAX+1 DUP(0)
bufSize DWORD ?
.code
main PROC
call InputTheString ; input the plain text
call TranslateBuffer ; encrypt the buffer
mov edx,OFFSET sEncrypt ; display encrypted message
call DisplayMessage
call TranslateBuffer ; decrypt the buffer
mov edx,OFFSET sDecrypt ; display decrypted message
call DisplayMessage
exit
main ENDP
;-----------------------------------------------------
InputTheString PROC
;
; Prompts user for a plaintext string. Saves the string
; and its length.
; Receives: nothing
; Returns: nothing
;-----------------------------------------------------
pushad ; save 32-bit registers
mov edx,OFFSET sPrompt ; display a prompt
call WriteString
mov ecx,BUFMAX ; maximum character count
mov edx,OFFSET buffer ; point to the buffer
call ReadString ; input the string
mov bufSize,eax ; save the length
call Crlf
popad
ret
InputTheString ENDP
;-----------------------------------------------------
DisplayMessage PROC
;
; Displays the encrypted or decrypted message.
; Receives: EDX points to the message
; Returns: nothing
;-----------------------------------------------------
pushad
call WriteString
mov edx,OFFSET buffer ; display the buffer
call WriteString
call Crlf
call Crlf
popad
ret
DisplayMessage ENDP
;-----------------------------------------------------
TranslateBuffer PROC
;
; Translates the string by exclusive-ORing each
; byte with the encryption key byte.
; Receives: nothing
; Returns: nothing
;-----------------------------------------------------
pushad
mov ecx,bufSize ; loop counter
mov esi,0 ; index 0 in buffer
L1:
xor buffer[esi],KEY ; translate a byte
inc esi ; point to next byte
loop L1
popad
ret
TranslateBuffer ENDP
END main

created 3 years ago

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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.

About Assembly

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.

Syntax help

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.

Variables

There are various define directives to allocate space for variables for both initialized and uninitialized data.

1. To allocate storage space to Initialized data

Syntax

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

2. To allocate storage space to un-initialized data

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

Constants can be defined using

1. equ

To define numeric constants

CONSTANT_NAME EQU regular-exp or value

2. %assign

To define numeric constants.

%assign constant_name value

3. %define

To define numeric or string constants.

%define constant_name value

Loops

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.

Procedures

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