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; Encryption Program (Encrypt.asm)

; This program demonstrates simple symmetric ; encryption using the XOR instruction.

INCLUDE Irvine32.inc 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 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

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