Encryption.asm

TITLE Encryption Program (Encrypt.asm)
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 ; 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