;Midterm Programming Question 5.asm
INCLUDE Irvine32.inc
;Cyril Farrar, CS310 System Programming, Midterm Programming Question5.asm
;---------------------------------------------------------------------------------------------------
;Program  Description: Write a program to demonstrate a simple symmetric encryption/decryption algorithm
;using the XOR instruction. Let the user enter a multi-byte encryption key consisting of multiple characters.
;Use this key to encrypt and decrypt the plaintext by XORing each character of the key against a corresponding byte in the message.
;Repeat the key as many times as necessary until all plain-text bytes are translated. Suppose, for example, the key equals “ABXmv#7”.
;This is how the key would align with the plain-text bytes:
;---------------------------------------------------------------------------------------------------
BUFMAX=128
KEYMAX=128
.data
strKey BYTE"Enter the key:",0
strText BYTE"Enter the plain text:",0
strEnc BYTE"The encrypted text is:",0
strDec BYTE"The decrypted text is:",0
buffer BYTE BUFMAX+1 DUP(0)
bufSize DWORD ?
key BYTE KEYMAX+1 DUP(0)
keySize DWORD ?
.code
main PROC
;Call procedures
call Clrscr	;Clears screen
call InputKey	;inputs key from the op
call InputPlainText	;inputs the plain text
call ConvertText
mov edx,OFFSET strEnc
call DisplayText	;display enc text
call ConvertText
mov edx,OFFSET strDEC
call DisplayText	;Displays dec text
exit
main ENDP

InputKey PROC USES ecx edx
;inputs the key from op
mov edx,OFFSET strKey
call WriteString	;prints strkey
call Crlf
mov ecx,KEYMAX	;max key length stored
mov edx,OFFSET key
call ReadString	;read to eax
mov keySize,eax; store input length
call Crlf
ret
InputKey ENDP

InputPlainText PROC USES ecx edx
;takes in plain text from user
mov edx,OFFSET strText
call WriteString	;prints strtext
call Crlf
mov ecx,BUFMAX
mov edx,OFFSET buffer
call ReadString
mov bufSize,eax
call Crlf
ret
InputPlainText ENDP

ConvertText PROC USES esi ecx edx
;encrypt plain text
mov ecx,bufSize
mov edx,0
mov esi,0
Label1:
xor buffer[esi],key[edx]		;converts the buf text (has an error on this line
;not letting the program finish. Stack over flow people stated it works for them
;I'm not sure how to fix error. People suggested changing the runtime
inc esi
inc edx
cmp edx,KeySize
je Label2		;jmp label2
jmp Label1
Label2:
mov edx,0		;index to 0
loop Label1
ret
ConvertText ENDP

DisplayText PROC USES edx
;display the enc and dec text
call WriteString
mov edx,OFFSET buffer		;print dec text
call WriteString
call Crlf
call Crlf
ret
DisplayText ENDP
END main