Question

MASM Assembly language -- Message Encryption Pgm

You are to write a program to input a text and a key and encrypt the text. I will supply you an encryption key consisting of multiple characters. Use this key to encrypt and decrypt the plain text by XOR-ing 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 were equal to "ABXmv#7", you would then repeat the key down the text until the entire text was encrypted. This is how the key would align with the plain text bytes:

Plain Text: T h i s   i s   a   P l a i n t e x t   m e s s a g e .
Key:        A B X m v # 7 A B X m v # 7 A B X m v # 7 A B X m v # 7

Input file will be a Text line (one line) next input is the key. The file will have several sets of inputs.

Example   Line One text
   abcKEY
   The second line of text
   KEYzyz133*$
   ...

Output:       print out line of original text,
   the encrypted text
   the decrypted text.
   A space line or two
   next lines...

Input file:

This is the first line of text to encrypt so have at it and have fun.
AV)$'932jk
My name is Secret Agent and I'm a counter spy for a large organization, the Itty Bitty Tinny Space ship to Mars program.
(9^54~JHT<>"]}aqd
The quarterly report in-house and confidential is being released on April 15 after the sun flair passes, Venus and mars are snuggle buddies and Haley’s comet passed Jupiter.
KiKjva*)(82199{}|x
The 2017 thru 2022 future accounting target is $204,000,000 in cost projections, with #100,500,000 in expenses showing $103,500,000 profit. I think this will be good for us and bad for them.
KHJ99*&$-";lkj:B:1](t&
I wish to conclude that this message will self destruct in 20 seconds and will have an impact for the next 25 years, 157 days, 16.5 hours, 23 minutes and then start over. Keep this under raps.
8&45$:Aa~

Answer 1

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

    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

    pushad

    call    WriteString

    mov edx,OFFSET buffer   ; display the buffer

    call    WriteString

    call    Crlf

    call    Crlf

    popad

    ret

DisplayMessage ENDP

    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