Question

Creating a Vigenère Cipher Encryption/Decryption in Assembly Language NASM/YASM

With these instructions so far we created some variables for the data and the outputs for them but are very confused on how to integrate the cipher portion so help would be appreciated.

As seen in the picture below we are supposed to use a Vigenere cipher which uses key and plaintext variables in nasm/yasm assembly. The key is extended to the size of the text and then for each letter the corresponding shift is due to the letter coinciding in the key variable (ex. O - A in ascii values equals 15, so the shift of the first character would increase by 15). The shifted letters are stored in a new variable and outputed once done. We know how to do this in python/c++ but are lacking the knowledge to understand/start it in assembly.

Our code is below:

Here we are asking the user for inputs for the plaintext and key. W do not know how to apply string concepts to convert each letter in the encryption.

Answer 1

begin:

mov ax,data

mov ds,ax

mov ax,stack

mov ss,ax

mov sp,stacktop

%define encoderkey 'sample'

%strlen encryption_key encoderkey

main:

         mov dx,detail1

         mov ah,9

         int 0x21

         mov dx,detail2

         mov ah,9

         int 0x21

         mov dx,blankline

         mov ah,9

         int 0x21

         mov dx,text1

         mov ah,9

         int 0x21

         mov dx,text2

         mov ah,9

         int 0x21

         mov dx,blankline

         mov ah,9

         int 0x21

select:

         mov ah,00

         int 16h

         mov dl,al

         cmp al, '1'          

         jz encryption

         cmp al,'2'

         jz decryption

encryption:

         mov dx,message3

         mov ah,9

         int 0x21

         mov dx,blankline

         mov ah,9

         int 0x21

main_encryption:

         mov ah,00

         int 16h

         mov dl,al

         cmp al, 'q'          

         jz done

         mov ah,encryption_key

         add dl, ah

         mov ah,2

         int 0x21

jmp main_encryption

decryption:

         mov dx,message4

         mov ah,9

         int 0x21

         mov dx,blankline

         mov ah,9

         int 0x21

main_decryption:

         mov ah,00

         int 16h

         mov dl,al

         cmp al, 'q'          

         jz done

         mov ah,-encryption_key

         add dl, ah

         mov ah,2

         int 0x21

jmp main_decryption

done:

         mov ax,0x4c00

         int 0x21