Ascii Vigenere Cipher Not Decrypting Properly

My Vigenere cipher program has all come down to two lists. One a list of ASCII numbers which represent the characters of the message to be encrypted/decrypted and the other is a li

Solution 1:

The problem is in the statement (x - y) % 26 because mod of a number is defined from 0 to m-1, in the above case 0 to 25, but while subtracting you get negative numbers, so in order to get the correct result you should do this (x - y + 26) % 26. If the negative value we get by (x-y) is lower than "-m" in the above case lower than -26, then the no still remains negative, then you have to make it positive, here is the pseudo code for that :

val = (x - y) % 26
while(val < 0) val += 26
val = val % 26