vigenere encryption probleme with 'spaces' on python

Question

i can t pinpoint the reason why the encryption goes wrong after 'spaces' here is the code:

def chiffre_vigenere(message,key):
        message = message.lower()
        key = key.lower()
        encrypted = []
        d = dict(a=0,b=1,c=2,d=3,e=4,f=5,g=6,h=7,i=8,j=9,k=10,l=11,m=12,n=13,o=14,p=15,q=16,r=17,s=18,t=19,u=20,v=21,w=22,x=23,y=24,z=25)
        long_key = key
        while len(message) > len(long_key) :
                long_key = long_key + key
        i=0
        while i < len(message) :
                decalage = d.get(long_key[i])
                if message[i].isalpha() :            
                        c = ord(message[i]) + decalage
                        if c > 122 :
                                c = c - 26
                        encrypted.append(chr(c))
                else :
                        encrypted.append(message[i])       
                i = i+1
        print(listToString(encrypted))

for example when i do: chiffre_vigenere('stack overflow','apple') it gives: sipno dkpvfadh the first word is encrypted right but after the space the encryption is wrong any help is appreciated thanks in advance

Answer 1

The problem is that you're using the same counter variable i to mark your place in two different strings: message and long_key . Whenever you encounter a non-alphabet character in message , you're stepping to the next character, but you're also stepping to the next character in long_key , which is not what you want to do.

If you just use i to count characters in long_key then the problem goes away. For example:

def chiffre_vigenere(message,key):
    message = message.lower()
    key = key.lower()
    encrypted = []
    d = dict(a=0,b=1,c=2,d=3,e=4,f=5,g=6,h=7,i=8,j=9,k=10,l=11,m=12,n=13,o=14,p=15,q=16,r=17,s=18,t=19,u=20,v=21,w=22,x=23,y=24,z=25)
    long_key = key
    while len(message) > len(long_key):
        long_key = long_key + key
    i=0
    for ch in message:
        decalage = d.get(long_key[i])
        if ch.isalpha() :        
            c = ord(ch) + decalage
            if c > 122 :
                c = c - 26
            encrypted.append(chr(c))
            i = i+1
        else :
            encrypted.append(ch)
    print(listToString(encrypted))

You might also consider simplifying your code a little. You can use the ord function to convert ASCII values to integers, and a modulus operator to cycle through the values of key without having to expand it to the size of the input message.

def chiffre_vigenere(message,key):
    key = key.lower()
    encrypted = ''
    i = 0
    for ch in message.lower():
        if 'a' <= ch <= 'z':
            a = ord(ch) + ord(key[i]) - ord('a')
            if a > ord('z'):
                a -= 26
            encrypted += chr(a)
            i = (i + 1) % len(key)
        else:
            encrypted += ch
    print(encrypted)