Vigenere Cipher program in Python using mod 37

Question

I'm trying to write a Vigenere Cipher program in Python using mod 37. I need help figuring out what the issue is.

alphabet= "abcdefghijklmnopqrstuvwxyz0123456789 "

def Let2Ind(x):
        return(ord(x)-ord("a"))

def Ind2Let(n):
    return(alphabet[n])

def encVigenere(key, plaintext):
    ciphertext=""
    L=len(key)
    for i in range(len(plaintext)):
        A=Let2Ind(key[i%L])
        B=Let2Ind(plaintext[i])
        C=(A+B)%37
        D=Ind2Let(C)
        ciphertext= ciphertext+D
    return(ciphertext)

def decVigenere(key, ciphertext):
    plaintext=""
    L=len(key)
    for i in range(len(ciphertext)):
        E=Let2Ind(key[i%L])
        F=Let2Ind(ciphertext[i])
        G=(F-E)%37
        H=Ind2Let(G)
        plaintext= plaintext+H
    return(plaintext)

Answer 1

One problem is that your Let2Ind() code does not handle digits or space properly. It will return a negative number for the digits (-49 or thereabouts for 0), and for space (-65).

You probably need something like:

def Let2Ind(x):
    return alphabet.index(x)