The Vigenère cipher

vigenre.py

#!/usr/bin/python

def chartable():
    """
    Define universe, i.e the character table available for encryption and decryption
    """
    universe = " " # space character
    universe += "0123456789" # numbers
    universe += "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz" # letters
    universe += "!#$%&'()*+,-./:;<=>?@[\]^_`{|}" # special characters
    return universe

def encrypt(msg, key, universe):
    """
    Encrypt a message using a "secret" key
    """
    # Initialize encrypted message
    enmsg = ""
    # Initialize key index
    j = 0
    # Loop over characters in input message to encrypt
    for i in range(0, len(msg)):
        # If the character is in the universe, it is replaced
        if msg[i] in universe:
            # Get the index of the message char `i` in universe
            ichrmsg = universe.index(msg[i])
            # Get the index of the key char `j` in universe
            ichrkey = universe.index(key[j%len(key)])
            # Calculate the index of the encrypted message char
            ichrencrypt = ( ichrmsg + ichrkey) % len(universe)
            # Add the new char in the encrypted message
            enmsg += universe[ichrencrypt]
            # Increment key index by 1
            j += 1
        # If the character is not in the universe, it is not replaced
        else:
            # Add the unchanged char in the encrypted message
            enmsg += msg[i]
    
    return enmsg
    
def decrypt(msg, key, universe):
    """
    Decrypt a message using a "secret" key
    """
    # Initialize decrypted message
    demsg = ""
    # Initialize key index
    j = 0
    # Loop over characters in input message to decrypt
    for i in range(0, len(msg)):
        # If the character is in the universe, it is replaced
        if msg[i] in universe:
            # Get the index of the message char `i` in universe
            ichrmsg = universe.index(msg[i])
            # Get the index of the key char `j` in universe
            ichrkey = universe.index(key[j%len(key)])
            # Calculate the index of the decrypted message char
            ichrencrypt = ( ichrmsg - ichrkey) % len(universe)
            # Add the new char in the decrypted message
            demsg += universe[ichrencrypt]
            # Increment key index by 1
            j += 1
        # If the character is not in the universe, it is not replaced
        else:
            # Add the unchanged char in the decrypted message
            demsg += msg[i]
    
    return demsg
  
if __name__ == "__main__":
    # Initialize character table
    universe = chartable()
    # Message to encrypt
    msg = "Happy New Year!"
    key = "The Dude"
    # Print message and key in terminal
    print(msg+' | '+key)
    # Encrypt message
    enmsg = encrypt(msg, key, universe)
    # Print encrypted message
    print('encrypt:: ' + enmsg)
    # Decrypt message
    demsg = decrypt(enmsg, key, universe)
    # Print decrypted message
    print('decrypt:: ' + demsg)