Vigenere cipher

vgnr.py

def vigenere(msg, key, crypt_flag=True):
    """
        vigenere cipher for small latin letters
        crypt_flag == True mean crypt
        crypt_flag == False mean decrypt
    """
    msg, key = list(map(ord, msg)), list(map(ord, key))
    msglen, keylen = len(msg), len(key)
    new_msg = ''
    for i in range(msglen):
        offset = key[i % keylen] - 97
        old_chr = msg[i] - 97
        new_chr = (old_chr + (offset if crypt else -offset)) % 26
        new_msg += chr(97 + new_chr)
    return new_msg