Python-сообщество

 tabula_recta = ["".join([chr(x) for x in range(97 + y, 123 + y) if x < 123] + [chr(x - 123 + 97) for x in range(97 + y, 123 + y) if x >= 123]) for y in range(26)]
def norm_key(key, word):
    return key * (len(word) // len(key)) + key[:len(word) % len(key)]
def coder(word, key):
    return "".join([tabula_recta[ord(x) - 97][ord(y) - 97] for (i, x) in enumerate(word) for (n, y) in enumerate(norm_key(key, word)) if n == i])
def decoder(word, key):
    return "".join([chr(tabula_recta[ord(x) - 97].index(y) + 97) for (i, x) in enumerate(norm_key(key, word)) for (n, y) in enumerate(word) if n == i])