BLKSZ = 5

ALPHSZ = 26

def modInverse(a, ALPHSZ):

a = a % ALPHSZ

for x in range(1, ALPHSZ):

if ((a * x) % ALPHSZ == 1):

return x

return 1

def translate(text, a, b, mode):

inv = modInverse(a, ALPHSZ)

if inv == 1:

raise ValueError("a and alphabet size must be coprime.")

chars = []

for c in text:

if c.isalnum():

orig = ord(c.lower()) - 97

if orig < 0:

chars.append(c)

continue

if mode == 0:

new = (a * orig + b) % ALPHSZ

elif mode == 1:

new = (inv * (orig - b)) % ALPHSZ

chars.append(chr(new + 97))

return ''.join(chars)

def encode(plain, a, b):

cipher = translate(plain, a, b, 0)

return " ".join([cipher[i:i + BLKSZ]

for i in range(0, len(cipher), BLKSZ)])

def decode(ciphered, a, b):

return translate(ciphered, a, b, 1)