double affine encryptor

doubleaffine.py

ALPHABET = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"

def encrypt(p,k):
    a = k[0]
    b = k[1]
    if len(p)%2 == 1:
        p += "X"
    c = ""
    for i in range(len(p)//2):
        value = ALPHABET.index(p[2*i])*26 + ALPHABET.index(p[2*i+1])
        value = (a*value + b)%(26*26)
        c += ALPHABET[value//26] + ALPHABET[value%26]
    return c

def decrypt(c,k):
    a = k[0]
    b = k[1]
    ainv = modular_inverse(a,26*26)
    p = ""
    for i in range(len(c)//2):
        value = ALPHABET.index(c[2*i])*26 + ALPHABET.index(c[2*i+1])
        value = (ainv*(value - b))%(26*26)
        p += ALPHABET[value//26] + ALPHABET[value%26]
    return p