caesar.py

#!/usr/bin/env python

from __future__ import unicode_literals


def check_cipher(plaintexts):
    winner = None
    winner_count = 0
    winner_shift = 0
    for plaintext, shift in plaintexts:
        e_count = 0
        for p in plaintext:
            if p == 'e':
                e_count += 1

        if e_count > winner_count:
            winner = plaintext
            winner_count = e_count
            winner_shift = shift

    print 'Plaintext: {}'.format(winner)
    print 'Shift: {}'.format(winner_shift)


def decrypt_caesar(ciphertext, shift):
    """return a decrypted version of a Caesar cipher encrypted ciphertext"""
    shift %= 26
    plaintext = []
    for c in ciphertext.lower():
        if c.isalpha():
            p = ord(c) + shift
            if p > ord('z'):
                p -= 26
            plaintext.append(chr(p))
        else:
            plaintext.append(c)
    return ''.join(plaintext)


def brute_caesar(cipher_encrypt):
    plaintexts = []
    for i in range(1, 26):
        plaintexts.append((decrypt_caesar(cipher_encrypt, i), i))
    return plaintexts


def main():
    cipher_encrypt = raw_input("Please enter encrypted text: ")
    shift = raw_input("Enter shift amount. Enter \"all\" for brute force: ")

    if shift == "all":
        possible_plaintexts = brute_caesar(cipher_encrypt)
        check_cipher(possible_plaintexts)
    else:
        print decrypt_caesar(cipher_encrypt, int(shift))


if __name__ == '__main__':
    main()