brute_force_caesar_cipher.py

"""
만약 우리에게 Caesar cipher 에 의해서 암호화된 단어 khoor이 주어졌고, 이를 풀어내야 한다.
하지만 우리는 key를 모른다. 따라서 암호를 해독하는데 가능한 가장 무식하고 쉬운 방법은 key로 
가능한 모든 수를 고려하여 암호를 해독해 보는 것이다. 이와같이 모든 경우의 수를 고려하는 것을 Brute force라 한다.
"""
from __future__ import print_function
def decrypt(message):
    """
    >>> decrypt('TMDETUX PMDVU')
    Decryption using Key #0: TMDETUX PMDVU
    Decryption using Key #1: SLCDSTW OLCUT
    Decryption using Key #2: RKBCRSV NKBTS
    Decryption using Key #3: QJABQRU MJASR
    Decryption using Key #4: PIZAPQT LIZRQ
    Decryption using Key #5: OHYZOPS KHYQP
    Decryption using Key #6: NGXYNOR JGXPO
    Decryption using Key #7: MFWXMNQ IFWON
    Decryption using Key #8: LEVWLMP HEVNM
    Decryption using Key #9: KDUVKLO GDUML
    Decryption using Key #10: JCTUJKN FCTLK
    Decryption using Key #11: IBSTIJM EBSKJ
    Decryption using Key #12: HARSHIL DARJI
    Decryption using Key #13: GZQRGHK CZQIH
    Decryption using Key #14: FYPQFGJ BYPHG
    Decryption using Key #15: EXOPEFI AXOGF
    Decryption using Key #16: DWNODEH ZWNFE
    Decryption using Key #17: CVMNCDG YVMED
    Decryption using Key #18: BULMBCF XULDC
    Decryption using Key #19: ATKLABE WTKCB
    Decryption using Key #20: ZSJKZAD VSJBA
    Decryption using Key #21: YRIJYZC URIAZ
    Decryption using Key #22: XQHIXYB TQHZY
    Decryption using Key #23: WPGHWXA SPGYX
    Decryption using Key #24: VOFGVWZ ROFXW
    Decryption using Key #25: UNEFUVY QNEWV
    """
    LETTERS = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
    for key in range(len(LETTERS)):
        translated = ""
        for symbol in message:
            if symbol in LETTERS:
                num = LETTERS.find(symbol)
                num = num - key
                if num < 0:
                    num = num + len(LETTERS)
                translated = translated + LETTERS[num]
            else:
                translated = translated + symbol
        print("Decryption using Key #%s: %s" % (key, translated))

def main():
    message = input("Encrypted message: ")
    message = message.upper()
    decrypt(message)

if __name__ == '__main__':
    import doctest
    doctest.testmod()
    main()