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    Simple Implementation of Caesar Cipher in Python

    What is the Caesar Cipher?

    Do you want to implement the Caesar Cipher in Python using functions and for-loops?

    But wait, we will also describe what the Caesar Cipher is – this will help you understand how to solve it with functions.

    Caesar Cipher
    • The Message Sender wants to send a message to the Message Receiver. Here an Emperor wants to send a message to his general out in the field.
    • A Messenger will take the encrypted message (the cipher) and transport it to the general (Message Receiver).
    • If the Enemy catches Messenger, then they do not understand the encrypted message (the cipher), and even better, then Messenger does not either.
    • When the encrypted message (the cipher) arrives at the general (Message Receiver) then they are the only ones that can decipher and understand the message.

    In the video, I will walk you through all you need including how to implement it (Part 7).

    Get an explanation of the code in this video.

    Step 1: Describing the Caesar Cipher

    Caesar Cipher Encryption

    The explanation of the encryption with the Caesar Cipher.

    • Step 1: First, you need to use an encryption key (a value from 0-25). In the case above, key 3 is used. You rotate the alphabet the number of times the key says. That is, you take the first letter and put it in the back of the alphabet, repeating that the number of times the key says.
    • Step 2: Then you take the first letter of the message you want to encrypt. Let’s say you want to encrypt YOU ARE AWESOME. Then you have the original alphabet below the rotated one. Find the letter Y and see what letter is above (here B). That is, Y is encrypted to B.
    • Step 3: Repeat Step 2 until you have the encrypted message. You should end up with the message BRX DUH DZHVRPH.
    Caesar Cipher Decryption

    You should see some similarities between encryption and decryption. The difference is how the encryption/decryption arrow points in Step 2.

    If we take the encrypted message (the cipher) BRX DUH DZHVRPH you should end up with the original message YOU ARE AWESOME.

    Step 2: Implement the encryption function

    Let’s implement a helper function encrypt_char(char, key) to encrypt a single character, char, with key. What this encrypt function does, is it makes the process above for us. Hence, we have this function to do the process of encrypting one letter.

    def encrypt_char(char, key):
        return chr(ord('A') + (ord(char) - ord('A') + key) % 26)
    
    Caesar cipher encryption breakdown

    Then we can implement encrypt_message(message, key), which uses the helper function. Hence, this function below is the process of encrypting a whole message.

    def encrypt_message(message, key):
        message = message.upper()
        cipher = ''
        for char in message:
            if char not in ' ,.':
                cipher += encrypt_char(char, key)
            else:
                cipher += char
        return cipher
    

    Let’s try it.

    encrypt_message("you are awesome.", 3)
    

    Which results in BRX DUH DZHVRPH.

    Step 3: Implement the decryption function

    This is quite similar.

    def decrypt_char(char, key):
        return chr(ord('A') + (ord(char) - ord('A') + 26 - key) % 26)
    def decrypt_message(cipher, key):
        cipher = cipher.upper()
        message = ''
        for char in cipher:
            if char not in ' ,.':
                message += decrypt_char(char, key)
            else:
                message += char
        return message
    

    Let’s try it.

    decrypt_message('BRX DUH DZHVRPH.', 3)
    

    Then you get YOU ARE AWESOME.

    Bonus: Hacking the Caesar Cipher

    There are only 26 possible keys, which makes a brute-force attack easy to make.

    Actually, we will re-use most of the code.

    def decrypt_char(char, key):
        return chr(ord('A') + (ord(char) - ord('A') + 26 - key) % 26)
    def decrypt_message(cipher, key):
        cipher = cipher.upper()
        message = ''
        for char in cipher:
            if char not in ' ,.':
                message += decrypt_char(char, key)
            else:
                message += char
        return message
    def hacking(cipher):
        for key in range(26):
            message = decrypt_message(cipher, key)
            print(key, message)
    

    Let’s try it.

    hacking("BRX DUH DZHVRPH")
    

    Which will give the following output.

    0 BRX DUH DZHVRPH
    1 AQW CTG CYGUQOG
    2 ZPV BSF BXFTPNF
    3 YOU ARE AWESOME
    4 XNT ZQD ZVDRNLD
    5 WMS YPC YUCQMKC
    6 VLR XOB XTBPLJB
    7 UKQ WNA WSAOKIA
    8 TJP VMZ VRZNJHZ
    9 SIO ULY UQYMIGY
    10 RHN TKX TPXLHFX
    11 QGM SJW SOWKGEW
    12 PFL RIV RNVJFDV
    13 OEK QHU QMUIECU
    14 NDJ PGT PLTHDBT
    15 MCI OFS OKSGCAS
    16 LBH NER NJRFBZR
    17 KAG MDQ MIQEAYQ
    18 JZF LCP LHPDZXP
    19 IYE KBO KGOCYWO
    20 HXD JAN JFNBXVN
    21 GWC IZM IEMAWUM
    22 FVB HYL HDLZVTL
    23 EUA GXK GCKYUSK
    24 DTZ FWJ FBJXTRJ
    25 CSY EVI EAIWSQI
    

    And we see the decrypted message at key 3, as expected.

    Note: The Caesar Cipher was “secure” back at the time of Caesar

    Back when the Caesar Cipher was used the theory of security was not evolved that far. And the security of the Caesar Cipher lies in the fact that the enemy (the attacker) does not know the encryption algorithm. That is, the enemy cannot make the above brute force attack, as she doesn’t know how the encryption or decryption is made.

    Therefore you can say that at the time of Caesar the Caesar Cipher was secure.

    What next?

    In the next lesson you will learn Python Dictionaries for Frequency Count

    If this is something you like and you want to get started with Python, then this is part of an 8 hours FREE video course with full explanations, projects on each level, and guided solutions.

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    • 17 video lessons teaching you everything you need to know to get started with Python.
    • 34 Jupyter Notebooks with lesson code and projects.
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    See the full FREE course page here.

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