Caesar cipher program in Python

A Caesar cipher replaces each letter with the letter a fixed number of places further along the alphabet. With a shift of 3, A becomes D, B becomes E, and at the end of the alphabet X, Y and Z go back to A, B and C.

The program

def caesar(text, shift):
    result = ""
    for ch in text:
        if "a" <= ch <= "z":
            start = ord("a")
        elif "A" <= ch <= "Z":
            start = ord("A")
        else:
            result += ch  # spaces, digits and punctuation stay the same
            continue
        position = ord(ch) - start
        result += chr(start + (position + shift) % 26)
    return result


message = input("Message: ")
shift = int(input("Shift: "))

secret = caesar(message, shift)
print("Encrypted:", secret)
print("Decrypted:", caesar(secret, -shift))

Typed in this example: Meet me at the Zoo at noon!, 3. Press Run to type your own.

Message: Meet me at the Zoo at noon!
Shift: 3
Encrypted: Phhw ph dw wkh Crr dw qrrq!
Decrypted: Meet me at the Zoo at noon!

How it works

• ord() gives the number for a character, and chr() turns a number back into a character. ord("a") is 97 and ord("d") is 100.
• position is where the letter is in the alphabet, from 0 for "a" to 25 for "z".
• % 26 makes the alphabet go round in a circle. "Z" is position 25, plus 3 is 28, and 28 % 26 is 2, which is "C".
• Decrypting is the same as encrypting with -shift. In Python, % with a positive number on the right never gives a negative answer: (0 - 3) % 26 is 23, so "a" goes back to "x".

The program checks for the 26 English letters only. ch.isalpha() would also be True for letters such as "é", which are not in the a to z range and would be changed into the wrong characters.

Try changing it

A shift of 13 is called ROT13. Because 13 + 13 = 26, using the same shift twice gives back the original text:

def caesar(text, shift):
    result = ""
    for ch in text:
        if "a" <= ch <= "z":
            start = ord("a")
        elif "A" <= ch <= "Z":
            start = ord("A")
        else:
            result += ch
            continue
        result += chr(start + (ord(ch) - start + shift) % 26)
    return result


once = caesar("Hello, Grace", 13)
print(once)
print(caesar(once, 13))

Uryyb, Tenpr
Hello, Grace

A Caesar cipher is easy to break, because there are only 25 useful shifts to try. Use it to learn about characters and %, not to protect real data.