ROT13

The ROT13 algorithm obscures text—it does not encrypt the text. The algorithm shifts each character back, or forward, 13 places.

Cipher code

ROT13 is a cipher algorithm that can deter unwanted examination. We implement it with Python—several Python language features are needed.

Example

We define a method called rot13(). We use the for-loop to iterate over the string characters. And then we call the ord() built-in function.

So We compare each integer representation against lowercase and uppercase letters. We subtract or add 13 to shift characters.

And We append to the result string the character representation of the integer. We call chr() to do this.

def rot13(s):
    result = ""

    # Loop over characters.
    for v in s:
        # Convert to number with ord.
        c = ord(v)

        # Shift number back or forward.
        if c >= ord('a') and c <= ord('z'):
            if c > ord('m'):
                c -= 13
            else:
                c += 13
        elif c >= ord('A') and c <= ord('Z'):
            if c > ord('M'):
                c -= 13
            else:
                c += 13

        # Append to result.
        result += chr(c)

    # Return transformation.
    return result

# Test method.
print(rot13("gandalf"))
print(rot13(rot13("gandalf")))tnaqnys
gandalf

Notes, tests

We test to make sure the rot13() method is correct. When we call rot13() on the result of rot13(), we again have our original string value.

Thus The rot13() algorithm round-trips its data correctly—the original data is not lost. This is correct.

Discussion

The ROT13 algorithm is an important one to implement. It is a handy way to add obscurity—text that is not easily readable. This is useful in certain situations.

Also The algorithm helps teach us how to treat characters as numbers. In Python we do this with ord() and chr(), two built-in functions.

Detail A faster implementation is possible using the translate method. We benchmark and implement one.

The ROT13 algorithm is well-known and can help us learn how to use core Python features. Ciphers, such as ROT13, are occasionally useful.