ROT13 Cipher

A stupidly simple reversible substitution cipher.

ROT13 was in use in the net.jokes newsgroup by the early 1980s. It is used to hide potentially offensive jokes, or to obscure an answer to a puzzle or other spoiler. - Wikipedia

How it works

Encoding

The ROT13 cipher will substitute each letter by the letter coming 13 places after it in the alphabet. According to this logic, N should map to the 27th character of the alphabet, but we only have 26 characters, so we loop the alphabet around. We now have the formula ROT13(c) = (c + 13) % 26, where c is the position in the alphabet of a given character.

Therefore, A => N, B => O, ..., M => Z, N => A

When encoding a string, we replace uppercase characters with their uppercase mirror, lowercase character with their lowercase mirror, and leave all other characters untouched.

To help us with the encoding process, we create a lookup table like this:

A	B	C	D	E	F	G	H	I	J	K	L	M
N	O	P	Q	R	S	T	U	V	W	X	Y	Z

N	O	P	Q	R	S	T	U	V	W	X	Y	Z
A	B	C	D	E	F	G	H	I	J	K	L	M

Example:

Let's encode the string Hello World!. We need to lookup each character in our table, and then write down its image.

H => U
e => r
l => y
...
_ => _ # space maps to space.
W => J
...
! => !

Our final encoded string is Urryb Jbeyq!.

Decoding

As you might have noticed from the lookup table, ROT13(ROT13(c)) = c. This cipher is therefore reversible, and to decode, we just need to go through the whole encoding process once more.

Example:

Our encoded output was Urryb Jbeyq!. We need to lookup each character in our table, and then write down its image.

U => H
r => e
y => l
...
_ => _ # space maps to space.
J => W
...
! => !

Our final decoded string is Hello World!.

Cons

• Frequency analysis can be used to decrypt the message as each letter is encrypted with the same letter every time.
• Very well known cipher, easy to recognize, easy to crack

Implementations

Language	Encrypt	Decrypt
Javascript	encrypt.js	decrypt.js
Python	encrypt.py	decrypt.py
Swift	lib.swift	lib.swift
C#	ROT13.cs	ROT13.cs

C# code needs fixing, please do not use.

Running the tests

Tests are automatically handled by Travis CI.

Contributing

Please read CONTRIBUTING.md for details on our code of conduct, and the process for submitting pull requests to us.

Versioning

We use SemVer for versioning. For the versions available, see the tags on this repository.

Authors

Made with ❤️ at CrypTools.

See also the list of contributors who participated in this project.

License

This project is licensed under the MIT License - see the LICENSE file for details