Rot13 encoder

An encoder/decoder for Rot13 written in Javascript with no dependencies

Continuing my theme of zero-dependency JS, I've written a very quick Rot13 encoder. This time, it has a test system, but even that is not a dependency, although it uses almost the same syntax as Mocha, just for familiarity.

What is Rot13

Rot13, or Rotate-by-13, is a nigh-on useless cipher used a long time ago when cryptography wasn't very advanced. It's not hard to break by using methods like:

• Letter frequency analysis
• Looking at punctuation to help form an idea of input (it doesn't encode anything but letters)
• Substituting letters for others until it just works - brute forcing

Basically, it sucks as a cipher, but it's ok for things like encoding the punchline to a joke on a forum to stop it being ruined by people looking at the punchline first, or encoding spoilers so that people can choose not to see them. It's just a bit of fun.

Rot13 works by offsetting the alphabet by 13 characters:

A	B	C	D	etc.
N	O	P	Q	etc.

The effect of which is two-fold:

• Each letter has another that it becomes when encoded
• Because we offset by 13, and the alphabet is 26 characters (or 2 * 13), encoding and decoding are the same action

Which is just a bit nifty, even if it is a useless cipher overall.

Usage

You need Node v10 or above installed and in your path.

CLI

The CLI has a hashbang, so should run as given on any system that has Node of a suitable version installed, POSIX environment needed though.

$ ./rot13 encode this
rapbqr guvf

Usage as a library

The file lib/rot13.js is a Node style module that you can include:

const { code } = require('path/to/rot13.js');

const codedMessage = code('message');

Tests

The repo has tests, which were basically the reason for writing the repo in the first place (to write a standalone basic unit test system that requires no dependencies.)

$ ./tests.js
Starting test
#code
  should encode a message correctly
    Success
  should decode the encoded message by the same process
    Success
  Success

The tests run by using the following functions:

• describe - Takes a title, and a predicate function, and runs the predicate function, keeping the output indented in nicely formatted blocks. Multiple describe blocks can be embedded.
• it - Syntactically identical to describe, but used to write tests that are more readable. E.g. it ('should work', testThatItWorks)
• assert - Takes a Boolean expression and a message. Throws the message if the boolean is false.

Uncompressed, the test functions are 390 bytes. The assert function would obviously struggle with complicated assertions, like comparing objects or arrays, but to be fair the assert function in mocha does too. chai does a good job of assertion though.