S-box Facts for Kids

In secret coding, an S-box (which stands for substitution-box) is a key part of secret codes that use the same key to lock and unlock messages. It performs a special kind of mixing called "substitution." In block codes, S-boxes are used to hide how the secret key changes the coded message. This helps make the code very confusing and hard to guess, a concept known as confusion. Think of it as a clever way to scramble information.

An S-box takes a certain number of input bits (let's say m bits) and changes them into a different number of output bits (let's say n bits). The number of input bits doesn't have to be the same as the number of output bits. An m×n S-box can be like a lookup table (a special chart) that has 2^m entries, each with n bits. Sometimes these tables are fixed and always the same, like in the Data Encryption Standard (DES). But in other codes, like Blowfish and Twofish, the tables are created new each time using the secret key.

How an S-box Works

Imagine an S-box as a mini-puzzle solver. It takes a small piece of your secret message (a few bits) and changes it into a completely different set of bits. This change isn't simple; it's designed to be very mixed up, so someone trying to break the code can't easily figure out the original message or the secret key.

Example: DES S-box

A great example of a fixed S-box is the S-box from the old DES (Data Encryption Standard) code. Let's look at S₅. This S-box takes 6 bits as input and gives you 4 bits as output.

S₅		Middle 4 bits of input
S₅		0000	0001	0010	0011	0100	0101	0110	0111	1000	1001	1010	1011	1100	1101	1110	1111
Outer bits	00	0010	1100	0100	0001	0111	1010	1011	0110	1000	0101	0011	1111	1101	0000	1110	1001
	01	1110	1011	0010	1100	0100	0111	1101	0001	0101	0000	1111	1010	0011	1001	1000	0110
	10	0100	0010	0001	1011	1010	1101	0111	1000	1111	1001	1100	0101	0110	0011	0000	1110
	11	1011	1000	1100	0111	0001	1110	0010	1101	0110	1111	0000	1001	1010	0100	0101	0011

To use this table, you take your 6-bit input. The first and last bits tell you which row to look at. The four bits in the middle tell you which column to pick.

For example, if your input is "011011":

The outer bits are "01". This means you look at the second row (the one highlighted in light green).
The inner bits are "1101". This means you look at the column highlighted in light orange.
Where the "01" row and "1101" column meet, you find the output: "1001".

So, the S-box changes "011011" into "1001". This simple step helps make the entire code much more secure!

S-box facts for kids

How an S-box Works

Example: DES S-box

See also