Substitution–permutation network

Imagine you have a big castle with a lot of rooms and you want to make sure nobody can steal anything from you. To do this, you need to create a secret code that only you and your trusted friends know.

A substitution-permutation network is like a secret code that you use to protect your castle. It's made up of two parts: substitution and permutation.

Substitution is when you take something and replace it with something else. For example, you might have a code where every letter of the alphabet is replaced with a different letter. So instead of writing "cat", you would write "dzu".

Permutation is when you take something and rearrange it. For example, you might have a code where you take the letters of a word and shuffle them around. So instead of writing "cat", you would mix up the letters and write "atc".

A substitution-permutation network combines these two techniques to make a very strong code. It first takes the message and substitutes certain parts of it with something else, and then it permutes it by shuffling it around. This makes it very difficult for anyone who intercepts the message to figure out what it says.

So when you want to protect your castle, you can use a substitution-permutation network to create a secret code that only you and your trusted friends know. That way, nobody can steal anything from you!