Closely related key

Imagine you have a secret book with all your secrets written down in it. This book is locked with a special key that only you have. Now, let's say you want to give your best friend access to some of your secrets but not all of them. Instead of giving them your special key that unlocks everything, you make a copy of the key that only works for the pages with the secrets you want to share. This copy is called a closely related key.

In cryptography (which is like secret code-making and code-breaking), a closely related key is a key that is very similar to another key. It's like having two keys that look almost exactly the same but only one will unlock a specific door. This is useful because it allows someone to access only certain information without giving them access to everything. It's like giving your friend the key to your diary, but only for the pages that talk about your crush, not the whole thing.