[Sylos]

"Security by obscurity" tries to keep the way that your encryption method works obscure, it does not try to keep a specific key obscure.

For example, if your way to encrypt works like this:

1) Shift all letters along by 5.

2) Cut out every second word and put them behind the message in order.

3) Whenever there's an f, s or y in a word, double up that word and shift the second word's letters by 7.

Then if your enemy figures out how your method works, you have to come up with a completely different method.

The opposite to security by obscurity would instead once come up with a method that entirely depends on a key. You can then publicize that method (or not), and if your enemy finds out your key, you just choose a new key and you're fine again.