Kerckhoffs' Principle (19th c.)

The security of a cryptosystem should not rely on a secret that's hard (or expensive) to change.

So don't have secret encryption methods.
Then what do we do?

Have a large class of encryption methods, instead.
Hopefully, they're all equally strong.

Make the class public information
Use a secret key to specify which one you're using
It's easy to change the key; it's usually just a smallish number.