[sroussey]

No, "Security through obscurity" is a valid and useful layer. A lot of weight hangs on your word “depends” though, in which case if it is the only layer then you will eventually have, uh, problems.

I’ve used it for a long long time. Like in 1999 I’d have a knock on certain ports in a certain order to unlock the ssh port.

And lots of weird stuff to stop forum spam. Which could work for weeks or months or even a year.

[pdpi]

Port knocking isn't security through obscurity. Given the knowledge that you have a port knocking system in place doesn't tell me what specific sequence of knocks will open up the service I want to target. Even just a two knock sequence gives you a key with 32 bits of entropy, which makes it trivial to block attempts at bruteforcing the key.

[ZoomZoomZoom]

I don't see how your argument makes sense. It's all just bits of entropy in the end, be it knowing a port to connect to or a character in your key.

[pdpi]

Yeah absolutely. That was precisely my point — Requiring a secret (be it a password or the private part of an asymmetric key) isn't security through obscurity, and finding the sequence of knocks is equivalent to finding a password of equivalent complexity.