[OJFord]

I'm aware. But Kerckhoff's principle is just saying that your mechanism of encryption shouldn't be obscured. It doesn't change that I can't define 'obscure' in a way that doesn't make it a mechanism (in itself inobscure) of obscuring data.

Also, there are plenty of historical ciphers that fall foul of Kerckhoff, I don't think we can say retrospectively that they weren't done for security, and in many cases were probably totally adequate for some time, if not their lifespan.