[kkl]

I certainly agree with you. I just think the "don't roll your own crypto" advice is overly ambiguous. Ironically, I think my original comment was ambiguous as well. Let me clarify. I'm not endorsing rolling your own cryptosystem (e.g. a replacement for RSA). Rather, I think the advice should often be paired with additional insight on what "rolling your own" means. When building some sort of software, not everyone (currently) has the luxury of a cryptographic library that handles everything painlessly.

For example, I think most would say that I'm not "rolling my own crypto" if I'm implementing some piece of functionality in my application leveraging the use of some API with "mac", "encrypt", and "decrypt" functions. There are still ways I can screw up using these functions, but I'm arguably not "rolling my own" crypto. So in this situation, the mantra is confusing at best.

[JoachimSchipper]

Maybe; but handing a MAC to generalist developers is going to end in a timing attack, like http://rdist.root.org/2009/05/28/timing-attack-in-google-key.... (And Keyczar is not amateur hour!)

[rosser]

I don't know anyone who has ever taken "don't roll your own crypto" to mean "don't write things that use crypto".