For years, it’s been the conventional wisdom in userid/password based authentication systems that the system provide the same error message for an invalid userid and/or invalid password. The idea being that you don’t want to let on to the “bad guys” that they guessed a valid userid and then proceed to repeatedly try passwords with the valid userid.
I watch the logs. And it turns out that while I’d really, really love for people to learn how to remember not one, but a combination of two, not-random strings, they often don’t. Or do, just the wrong combination for that particular tool.
userid/password authentication schemes are already bad from a security perspective — it’s not like obfuscating the result of mistyping a not-random string improves upon that much at all, and by golly, it should would save the users some time “did I mistype my password? did I forgot my password? WHAT DO YOU MEAN INVALID ID?”
So from now on, goodbye conventional wisdom, I’m actually going to start telling people in every uid/pass dialog I write which string they got wrong.