Shit, you're right. (PBKDF2 generates arbitrary-length output, which is what you want from a key derivation function, but maybe not something you care about for a password hash).

Here's a version that's much easier to read than the spec:

https://github.com/emerose/pbkdf2-ruby/blob/master/lib/pbkdf...

I should stop saying PBKDF2 and just go back to saying PBKDF.

PBKDF2 also protects against entropy loss from repeated iteration, although that doesn't really matter unless you're using an unreasonably short hash.