You didn't give enough information. What OS are you using? In particular, what browser are you using? What exactly are trying to do?

I'll assume that you use digest authentication. There are some things that you need to be aware of. First, it's still somewhat experimental. Second, it's more secure than basic authentication, but not all browsers support it. Amaya, Kongueror, MSIE for Mac OX, and Windows IE (but that can fail with GET requests with query strings) support it. Mozilla, Netscape 7, Opera, Safari, and lynx do not support it. In other words, you should only use it in an environment where all users have a supporting browser.

As to what the error is, htdigest -b should probably be htdigest -c. I couldn't find anything in the docs for -b.