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You could do that with something called a Zero-knowledge password proof[1], of which SRP[2] is an example. As far as I know, SRP is patented. It is extremely clever, though.

[1] http://en.wikipedia.org/wiki/Zero-knowledge_password_proof [2] http://en.wikipedia.org/wiki/Secure_remote_password_protocol

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To the best of my knowledge the SRP patents are not a big deal, and SRP is used in a number of commercial products. The bigger problem with SRP is that it's dangerous; it is difficult to screw bcrypt up, but it's very easy to screw SRP up; if you have to implement it yourself, dollars/donuts you're going to end up with an implementation that allows me to log in without a password.

I agree. I should just clarify that I'm all for bcrypt, having used it and recommended to friends/clients/employers. When I found out about SRP, I was researching on ways to do exactly what tzs asked, i.e., verifying a password without sending the plaintext (or plaintext-equivalent). In my case, the client had a login form that couldn't be placed behind SSL (don't ask why). Do you know of any solutions to this problem?

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