[tomp]

How so?

A simple way to do this would be to send a unique, random salt S for every login, and the user would reply with e.g. sha1(password + S). However, to be able to check that the answer is correct, you would need to know the user's password, in plain text, which brings you back to square one.

To securely do this, you would need commutative hashing functions, i.e. hashing functions f(x) and g(x) such that f(g(x)) = g(f(x)). Actually, to be completely safe, you would need to be able to generate a whole (preferably infinite) family of commutative hash functions g(x), a random one for each login. I have no idea if such functions exist, more importantly, if they are known, it's an interesting idea actually.

[gizzlon]

The web is not exactly the state-of-the-art in network security and encryption. What GP is suggesting is common in other scenarios.

Since you can't trust the server in this scenario, you can't implement in in (normal) javascript.