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You could scrypt on the client.

You're still passing the output to the server, which can be repeated. Certainly better than just a password, but not as good as public key crypto.

In the end, I'd probably prefer something like SRP

> You're still passing the output to the server, which can be repeated.

Not if the server generates a unique ID for each attempt, and that ID must be used in hashing. It can be as simple as a timestamp.

Which is then going beyond just using scrypt on the client. What you explain is simply challenge-response authentication.