Saving a hashed hash not to transmit passwords in clear?

Question

Ok, so here's the deal: (This is a hypothetical scenario)

Say for some reason I had a server (e.g. a legacy one) that can not use encryption in transmitting passwords over the net

I will still want to store my passwords in the database safely, e.g. as a dedicated password hash like bcrypt. That of course means I can't hash them on the client side and compare the two hashes server side.

Now here's the idea: If I stored a bcrypt hash of e.g. a SHA-256 hash of the cleartext password in the database, I could then compute the SHA-256 hash of the cleartext password over the net and still compare that to the bcrypt hash of the SHA-256 hash on the server side.

The big questions are:

1. Is this safe, or is it opening me up to some sort of attack I'm not thinking off right now?
2. Would this approach actually gain me anything, or would I just be chasing my own tail?

Answer 1

The problem here is, if you want to compare hashes as I described, you must by definition use a deterministic hashing function, which means there can not be a variable element like there is in a proper password hash like BCrypt. Every time you authenticated, the hash you send would have to be the same.

That means that approach will protect the user's clear text password to a degreee if the communication is observed, and it protects the user in case the password database is lifted, but it does not solve the problem of transit security.

An unencrypted connection means your communication can potentially be observed. Since you would be sending the same hash value over the net with every login, anyone who managed to intercept one communication would still be able to impersonate you, and gain access to the system in question.