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In my database, I have a PBKDF2 hash + salt for every user. My objective is for the user to be able to log in without stating the user's password (in case somebody is logging the network traffic somehow).

I was wondering if the following scheme is secure, or if there are any fallacies.

  1. Client starts the login process by stating the username
  2. Server returns the salt together with a string of non-pseudo random bytes (a challenge)
  3. Client uses the salt to derive the hash. The challenge bytes are hashed together with the derived password hash and sent back to the server. (should I use some random salt during this process?)
  4. Server verifies that the challenge hashed together with the hash in the database matches what the client sent.
  5. Success! Server has implicitly verified the password without the client stating the password, and the input is never the same (cannot be reused by an attacker).

Is this a good idea? Are there any security issues with giving up the salt? In case the user doesn't exist and I don't want to give up that information, I could invent some fake salt based on some hash of the username.

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