.. when the user tries to log in, a hash of his password is sent
No. When the user tries to log in, the password is sent (encrypted
in transit, then decrypted (in memory only) to clear text on the server).
Given this, how do you propose the password is re-hashed without having the original password to work from?
At next successful login. Add password expiry functionality (i.e. max 30 days), and we can ensure that all passwords are either
- invalid, or
- re-hashed with increased cost over the next 30 days.
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No matter how great and destructive your problems may seem now, remember, you've probably only seen the tip of them. [1]
