> I thought the standard way of migrating your PW hashing function was that you could only do it during a login, because that's the only time you have the PW in

> I thought the standard way of migrating your PW hashing function was that you could only do it during a login, because that's the only time you have the PW in plain text. No? No. Well, I mean, it's common to do that, but it's a bad idea and it's not necessary to keep all of those vulnerable hashes around. Say you have a bunch of MD5 password hashes stored, and you want to upgrade your password hashing to BCrypt. Don't wait for the user to login and rehash their plain text password. Instead, Bcrypt all the MD5s. The authentication mechanism is now BCrypt(MD5(plaintext)) == stored_bcrypt. Upgrade them to straight bcrypt on login.