What will happen if they changed PASSWORD_DEFAULT in PHP Password library?

Question

Consider this line of code by using PHP:

$password = password_hash($password, PASSWORD_DEFAULT);

What will happen if they changed the default password hashing algorithm? I mean I will be having hashed password inside the database. Then, from my own understanding, it will be impossible to check the password because the hashing algorithm will be totally changed.

Answer 1

What would happen is that newly-hashed passwords will be using the new algorithm - obviously.

However, you shouldn't be concerned about this, because the whole thing is designed with forward-compatibility in mind - your code won't be broken when the default algorithm changes, as long as you're using the password_*() functions correctly.
By correctly, I mean use password_verify().

password_verify() accepts a plain-text password and a hash, and it can easily determine what the used algorithm is by looking at the hash that you feed it. Therefore, it would also still be able to verify a password that has been hashed using an old algorithm - not just the previous one, but any algorithm that is supported.

In fact, the sole purpose of the PASSWORD_DEFAULT constant is that you can easily migrate older hashes to a new algorithm (once one is added). This happens the following way:

• When a user logs in, you verify their password via password_verify() (any hashing algorithm that has a PASSWORD_<name> constant will work).
• You call password_needs_rehash(), and if the password you just verified is using an older algorithm (or a lower 'cost' paramater) - it will return boolean TRUE.
  • If boolean TRUE was indeed returned, you can now replace the old hash with one that uses the new algorithm; you can do that during a login, because the user just gave you the password and you verified that it is correct.

In summary - it's a really, really well-designed API and it solves problems for you that you haven't even thought about. Don't worry about it.

Edit (noted in the comments):

It should be noted, however, that new algorithms will quite probly result in longer hash lengths, so if you're storing the passwords in a database - don't limit the field's length (i.e. use a varchar(255) field).