Function to strengthen passwords

[Vlix]

A nice additional functionality might be a way to create new hashes after validating a password.

This would be a modified version of checkPassword that returns a new hash that has new parameters. (Using Argon2 as a concrete example, the idea would be implemented for all algorithms)

Data.Argon2.checkPasswordRenew
    :: Argon2Params
    -> Password
    -> PasswordHash Argon2
    -> Maybe (PasswordHash Argon2)
checkPasswordRenew currentParams pw pwh mNewPw

This would have some way of indicating that the check was successful or not, and if successful it would return the new hash that can be used to overwrite the previously stored hash.

Not quite sure if we'd want to limit it to strengthening passwords, with an opt-in to be able to make passwords weaker; or just to put the responsibility on the user, since there are legitimate reasons to decrease the hashing cost programmatically.

[Vlix]

Of course the parameters should be compared to the current hash and there should be some way to indicate the password was correct, but also that the hash might not need to be updated (i.e. because the current hash already matches the parameters given)

[Vlix]

I've been thinking about this for a while now, and some points that came up were:

• PBKDF2 and bcrypt are easily "strengthened", but scrypt and Argon2 are a bit ambiguous when determining if one set of parameters are "stronger" than another.
• We could either add functions that just return a new hash with the given parameters if the given hash was different, basically leaving the usage to the user (maybe returning the original parameters of the given hash so the user can decide what to do)
• This would result in double hashing of a password before returning the function, which increases the time end-users would have to wait for a confirmation of their password being correct or not. Maybe we can include a way to have the rehashing ("strengthening") of the hash happen in another thread? Like have the user give a callback that (if a new hash is made, this will be handled in a different thread)?