[Cryptography] Password rules and salt

[lists at notatla.org.uk]

Jerry Leichter <leichter lrw.com> wrote

> He suggested that instead, any string is valid as a
> password, as long as it has never been used before as a
> password by anyone ...
> ...  Keep track of used passwords in a Bloom
> filter, *using cryptographically secure hash functions*
> to prove the filter.  This has the following properties:
> If a password has been used, it will definitely be caught
> by the filter; if the password hasn't been used, there
> is a small probability it will be caught anyway; even
> someone who sees the individual updates to the filter
> can't determine what's been added to it.

When your secure hash function reaches end of life and
you pick a new one I suppose that means starting a new
dataset.  You could test new passwords against both the
old and new filters; storing them in the new one if they
appeared to be in the old.  Eventually you'd retire the
old version.