Re: Crypto agility in SCRAM + draft-josefsson-password-auth?

[Tony Finch <dot@xxxxxxxx>]

On Tue, 18 Mar 2008, Nicolas Williams wrote:
>
> One day your servers all have SCRAM HMAC-MD5 verifiers for your users'
> names and passwords.  The next you want to move to SCRAM HMAC-SHA-224,
> but you don't have any of those verifiers.  How do you complete the
> migration, operationally?

The usual way to avoid too much interaction with users is to instrument
the authentication process to capture passwords verified by the old
mechanism and re-encode them using the new mechanism.

> > That's a problem with SASL deployment in general though.  When the day
> > comes for a site to replace SCRAM-HMAC-MD5, it might be in favor of a
> > non-SCRAM mechanism.  Then it doesn't matter whether or not SCRAM
> > supports hash algorithm negotiation.
>
> Putting the hash negotiation inside SCRAM doesn't help you with that.

It depends on whether clients are good at automatic fail-over from one
SASL mechanism to another. If you negotiate the algorithm after you know
who the user is, the server can advertise different algorithms for
different users, and advertise only the algorithms that have a chance of
working, so client-side fail-over isn't required.

Tony.
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