Re: [TLS] TLS-OBC proposal

Nico Williams <nico@cryptonector.com> Wed, 07 September 2011 22:15 UTC

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Date: Wed, 7 Sep 2011 17:17:29 -0500
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From: Nico Williams <nico@cryptonector.com>
To: Dirk Balfanz <balfanz@google.com>
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Cc: tls@ietf.org
Subject: Re: [TLS] TLS-OBC proposal
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I think the cookie hardening aspect of this is the key feature of your
proposal.  Alternatively you'd need a certificate enrollment facility
by which to associate one of these certs with a user account.  Either
way, other user authentication mechanisms are still required (always
at login time w/o cert enrollment, or just at initial cert enrollment
time, but with a more complete key rollover mechanism required to
avoid having to authenticate again at key rollover time.

I'd also note that in all TLS user cert schemes TLS session resumption
(without server-side state) becomes even more important for
performance.

I believe logout is always a difficult problem.  Specifically: proving
to one's peer that state has been torn down is difficult.  But it is
more difficult when logout requires layer crossing.  In this case,
because cookies would still be used for web session identification,
you'd avoid the difficulties of implementing logout in other TLS user
cert applications.  This is good.

The main benefit of binding cookies to user certs rather than server
certs is that cookie leakage is made much less harmful: leaked cookies
would be useful for nothing more than traffic analysis provided that
cookies bear no sensitive information in cleartext.  This is a very
good thing.

Looking at usability, my main concern is that we still need to address
user authentication issues, and/or user cert enrollment and key
rollover.

I'm not a fan of TLS user certs for user authentication: we'll end up
building SACRED-like protocols to deal with the need to use certs from
multiple devices, and so on.  But the HTTP cookie protection aspect of
TLS-OBC is quite enticing.

Nico
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