Re: [TLS] Forward secrecy with resumption, and 0-RTT security

Benjamin Kaduk <bkaduk@akamai.com> Tue, 08 December 2015 23:26 UTC

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To: Bill Cox <waywardgeek@google.com>, "tls@ietf.org" <TLS@ietf.org>
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From: Benjamin Kaduk <bkaduk@akamai.com>
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Date: Tue, 8 Dec 2015 17:26:20 -0600
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Subject: Re: [TLS] Forward secrecy with resumption, and 0-RTT security
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On 12/06/2015 08:50 AM, Bill Cox wrote:
> I think the current spec does not describe well enough how to
> implement secure 0-RTT infrastructure.  Instead, it seems to recommend
> against using 0-RTT, with a pretty dire warning about the insecurity
> of 0-RTT.  I think that nearly the entire world will switch to 0-RTT,
> regardless of security limitations and warnings.  Companies won’t be
> able to

Perhaps I am living under a rock, but I do not have the sense that
"nearly the the entire world will switch to 0-RTT".

> resist the improved connection time.  IMO, the spec should describe
> how to address the security issues with 0-RTT, rather than just
> warning about security issues.
>
> So, here are my dumb thoughts about practical 0-RTT security
> infrastructure...
>
>
> One way is to bind resumption to orbits, by having only the orbit that
> issues a PSK remember the secret key.  Something like this is needed
> in any case to provide 0-RTT replay protection.  Without at least
> orbit-based replay protection, I think it does not make sense to allow
> a ClientVerify in a 0-RTT connection.
>
>
> There was a TODO in the last version I read asking what the client
> would sign in a 0-RTT ClientVerify.  If orbit-based replay protection
> is in place using PSKs, then signing anything that identifies what PSK
> to use is enough. However, the handshake has to somehow be bound to
> the orbit to prevent the signature from being replayed in a MitM
> attack. Having a ticket that can only be decrypted using the
> orbit-specific ticket decryption key would work. Using a source
> address token bound to the orbit would also work.  Signing the server
> Finish is not required to insure the signature will only be accepted once.
>
>
> There are two parts to a proof-of-possession: uniqueness and
> freshness.  Uniqueness can be had using orbit-bound PSKs.  Insuring
> freshness is harder.  This could be done by adding timestamps to the
> material signed.  With this, we would not get a complete proof of
> possession, because an attacker can attack the client's clock, unlike
> a normal proof of possession.  However, if steps are taken to secure
> clocks, which is needed for TLS in any case, then we can get what I
> think of as "compelling evidence" of possession, which might be good
> enough.
>
>

Perhaps I do not fully understand the proposal, but binding to source
address and signing timestamps sound an awful lot like things that have
been causing headaches for *decades* in the Kerberos community.  I would
be wary of introducing them into TLS.

-Ben