Re: [TLS] Negotiate only symmetric cipher via cipher suites (was: Ala Carte Cipher suites - was: DSA should die)

Daniel Kahn Gillmor <> Mon, 13 April 2015 20:45 UTC

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From: Daniel Kahn Gillmor <>
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Date: Mon, 13 Apr 2015 16:45:25 -0400
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Subject: Re: [TLS] Negotiate only symmetric cipher via cipher suites (was: Ala Carte Cipher suites - was: DSA should die)
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On Mon 2015-04-13 14:55:05 -0400, Ilari Liusvaara wrote:
> On Mon, Apr 13, 2015 at 02:29:49PM -0400, Daniel Kahn Gillmor wrote:
>> What do you think an implementation should do when a peer tries to
>> negotiate the combination above?  Require an RSA-PSS signature from the
>> server over some material derived in part from the PSK key agreement
>> mechanism?
> To me it seems like that combo is quite insecure, unless one handles it as
> a special case (in which case it is just equivalent to PSK in security,
> just wastes time with a certificate).

Can you explain why it's "quite insecure"?

It doesn't seem any worse than normal (non-DHE) PSK other than the cost
of signing and verifying.  I suppose that it also proves that an
attacker has access to both the PSK and the secret key material
belonging to the cert.

So, for example, you could give a dozen clients the same PSK (maybe to
provide them with indistinguishability from each other to a network
observer?), but clients that want to be sure they're talking to the
server (instead of each other) can rely on the server's cert.

> As list, the key exchange methods TLS v1.2 actually supports (after merging
> some non-significant distinctions) are:
> - RSA [going away]
> - RSA_EXPORT [you have deimplemented this already, right?]
> - DH_CERT [going away]
> - DHE_CERT [supported]
> - DHE_ANON [???]
> - KRB5 [going away]
> - PSK [???]
> - DHE_PSK [???]
> - RSA_PSK [going away]
> - SRP [???]
> - SRP_CERT [???]

I think this list mixes up key exchange with authentication, which was
what Dave was trying to tease apart.  For key exchange, there is no
difference between DHE_CERT and DHE_ANON and DHE_PSK -- they're all DHE.
the ANON/CERT/PSK parts would be negotiated separately in the
authentication side of things (via the signature_algorithms extension).

(And if we're using supported_groups, the distinctions between different
 flavors of DHE (FFDHE2048, ECDHEP256, ECDHEX25519, etc) are hardly
 non-significant, but i guess that's a slightly different discussion.)