[TLS] New Problem (Was: Last Call: draft-altman-tls-channel-bindings)

Nicolas Williams <Nicolas.Williams@sun.com> Wed, 04 November 2009 22:58 UTC

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Date: Wed, 04 Nov 2009 16:47:42 -0600
From: Nicolas Williams <Nicolas.Williams@sun.com>
To: Larry Zhu <larry.zhu@microsoft.com>
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Subject: [TLS] New Problem (Was: Last Call: draft-altman-tls-channel-bindings)
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On Wed, Nov 04, 2009 at 10:13:46PM +0000, Larry Zhu wrote:
> The proposed looks fine. Thanks,


HOWEVER, Martin's post to the TLS WG list about MITM attacks in
re-negotiations is relevant.

Re-negotiations have no real binding between inner and outer
connections.  Clients can enforce that the server end-point is the same
(has the same certificate, whatever) for both connections: inner and
outer.  Servers can also force the inner connection to change cipher
specs.  But suppose that the outer connection used an TLS_DH_anon_*
cipher suite!  Then there is no binding whatsoever between the inner and
outer connection.  And then we have a real problem for tls-unique.

We need at least a security considerations note about this.  But we
should also consider changing tls-unique to be the client's Finished
message for the _inner-most_ TLS connection, not outer-most.
(Outer-most is OK IFF there's a binding between each channel.)