Re: [TLS] Technical comment on design draft-rhrd-tls-tls13-visibility-00: why also break integrity/authentication?

Hi Richard,

Thanks for the pointer, I had missed your earlier observation of
essentially the same thing in the large mail threads.

Personally, I think it is a substantial and unmotivated loss in security to
also give up authentication/integrity.

Note that any changes towards PERC would require some significant changes
in the security analyses; for mctls, I expect it would be a completely new
analysis. (I haven't seen any analyses of rhrd, but of the three, it is of
course closest to the current setup.)

Best,

Cas

On Fri, Nov 3, 2017 at 2:00 PM, Richard Barnes <rlb@ipv.sx> wrote:

> Hey Cas,
>
> This question is a good one.  Earlier I brought up mcTLS, which some folks
> have been working on to provide even more granular separations than you
> suggest:
>
> https://mctls.org/
>
> I think the authors of draft-rhrd are trying for a more lightweight change
> to TLS.  That said, there might be some intermediate points on the spectrum
> here.  For example, you could define a TLS ciphersuite akin to what we
> defined in PERC when we wanted to break integrity partly and
> confidentiality not at all.
>
> https://tools.ietf.org/html/draft-ietf-perc-double-07
>
> That would be less invasive than mcTLS, while still getting the properties
> you describe.
>
> --Richard
>
>
> On Nov 3, 2017 09:43, "Cas Cremers" <cas.cremers@cs.ox.ac.uk> wrote:
>
> Dear all,
> 
> Disclaimer: I am not a proponent of the idea behind draft
> visibility/green/rhrd; I think such a mechanism should not be part of the
> TLS 1.3 standard.
> 
> I have a technical problem with the current design, whose goal is to
> allow eavesdropping for inspection, i.e., selectively decreasing
> confidentiality.
> 
> However, the design in the draft also enables arbitrary traffic
> modification/insertion, additionally breaking all authentication and
> integrity guarantees. Once someone has the session keys, they can not only
> eavesdrop but can also start to insert/modify traffic. This additional
> decrease in security is entirely unmotivated by the cited use cases.
> 
> It is possible to offer authentication and integrity, while selectively
> giving up confidentiality. For example, one could replace the AEAD by (i) a
> mechanism for authentication and (ii) a separate mechanism for
> confidentiality, and then possibly reveal the keys used for (ii), but make
> sure only the real endpoint has the keys for (i). That seems more rational
> to me, and may retain the authentication/integrity guarantees. However, it
> would require a much more invasive change.
> 
> Best,
> 
> Cas
>
>
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