Re: [TLS] WGLC for draft-ietf-tls-exported-authenticator

Eric Rescorla <ekr@rtfm.com> Thu, 10 May 2018 04:07 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Wed, 09 May 2018 21:06:42 -0700
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To: Martin Thomson <martin.thomson@gmail.com>
Cc: R du Toit <r@nerd.ninja>, "<tls@ietf.org>" <tls@ietf.org>
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Subject: Re: [TLS] WGLC for draft-ietf-tls-exported-authenticator
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On Wed, May 9, 2018 at 7:15 PM, Martin Thomson <martin.thomson@gmail.com>
wrote:

> I'm not that concerned about this, though I will concede that it's worth
> pointing out.
>
> Failing to validate a secondary certificate for a server shouldn't be cause
> for terminating an otherwise usable connection.


To be clear: it's not failing to validate the *certificate* it's that the
CertificateVerify
to validate. This should ordinarily never happen, so I definitely think we
should
call that out.

-Ekr

The same goes for clients
> that authenticate using this.  As long as users of exported authenticators
> are aware of this, then they can design for it.
>
> If those users wanted confirmation, then we could provide a
> confirmation-of-mechanism-viability in the application layer.  That could
> just be by including a value derived from the exporter in the negotiation.
>
> In h2, we could do as much as 4 octets of confirmation for cheap, and maybe
> we should.
> https://github.com/httpwg/http-extensions/issues/617
> On Wed, May 9, 2018 at 11:28 PM Eric Rescorla <ekr@rtfm.com> wrote:
>
> > Regardless of where it goes in the document, I think there's a real
> deployment
> > consideration here: if you run this mechanism through a conventional MITM
> > proxy, what will happen will be that the secondary cert auth will appear
> to just
> > fail with a bogus signature. If clients respond to that by terminating
> the connection,
> > then we're going to be in pretty bad shape [0]. So I think we either
> need:
>
> > 1. To negotiate this with an extension
> > 2. To tell people not to treat signature failures as a hard failure.
> > 3. To have some way to indicate that there was a MITM on-path (e.g., have
> the
> >      application level mechanism include some separate value that's tied
> to
> >      the channel that lets the endpoints detect when its two separate
> channels).
>
> > -Ekr
>
>
> > [0] Note that this also applies to post-handshake authentication, but
> that's negotiated
> > with an extension, so S 9.3 applies.
>
>
> > On Wed, May 9, 2018 at 12:39 AM, Martin Thomson <
> martin.thomson@gmail.com>
> wrote:
>
> >> This isn't really a security consideration though, it's a truism.  A
> MitM
> >> can break things that depend on end-to-end integrity of the connection.
> >> On Wed, May 9, 2018 at 11:25 AM Roelof duToit <r@nerd.ninja> wrote:
>
> >> > If the use of the mechanism is not negotiated on the TLS level then I
> >> would appreciate it if the “Security Considerations” section of the
> draft
> >> could be amended to include a paragraph that warns potential
> implementors
> >> that protocol-agnostic middleboxes will break the mechanism without any
> >> clear failure indicators.
>
> >> > > On May 8, 2018, at 8:13 PM, Martin Thomson <
> martin.thomson@gmail.com>
> >> wrote:
> >> > >
> >> > > On Wed, May 9, 2018 at 2:20 AM Roelof duToit <r@nerd.ninja> wrote:
> >> > >
> >> > >> I understand that there is not really anything to negotiate per se,
> but
> >> > > would it not be prudent to add a TLS extension to negotiate support
> for
> >> > > exported-authenticator in the TLS layer prior to using it in the
> >> > > application layer?
> >> > >
> >> > > We don't signal the potential need for exporters.  I see no reason
> to
> >> > > signal this either.  Any signaling necessary really belongs at the
> >> higher
> >> > > layer.
>