Re: [TLS] ESNIKeys.public_name vs. cleartext SNI

Ben Schwartz <bemasc@google.com> Mon, 29 July 2019 15:07 UTC

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From: Ben Schwartz <bemasc@google.com>
Date: Mon, 29 Jul 2019 11:07:00 -0400
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To: Stephen Farrell <stephen.farrell@cs.tcd.ie>
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Subject: Re: [TLS] ESNIKeys.public_name vs. cleartext SNI
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On Mon, Jul 29, 2019 at 10:39 AM Stephen Farrell <stephen.farrell@cs.tcd.ie>
wrote:

>
> Hiya,
>
> On 29/07/2019 14:35, Ben Schwartz wrote:
> >> In which case I must've explained it badly, because it's
> >> entirely practical:-) Let me try again...
> >>
> >> What I'm asking is that we not insist that the cleartext
> >> SNI is the ESNIKeys.public_name even though those will be
> >> the same value in almost all cases.
> >>
> >> I don't think there's any major security benefit to insisting
> >> that they be the same and there are cases where e.g. a command
> >> line tool like curl or wget can easily supply a different
> >> cleartext SNI value as an override, should that ever be needed
> >> or useful.
> >>
> > It sounds like you are arguing for replacing a MUST with a SHOULD.
>
> Sure, that'd be good enough, though I think a MAY is actually all
> that's needed. I think the way I'd phrase it might be "clients
> are advised to include the value of ESNIKeys.public_name in the
> TLS handshake in the cleartext SNI extension; servers need to be
> aware that some clients might not do so, either omitting cleartext
> SNI entirely or including some other value that may or may not be
> meaningful at that server; successful ESNI decryption means ignoring
> any cleartext SNI value provided" and avoid use of 2119 terminology,
> but I'm sure various possible wordings would be fine.
>
> > I think our best option is probably to clarify who is supposed to do
> what,
> > and why.  For example, we could say that a compliant client MUST set sni
> =
> > public_name so that fallback can work on servers with multiple public
> > names,
>
> I'm pretty sure I disagree with the above MUST as being necessary.
>
> But what do you mean by "so that fallback can work"?


I mean "so that esni_retry_requested can work".  This is the ESNI fallback
mechanism, where the server delivers a fresh ESNIKeys in-band, and the
client closes the socket and tries again.  The client will only accept the
new ESNIKeys if the server's provided certificate validates for
public_name, but if the server has multiple public names, it will not in
general know which certificate to serve unless the client sets sni =
public_name.

Admittedly, having multiple public names in separate certificates may not
be a common case, but as long as we view it as in-scope, we have to require
sni = public_name for general-purpose clients.  Special-purpose clients
that have additional out-of-band information (like knowing that the server
only has one public name) are of course able to do whatever they want;
they're no longer tightly bound by the standard.


> ISTM that if
> ESNI doesn't work, then the right thing is for the server to fail
> or continue the TLS h/s based on the cleartext SNI as normal, or
> to default to something if no cleartext SNI is provided, again as
> normal. A client that's not only greasing MUST of course make it
> appear to the client application that the TLS h/s failed, as ESNI
> has failed.
>
> > but the server MAY accept connections with missing or unrecognized
> > SNI to enable connections from non-compliant clients that prefer not to
> > include the public_name.
>
> Right. I'd expect some variation in all this and would like that
> we can make ESNI work even for clients with somewhat outdated or
> incorrect information, if there's no security downside in so
> doing.
>
> Cheers,
> S.
>
>
> >
>