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>
Cc: Christian Huitema <huitema@huitema.net>, "tls@ietf.org" <tls@ietf.org>
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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. > > > > >
- [TLS] ESNIKeys.public_name vs. cleartext SNI Stephen Farrell
- Re: [TLS] ESNIKeys.public_name vs. cleartext SNI Dmitry Belyavsky
- Re: [TLS] ESNIKeys.public_name vs. cleartext SNI Stephen Farrell
- Re: [TLS] ESNIKeys.public_name vs. cleartext SNI Christian Huitema
- Re: [TLS] ESNIKeys.public_name vs. cleartext SNI Stephen Farrell
- Re: [TLS] ESNIKeys.public_name vs. cleartext SNI Ben Schwartz
- Re: [TLS] ESNIKeys.public_name vs. cleartext SNI Stephen Farrell
- Re: [TLS] ESNIKeys.public_name vs. cleartext SNI Ben Schwartz
- Re: [TLS] ESNIKeys.public_name vs. cleartext SNI Stephen Farrell