Re: [TLS] WGLC for draft-ietf-tls-cross-sni-resumption

Carrick Bartle <cbartle891@icloud.com> Wed, 11 August 2021 22:51 UTC

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From: Carrick Bartle <cbartle891@icloud.com>
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Date: Wed, 11 Aug 2021 15:50:54 -0700
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Cc: Christopher Wood <caw@heapingbits.net>, "TLS@ietf.org" <tls@ietf.org>
To: David Benjamin <davidben@chromium.org>
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Subject: Re: [TLS] WGLC for draft-ietf-tls-cross-sni-resumption
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Okay, in that case, I wouldn't use the word "re-validated," since to me that sounds like the certificate is to be completely validated again (e.g. checking expiration). Instead I would say something like "attempt resumption only if the certificate is valid for the new SNI," ideally with a reference to the current best practice of how to do that.



> On Aug 11, 2021, at 3:25 PM, David Benjamin <davidben@chromium.org> wrote:
> 
> On Wed, Aug 11, 2021 at 5:49 PM Carrick Bartle <cbartle891@icloud.com <mailto:cbartle891@icloud.com>> wrote:
>> - Ticket-based PSKs carry over the server certificate from the previous connection
> 
> What does "carry over" mean here? That the client literally holds on to the certificate and re-evaluates it before resumption? Or just that the trust from evaluating the certificate during the initial handshake also applies to the PSK? Because, AFAICT, the literal ticket isn't required to contain the server certificate.
> 
> I meant the latter. Though every TLS stack I've seen does actually retain the peer certificate. It's not in the literal ticket (that wouldn't work since it's issued by the server), but in the session state the client stores alongside the ticket, just like the PSK and other state. This is because TLS APIs typically have some kind of function to get the peer certificate, and applications typically expect that function to work consistently for all connections. That stuff is mostly not in the RFCs because it's local state and TLS doesn't define an API.
> 
> Anyway, as with any other use of resumption, in order to offer a ticket, you need to have retained enough information locally to know that the trust from the initial handshake is also good for this connection. This could be remembering application context (perhaps by way of separate session caches). This could be remembering the whole certificate. This could be remembering smaller amounts of information from the certificate. The exact details here I don't think TLS should specify, only the conditions on when using a session is okay.
> 
> David
>  
>> On Aug 11, 2021, at 2:13 PM, David Benjamin <davidben@chromium.org <mailto:davidben@chromium.org>> wrote:
>> 
>> On Wed, Aug 11, 2021 at 5:00 PM Carrick Bartle <cbartle891=40icloud.com@dmarc.ietf.org <mailto:40icloud.com@dmarc.ietf.org>> wrote:
>> >  Notably, it still relies on the server certificate being re-validated against the new SNI at the
>> >  session resumption time.
>> 
>> Where is this specified? I can't find it in RFC 8446. (Sorry if I missed it.)
>> 
>> Does RFC 8446 actually say this? I haven't looked carefully, but I suspect, if it says anything useful, it's implicit in how resumption works:
>> 
>> - If the client offers a PSK, it must be okay with the server authenticating as that PSK for this connection
>> - Ticket-based PSKs carry over the server certificate from the previous connection
>> - Therefore, in order to offer a ticket in a connection, the client must be okay with that previous server certificate in the context of that connection. Server name, trust anchors, and all.
>> 
>> This is another one of those cases where cross-SNI resumption is just a more obvious example of a general principle that needs to be written down somewhere in TLS proper. (Even with the same SNI, suppose two different parts of my application use different trust stores. My session resumption decisions must be consistent with that.)
>>  
>> >  However, in the absence of additional signals, it discourages using a session ticket when the SNI value > does not match ([RFC8446], Section 4.6.1), as there is normally no reason to assume that all servers
>> > sharing the same certificate would also share the same session keys.
>> 
>> It'd be helpful to describe under what circumstances there is reason to assume that servers that share the same certificate also share the same session keys (and are able to take advantage of cross-SNI resumption).
>> 
>> 
>> > On Jul 30, 2021, at 6:57 PM, Christopher Wood <caw@heapingbits.net <mailto:caw@heapingbits.net>> wrote:
>> > 
>> > Given the few responses received thus far, we're going to extend this WGLC for another two weeks. It will now conclude on August 13.
>> > 
>> > Best,
>> > Chris, for the chairs
>> > 
>> > On Fri, Jul 16, 2021, at 4:55 PM, Christopher Wood wrote:
>> >> This is the working group last call for the "Transport Layer Security 
>> >> (TLS) Resumption across Server Names" draft, available here:
>> >> 
>> >>    https://datatracker.ietf.org/doc/draft-ietf-tls-cross-sni-resumption/ <https://datatracker.ietf.org/doc/draft-ietf-tls-cross-sni-resumption/>
>> >> 
>> >> Please review this document and send your comments to the list by July 
>> >> 30, 2021. The GitHub repository for this draft is available here:
>> >> 
>> >>    https://github.com/vasilvv/tls-cross-sni-resumption <https://github.com/vasilvv/tls-cross-sni-resumption>
>> >> 
>> >> Thanks,
>> >> Chris, on behalf of the chairs
>> >> 
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>> > 
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