Re: [TLS] Making post-handshake messages optional in TLS 1.3 (#676)

David Benjamin <davidben@chromium.org> Sat, 08 October 2016 15:06 UTC

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From: David Benjamin <davidben@chromium.org>
Date: Sat, 08 Oct 2016 15:06:28 +0000
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To: Ilari Liusvaara <ilariliusvaara@welho.com>, Nick Sullivan <nicholas.sullivan@gmail.com>
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Subject: Re: [TLS] Making post-handshake messages optional in TLS 1.3 (#676)
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On Sat, Oct 8, 2016 at 5:03 AM Ilari Liusvaara <ilariliusvaara@welho.com>;
wrote:

On Sat, Oct 08, 2016 at 01:03:21AM +0000, Nick Sullivan wrote:
> There has been a lot of discussion lately about post-handshake messages
> that do not contain application data and how to handle them. This PR is an
> attempt to make the story more explicit by adding a new post_handshake
> extension to TLS 1.3.
>
> Supporting all types of post-handshake messages can require extra
> complexity and logic, even when the features that these messages enable
are
> not needed. Some types of connections/implementations don't need to
support
> key updates (some unidirectional connections), session tickets (pure PSK
> implementations) and post-handshake client auth (most browsers). These are
> all currently SHOULDs in the spec and they don't need to be.

Post-handshake authentication is the only one of these that is genuinely
annoying. This is because you can't even reject it without a MAC, that
additionally continues the handshake hash.

KeyUpdate is rather simple, and NST can just be ignored (leading to some
waste in bandwidth).


Yeah, after the fix to how KeyUpdate is acked, I don't think we'd have
problems with either of the way, while post-handshake auth is indeed
horrific.


Furthermore, the post-handshake authentication mechanism doesn't look to
be featureful enough for kind of post-handshake auth e.g. HTTP/2 wants,
And there are serious questions about how it should interact with
applications.


An extension also doesn't really capture things if we intend for, say, this
to be used for legacy protocols like HTTP/1.1 (where we don't have as rich
a framing layer) but not HTTP/2 (where we do and can use it as a substrate
for all this silliness). I was anticipating that, if this ends up being
used in the HTTP world like renego is, it would be like our renego stuff:
off by default, forbidden in HTTP/2, and with all interleave forbidden.

Further, what useful thing could a server even do with this extension?
Decline to do post-handshake auth doesn't work. Either the application
protocol doesn't use post-handshake auth (please pick this one) or it does.
One doesn't need to send a client_writes_first extension in HTTP/1.1.

Post-handshake auth another knob on the TLS <-> application protocol
boundary. This means each application protocol must specify for itself what
post-handshake auth means: what to do with the context field, when it may
be received, what it does to application flow, etc. Then the spec must be
clear that if the application protocol does not opt in, CertificateRequest
is forbidden. A CertificateRequest at an unexpected point (say, half-way
through a HTTP/1.1 body) is also forbidden. Also make it clear this is for
legacy protocols and new ones do not use it.

This is analogous to how HTTP/2 forbids renego at the spec level. (TLS 1.2
got the "defaults" for renego wrong.) No one ever wrote down the exact
rules for the client-auth/renego hack in HTTP/1.1, but whatever spec does
this for post-handshake auth should do the same for renego.

It is a lot of spec work for such a tiny-seeming feature, but this is
consistent with how messy the feature actually is.

David