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

Nick Sullivan <nicholas.sullivan@gmail.com> Sat, 08 October 2016 20:03 UTC

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From: Nick Sullivan <nicholas.sullivan@gmail.com>
Date: Sat, 08 Oct 2016 20:02:52 +0000
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To: Eric Rescorla <ekr@rtfm.com>
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Subject: Re: [TLS] Making post-handshake messages optional in TLS 1.3 (#676)
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I agree that "I don't like NST or KU" is not a very useful thing to add to
the spec. I added them as part of a general move towards clarity and
conservatism about which types of post-handshake messages are permissible
in TLS 1.3. Right now the spec is ambiguous about what each side of the
connection is supposed to do when it receives an unexpected or unknown
post-handshake message. This change hopefully makes it crystal clear: any
post-handshake message that wasn't agreed upon by both parties should be
fatal.

David suggested that the behavior with respect to post-handshake messages
should be negotiated at the application layer. This also seems reasonable,
but I worry about how this policy would be communicated from the
application layer to the TLS stack. Would the application layer be alerted
of incoming CertificateRequests and either terminate the connection or
reply with an empty certificate and finished message? If we want to support
"2018" post-handshake messages, will we just end up re-inventing this
post_handshake extension in a later draft to advertise support? Different
post-handshake messages have different uses, so I see how this more
case-by-case application-dependent policy could be more expressive than
what I proposed, but I worry we may end up with something more complex than
necessary.

In any case, post-handshake authentication as currently described is
problematic. I'd be open to seeing a text change along the lines of what
David proposed instead of the wire change I'm proposing as long as it
includes some guidance around how it should interact with policies from the
application layer.



On Sat, Oct 8, 2016 at 9:55 AM Eric Rescorla <ekr@rtfm.com>; wrote:

> I could go either way on this. It seems like this pushes complexity from
> the client to the server.
>
> Consider the case of NST. Presently, a client which doesn't want
> resumption can just ignore NST,
> but in your proposed change, the server needs to read this extension and
> then conditionally send
> NST, and the client still needs the logic to detect unexpected handshake
> messages and abort
> on them.
>
> As I said, I could go either way, and I do think it's potentially useful
> to be able to say "I won't do
> post-handshake client auth" and even more useful to be able to say "I
> would do post handshake
> message X which will be invented in 2018" (but of course we can register
> an extension for
> that then), but less useful to say "I don't like NST or KU"
>
> -Ekr
>
>
> On Sat, Oct 8, 2016 at 9:32 AM, Nick Sullivan <nicholas.sullivan@gmail.com
> > wrote:
>
> I'm not proposing any new post-handshake authentication mechanisms or
> anything relating to HTTP/2 renegotiation in this change. I'm simply making
> support for the existing post-handshake messages optional.
>
> With this change, if the client does not opt in, unexpected
> CertificateRequests are fatal to the connection. Same with unexpected
> KeyUpdates and SessionTickets. This will hopefully reduce the complexity of
> TLS 1.3 implementations that don't need these features.
>
> Nick
>
> On Sat, Oct 8, 2016 at 8:06 AM David Benjamin <davidben@chromium.org>;
> wrote:
>
> 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
>
>
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