Re: [TLS] close_notify and TLS 1.3

[Eric Rescorla <[e-mail]>]

Initial inspection suggests that NSS behaves the same way, so I would be
fine with this change.

-Ekr


On Sat, Nov 11, 2017 at 3:46 PM, David Benjamin <[e-mail]>
wrote:

> I think this change is a good idea.
>
> Our implementation actually does this already anyway. We are happy to
> continue servicing writes even when the read half has consumed a
> close_notify. I believe we inherited this behavior from OpenSSL, so it
> should be there too. Go's crypto/tls implementation appears to also already
> do this.
>
> We don't have a particular need for the half-close semantics that I know
> of, but I don't care for the current spec text (it requires yet another
> undesirable read/write sync point). Aligning with TCP's semantics is also
> generally a good default.
>
> On Sat, Nov 11, 2017 at 11:18 PM Martin Thomson <[e-mail]>
> wrote:
>
>> This seems like it might be worth looking at.  This seems to be
>> something that harks back to SSL3 or even earlier.  We aren't going to
>> make it so that you can rely on this behaviour, but we might be able
>> to make it possible to half-close, which for new protocols using TLS
>> could be hugely useful.
>>
>> On Sat, Nov 11, 2017 at 5:21 PM, David Schinazi <[e-mail]>
>> wrote:
>> > Hello all,
>> >
>> > Currently TLS 1.3 specifies close_notify in the same way that TLS 1.2
>> did.
>> > I believe that has issues and this might be the right time to fix them.
>> > The purpose of close_notify is to protect against data truncation
>> attacks,
>> > each side is required to send close_notify before closing the write
>> side of
>> > the transport connection so the other side knows that the data was not
>> > truncated.
>> > As such, close_notify only needs half-close semantics to prevent
>> truncation.
>> >
>> > However, the specification contains the following text:
>> > << Each party MUST send a “close_notify” alert before closing the write
>> side
>> >     of the connection, unless some other fatal alert has been
>> transmitted.
>> >     The other party MUST respond with a “close_notify” alert of its own
>> and
>> > close
>> >     down the connection immediately, discarding any pending writes. >>
>> >
>> > This means that an application-layer client can't send a query then
>> close
>> > their
>> > write transport when they know that they're done, because the server
>> would
>> > terminate the TLS session before sending the reply. On top of this, when
>> > the server receives the close_notify, it may have already sent part of
>> the
>> > reply
>> > (or wrote it to the socket send buffer) so the responding close_notify
>> would
>> > in effect be inflicting a truncation attack on the client.
>> >
>> > This doesn't make much difference for HTTP because clients already
>> > don't close their write transport after sending a reply, however having
>> the
>> > option do do this could allow innovation in new protocols that can
>> define
>> > the semantics of when they use close_notify. An example is DNS PUSH:
>> > https://tools.ietf.org/html/draft-ietf-dnssd-push
>> >
>> > A proposal to solve this problem would be to give close_notify
>> half-close
>> > semantics: we keep the requirements that a close_notify be sent before
>> > closing the transport, and that any data received after a close_notify
>> is
>> > ignored, but we simply remove the requirement to immediately reply
>> > with a close_notify. This has the advantage that current implementations
>> > are already compliant but future ones can leverage this improvement.
>> >
>> > What do you think? Is this worth discussing on Thursday?
>> >
>> > Thanks,
>> > David Schinazi
>> >
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>> >
>>
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