Re: [TLS] close_notify and TLS 1.3

[Martin Thomson <[e-mail]>]

We suppress the sending of a close_notify if we have received one, but
that's a fairly simple thing to correct.

On Sun, Nov 12, 2017 at 9:13 AM, Eric Rescorla <[e-mail]> wrote:
> 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
>>> >
>>> > _______________________________________________
>>> > TLS mailing list
>>> > [e-mail]
>>> > https://www.ietf.org/mailman/listinfo/tls
>>> >
>>>
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>>
>>
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