Re: [TLS] close_notify and TLS 1.3
Eric Rescorla <ekr@rtfm.com> Mon, 13 November 2017 12:25 UTC
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From: Eric Rescorla <ekr@rtfm.com>
Date: Mon, 13 Nov 2017 12:25:07 +0000
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To: Hubert Kario <hkario@redhat.com>
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Subject: Re: [TLS] close_notify and TLS 1.3
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On Mon, Nov 13, 2017 at 11:37 AM, Hubert Kario <hkario@redhat.com> wrote: > On Saturday, 11 November 2017 10:21:11 CET David Schinazi 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 > > <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? > > what about alerts? > if you half-closed the connection for write and then received key-update, I assume you mean with update_requested set. in any case, you do nothing: If the request_update field is set to "update_requested" then the receiver MUST send a KeyUpdate of its own with request_update set to "update_not_requested" prior to sending its next application data record. > or > message with an invalid tag, how are you supposed to react to it? > I think it would be fine to either silently tear down the connection or to send an alert. -Ekr > > -- > Regards, > Hubert Kario > Senior Quality Engineer, QE BaseOS Security team > Web: www.cz.redhat.com > Red Hat Czech s.r.o., Purkyňova 115, 612 00 Brno, Czech Republic > _______________________________________________ > TLS mailing list > TLS@ietf.org > https://www.ietf.org/mailman/listinfo/tls > >
- [TLS] close_notify and TLS 1.3 David Schinazi
- Re: [TLS] close_notify and TLS 1.3 Martin Thomson
- Re: [TLS] close_notify and TLS 1.3 David Benjamin
- Re: [TLS] close_notify and TLS 1.3 Eric Rescorla
- Re: [TLS] close_notify and TLS 1.3 David Schinazi
- Re: [TLS] close_notify and TLS 1.3 Eric Rescorla
- Re: [TLS] close_notify and TLS 1.3 Martin Thomson
- Re: [TLS] close_notify and TLS 1.3 David Schinazi
- Re: [TLS] close_notify and TLS 1.3 Ilari Liusvaara
- Re: [TLS] close_notify and TLS 1.3 Hubert Kario
- Re: [TLS] close_notify and TLS 1.3 Eric Rescorla
- Re: [TLS] close_notify and TLS 1.3 David Benjamin
- Re: [TLS] close_notify and TLS 1.3 Hubert Kario
- Re: [TLS] close_notify and TLS 1.3 Eric Rescorla