Re: [TLS] I-D Action: draft-ietf-tls-ticketrequests-02.txt

Daniel Migault <daniel.migault@ericsson.com> Tue, 08 October 2019 14:04 UTC

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From: Daniel Migault <daniel.migault@ericsson.com>
Date: Tue, 8 Oct 2019 10:04:18 -0400
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To: Hubert Kario <hkario@redhat.com>
Cc: tls <tls@ietf.org>
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Subject: Re: [TLS] I-D Action: draft-ietf-tls-ticketrequests-02.txt
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If we suppose all tickets are sent by the server at once, I am wondering if
we have any case where the server will send more tickets that the number
provided by the hint.

Yours,
Daniel

On Mon, Oct 7, 2019 at 10:46 AM Hubert Kario <hkario@redhat.com> wrote:

> On Saturday, 5 October 2019 14:08:45 CEST Christopher Wood wrote:
> > On Fri, Oct 4, 2019, at 6:17 AM, Hubert Kario wrote:
> > > On Thursday, 3 October 2019 22:15:14 CEST Daniel Migault wrote:
> > > > On Thu, Oct 3, 2019 at 7:56 AM Hubert Kario <hkario@redhat.com>
> wrote:
> > > > > On Wednesday, 2 October 2019 22:42:32 CEST Daniel Migault wrote:
> > > > > > I understand the meaning of count is the higher limit of ticket
> and
> > > > > > the
> > > > > > server can provides any tickets between 0 and count. If that is
> > > > > > correct,
> > > > > > this could be clearly stated and indication to chose an
> appropriated
> > > > >
> > > > > value
> > > > >
> > > > > > for each cases may be provided.
> > > > > >
> > > > > > I would rather see the count value as a hard line from the server
> > > > > > with a
> > > > > > MUST NOT instead of a SHOULD NOT statement.
> > > > >
> > > > > see my previous comments: this ignores existence of post handshake
> > > > > authentication, ticket lifetimes and KeyUpdate
> > > >
> > > > I think we agree on the problem which is it is hard to have a
> specific
> > > > count as tickets may be sent at multiple time during the key
> exchange or
> > > > the later during the session. If the "SHOULD" is addressing this
> aspect
> > > > I
> > > > believe that more text is needed.. From my perspective, what I
> proposed
> > > > I
> > > > imagined that count was related to the maximum number of ticket
> provided
> > > > **each time the server is sending tickets**. The reason for having
> the
> > > > same
> > > > number is that the server does not know how the client will use these
> > > > tickets and the client does not know precisely when the server sends
> the
> > > > tickets. So at the end the number of tickets sent is likely to be
> > > > n*count
> > > > when n represents the number of times during the session tickets are
> > > > provided.
> > > >
> > > > My understanding of the SHOULD is that it makes legitimate for a
> server
> > > > to
> > > > ignore the count value provided by the client.
> > >
> > > yes, it looks like we are in agreement
> > >
> > > Sounds like something that should be spelled out explicitly in the
> draft.
> > > I.e. it should talk about groups of tickets, not tickets in connection,
> > > and it should definitely not specify the maximum number of tickets a
> > > server can send in a connection.
> >
> > Since it's meant as a hint, removing that clause (maximum number of
> tickets
> > a server can send in a connection) is reasonable, and sounds like it
> should
> > address the concerns here. (Assuming we also include text which says
> > servers should obviously place a cap on what they send, as they do
> today.)
> > Would you agree? I'm really trying to avoid this becoming overly complex,
> > as it's a very small feature.
>
> yes, having a "server SHOULD place a cap on the number of tickets it sends
> at
> once" with a note in security section that "not having such a limit may
> expose
> the server to DoS or amplification attacks" would be enough
>
> (note that without client certificate authentication, the server can use
> early
> exit from handshake method and thus send the extensions just as a result
> of a
> single ClientHello message)
> --
> 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
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