Re: [TLS] DTLS 1.3

"Fossati, Thomas (Nokia - GB)" <thomas.fossati@nokia.com> Fri, 08 July 2016 11:06 UTC

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From: "Fossati, Thomas (Nokia - GB)" <thomas.fossati@nokia.com>
To: Nikos Mavrogiannopoulos <nmav@redhat.com>, "Fossati, Thomas (Nokia - GB)" <thomas.fossati@nokia.com>, Stephen Farrell <stephen.farrell@cs.tcd.ie>
Thread-Topic: [TLS] DTLS 1.3
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Date: Fri, 08 Jul 2016 11:06:46 +0000
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Cc: tls <tls@ietf.org>
Subject: Re: [TLS] DTLS 1.3
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Hi Nikos,

On 08/07/2016 10:55, "Nikos Mavrogiannopoulos" <nmav@redhat.com> wrote:
>On Fri, 2016-07-08 at 09:29 +0000, Fossati, Thomas (Nokia - GB) wrote:
>
>> > > > How would the hash chain matching work for a server handling
>> > > > multiple
>> > > > clients?
>> > > Sorry, I'm not sure I understand the question.  Are you asking
>> > > what
>> > > happens if there is an Id collision between two separate hash
>> > > chains?
>> > No, my question is much simpler. How would a server handling for
>> > example 20000 clients, will figure to which chain a hash of H(x)
>> > belongs to? Will it have to iterate through all the chains (client
>> > states) and test for matching or there is something more clever
>> > than
>> > that?
>> Ah! The hash chain would be computed at the end of the handshake, so
>> all L
>> Ids can be put in a hash table that maps them to the same DTLS
>> context.
>> When a data record comes in, its Id can be used to look up the
>> context in
>> O(1).
>> Clearly the server needs to negotiate a sensible L if it doesn't want
>> to blow up.
>
>As long as both the client and the server are able to notify each other
>that they only support L=1 I'd be content with it.

In the straw man I proposed above, it is sufficient for the server to say
L=1.  In that case the client will trigger a re-handshake when its Id
rollover policy says so.

>However, I'd prefer
>a simple solution and defer the hash chain negotiation for a separate
>extension, since we cannot foresee how and if such an extension will be
>used. As we saw with heartbleed any unused complexity is not necessary
>adding value.

I wholeheartedly agree on the implementation simplicity goal.  I think the
proposal allows both sides to be as simple as they want to.

Cheers, t