Re: [TLS] Certificate compression draft

[Victor Vasiliev <[e-mail]>]

Hi Vlad,

This is still an open issue:
https://github.com/ghedo/tls-certificate-compression/issues/2

The problem here is creating a dictionary that is both neutral with respect
to
the certificate's issuing authority, and actually has a noticeable effect.
So
far my personal attempts at making such a dictionary have not been very
successful, but this might change.  Even if we get a dictionary, I do not
expect the effect to be large compared to the effect of just compressing the
chain in the first place.

  -- Victor.

On Mon, Mar 6, 2017 at 6:32 PM, Vlad Krasnov <[e-mail]> wrote:

> Hi Victor,
>
> Have you considered creating a common dictionary, similarly to what SPDY
> did for header compression?
>
> Cheers,
> Vlad
>
>
> On Mar 6, 2017, at 3:23 PM, Victor Vasiliev <[e-mail]> wrote:
>
> Hi Martin,
>
> I've measured the effect of compression on a corpus of popular website
> certificate chains I had lying around (Alexa Top 100k from a few years
> ago),
> and the effect seems to be about -30% of size at the median and -48% at
> 95th
> percentile (with Brotli, subtract 3-5% for zlib).
>
> I think the most dramatic effect from the compression is observed for the
> certificates with a lot of SNI values, which is not uncommon.
>
>   -- Victor.
>
> On Mon, Mar 6, 2017 at 6:06 PM, Martin Thomson <[e-mail]>
> wrote:
>
>> Hi Victor,
>>
>> Do you have any evidence to suggest that this reduces size in any
>> meaningful way?  Certificates tend to include both repetitious values
>> (OIDs), and non-repetitious values (keys).
>>
>> On 7 March 2017 at 09:58, Victor Vasiliev <[e-mail]> wrote:
>> > Certificate compression has been discussed on this list briefly before,
>> and
>> > there was some interest in at least considering a draft for it.  The
>> draft
>> > now
>> > exists (co-authored by Alessandro and myself), and it can be found at:
>> >
>> > https://datatracker.ietf.org/doc/draft-ghedini-tls-certifica
>> te-compression/
>> >   [ GitHub repo: https://github.com/ghedo/tls-certificate-compression ]
>> >
>> > The proposed scheme allows a client and a server to negotiate a
>> compression
>> > algorithm for the server certificate message.  The scheme is purely
>> opt-in
>> > on
>> > both sides.  The current version of the draft defines zlib and Brotli
>> > compression, both of which are well-specified formats with an existing
>> > deployment experience.
>> >
>> > There are multiple motivations to compress certificates.  The first one
>> is
>> > that
>> > the smaller they are, the faster they arrive (both due to the transfer
>> time
>> > and
>> > a decreased chance of packet loss).
>> >
>> > The second, and more interesting one, is that having small certificates
>> is
>> > important for QUIC in order to achieve 1-RTT handshakes while limiting
>> the
>> > opportunities for amplification attacks.  Currently, TLS 1.3 over TCP
>> > without
>> > client auth looks like this:
>> >
>> >   Round trip 1: client sends SYN, server sends SYN ACK
>> >     Here, the server provides its own random value which client will
>> >     have to echo in the future.
>> >   Round trip 2: client sends ACK, ClientHello, server sends
>> > ServerHello...Finished
>> >     Here, ACK confirms to server that the client can receive packets
>> and is
>> > not
>> >     just spoofing its source address.  Server can send the entire
>> > ServerHello to
>> >     Finished flight.
>> >
>> > In QUIC, we are trying to merge those two rounds into one.  The problem,
>> > however, is that the ClientHello is one packet, and
>> ServerHello...Finished
>> > can
>> > span multiple packets, meaning that this could be used as an
>> amplification
>> > attack vector since the client's address is not yet authenticated at
>> this
>> > point.
>> > In order to address this, the server has to limit the number of packets
>> it
>> > sends
>> > during the first flight (i.e. ServerHello...Finished flight).  Since
>> > certificates make up the majority of data in that flight, making them
>> > smaller
>> > can push them under the limit and save a round-trip.
>> >
>> > Cheers,
>> >   Victor.
>> >
>> > _______________________________________________
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>> >
>>
>
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