Re: [TLS] Certificate compression draft

Sankalp Bagaria <> Wed, 05 April 2017 05:36 UTC

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Date: Wed, 5 Apr 2017 11:05:46 +0530 (IST)
From: Sankalp Bagaria <>
Reply-To: Sankalp Bagaria <>
Cc: R Balaji <>, "Bagaria, Sankalp" <>
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Subject: Re: [TLS] Certificate compression draft
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How is Certificate Compression advantageous over tls cached-info extension?
Only case I can think of is - when the certificate is being sent for the first
it can be compressed. Since the client doesn't have a copy of the certificate,
cached-info can't be used. Are there more cases where compression is useful?

Another point that need to be seen is that the time required to compress/
decompress is not more than the savings got in transmitting a compressed
certificate (smaller in size than complete certificate).

Thanks and Regards,
Sankalp Bagaria.

On Mon, Mar 6, 2017 at 2:58 PM, Victor Vasiliev <
<> > 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:
> <>
>  certificate - compression /
>   [ GitHub repo:
> <>  ]
> 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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