Re: [TLS] Certificate compression draft

Benjamin Kaduk <bkaduk@akamai.com> Wed, 05 April 2017 16:14 UTC

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To: Eric Rescorla <ekr@rtfm.com>, Ryan Sleevi <ryan-ietftls@sleevi.com>
References: <1417765250.1980.1491370546230.JavaMail.open-xchange@webmail.cdac.in> <CAErg=HFM_QDXSQHir8+eViT9+H7t5G2_pLbFhGspzZ+vH6Q-Fg@mail.gmail.com> <CABcZeBMQ5wwkW2u7BToo+o86H9omL5wH7+595LXz+fjtYLa8NQ@mail.gmail.com>
Cc: R Balaji <balaji@cdac.in>, Sankalp Bagaria <sankalp@cdac.in>, "Bagaria, Sankalp" <sankalp.nitt@gmail.com>, "tls@ietf.org" <tls@ietf.org>
From: Benjamin Kaduk <bkaduk@akamai.com>
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Date: Wed, 5 Apr 2017 11:14:26 -0500
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Subject: Re: [TLS] Certificate compression draft
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On 04/05/2017 09:21 AM, Eric Rescorla wrote:
>
>
> On Wed, Apr 5, 2017 at 7:12 AM, Ryan Sleevi <ryan-ietftls@sleevi.com
> <mailto:ryan-ietftls@sleevi.com>> wrote:
>
>
>
>     On Wed, Apr 5, 2017 at 1:35 AM, Sankalp Bagaria <sankalp@cdac.in
>     <mailto:sankalp@cdac.in>> wrote:
>
>         Hello,
>          
>         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 time,
>         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?
>
>

The authors presented work noting that reducing the certificate size
when the client is first talking to a server can be useful to keep the
server's first flight into a certain number of packets, useful to avoid
hitting congestion control backoff and/or a cap on (reply size / request
size) to avoid DDOS amplification attacks, as for QUIC.

>     Does cached-info not represent a privacy info-leak by disclosing
>     past sessions prior to authenticating the new session? Versus
>     compression, which limits it to the session and thus reveals no
>     new/additional information. That was certainly true for TLS1.2
>
>
> This will also be true in TLS 1.3, even with encrypted certificates
> because (hopefully) they
> will be a lot smaller. Though you could of course pad out to the same
> size :)
>

The elephant in the room being that an attacker watching the network can
replay the observed ClientHello but using its own KeyShare and read the
resulting encrypted certificate reply.  (h/t davidben)

-Ben