Re: [TLS] ESNI padding the Certificate message

Eric Rescorla <ekr@rtfm.com> Thu, 13 December 2018 16:54 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Thu, 13 Dec 2018 08:53:33 -0800
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Subject: Re: [TLS] ESNI padding the Certificate message
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On Thu, Dec 13, 2018 at 8:03 AM Viktor Dukhovni <ietf-dane@dukhovni.org>;
wrote:

> On Thu, Dec 13, 2018 at 07:51:17AM -0800, Eric Rescorla wrote:
>
> > Random padding does poorly with repeated trials. So, for instance,
> > if I get to observe a large number of requests from the same client
> > to the same server, you can gradually infer the length of the cert
> > chain.
>
> Yes, I was aware of this.  But one might also note that may services
> are not behind a CDN, and that data traffic patterns after the
> handshake can also fingerprint the target service.
>

Yes. But the idea here is to gradually remove fingerprinting surface, and so
the mitigations we recommend should work.

-Ekr


Session resumption helps to reduce the number of full handshakes
> that leak some information about the certificate size.
>
> Users who really want (better) protection against traffic analysis
> should use Tor.  TLS traffic analysis protection from ESNI et. al.
> is IMHO best-effort protection for casual use.
>
> If a user visits just one site (and no others) at a particular CDN,
> absent traffic flow obfuscation ala Tor, the user's TLS traffic
> will eventually stand out from the noise.
>
> --
>         Viktor.
>
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