Re: [TLS] ESNI padding the Certificate message

Viktor Dukhovni <> Thu, 13 December 2018 16:02 UTC

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Subject: Re: [TLS] ESNI padding the Certificate message
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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.

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.