Re: [tcpm] Privacy problems of TCP Fast Open

Michael Tuexen <> Tue, 21 May 2019 16:34 UTC

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From: Michael Tuexen <>
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Date: Tue, 21 May 2019 18:34:38 +0200
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Subject: Re: [tcpm] Privacy problems of TCP Fast Open
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> On 21. May 2019, at 14:25, Erik Sy <> wrote:
> On 5/21/19 12:18, Michael Tuexen wrote:
>>> On 21. May 2019, at 09:52, Erik Sy <> wrote:
>>> Hi Michael,
>>> thanks for this question!
>>> Yes, TFO cookies are bound to the clients (local) IP address. However, a
>>> client with a static local IP address in a home network will use the
>>> same TFO cookie independently of it's publicly visible IP address. As a
>>> result, TFO cookies present an independent tracking mechanism, which
>>> does not necessarily rely on the client's publicly visible IP address.
>> How often do the public addresses change?
> I do not have a general answer to your question. In the case of my home
> network, my ISP assigns me at least every 24 hours a new IPv4 address.
I also had this on my DSL line (I guess we both live in Germany),
but since my telephone line was moved to "all IP", the assignment stays
up for months.
> Additionally, I can initiate a change of my network's public IP address
> at anytime.
> TFO cookies allow basically unlimited tracking periods because they do
> not have an expiration mechanism. Thus, even infrequently changed IP
> addresses can be correlated.
An implementation can do implement such a thing and even allow an API for it.
For testing I'm flushing the cookie cache quite often...
>> One could extend the TFO API in
>> a way that the application can request a new cookie by only sending 
>> a cookie request.
> I do not think this is an appropriate countermeasure.
> From my perspective, caching TFO cookies in the kernel is a more
> fundamental privacy problem. This design requires applications to share
> a pool of TFO cookies, which allows tracking across several
> applications. For example, this prevents user's to separate their online
> activities across different browsers.
They would share the IP address. If they decide to trigger a new address
binding on the access router, why couldn't they trigger flushing the

Best regards
>>> Returning to your example, onion routing does not necessarily protect
>>> you against tracking via TFO cookies.
>> Yepp, that is what I wanted to say. 
>> But using TFO in that case doesn't
>> make much sense.
> Yes, TFO does not make sense if user privacy is at stake. Thus, we
> should warn users about these risks of RFC 7413.
> Best regards,
> Erik