Re: [saag] Liking Linkability

Ben Laurie <benl@google.com> Fri, 19 October 2012 13:52 UTC

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References: <CCA5E789.2083A%Josh.Howlett@ja.net> <tslzk3jsjv8.fsf@mit.edu> <201210181904.PAA07773@Sparkle.Rodents-Montreal.ORG> <FB9E461D-CA62-4806-9599-054DF24C3FD9@bblfish.net> <CAG5KPzxGz+4MywjP4knfbDr2gyvqUZc1HEBXgtaDfYT+DPg5yg@mail.gmail.com> <8AB0C205-87AE-4F76-AA67-BC328E34AF5E@bblfish.net> <CABrd9SQghpi6_rVQKxYXZDtM5HwvE7Kq7SUw5zi41ZRd3y2h9A@mail.gmail.com> <4324B524-7140-49C0-8165-34830DD0F13B@bblfish.net>
Date: Fri, 19 Oct 2012 14:52:25 +0100
Message-ID: <CABrd9SQU1uYVaVPedokHxeYkT=759rkPFfimWK1Z8ATzo3yNFA@mail.gmail.com>
From: Ben Laurie <benl@google.com>
To: Henry Story <henry.story@bblfish.net>
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Subject: Re: [saag] Liking Linkability
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On 19 October 2012 14:46, Henry Story <henry.story@bblfish.net> wrote:
>
> On 19 Oct 2012, at 15:31, Ben Laurie <benl@google.com> wrote:
>
>> On 19 October 2012 13:01, Henry Story <henry.story@bblfish.net> wrote:
>>>
>>> On 18 Oct 2012, at 21:29, Ben Laurie <ben@links.org> wrote:
>>>
>>>> On Thu, Oct 18, 2012 at 8:20 PM, Henry Story <henry.story@bblfish.net> wrote:
>>>>>
>>>>> On 18 Oct 2012, at 21:04, Mouse <mouse@Rodents-Montreal.ORG> wrote:
>>>>>
>>>>>>> [...]
>>>>>>> Unfortunately, I think that's too high of a price to pay for
>>>>>>> unlinkability.
>>>>>>> So I've come to the conclusion that anonymity will depend on
>>>>>>> protocols like TOR specifically designed for it.
>>>>>>
>>>>>> Is it my imagination, or is this stuff confusing anonymity with
>>>>>> pseudonymity?  I feel reasonably sure I've missed some of the thread,
>>>>>> but what I have seem does seem to be confusing the two.
>>>>>>
>>>>>> This whole thing about linking, for example, seems to be based on
>>>>>> linking identities of some sort, implying that the systems in question
>>>>>> *have* identities, in which case they are (at best) pseudonymous, not
>>>>>> anonymous.
>>>>>
>>>>> With WebID ( http://webid.info/ ) you have a pseudonymous global identifier,
>>>>> that is tied to a document on the Web that need only reveal your public key.
>>>>> That WebID can then link to further information that is access controlled,
>>>>> so that only your friends would be able to see it.
>>>>>
>>>>> The first diagram in the spec shows this well
>>>>>
>>>>> http://webid.info/spec/#publishing-the-webid-profile-document
>>>>>
>>>>> If you put WebID behind TOR and only have .onion WebIDs - something that
>>>>> should be possible to do - then nobody would know WHERE the box hosting your
>>>>> profile is, so they would not be able to just find your home location
>>>>> from your ip-address. But you would still be able to link up in an access
>>>>> controlled manner to your friends ( who may or may not be serving their pages
>>>>> behind Tor ).
>>>>>
>>>>> You would then be unlinkable in the sense of
>>>>> http://tools.ietf.org/html/draft-iab-privacy-considerations-03
>>>>>
>>>>> [[
>>>>>     Within a particular set of information, the
>>>>>     inability of an observer or attacker to distinguish whether two
>>>>>     items of interest are related or not (with a high enough degree of
>>>>>     probability to be useful to the observer or attacker).
>>>>> ]]
>>>>>
>>>>> from any person that was not able to access the resources. But you would
>>>>> be linkable by your friends. I think you want both. Linkability by those
>>>>> authorized, unlinkability for those unauthorized. Hence linkability is not
>>>>> just a negative.
>>>>
>>>> I really feel like I am beating a dead horse at this point, but
>>>> perhaps you'll eventually admit it. Your public key links you.
>>>
>>> The question is to whom? What is the scenario you are imagining, and who is
>>> the attacker there?
>>>
>>>> Access
>>>> control on the rest of the information is irrelevant. Indeed, access
>>>> control on the public key is irrelevant, since you must reveal it when
>>>> you use the client cert.
>>>
>>> You are imagining that the server I am connecting to, and that I have
>>> decided to identify myself to, is the one that is attacking me? Right?
>>> Because otherwise I cannot understand your issue.
>>>
>>> But then I still do not understand your issue, since I deliberately
>>> did connect to that site in an identifiable manner with a global id.
>>> I could have created a locally valid ID only, had I wanted to not
>>> connect with a globally valid one.
>>>
>>> So your issue boils down to this: if I connect to a web site deliberately
>>> with a global identifier, then I am globally identified by that web site.
>>> Which is what I wanted.
>>>
>>> So perhaps it is up to you to answer: why should I not want that?
>>
>> I am not saying you should not want that, I am saying that ACLs on the
>> resources do not achieve unlinkability.
>
> Can you expand on what the dangers are?
>
>>
>>>> Incidentally, to observers as well as the
>>>> server you connect to.
>>>
>>> Not when you re-negotiation I think.
>>
>> That's true, but is not specified in WebID, right? Also, because of
>> the renegotiation attack, this is currently insecure in many cases.
>
> WebID on TLS does rely on TLS. Security is not a goal one can reach,
> it is a way of travelling. So I do expect every security protocol to
> have issues. These ones are being fixed, and if more people build on
> them, the priority of the need to fix them will grow faster.
>
>>
>>> And certainly not if you use Tor, right?
>>
>> Tor has no impact on the visibility of the communication at the server end.
>
> You really need to expand on what the danger is. Because again
> I think you are thinking of the site I am connecting to as the attacker.
> But I may be wrong.

I'm getting quite tired of this: the point is, you cannot achieve
unlinkability with WebID except by using a different WebIDs. You made
the claim that ACLs on resources achieve unlinkability. This is
incorrect.

So yes, the scenario is there are two sites that I connect to using
WebID and I want each of them to not be able to link my connections to
the other. To do this, I need two WebIDs, one for each site. ACLs do
not assist.

>
>>
>>>
>>>
>>> Social Web Architect
>>> http://bblfish.net/
>>>
>>>
>>> _______________________________________________
>>> saag mailing list
>>> saag@ietf.org
>>> https://www.ietf.org/mailman/listinfo/saag
>>>
>
> Social Web Architect
> http://bblfish.net/
>