Re: [saag] Liking Linkability

Ben Laurie <benl@google.com> Fri, 19 October 2012 13:31 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>
Date: Fri, 19 Oct 2012 14:31:08 +0100
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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 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.

>> 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.

> And certainly not if you use Tor, right?

Tor has no impact on the visibility of the communication at the server end.

>
>
> Social Web Architect
> http://bblfish.net/
>
>
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