W3C home > Mailing lists > Public > public-webid@w3.org > October 2012

Re: unlinkability

From: Melvin Carvalho <melvincarvalho@gmail.com>
Date: Sun, 7 Oct 2012 21:27:32 +0200
Message-ID: <CAKaEYhLx73aBH3T-qn6YwxiavBigdyttxxHE89=1PnGxSt2t7Q@mail.gmail.com>
To: Henry Story <henry.story@bblfish.net>
Cc: "public-webid@w3.org" <public-webid@w3.org>
On 7 October 2012 21:16, Melvin Carvalho <melvincarvalho@gmail.com> wrote:

>
>
> On 6 October 2012 12:03, Henry Story <henry.story@bblfish.net> wrote:
>
>>
>> On 6 Oct 2012, at 12:01, Melvin Carvalho <melvincarvalho@gmail.com>
>> wrote:
>>
>>
>>
>> On 6 October 2012 11:42, Henry Story <henry.story@bblfish.net> wrote:
>>
>>>
>>> On 6 Oct 2012, at 11:39, Melvin Carvalho <melvincarvalho@gmail.com>
>>> wrote:
>>>
>>>
>>>
>>> On 6 October 2012 11:25, Henry Story <henry.story@bblfish.net> wrote:
>>>
>>>> >>
>>>> >> (1) I think solves the unlinkability problem
>>>> >
>>>> > Can you explain what the unlinkeability problem is? Or for who it is
>>>> a problem?
>>>> >
>>>> > 4.  Unlinkability
>>>> >
>>>> >    Definition:  Unlinkability of two or more Items Of Interest (e.g.,
>>>> >       subjects, messages, actions, ...) from an attacker's perspective
>>>> >       means that within a particular set of information, the attacker
>>>> >       cannot distinguish whether these IOIs are related or not (with a
>>>> >       high enough degree of probability to be useful).
>>>> >
>>>> > This is something Harry brought up.
>>>>
>>>> Can you explain why it is problematic. It is not because he brought it
>>>> up
>>>> that it is problematic right? Or is he someone who sets the standards
>>>> of what is or is not problematic? Through what authority?
>>>>
>>>
>>> Harry stressed that this was a key consideration to him.  As an
>>> influential member of the social web (he was chair of the W3C Social Web
>>> XG), I would consider his opinions important.  His complain was that he
>>> raised this before, and that the webid group did not look at it.
>>>
>>>
>>> But you have not summarised in your own words what his complaint is. So
>>> how do you know we did not answer it?
>>>
>>>
>>> If we, as a group, are able to address such concerns, or show that we
>>> have evaluated them and proven then are non issues (for example in a FAQ),
>>> it may help bring the benefits of WebID to a wider audience.
>>>
>>>
>>> That is why I ask you to express in your words what the problem is, and
>>> see if you can come up with an answer to the
>>> problem. And indeed we should add this on a list of question and answers
>>> that comes up.
>>>
>>
>> I have quoted the passage cited by Hannes, Harry and others.
>>
>>
>> yes, but you have to develop that passage and see how it applies to
>> WebID. It is not an obvious passage at all, and it is not clear it applies
>> at all to WebID.
>>
>> It's something we (as a group) have been asked to look at.  In truth,
>> it's been quite a hard conversation to follow as there were many replies
>> and points raised in a short period of time.  I dont know if unlinking the
>> public key from the URI provides more 'unlinkability', it was just a
>> suggestion.
>>
>>
>>
>> But it seems unclear to me that the concerns have been addressed.
>>
>>
>> Well I did in fact answer that mail. But I am going to send out a new
>> mail right now, to make sure it is clear.
>>
>> Certainly there was no acknowledgement of that.
>>
>>
>> By whome? By Harry? He never acknowledges mails that don't go in his
>> direction.
>>
>
> OK, I've managed to look through a lot of this now.
>
> Unlinkability seems to be useful when you want to provide anonymity or
> pseudo anonymity.
>
> Both valuable use cases.
>
> I am guessing the perception of those that have never tried webid may be
> that the certificate is sent *every* time.
>
> This can be avoided as follows:
>
> - Do not send a cert when the popup arises
> - Use a different browser
> - We create a public cert at http://webid.info/#anon
>

One other thing Joe Presbrey has done for anonymity is to write the
mod_auth_everyone apache modules which allows you to view a webpage
anonymously if you dont send a certificate.

There are more sophisticated techniques for unlinkability such as blind
signing [1] and knowledge proofs [2] which it may be valuable to look at,
depending on the use case (eg for DC nets or AV net).

[1] http://en.wikipedia.org/wiki/Blind_signature
[2] http://en.wikipedia.org/wiki/Proof_of_knowledge


>
> Pseudo anonymous identifies can be provisioned by WebID
>
> - One cert per identity
>
> Linkabiity is desirable in many cases as stated in the final paragraph of
> the IETF draft.
>
> BrowserID aka persona seems not to solve this issue as the cert sends:
>
> - The user's email address.
> - The user's public key for that address on that browser.
> - The time that the certificate was issued.
> - The time that the certificate expires.
> - The IdP's domain name.
>
> Additionally your webmail provider and/or mozilla can impersonate you as
> they control your private key server side.  By extension any agency that
> requests information from your webmail provider or mozilla can view your
> external data.
>
> Furthermore, your webmail provider and/or mozilla can sign you up for any
> services offered by a relying party *without you even knowing*.  This is
> quite scary in privacy terms and has me thinking twice whether I want to
> use BrowserID as a fallback to WebID, as was my original intention.
> Perhaps let the user decide.
>
> Maybe we should add these points to an FAQ
>
>
>>
>> Perhaps it is the nature of mailing lists that it can be challenging to
>> know when a consensus is reached or a problem has been solved.
>>
>>
>>>
>>> Henry
>>>
>>>
>>>
>>>
>>>>
>>>> Henry
>>>>
>>>> Social Web Architect
>>>> http://bblfish.net/
>>>>
>>>>
>>>
>>>  Social Web Architect
>>> http://bblfish.net/
>>>
>>>
>>
>>   Social Web Architect
>> http://bblfish.net/
>>
>>
>
Received on Sunday, 7 October 2012 19:28:00 UTC

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