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[liberationtech] Announcing a privacy preserving authentication protocol

Guido Witmond guido at witmond.nl
Wed Mar 13 03:36:57 PDT 2013


Thanks for bringing up these points.

On 03/13/2013 01:53 AM, Steve Weis wrote:
> At its core of this proposal, sites run their own CAs and users 
> install site-specific client-side certificates. Many organizations 
> have been doing this for years. For example, MIT: 
> http://ist.mit.edu/certificates .
>

In these organisations the people need to know each other by name. MIT 
is not going to sign any certificate unless they know the person is 
affiliated to them. By signing they state that the person belongs to  
MIT. It's a different (and good use of client certificates).

Eccentric offers client certificates to stay anonymous while creating an 
account. It's like the difference between email accounts at google.com 
and gmail.com. I assume that an email from the first is a Google 
employee but the second is not.

> I like client certificates as an additional factor in general, but 
> user enrollment across multiple devices, browser and platform 
> compatibility, and revocation of lost devices are a pain. I think the 
> biggest adoption of client certificates has been in large 
> organizations with managed devices and support staff.
>

Current browsers are very lousy with handling client certificates. I 
believe that to be the reason that only large organisations can deploy 
them successfully.

I also start to believe that the browsers are the biggest trojan horse 
on the computer. They're being written by advertisers and software 
companies that want world domination over our privacy. And people wonder 
why there is no privacy? :-)

> Incidentally, there have been attacks to use client certificates as 
> persistent "supercookies" to track users, but I don't know the current 
> state of how browsers handle this. Here's an old PoC: 
> http://0x90.eu/ff_tls_poc.html . Firefox 4 at least prompts you before 
> dumping your cert to https://www.apache-ssl.org/cgi/cert-export .

This is quite a big concern.

But I counter that risk by specifying that the browser must not log in 
automatically to any site. To log in a user decision. My ideas: only 
when the user browses to a site, ( a deliberate action) may the browser 
log in automatically when the user has specified that.
When the site is just linked from another page, it must prevent 
linkability. See: 
https://blog.torproject.org/blog/improving-private-browsing-modes-do-not-track-vs-real-privacy-design.

(there is so much to write before this becomes a real protocol).

>
> The author also makes claims this could prevent cross-site scripting 
> with a "cryptographic same origin policy". I don't buy that, since XSS 
> attacks could still be served from sites with valid certificates. If 
> someone has a vulnerable web app, it's still going to be vulnerable.
>

The problem comes from the 'we trust the world' model in browsers.

The fact that a javascript comes in from a site that has a certificate 
does not imply it is trusted. That's the mistake that current browsers 
make.

The Cryptographic Same Origin Policy states that all servers with 
certificates from the local CA are a single trust-domain. Resources 
served from other sites (signed by a different CA) have a different 
trust-domain.

The browser is able to distinguish between the two trust domains. If it 
doesn't take heed, it's broken.


> Finally, this proposal requires changes on server-side authentication 
> and potentially in browsers themselves. Sites don't typically change 
> their authentication system unless it drives user adoption (e.g. 
> OpenID or Facebook Connect) or is needed for security (e.g. 2-factor 
> auth). I don't see any incentives for adoption here.
>

Indeed, it needs changes at the server, and many changes at the browser.
Driving it to adoption is the difficult part (as always). That's why I 
brought it to this list.


With regards, Guido Witmond.



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