[Security] Gajim 0.12's E2E encryption UI

Simon Josefsson simon at josefsson.org
Thu Aug 21 11:22:46 CDT 2008

Peter Saint-Andre <stpeter at stpeter.im> writes:

> Simon Josefsson wrote:
>> Brendan Taylor <whateley at gmail.com> writes:
>>> I've posted a description (with screenshots) of Gajim 0.12's end-to-end
>>> encryption UI: <http://necronomicorp.com/lab/gajim-0.12-esessions-ui>
>>> I think it's generally a good model and would like to be able to do
>>> something similar, whatever system we end up with.
>> That's useful, it looks like a fairly good user experience.
>> XMPP could use TLS and OpenPGP and achieve a similar user experience,
>> here's how:
>> Each client generate an OpenPGP key for the user when she creates an
>> account.  
> Or, presumably, a self-signed DSA or RSA key?

You mean a X.509 certificate?  Sure.

>> Instead of verifying a SAS in your example above, the users
>> needs to verify the OpenPGP fingerprint.  If a SHA-1 hash is too
>> techno-babbly, a human-readable transformation of the fingerprint could
>> be used.  Advanced users can configure the client to use their already
>> existing OpenPGP key if they want to re-use it for XMPP, which allows
>> for re-use of the existing web of trust.
> Right, or re-use an existing X.509 cert (some organizations issue the
> latter to their employees) or obtain such a cert from a CA (e.g., the
> one we run at xmpp.net).

The difference is that there is no simple web-of-trust mechanism that
allows client X.509 certificates to cross-sign each other.  You can
specify one, but then it begs the question why you don't use OpenPGP

>> Advanced clients could notice when the remote's OpenPGP key is already
>> trusted via the web-of-trust, and then print both the OpenPGP
>> fingerprint and the names of all keys in the OpenPGP trust path.  This
>> allows users to have more confidence of the remote identity before
>> verifying the OpenPGP fingerprint herself.
> Right.
> As far as I can see, we would treat all of the following in roughly
> the same way:
> - X.509 cert
> - OpenPGP key


> - DSA key
> - RSA key

Using raw RSA/DSA keys are often more complicated than it sounds: you
need to invent several thing (e.g., naming, signing hierarchies, and
algorithm agility) that is already done in OpenPGP and X.509.

> Some of these might be more trusted than others (e.g., CA-issued cert,
> OpenPGP key that's in my WoT), but all of them can be used to show a
> fingerprint (or potentially SAS, or shared password a la SRP) to the
> user.


> The first interaction might involve a leap of faith (in the case of
> self-signed keys). Or if we can figure out a way to check fingerprints
> with other trusted entities on the network (e.g., people in my contact
> list), the leap of faith might be slightly less scary (e.g. this is
> what some people do now for ssh -- ask the server admin who creates
> your account what the fingerprint should be).

Yes.  I believe a leap-of-faith ssh-style approach for simplicity
combined with a web-of-trust mechanism for re-use of earlier
leap-of-faith assumptions would be ideal.


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