Re: Trust and public keys

[Jens Lechtenboerger]

On 2015-11-15, at 21:07, Uwe Brauer wrote:

> If I had to communicate something really secret say with Ed Snowden, I
> would use of course use gpg[1] and not smime, ,
> then I would try somehow to compare the fingerprints of the keys by some
> secure means (a secure chat).
>
> Now if you say that all the above scenarios are usually out of reach of
> «normal» attackers,

That came out wrong, then.  Part of my problem would be to figure
out the “real” e-mail address of “Ed Snowden”.  If you registered
the fresh e-mail address “ed.snowden@gmail.com” and uploaded a
matching key to usual keyservers, then I might fall for that.  No
special attack skills required.

I don’t know too much about CAs that issue e-mail certificates for
free.  However, based on your description of Comodo I guess that you
could also obtain an S/MIME certificate in the above case (for
ed.snowden@gmail.com after registering that address).  So the
“trust” built into S/MIME seems worthless.

> When I apply for a certificate the private key is generated by the crypt
> module of my browser. Are you suggesting that this is also hacked? That
> indeed would be disastrous. Then indeed the intruder could obtain a copy
> of my private key and sell it to some sinister organisation.

For me as malicious CA (or intruder into a CA) there is no reason to
steal the private key as I could generate a certificate with
matching private key in your name for your e-mail address, which is
“trusted”.  Then I could send signed e-mails in your name.  That
alone might get you into trouble, but you might receive responses
that alert you about some ongoing attack.  If I was a powerful
attacker, able to replace e-mails on the way, I could additionally
re-encrypt (modified) responses to your real certificate (or drop
messages entirely), and you would never know I was there.

If I cannot replace e-mails on the way, I can still send “trusted”
signed e-mails in your name and tell the recipients to switch to
different e-mail addresses with “trusted” certificates.  Then,
again, I can re-encrypt responses to your real certificate and
e-mail address.

Best wishes
Jens