Deniability

[Robert J. Hansen]

> What stops her from sending me  real messages with this kind of content?
> Even  non-encrypted? I  could reply  "I don't  know what  you're talking
> about", but how  does the prosecutor care?

If the prosecutor has plaintext of the emails, it makes your claims of
innocence much easier to believe.  It's when the prosecutor cannot know
what the plaintext is that the prosecutor has an incentive to ramp up the
pressure immensely.

> The only way  I could get out
> of it is to show I don't have any connection with Alice

Not at all.  Imagine if you were using a non-deniable system, such as
plain-vanilla OpenPGP.  "This message was sent via a non-deniable system. 
There, see?  That's a correct signature from Alice, and it was encrypted
with my certificate.  There!  See?  She was just sending me a recipe for
potato chip dip for my Super Bowl party!"

The prosecutor is going to be afraid of what she can't see.  She has
Alice, saying you're in it up to your eyeballs: she has you, claiming
innocence: she has a bunch of messages which you say are deniable and you
can't prove anything but which Alice says "he's lying to you."  Really, I
feel sympathy for Paula: she's in a terrible spot.  Being able to present
your messages is a good way of breaking that logjam: suddenly, Paula's
wrath turns on Alice for her deceptiveness and deceit.

> way I could ever do that -- as Sven mention off-list, the mere existence
> of deniable systems gives me this danger.

Not as much as you might think.  You could also say that the evidence of
disk wiping programs makes it hard for you to claim, "but I never had that
data in the first place!"  In reality, if the cops search your hard drive
and see Evidence Eliminator, they're going to strongly suspect you of
trying to destroy something important: but if the forensicist comes back
and says, "nope, no evidence he ever downloaded a file wiper," it gives
your claims of innocence more weight.

> Also, when did Alice turn evil? :)

She and Bob have been overthrowing governments, committing securities
fraud, carrying on a torrid affair without their spouses' knowledge, etc.,
for a very long time, all despite the fact they've never met face to face,
they don't trust each other, and know they're under surveillance by the
secret police.

As one wag said, "a cryptographer is someone who doesn't think Alice and
Bob are crazy."