RE: secure sign & encrypt

[Terje Braaten <[e-mail]>]

Derek Atkins <[e-mail]> wrote:
> I'm not sure exactly what you mean by when you say Alice saves a copy
> of the session key... How does Alice get that key to Charlie?  Also
> keep in mind that the interior and exterior encryptions SHOULD be
> using different session keys.  So, I don't understand what you mean?

She could send it to Charlie in a different mail, or add it on the outside
of the signature (ES) packet before she encrypt and send it to Charlie.
And since she control the building of the message, another solution
would be that she could also use the same session key in the interior and
exterior encryptions no matter what the protocol says should be done.

> 
> Can you show the packets that Charlie sees?  I don't see any way
> to add a new ESK on the interior message without invalidating the
> signature....

Charlie sees after decrypting the first layer
 PreSig[Alice]{ESK [Bob] Enc { Literal { Message } } }PostSig[Alice]

In addition he has, or can make ESK[Charlie]. This information he can
claim he must have got from Bob, since he is the only original recipient.

-- 
Terje Bråten