Show that an encrypted message was signed, without decrypting it

[Stefan Claas]

Helmut Waitzmann Anti-Spam-Ticket.b.qc3c wrote:
 
> Stefan Claas <sac at 300baud.de>:
> >Helmut Waitzmann Anti-Spam-Ticket.b.qc3c wrote:
> >> Stefan Claas <sac at 300baud.de>:
> 
> [The ability to check that an encrypted message has been signed.]  
> 
> 
> >It would allow Alice (in her organization), or others, to do a 
> >pre-check, with procmail etc., to set-up an auto-responder, 
> >informing Bob that he did not signed his message and that his 
> >message will be discarded.
> 
> >>> And is this optional in GnuPG, in case it is already 
> >>> implemented?
> >>
> >> As far as I know the order “first sign, then encrypt” is 
> >> mandatory, so there is no way for GnuPG to deviate from it. 
> >>
> >> And this is a good thing, as it thwarts Eve eavesdropping on the 
> >> originator's identity (i. e. Bob) of a message sent to Alice. 
> >
> >It should be not a mandatory feature and it should only append 
> >secured bytes, which are stating that Bob's message contains a 
> >signature (yes|no bytes), without revealing his identity.
> 
> What do you mean by the term “secured bytes”? 

Well, there should be a way that appended bytes to a signed and
encrypted message could not be exchanged by third parties, to
allow a pre-check (procmail etc.), like I explained, without the
need that Alice has to decrypt the message manually and then check
if the message was signed.

While not being off-topic, how does for example Zero Knowledge Proof
Encryption does a check that the identity of a user is proven, while
the user does not have to reveal his actual age? He only proves with
that, that he his over 18 years of age.

While I am no programmer or cryptographer, I think if this is possible
than something that I asked for should be somehow possible too, or not?

Regards
Stefan

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