How to decrypt a message while preserving the signature?

[Mark H Weaver]

I wrote:
> I'm reading RFC 4880 now, to get my own answers.  Still, I would be
> grateful if someone with deeper knowledge would answer my question.

Quoting the last paragraph of section 2.1 of RFC 4880:

   Both digital signature and confidentiality services may be applied to
   the same message.  First, a signature is generated for the message
   and attached to the message.  Then the message plus signature is
   encrypted using a symmetric session key.  Finally, the session key is
   encrypted using public-key encryption and prefixed to the encrypted
   block.

Exactly as I said.  So what I'm asking for can certainly be done.

      Mark