passing information among several users

[Klaus Slott]

Hi

On Mon, 21 Jan 2013 14:43:35 -0500
Jean-David Beyer <jeandavid8 at verizon.net> wrote:

> On 01/21/2013 11:56 AM, Rita wrote:
> > Masteruser will be generating data and I would like userA and userC
> > be able to decypt the data and others not to. However, in the
> > future I would like to add userE to decrypt the data and remove
> > userA (any old data she has is fine). I was wondering how I can
> > achieve this using gpg
> > 
> 1.) get gnupg software. http://gnupg.org/
> Install it, generate your keys and your revocation certificate (you
> never know when you will need ont.
> 2.) Upload your public key to a keyserver.
> 3, Have A, and C do the same.
> 3a.) If you want to anytime, have B and D do it too.
> 4.) When you want to send data get the public keys for A and C.
> 5.) Encrypt these data with the public the public keys of A and C.
> 
> Am I missing something?

I suspect that Rita was aiming at user E who at a later stage is added
to the allowed readers. User E will still not be able to read earlier
generated data. To my knowledge it is not possible to add another key
to a already encrypted message.
You would have to decode the original data and encode it again, this
time using user E and user C public keys. 

-- 
Klaus