RFE: Unsignable keys

[Gregor Zattler]

Hi David,
* David Shaw <dshaw at jabberwocky.com> [01. Jan. 2005]:
> On Sat, Jan 01, 2005 at 01:42:57PM +0000, Neil Williams wrote:
> > Request for Enhancement / Comments: GnuPG.
> > 
> > Would it be possible to create an --expert option to generate a key that 
> > CANNOT be signed (under any circumstances) unless BOTH secret keys are 
> > available? (signer has to have secret key anyway, these special keys would 
> > also need the signee secret key in the same keyring).
[...]
> What you suggest is not impossible, but has a number of caveats when
> done as part of OpenPGP.  The signature math of OpenPGP does not cover
> this sort of case, so such a flag would need to be somewhat advisory.

But then it is possible to add such signature with some knowledge
and a hex editor or an older implementation?

> This isn't to say that advisory flags are useless: most things like
> this in OpenPGP are advisory, and they work fairly well.
> 
> There are quite a few ways to do this, each with their plusses and
> minuses, but it comes down to the interoperability question.  It would
> have to be part of OpenPGP (and not GnuPG-specific) if it was to
> really work, and some consideration would have to be given to what the
> semantics were when an old implementation ignored the flag and signed
> anyway.

This special signature needs a special marker an older
implementation does not provide.  Newer implementation ignores
such signatures without correct marker.

But isn't this an example where shared secrets are useful?  Is
there any hope of shared secrets being part of OpenPGP in the
future?

Gregor