RFE: Unsignable keys

[David Shaw]

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).

[..]

> This could be useful for corporate and verification keys that would then be 
> used to sign other keys but could only be signed by keys owned by the 
> original key owner. These signatures could be used to bring the key into the 
> WoT without allowing any of the noise that pollutes current robot / 
> impersonal keys.
> 
> If the PGP Global Directory Verification Key was unsignable, only those with 
> access to the secret key within PGP GD would be able to sign it.
> 
> Anyone else would get a telling off from GnuPG:
> "This is a verification key - it cannot be verified or signed without access
> to it's secret key. Your request to sign this key has been ignored."

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.
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.

David