From: "Bonatti Chris" <bonattic(_at_)ieca(_dot_)com>
This was one of the reasons that the attribute certificate
emerged in X.509. (There were other reasons too.) It allowed a
wide range of this "non-principal" binding data to be abstracted
from the main structure.
Yes, this capability "tag" model is also the heart of SPKI.
Certification tags are signed separately. The principal stays the same.
There is great strength in this kind of modularity. We
should consider this concept in Open-PGP.
PGP already has this modularity. Each username is signed separately.
A particular username signature can be separately expire or be revoked.
The fields are all there!
WSimpson(_at_)UMich(_dot_)edu
Key fingerprint = 17 40 5E 67 15 6F 31 26 DD 0D B9 9B 6A 15 2C 32
BSimpson(_at_)MorningStar(_dot_)com
Key fingerprint = 2E 07 23 03 C5 62 70 D3 59 B1 4F 5E 1D C2 C1 A2