Beautiful Security

Enhancements to the Original Web of Trust Model

Although the model described earlier in this chapter is classic and fairly well known, few people know about the many extra features and enhanced sophistication that has been added to various later versions of PGP. As we recognized the limitations of the basic model, we added these elements to improve scaling and smooth over the rough edges.

The overarching areas of concern that came up again and again concerned revocation (or other reasons for keys becoming invalid), scaling problems, and the bloat caused as outdated signatures built up in keys. This section addresses each of those areas and a few other interesting enhancements.

Revocation

All PKIs need a way to revoke certificates. People are fallible creatures and they sometimes lose control of computers and keys. Systems can be lost or compromised, and the keys and certificates have to be declared invalid before they expire on their own.

Revocation is theoretically simple (although often still hard to propagate) in a hierarchical PKI such as X.509. Central authorities distribute revocations using the same channels they use to distribute the original authorizations to use keys.

The basic model for revocation

The original PGP Web of Trust described in the previous section offered two mechanisms for revocation:

Key revocation

Someone who has lost control of her key must be able to revoke the whole thing; she can’t depend on getting everyone who has signed it to revoke their signatures. This ...