Certificate revocation is a well recognized problem with the existing Public Key Infrastructure (PKI). Several proposals address this problem. We briefly review these proposals and compare them to the SEM architecture. For each proposal we describe how it applies to signatures and to encryption. For simplicity we use signed and encrypted Email as an example application. We refer to the entity validating and revoking certificates as the Validation Authority (VA). Typically, the VA is the same entity as the Certificate Authority (CA). However, in some cases these are separate organizations.
A note on timestamping. Binding signature semantics (Section 1.3) for signature verification states that a signature is considered valid if the key used to generate the signature was valid at the time signature generation. Consequently, a verifier must establish exactly when a signature was generated. Hence, when signing a message, the signer must interact with a trusted timestamping service to obtain a trusted timestamp and a signature over the user's (signed) message. This proves to any verifier that a signature was generated at a specific time. All the techniques discussed below require a signature to contain a timestamp indicating when a signature was issued. We implicitly assume this service. As we will see, there is no need for a trusted time service to implement binding signature semantics with the SEM architecture.