In order for a public key scheme to be successful, users must be guaranteed that the public key of another user truly belongs to that user. The means by which a public key is bound to a user is the public key certificate. The public key certificate is a collection of information issued and signed by a CA (Certification Authority). A certificate contains:
When a CA issues a certificate to a user, the CA provides a copy of its public key. With this key the user can validate certificates issued to other users subscribing to the same authority. To validate certificates issued by different authorities, a certification path (i.e., a path of trusted certificates) must be constructed between the two users. For example, if User1 subscribes to CA1, User2 subscribes to CA2, and both CA1 and CA2 have a trusted relationship with CA3, the certification path (C1, C3, C2) can be constructed, allowing User1 and User2 to obtain each other's public key. This concept of CAs generating certificates for other CAs is called cross certification. Figure 11.10 illustrates a hierarchical CA structure for cross certification. In the hierarchical model, CAs only generate certificates for the entities (i.e., CAs or users) below them.
An asymmetric key management scheme is presented in the directory system authentication framework, described in Recommendation X.509 [CCI88d]. The directory can be used to store public key certificates for MHS entities. These certificates can be accessed by other MHS entities to compute and/or validate the integrity and confidentiality of MHS messages.
Within the MHS, public key certificates may be conveyed by several methods. When an MHS entity initiates a connection to a peer, it may transfer its public key certificate for the peer to use to validate its credentials. Certificates can be registered with MHS entities, and transferred in MHS messages and reports. MHS entities can also obtain public key certificates by some means outside the MHS, such as accessing the X.500 directory.
Figure 11.10: Hierarchical Model for Certification Authorities.
An important aspect of key management is the management of certificate revocation. Certificates may be revoked for a number of reasons including: