This interface defines the methods required of a certificate attribute. Examples of X.509 certificate attributes are Validity, Issuer_Name, and Subject Name. A CertAttrSet may comprise one attribute or many attributes. <p> A CertAttrSet itself can also be comprised of other sub-sets. In the case of X.509 V3 certificates, for example, the "extensions" attribute has subattributes, such as those for KeyUsage and AuthorityKeyIdentifier.