Non-PKI methods for public key distribution

The X.509 certification authority-based (CA) public key infrastructure (PKI) is a widely accepted PKI standard which defines data formats and procedures related to the distribution of public keys via public key certificates that are digitally signed by CAs. However, X.509 requires a huge and expensive infrastructure with complex operations. This overhead may be tolerable in some cases, but it is highly desirable to find other solutions. The objective of this paper is to present alternative simpler solutions to the X.509 PKI to save storage, bandwidth and to reduce the complexity of the operations. We offer three such solutions. They rely on the existence of passwords that are known to both users and service providers.