A Public Key Infrastructure is a combination of policies, procedures, and computer hardware and software products providing a controlled framework for managing private and public key pairs. An effective Public Key Infrastructure is primarily focused on management rather than just the technology. A PKI also provides access to identifiers known as Public-Key Certificates.
A Public-Key Certificate is an electronic data structure that binds an entity (e.g. user) to a public key. While public keys must be published and highly available, changes to the public encryption keys must not be allowed, otherwise an attacker could replace a recipient’s public key with his own. The sender would then mistakenly encrypt the message for the attacker instead of for the intended recipient. Many of the regulations, means, and infrastructure installations are established to protect the authenticity and integrity of the public keys.