Public Key Infrastructure (PKI) is a system that manages digital keys and certificates to ensure secure communication over the internet. It uses a pair of keys: a public key, which can be shared with anyone, and a private key, which is kept secret. This setup allows users to encrypt messages and verify identities, ensuring that data remains confidential and authentic. PKI relies on trusted entities called Certificate Authorities (CAs) to issue and manage digital certificates. These certificates confirm the ownership of a public key, helping users trust the identity of the parties they are communicating with. This infrastructure is essential for secure online transactions and communications.