The main purpose of a public key infrastructure (PKI) is to provide a secure framework for managing digital certificates and public-key encryption. PKI enables the secure exchange of information over the internet by ensuring the identity of users and devices. It encompasses the generation, distribution, and management of digital certificates that verify public keys, which are crucial for secure communications and transactions, such as SSL/TLS connections. PKI plays a vital role in various applications, including email encryption, secure website access, and protecting sensitive data during transmission.