Public Key Infrastructure (PKI) and Transport Layer Security (TLS/SSL) form the foundation of secure digital communications, enabling encrypted connections and identity verification across the internet. PKI provides the organizational framework for managing digital certificates and cryptographic keys, while TLS/SSL protocols use these certificates to establish secure, authenticated channels between clients and servers. Understanding certificate types, validation mechanisms, and the TLS handshake process is essential for implementing robust security architectures, from basic HTTPS websites to complex zero-trust environments requiring mutual authentication.