This document discusses secure communications using cryptography. It explains how encryption works to secure messages by transforming them in a way that can only be decrypted by someone with the proper key. Symmetric encryption uses the same key to encrypt and decrypt, while asymmetric encryption uses separate public and private keys. The document outlines how public key infrastructure (PKI) allows people to securely communicate over the internet using digital certificates from a certification authority to verify identities and encrypt messages that can only be read by the intended recipient.