Definition
Perfect forward secrecy (PFS) is a security feature ensuring that the compromise of a server's long-term private key does not compromise past communication sessions. It achieves this by generating unique, ephemeral session keys for each connection, derived from a temporary key exchange. This means even if an attacker obtains the server's main private key later, they cannot decrypt previously recorded encrypted traffic. For example, when you connect to a website using HTTPS, PFS ensures that if the website's server key is stolen, your past browsing history remains secure. This differs from older methods where a single long-term key might decrypt all historical data. PFS is a critical concept covered in cybersecurity certifications like CompTIA Security+ and CISSP.