Secure Sockets Layer (SSL)
Secure Sockets Layer (SSL) is a deprecated protocol for establishing secure communication between computers.
SSL was deprecated in favor of TLS due to serious security flaws present in all versions of SSL.
Public-Key Infrastructure (PKI)
Public-Key Infrastructure (PKI) is a system that helps entities secure transactions and communications over networks.
Commonly, it involves a trusted 3rd party called a certificate authority which verifies ownership of a server’s public key by validating an entity’s certificate.