The document discusses Pretty Good Privacy (PGP) and S/MIME, which are protocols for securing email communications. PGP uses public/private key encryption and digital signatures to provide confidentiality, authentication, integrity and non-repudiation of messages. It operates by encrypting messages and signatures with session keys, and attaching encrypted session keys. S/MIME also provides security features like encryption and signatures, using X.509 certificates managed through a combination of certificate authorities and PGP's web of trust model. Both aim to protect the confidentiality, authenticity and integrity of email messages.

1. Cryptography and Network Security Third Edition by William Stallings Lecture slides by Lawrie Brown

11. PGP Operation – Summary