Protocols & PKIAcceptable
PGP/OpenPGP and S/MIME
End-to-end email and file encryption and signing
How it works
Both use hybrid encryption: a random symmetric key encrypts the message, and that key is encrypted with each recipient's public key; the sender signs with their private key. OpenPGP uses a decentralised 'web of trust' (or direct key verification), while S/MIME uses X.509 certificates from a CA hierarchy.
Status: Sound cryptography, but usability and client handling are the weak points.
Strengths
- End-to-end confidentiality that the mail server cannot read
- Signatures provide integrity and non-repudiation
Weaknesses and known attacks
- EFAIL (2018) showed some mail clients could be tricked into leaking decrypted content through HTML features.
- Key distribution and verification are hard for ordinary users.
- Encrypted messages generally lack forward secrecy, and subject lines and metadata are often not protected.
Use it for
- Signing and encrypting sensitive email and files
- Signing software releases (OpenPGP)
Don't use it for
- Assuming metadata such as sender, recipient or subject is hidden
Exam tip
PGP = web of trust; S/MIME = CA-issued certificates. Both are hybrid: symmetric for the message, asymmetric for the key and the signature.