PGP in Practice, Part 1: What it is and why you’d use it

If you've ever seen a message signed -----BEGIN PGP MESSAGE----- and wondered whether it was worth the friction, this series is for you. Over seven parts we'll walk through what PGP actually does, when it helps, when it doesn't, and how to use it from the four mail clients most operators reach for: Thunderbird, Gmail, Apple Mail, and Outlook. Then we'll cover key generation and the threat-model edges where PGP stops earning its keep.

This first part is the concept tax. Skip it if you're already comfortable with public-key cryptography and head straight to Part 2 (Thunderbird) — but the threat-model section at the end is worth a skim either way, because most criticism of PGP comes from misunderstanding what the protocol claims to protect.

PGP — Pretty Good Privacy, originally Phil Zimmermann's 1991 implementation, now standardised as OpenPGP — gives you two distinct operations that get confused often: encryption and signing.

Encryption lets someone lock a message so that only you can open it. The mental model that holds up: think of a padlock and a key. You hand out copies of your padlock (your public key) to anyone who might want to send you mail. Anyone can snap your padlock shut around their message. Only your private key opens it. The padlock copies are public; the key never leaves your machine.

Signing is the inverse operation. You lock something with your private key — the lock is forgery-resistant because only you have the private half. Anyone with your public key can verify that the lock is yours, and that the message hasn't been tampered with since you locked it. They can't forge the lock without your private key.

In practice an encrypted email usually does both at once: signed-and-encrypted means "this is from me, I can prove it, and only the recipient can read it." That's the default mode for any sensitive correspondence. Encryption-without-signing is unusual — it leaves the recipient unable to confirm the sender. Signing-without-encrypting is common for code commits, software releases, and public statements where the message itself is meant to be public but the authenticity matters.

Under the hood, a single email uses several primitives: a fresh symmetric key for the message body (because public-key crypto is too slow for long content), the recipient's public key to encrypt that symmetric key, and a hash of the message signed with the sender's private key. The user-facing tools hide this — you click "Encrypt + Sign" and the client orchestrates it.

PGP defends against a specific set of threats:

• Eavesdropping by mail providers and network operators. Google, Microsoft, your ISP, and any nation-state operating on the wire between you and your recipient see only ciphertext when both ends are using PGP. Without PGP, your provider reads everything (because that's how mail delivery works — the message body lands in their database in plaintext) and the wire-level encryption (TLS between mail servers) protects only against passive eavesdroppers, not the providers themselves.