The Guardian takes source protection seriously. If you want to email a Guardian journalist, but you're worried that your email could be read by someone else, public key encryption can add an extra layer of security.
A safely encrypted email cannot be read in transit by email or internet service providers, or by other people who somehow gain access to the message. All being well, it can only be decrypted by the people you encrypted it for.
You do this by means of a person's "public key", which, as the name suggests, can be used by anyone. It allows you to encrypt material specifically for whoever owns a corresponding "private key". That private key is secret to the recipient of the encrypted material, and is used to decrypt the message back into something they can read.
This page carries links to the public keys of several Guardian journalists. You can use them to encrypt both messages and attachments that you wish to send to those journalists.
To correspond back and forth using public key encryption, you will also need to create your own private and public keys. You would keep the former to yourself, and share the latter with the Guardian journalist so they can encrypt their messages to you.
There are many online sources explaining how to make keys and how to encrypt messages and documents. The most popular software is free to use. The Guardian does not endorse any particular application.