The use of strong public encryption has always been popular among geeks. Perhaps the most commonly used and most beloved encryption for e-mail is Pretty Good Privacy (PGP); started as a free method for protecting emails or other sensitive information, later turned into a cornerstone for a large company. As PGP became more corporate, costly and used patented algorithms, another project, GnuPG, sprung up to continue to offer strong encryption to the masses.

One foundation of reliable encryption is trust. The use of encryption between two or more people relies on you being sure that the message you sent is properly encrypted to and able to be decrypted solely by the intended recipient. When using a friend's GPG key, you must be sure that the key was created by and belongs solely to your friend. Otherwise, you may send mail that your friend cannot read (if they don't have the key you encrypted to), or worse, mail that some other party can read (if that party does have the key you encrypted to).