Today, Facebook have added a new encryption features to the users profiles for enhancing email security. As users always get the email notification from Facebook side and these emails also include content of message, so this should be secure and private.
What is PGP Key ?
Facebook is using PGP (Pretty Good Privacy), an open standard widely used for email encryption. Each user has a public key and a private key; both are similar to what you might see generated from password platforms such as LastPass. Anyone with your public key can send you a message, and you use your private key to view it.
This new tool allows users to encrypt their email notification with there own PGP keys. It is always recommend not to share your Private key as this is the most important point for the security. If you are looking to use the PGP key then you have to used desktop but soon Facebook will add the same features within mobile version also.
This is a another great move from the Facebook team towards the users security and this makes users feel safer sharing information on a social network