ProtonMail uses PGP for end-to-end encryption (learn more what is encrypted here: https://protonmail.com/support/knowledge-base/what-is-encrypted/). This means that your account has two keys, one public and one private. You can share your public key with other contacts outside of ProtonMail, so they can send you secure messages.
- Go to Settings
- Open the Keys tab
- Click on “Download Public Key”
Your public key will be downloaded in the format “protonmail_public_your.username.txt” locally. You can share this key with other people, or use it with Facebook by following the steps on this article Using ProtonMail with Facebook PGP emails