This new feature will enable ProtonMail users to communicate with other people that may not use ProtonMail, but another service with PGP encryption, in a more secure way. Because the contact fields can now be digitally signed, that means attackers can’t change a contact’s PGP public key, either. For instance, the contact manager will be expanded to store PGP public keys securely. ProtonMail said that the newly announced changes also pave the way for other security enhancements in 2018. The signing of the contacts is done with your private key, and the integrity of the contacts’ information is checked every time you access it. The contact fields are encrypted with your contact’s public key and can be decrypted only with your own corresponding private key. This also means ProtonMail doesn’t have access to the private key. The key is generated as a derivative of the user’s password, to which ProtonMail also doesn’t have access. ProtonMail generates a new private/public key pair in the user’s browser that is used exclusively for contact signing.

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