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There is a service called ProtonMail.

It encrypts the mail while sending between two proton-mail users, but how does it encrypts and decrypts while sending the to the non proton-mail services providers.

Also from version 3.6 there is only single password, no 2 passwords so how does it work?

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Based on the limited information available on their site that says they're using OpenPGP, here's my guess:

When sending email to another Protonmail account the body of the email is encrypted using the recipients public key (which is public, non-sensitive information).

When reading email, your password is used to generate an encryption key, possibly via PBKDF2, this is in turn used to decrypt the private key - hopefully locally on your device - which can then be used to decrypt the message.

This is pretty much how PGP works and has been used to encrypt email for quite some time, though there has never been a good, commercially available solution that I know of. As for encryption to non-Protonmail users, they write here that they don't support PGP (which is a bit odd since they seem to be using OpenPGP), so you can't send encrypted email to recipients outside of Protonmail.

Note that the messages themselves probably aren't encrypted/decrypted with the public/private keys, but instead encrypted with a randomly generated symmetric encryption key. The encryption key is then encrypted/decrypted with the asymmetric keys. This is done for performance and space-saving reasons.

As for their feature of sending encrypted content to other users via links, it's really no different to sending password-protected Zip files, Word documents or password protected links on Dropbox - though it may be more convenient since it's integrated into the email client.

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  • According to this now there is a way for premium customers of Protonmail how to send an encrypted message to users of other mail services. It seems that the link to display the message is send in the e-mail, not the message itself .
    – Ktator
    Aug 14, 2018 at 7:25
  • ...and you can use PGP with external parties.
    – Ktator
    Aug 14, 2018 at 7:32

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