This is the system used by p≡p:

The full set of Privacy Status’ are:

• Gray/Unknown/Unsecure/Unreliable Security

Unknown is commonly for outgoing messages where no contact or address has yet been added to the To, Cc or Bcc fields of an email or message.

Unsecure or Unsecure for Some means that p≡p cannot find a way of sending or receiving the communication with any form of encryption (to all recipients if Unsecure for Some). This represents the default situation today which, in the case of email, usually must be considered as “secure” as sending a physical post card.

Unreliable means that p≡p cannot find a way of sending or receiving the communication reliably. So, for example, the communication could have been sent using S/MIME. With S/MIME it's known that if one public Certificate Authority (CA) is subverted then the security of the entire system is lost — potentially subverting all the entities that trust the compromised CA.

• Yellow/Secure:

The communication is encrypted using state-of-the-art technology. However, your communication partner still needs to be trusted by completing a handshake.

• Green/Secure & Trusted:

The communication is encrypted using state-of-the-art technology and your communication partner is trusted. Trust is confirmed with a handshake where, using a side-channel (e. g. by phone call), communication partners verify they are each who they say they are and the communication can be fully trusted by all reasonable means expected from a regular user.

• Red/Mistrusted, Under Attack:

Mistrusted means that you have previously failed a handshake. You cannot trust that your communication partner is who they say they are.

Under Attack means that either a man-in-the-middle (MITM) attack has to be assumed or another (serious) cryptographic error occurred. The communication channel must be considered unsecure and any exchanged information not private.