Private Email supports two methods of second-factor authentication, which are among the toughest technologies in the world today. TOTP (Time-based One-Time Password) is a standardized method that generates a changing temporary code that is unique each time, and expires as soon as the new one has been generated. U2F (Universal 2nd Factor) is a method that doesn't require any codes or software applications because it is hardware-based. Authentication happens with the help of a small USB or NFC device like YubiKey that stores your encrypted data.