not shared, and moreover, use of the passphrase is strictly confined to the client system. Authentication via SSH is an example of passwordless authentication since it avoids the transmission of a shared secret over the network. In fact, SSH authentication does not require a shared secret at all. FIDO2 The FIDO U2F protocol standard became the starting point for the FIDO2 Project, a joint effort between the World Wide Web Consortium (W3C) and the FIDO Alliance. Project deliverables include the W3C Web Authentication (WebAuthn) standard and the FIDO Client to Authenticator Protocol (CTAP). Together WebAuthn and CTAP provide a strong authentication solution for the web. A FIDO2 authenticator, also called a WebAuthn authenticator, uses public-key cryptography to interoperate with a WebAuthn client, that is, a conforming web user agent that implements the WebAuthn JavaScript API. The authenticator may be a platform authenticator, a roaming authenticator, or some combination of the two. For example, a FIDO2 authenticator that implements the CTAP2 protocol is a roaming authenticator that communicates with a WebAuthn client via one or more of the following transport options: USB, near-field communication (NFC), or Bluetooth Low Energy (BLE). Concrete examples of FIDO2 platform authenticators include Windows Hello and the Android operating system. A FIDO2 authenticator may be used in either single-factor mode or multi-factor mode. In single-factor mode, the authenticator is activated by a simple test of user presence (e.g., a button push). In multi-factor mode, the authenticator (something that one has) is activated by either a PIN (something that one knows) or a biometric ("something that is unique to oneself"). Security code First and foremost, strong authentication begins with multi-factor authentication. The best thing one can do to protect a personal online account is to enable multi-factor authentication. There are two ways to achieve multi-factor authentication: Use a multi-factor authenticator Use a combination of two or more single-factor authenticators In practice, a common approach is to combine a password authenticator (something that one knows) with some other authenticator (something that one has) such as a cryptographic authenticator. Generally speaking, a cryptographic authenticator is preferred over an authenticator that does not use cryptographic methods. All else being equal, a cryptographic authenticator that uses public-key cryptography is better than one that uses symmetric-key cryptography since the latter requires shared keys (which may be stolen or misused). Again all else being equal, a hardware-based authenticator is better than a software-based authenticator