Terms

Here's a list of terms which are used frequently throughout this site and in discussions about passkeys, FIDO2, and WebAuthn.

2FA user

A user whose account has 2FA turned on, i.e., who must present 2 authentication factors during sign-in.

2-Factor Authentication (2FA)

also sometimes referred to as MFA: multi-factor authentication or 2SV: two-step verification

This refers to a contract between a user and a Relying Party (RP) where the RP must collect at least two distinct authentication factors from the user during a bootstrap sign-in.

Account bootstrapping

A Relying Party (RP) authenticates a user without any prior knowledge of who the user is. This means that the RP not only has to verify the identity of the user (checking the password, verifying cryptographic signatures, etc), it also has to establish the identity of the user (figure out the user id, username, etc. of the user who’s signing in). This may happen when a user signs into an existing account for the first time on a newly-purchased device; or when a user logs into a website for the first time in a given browser instance. Or when a user logs into a website in a private browsing session. Or when a user signs into a mobile app for the first time on a given device (contrast this with reauthentication below).

Note that this is different from creating an account with a service in the first place.

Authentication factor

Information provided by a user (or one of the user’s devices) for purposes of authentication, usually in response to a login challenge. Often categorized into “knowledge factors” (e.g. passwords), “something you have” factors (e.g. another already signed-in device), and “something you are” factors (e.g. biometrics). Note that a single login challenge may collect multiple factors simultaneously.

Autofill UI

A privacy preserving list UI element that is rendered by the browser (or the OS platform in the case of native apps), in cooperation with the platform authenticator, on username and/or password fields that have the webauthn value included in the autocomplete attribute.

This UI element provides a list of passkeys that are available for the Relying Party (RP) on the local device, and may also provide an option to kick off Cross-Device Authentication (CDA) or use a FIDO2 security key.

A generic example of an autofill UI for passkeys is shown below:

Image

The technical name for this feature in the WebAuthn and Credential Management specifications is “Conditional Mediation”.

Cross-Device Authentication (CDA)

FIDO Cross-Device Authentication (CDA) allows a passkey from one device to be used to sign in on another device. For example, your phone can be linked to your laptop, allowing you to use a passkey from your phone to sign into a service on your laptop.

CDA is powered by the FIDO Client-to-Authenticator Protocol (CTAP) using “hybrid” transport. CTAP is implemented by authenticators and client platforms, not Relying Parties.

CDA Client

The client in a cross-device authentication flow is the device where the relying party is being actively accessed.

CDA Authenticator

The authenticator in a cross-device authentication flow is the device generating the FIDO assertion.

Conditional Mediation

See Autofill UI

Conditional UI

See Autofill UI

Device Public Key (DPK)

A Device Public Key (DPK) is a device-bound key that can be requested by a Relying Party (RP) for higher assurance scenarios where device continuity signals are desired. If supported by the authenticator, a signature from both the passkey and DPK are returned.

Example: Say that a sign-in request appears at a website along with some geolocation signal that has not been seen for this user account before, and is outside of the typical usage hours observed for the account. The risk may be deemed high enough not to allow the request, even with an assertion by a passkey on its own. But if a signature by a device-bound key that is well established for this user can also be presented, then that may tip the balance.

Just like a passkey, DPKs are unique to each RP.

Discoverable Credential

A Discoverable Credential (previously known as a “resident credential” or “resident key”) is a FIDO2/WebAuthn credential that is entirely stored in the authenticator (private key, credential ID, user handle, and other metadata). The Relying Party (RP) also stores a copy of the public key and credential ID

Passkeys (and single-device passkeys) are Discoverable Credentials.

Login challenge

A prompt served to the user that they need to satisfy.

Examples:

  • a prompt asking the user for their password
  • a prompt asking the user to confirm sign-in on another device (e.g., their phone)
  • a prompt asking the user to insert and activate their security key

Account bootstrapping and reauthentication usually consist of serving the user one or more login challenges.

Logging in

see Signing in.

Passkey

sometimes referred to as a multi-device passkey

Short version:

A passkey is a FIDO2 Discoverable Credential that requires user verification and is protected against device loss.

Longer version:

A FIDO2/WebAuthn credential that can reliably be used for bootstrap sign-in, without requiring other login challenges such as passwords. “Reliable” here means that the passkey should be available to, and usable by, the user whenever they need to sign in. This availability can be achieved through different means: for example, platforms could restore passkeys from a backup whenever a user sets up a new device, offer passkeys across different contexts (a passkey established from an app can be used in the browser when visiting the app’s website), or allow users to exercise passkeys across devices (by, say, using the passkey from a nearby phone when signing in from a laptop).

The important thing is that a passkey is there when the user needs it, and that it can be used without other login challenges. A WebAuthn credential that was created in a private browsing context and disappears when that browsing context is dismissed would not be considered a passkey (since it won’t be there for the user next time they’re trying to sign in). Nor would a U2F credential on a security key (since it requires additional factors for sign-in).

Platform authenticator

A FIDO authenticator that is built-in to a user’s device.

Reauthentication

Reauthentication happens when a Relying Party (RP) already knows who the user is, but would like to reconfirm this.

For example, this can happen before making sensitive changes to an account (adding a recovery email address, changing authentication methods, etc.): a relying party would typically ask the user to re-enter their password or perform some other action to reconfirm their control of the session. Likewise, when a mobile app asks the user to sign in every time the app starts (or a web site asks the user to sign in again after a period of inactivity), this is technically a reauthentication, since the app or web site can choose to remember the user’s identity after the account has been bootstrapped on the device (using things like cookies or local storage).

Relying Party (RP)

The website that is trying to ascertain and verify the identity of the user or perform FIDO authentication.

Roaming authenticator

A FIDO authenticator usable with any device the user is trying to sign-in from. Roaming authenticators attach to users’ devices in using USB, NFC, and/or Bluetooth. These authenticators are often referred to as “security keys”. A smartphone can also act as a roaming authenticator using FIDO Cross-Device Authentication.

Signing in

This can refer to either account bootstrapping or reauthentication.

Single-device passkey

A FIDO2 Discoverable Credential that requires user verification and is bound to a single authenticator. For example, FIDO2 security keys hold single-device passkeys as the credential cannot leave the device.

User Presence (UP)

A test of User Presence (UP) is used to ensure the user is in local proximity to the authenticator during an authentication or credential creation ceremony. UP is often satisfied by pressing a button or metallic area of a security key, or interacting with a platform authenticator on a device.

User Verification (UV)

User Verification (UV) requires the user to either perform a biometric gesture or enter the device PIN for the authenticator to authorize creation and use of the credential. In some cases, UV also satisfies a test of User Presence (such as when the authenticator itself has a biometric built-in or the device PIN is entered directly into the authenticator).

User-Verifying Roaming Authenticator

A User-Verifying Roaming Authentication (UVRA), also known as a first-factor roaming authenticator, can verify individual users through the use of biometrics, or through the user entering a device PIN. An important class of UVRAs are smartphones, in which case the “attachment” typically happens over a wireless connection.