3.1.3.1 Out-of-Band Authenticators

Before · SP 800-63B

After · SP 800-63B-4

Out-of-Band Authenticators

Before · SP 800-63B

After · SP 800-63B-4

The out-of-band authenticator SHALL establish a separate channel with the verifier to retrieve the out-of-band secret or authentication request. This channel is considered to be out-of-band with respect to the primary communication channel (even if it terminates on the same device), provided that the device does not leak information from one channel to the other without the claimant’s participation.

The out-of-band device SHOULD be uniquely addressable by the verifier. Communication over the secondary channel SHALL use approved encryption unless sent via the public switched telephone network (PSTN). For additional authenticator requirements that are specific to using the PSTN for out-of-band authentication, see Sec. 3.1.3.3 .

Email SHALL NOT be used for out-of-band authentication because it may be vulnerable to:

• Access using only a password
• Interception in transit or at intermediate mail servers
• Rerouting attacks, such as those caused by Domain Name System (DNS) spoofing

Confirmation codes that are sent to validate email addresses or are issued as recovery codes (see Sec. 4.2.1.2 ) are not authentication processes and not affected by the above prohibition.

The out-of-band authenticator SHALL uniquely authenticate itself in one of the following ways when communicating with the verifier:
• Using approved cryptography, establish a mutually authenticated protected channel (e.g., client-authenticated transport layer security (TLS) [RFC8446] ) with the verifier. Communication between the out-of-band authenticator and the verifier MAY use a trusted intermediary service to which each authenticates. The key used to establish the channel SHALL be provisioned in a mutually authenticated session during authenticator binding, as described in Sec. 4.1 .

• Authenticate to a public mobile telephone network using a SIM card or equivalent secret that uniquely identifies the subscriber. This method SHALL only be used if a secret is sent from the verifier to the out-of-band device via the PSTN (i.e., SMS or voice) or an encrypted instant messaging service.

• Use a wired connection to the PSTN that the verifier can call and dictate the out- of-band secret. For the purposes of this definition, “wired connection” includes services such as cable providers that offer PSTN services through other wired media and fiber via analog telephone adapters.

For single-factor out-of-band authenticators, if a secret is sent by the verifier to the out-of-band device, the device SHOULD NOT display the authentication secret while it is locked by the owner. Rather, the device SHOULD require the presentation and verification of a PIN, passcode, or biometric characteristic to view the secret. However, authenticators SHOULD indicate the receipt of an authentication secret on a locked device.

If the out-of-band authenticator requests approval over the secondary communication channel rather than by presenting a secret that the claimant transfers to the primary communication channel, it SHALL accept a transfer of the secret from the primary channel and send it to the verifier over the secondary channel to associate the approval with the authentication transaction. The claimant MAY perform the transfer manually or with the assistance of a representation, such as a barcode or quick response (QR) code.