3.1.3.2 Out-of-Band Verifiers

Before · SP 800-63B

After · SP 800-63B-4

Out-of-Band Verifiers

Before · SP 800-63B

After · SP 800-63B-4

The verifier waits for an authenticated protected channel to be established with the out-of-band authenticator and verifies its identifying key. The verifier SHALL NOT store the identifying key itself but SHALL use a verification method (e.g., an approved hash function or proof of possession of the identifying key) to uniquely identify the authenticator. Once authenticated, the verifier transmits the authentication secret to the authenticator. The connection with the out-of-band authenticator MAY be either manually initiated or prompted by a mechanism, such as a push notification.

Depending on the type of out-of-band authenticator, one of the following SHALL take place:

• Transfer of the secret from the secondary to the primary channel . As shown in Fig. 1 , the verifier MAY signal the device that contains the subscriber’s authenticator to indicate a readiness to authenticate. It SHALL then transmit a random secret to the out-of-band authenticator and wait for the secret to be returned via the primary communication channel.

• Transfer of the secret from the primary to the secondary channel . As shown in Fig. 2 , the verifier SHALL transmit a random authentication secret to the claimant via the primary channel. It SHALL then wait for the secret to be returned via the secondary channel from the claimant’s out-of-band authenticator. The verifier MAY additionally display an address, such as a phone number or VoIP address, for the claimant to use in addressing its response to the verifier.

In all cases, the authentication SHALL be considered invalid unless completed within 10 minutes. Verifiers SHALL accept a given authentication secret as valid only once during the validity period to provide replay resistance , as described in Sec. 3.2.7 .

The verifier SHALL generate random authentication secrets that are at least six decimal digits (or equivalent) in length using an approved random bit generator as described in Sec. 3.2.12 . If the authentication secret is less than 64 bits long, the verifier SHALL implement a rate-limiting mechanism that effectively limits the total number of consecutive failed authentication attempts that can be made on the subscriber account as described in Sec. 3.2.2 . Generating a new authentication secret SHALL NOT reset the failed authentication count.

Out-of-band verifiers that send a push notification to a subscriber device SHOULD implement a reasonable limit on the rate or total number of push notifications that will be sent since the last successful authentication.

For additional verification requirements that are specific to the PSTN, see Sec. 3.1.3.3 .