3.1.5 Multi-Factor OTPs

Multi-factor OTP authenticators operate similarly to single-factor OTP authenticators (see Sec. 3.1.4.1), except they require the presentation and verification of an activation factor (i.e., a password or a biometric characteristic) to obtain the OTP from the authenticator. Each use of the authenticator SHALL require the input of the activation factor.

In addition to activation information, multi-factor OTP authenticators and verifiers contain two persistent values: 1) a symmetric key that persists for the authenticator’s lifetime and 2) a nonce that is either changed each time the authenticator is used or based on a real-time clock.

The secret key and its algorithm SHALL provide at least the minimum security strength specified in the latest revision of [SP800-131A] (i.e., 112 bits as of the date of this publication). The nonce SHALL be of sufficient length to ensure that it is unique for each operation of the authenticator over its lifetime. If a subscriber needs to change the device on which a software-based OTP authenticator resides, they SHOULD bind the authenticator application on the new device to their subscriber account, as described in Sec. 4.1.2, and invalidate the authenticator application that will no longer be used. Alternatively, the subscriber MAY export the secret key and store it in a sync fabric that meets the requirements in Appendix B.2 and retrieve the key with their new device.

The authenticator output is obtained using an approved cryptography block cipher or hash function to securely combine the key and nonce. In coordination with the verifier, the authenticator MAY truncate its output to as few as six decimal digits or equivalent.

If the nonce used to generate the authenticator output is based on a real-time clock, the nonce SHALL be changed at least once every two minutes.

Authenticator activation secrets SHALL meet the requirements of Sec. 3.2.10. A biometric activation factor SHALL meet the requirements of Sec. 3.2.3, including limits on the number of consecutive authentication failures. The unencrypted key and activation secret or biometric sample and any biometric data derived from the biometric sample (e.g., a fingerprint image and feature locations produced by a fingerprint feature extractor) SHALL be erased immediately after an OTP has been generated.

Multi-factor OTP verifiers effectively duplicate the process of generating the OTP used by the authenticator without requiring a second authentication factor. As such, the symmetric keys used by authenticators SHALL be strongly protected against unauthorized disclosure by access controls that limit access to the keys to only those software components that require access.

When binding a multi-factor OTP authenticator to a subscriber account, the verifier or associated CSP SHALL use approved cryptography for key establishment to generate and exchange keys or to obtain the secrets required to duplicate the authenticator output.

The verifier SHALL use approved encryption and an authenticated protected channel when collecting the OTP. Verifiers SHALL accept a given OTP only once while it is valid to provide replay resistance, as described in Sec. 3.2.7. If a claimant’s authentication is denied due to the duplicate use of an OTP, verifiers MAY warn the claimant if an attacker has been able to authenticate in advance. Verifiers MAY also warn a subscriber in an existing session of the attempted duplicate use of an OTP.

Time-based OTPs [TOTP] SHALL have a defined lifetime that is determined by the expected clock drift in either direction of the authenticator over its lifetime plus an allowance for network delay and claimant entry of the OTP.

The verifier SHALL implement a rate-limiting mechanism that effectively limits the number of consecutive failed authentication attempts that can be made on the subscriber account, as required by Sec. 3.2.10.