3.1.7 Multi-Factor Cryptographic Authentication

Multi-factor cryptographic authenticators encapsulate one or more authentication keys that SHALL only be accessible through the presentation and verification of an activation factor (i.e., a password or a biometric characteristic). Non-exportable authentication keys, suitable for use at AAL3, SHALL be stored in an isolated execution environment that is protected by hardware or in a separate processor with a controlled interface to the central processing unit of the user endpoint. Exportable authentication keys (usable at AAL2 or below) SHOULD be stored in appropriate storage that is available to the authenticator (e.g., keychain storage). If accessible to the endpoint being authenticated, authentication keys SHALL be strongly protected against unauthorized disclosure by using access controls that limit access to the authentication keys to only those software components that require access.

External (non-embedded) cryptographic authenticators SHALL meet the requirements for connected authenticators in Sec. 3.2.11. Each authentication event SHALL require input and verification of the local activation factor. 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 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 after an authentication transaction.

The requirements for a multi-factor cryptographic verifier are identical to those for a single-factor cryptographic verifier, as described in Sec. 3.1.6.2. Some multi-factor authenticators include flags to indicate whether an activation factor was used. If such a flag is present and indicates that an activation factor was not used, the authentication SHALL be treated as single-factor. Otherwise, verification of the output from a multi-factor cryptographic authenticator indicates that the activation factor was used.

A specific form of multi-factor cryptographic authentication is a subscriber-controlled wallet on the subscriber’s device, as described in Sec. 5 of [SP800-63C]. After the claimant first unlocks the wallet using an activation factor, the authentication process uses a federation protocol, as detailed in [SP800-63C]. The assertion contents and presentation requirements of the federation protocol provide the security characteristics required of cryptographic authenticators. As such, subscriber-controlled wallets on the subscriber’s device can be considered multi-factor authenticators through the activation factor and the presentation and validation of an assertion generated by the wallet.

Cloud-hosted wallets are not considered cryptographic multi-factor authenticators because access is maintained through authentication over the internet rather than locally. All authentication information from hosted wallets is treated as an assertion, as addressed in Sec. 5 of [SP800-63C].

Access to the private key SHALL require an activation factor. Authenticator activation secrets SHALL meet the requirements of Sec. 3.2.10. Biometric activation factors SHALL meet the requirements of Sec. 3.2.3, including limits on the number of consecutive authentication failures. The password or biometric sample used for activation and any biometric data derived from the biometric sample SHALL be erased immediately after an authentication transaction.

Authentication processes using subscriber-controlled wallets SHALL be used with a federation process as detailed in Sec. 5 of [SP800-63C]. Signed audience-restricted assertions that are generated by subscriber-controlled wallets are considered phishing-resistant because they prevent an assertion presented to an impostor RP from being used by the legitimate one. Assertions that lack a valid signature from the wallet or an audience restriction SHALL NOT be considered phishing-resistant. Assertions SHALL also include sufficient information to determine the nature of the activation method used to activate the wallet.

Some cryptographic authenticators allow the subscriber to copy (i.e., clone) the authentication secret to additional devices, usually via a sync fabric. This eases the burden for subscribers who want to use additional devices to authenticate. Specific requirements for syncable authenticators and the sync fabric are given in Appendix B.