since the authenticator secret is presumably better protected in hardware. This preference is reflected in the NIST requirements outlined in the next section. NIST authenticator assurance levels NIST defines three levels of assurance with respect to authenticators. The highest authenticator assurance level (AAL3) requires multi-factor authentication using either a multi-factor authenticator or an appropriate combination of single-factor authenticators. At AAL3, at least one of the authenticators must be a cryptographic hardware-based authenticator. Given these basic requirements, possible authenticator combinations used at AAL3 include: A multi-factor cryptographic hardware-based authenticator A single-factor cryptographic hardware-based authenticator used in conjunction with some other authenticator (such as a password authenticator) See the NIST Digital Identity Guidelines for further discussion of authenticator assurance levels. Restricted authenticators Like authenticator assurance levels, the notion of a restricted authenticator is a NIST concept. The term refers to an authenticator with a demonstrated inability to resist attacks, which puts the reliability of the authenticator in doubt. Federal agencies mitigate the use a restricted authenticator by offering subscribers an alternative authenticator that is not restricted and by developing a migration plan in the event that a restricted authenticator is prohibited from use at some point in the future. Currently, the use of the public switched telephone network is restricted by NIST. In particular, the out-of-band transmission of one-time passwords (OTPs) via recorded voice messages or SMS messages is restricted. Moreover, if an agency chooses to use voice- or SMS-based OTPs, that agency must verify that the OTP is being transmitted to a phone and not an IP address since Voice over IP (VoIP) accounts are not routinely protected with multi-factor authentication. Comparison It is convenient to use passwords as a basis for comparison since it is widely understood how to use a password. On computer systems, passwords have been used since at least the early 1960s. More generally, passwords have been used since ancient times. In 2012, Bonneau et al. evaluated two decades of proposals to replace passwords by systematically comparing web passwords to 35 competing authentication schemes in terms of their usability, deployability, and security. (The cited technical report is an extended version of the peer-reviewed paper by the same name.) They found that most schemes do better than passwords on security while every scheme does worse than passwords on deployability. In terms of usability, some schemes do better and some schemes do worse than passwords. Google used the evaluation