strong authentication

Strong authentication is  any method of verifying the identity of a user or device that is intrinsically stringent enough to ensure the security of the system it protects by withstanding any attacks it is likely to encounter.  

Strong authentication is a commonly-used term that is largely without a standardized definition. According to the European Central Bank (and the many organizations that follow its guidelines), strong authentication combines at least two mutually-independent factors so that the compromise of one method should not lead to the compromise of the second. Additionally, the authentication method must include one non-reusable element, which cannot easily be reproduced or stolen from the Internet.

The term strong authentication is often used synonymously with two-factor authentication (2FA) or multifactor authentication (MFA).  However, that usage is misleading because some types of very secure authentication rely on a single authentication factor.

In some cases, for example, systems using multiple challenge / response answers are considered strong authentication. However, such systems are based on multiple instances of the knowledge factor, rather than multiple independent factors and as such are single-factor authentication (SFA). Similarly, some types of biometric authentication are reliable enough when used alone that they are considered strong authentication.