What is the most basic form of two-factor authentication?

The most basic form of two-factor authentication is typically understood to involve the combination of something you know (like a password) and something you have or are (like a token, smart card, or biometric). In this context, biometrics refers to the use of unique physical characteristics, such as fingerprints or retinal scans, to verify identity.

While password-based authentication alone is single-factor since it relies only on something you know, combining that with biometrics introduces an additional layer of security, meeting the criteria for two-factor authentication. This method significantly enhances security since it's much more difficult for an unauthorized user to replicate physical traits compared to just stealing or cracking a password.

In contrast, tokens, smart cards, and other forms of authentication can also be components of two-factor authentication but typically require an additional element like a password or PIN to function effectively. Thus, while each option has its benefits, biometrics represents the fundamental concept of using something inherently unique to the individual, paired with something they know, to create a robust authentication process.