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Definition: two-factor authentication
The use of two independent mechanisms to verify the identity of a user. Two-factor authentication (2FA) is also called "2-step verification" (2SV). There are four factors:
One Each From Any Two
2FA requires one factor from any two of the above four categories; for example, a password and phone (#1 and #2) or a password and fingerprint scan (#1 and #3). A password plus a question such as "what city were you born" may be two factors, but they both fall into category #1. See FIDO, multi-factor authentication, authentication, smart card, password and one-time password.
Cellphone Second Factor
A Backup for 2FA
One Each From Any Two
2FA requires one factor from any two of the above four categories; for example, a password and phone (#1 and #2) or a password and fingerprint scan (#1 and #3). A password plus a question such as "what city were you born" may be two factors, but they both fall into category #1. See FIDO, multi-factor authentication, authentication, smart card, password and one-time password.
A common two-factor method is a number texted to the user's cellphone after logging into a site with username and password. The number must be copied to the website as in this example to complete the login. In this case, username and password are factor #1, and the phone is #2.
In this example, users are given a temporary authentication code in case the phone were lost.