Understanding The Basics Of Two-Factor Authentication

Knowing a PIN or TAN code (ATM withdrawals, money transfers)

Having access to an email account (when verification codes are sent by mail)

Secret questions (often frowned upon as they are sometimes easy to guess)

Physical keys (card readers, USB keys)

Biometrics (fingerprint readers, iris scanners)

Mobile devices that can scan barcodes or QR codes and calculate a login code for one time use (Authy,  Google Authenticator)

Single Sign On (SSO): this is mostly used as a method to dampen the impact of using 2FA methods, particularly when given an authenticated user access to several resources. The idea is that once the user has been identified and approved, the SSO software provides access to all platforms tied to the SSO. Given the possible impact of a breach the login procedure for a SSO system is usually done by using a MFA procedure. Another consideration when choosing a SSO system is the consequences of a failure. If the SSO software goes offline, will this block the user from all the underlying resources?

Time-based One-time Password (TOTP): this is a special authentication method that uses an algorithm that calculates a one-time login code based on the time. The server and the user that wants to login both run simultaneous calculations with the same seed and time-stamp. If the results match, the user is granted access. Obviously the clocks need to be synchronized, although there usually is some leniency built into the procedure (up to a one minute difference is generally allowed). Since losing the machine that runs the algorithm or any other way that leaks the algorithm could allow access to the wrong person, this method is generally used as one factor in a MFA method.

Token Authentication: besides physical tokens, other tokens can be used as a means of authentication. Consider, for example, apps that run on your smartphone and can show an image to your webcam or play a sound which can be compared to an original. As this is not a very strong authentication method (for now) it is advisable to be used as one of the authentication factors and not the sole one.