What is Two-factor Authentication (2FA)? How does it work? | Fortinet (2024)

Hardware tokens are one of the original types of 2FA formats. They are typically small key-fob devices that generate a unique numerical code every 30 seconds. When a user submits their first authentication request, they can head over to the key fob and issue the code it is displaying. Other forms of hardware tokens include universal serial bus (USB) devices that, when inserted into a computer, automatically transfer an authentication code.

An example of this is YubiKey, which is short for ubiquitous key, a security key that enables users to add a second factor of authentication to services like Amazon, Google, Microsoft, and Salesforce. The USB device is used when users log in to a service that supports one-time passwords (OTPs), such as GitHub, Gmail, or WordPress. The user plugs the YubiKey into their USB port, enters their password, clicks the YubiKey field, and touches a button on the device. It generates a 44-character OTP and automatically enters it on the user’s device to verify them with a possession 2FA factor.

Hardware token devices are generally expensive for organizations to distribute. Furthermore, they are easily lost by users and can themselves be cracked by hackers, making them an insecure authentication option.

Short message service (SMS) and text message 2FA factors are generated when a user attempts to log in to an application or service. An SMS message will be sent to their mobile device containing a unique code that the user then enters into the application or service. This 2FA factor type has been used by banks and financial services to verify purchases or changes that customers made to their online banking accounts. However, they are generally moving away from this option, given the ease with which text messages can be intercepted.

Similar to the SMS factor is voice call 2FA. When a user enters their login credentials, they will receive a call to their mobile device that tells them the 2FA code they need to enter. This factor is used less frequently but is deployed by organizations in countries that have low smartphone usage levels.

A more commonly used passwordless two-step authentication format is push notifications. Rather than receiving a code on their mobile device via SMS or voice, which can be hacked, users can instead be sent a push notification to a secure app on the device registered to the authentication system. The notification informs the user of the action that has been requested and alerts them that an authentication attempt has taken place. Then, they simply approve or deny the access request.

This authentication format creates a connection between the app or service the user is attempting to access, the 2FA service provider, the user themselves, and their device. It is user-friendly and reduces the possibility of security risks like phishing, man-in-the-middle (MITM) attacks, social engineering, and unauthorized access attempts.

This authentication format is more secure than SMS or voice calls but still carries risks. For example, it is easy for a user to accidentally confirm an authentication request that has been fraudulently requested by quickly tapping the approve button when the push notification appears.

Smartphones offer a variety of possibilities for 2FA, enabling companies to use what works best for them. Some devices are capable of recognizing fingerprints. A built-in camera can be used for facial recognition or iris scanning, and the microphone can be used for voice recognition. Smartphones equipped with a Global Positioning System (GPS) can verify location as an additional factor. Voice or SMS may also be used as a channel forout-of-band authentication.

A trusted phone number can be used to receive verification codes by text message or automated phone call. A user has to verify at least one trusted phone number to enroll in 2FA.Apple iOS, Google Android, and Windows 10 all have applications that support 2FA, enabling the phone itself to serve as the physical device to satisfy the possession factor.

Ann Arbor, Michigan-based Duo Security, which was purchased by Cisco in 2018 for $2.35 billion, is a 2FA platform vendor whose product enables customers to use their trusted devices for 2FA. Duo's platform first establishes that a user is trusted before verifying that the mobile device can also be trusted for authenticating the user.

Authenticator applications replace the need to obtain a verification code via text, voice call, or email. For example, to access a website or web-based service that supports Google Authenticator, users type in their username and password—a knowledge factor. Users are then prompted to enter a six-digit number. Instead of having to wait a few seconds to receive a text message, an authenticator generates the number for them. These numbers change every 30 seconds and are different for every login. By entering the correct number, users complete the verification process and prove possession of the correct device—an ownership factor.

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