What is two-factor authentication and why is it used?

Two-factor authentication (2FA), sometimes referred to as two-step verification or dual-factor authentication, is a security process in which users provide two different authentication factors to verify themselves.

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2FA is implemented to better protect both a user"s credentials and the resources the user can access. Two-factor authentication provides a higher level of security than authentication methods that depend on single-factor authentication (SFA), in which the user provides only one factor -- typically, a password or passcode. Two-factor authentication methods rely on a user providing a password as the first factor and a second, different factor -- usually either a security token or a biometric factor, such as a fingerprint or facial scan.

Two-factor authentication adds an additional layer of security to the authentication process by making it harder for attackers to gain access to a person"s devices or online accounts because, even if the victim"s password is hacked, a password alone is not enough to pass the authentication check.

Two-factor authentication has long been used to control access to sensitive systems and data. Online service providers are increasingly using 2FA to protect their users" credentials from being used by hackers who stole a password database or used phishing campaigns to obtain user passwords.

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What is identity and access management? Guide to IAM

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What are authentication factors?

There are several ways in which someone can be authenticated using more than one authentication method. Currently, most authentication methods rely on knowledge factors, such as a traditional password, while two-factor authentication methods add either a possession factor or an inherence factor.

Authentication factors, listed in approximate order of adoption for computing, include the following:

A time factor restricts user authentication to a specific time window in which logging on is permitted and restricts access to the system outside of that window.

The vast majority of two-factor authentication methods rely on the first three authentication factors, though systems requiring greater security may use them to implement multifactor authentication (MFA), which can rely on two or more independent credentials for more secure authentication.

How does two-factor authentication work?

Enabling two-factor authentication varies depending on the specific application or vendor. However, two-factor authentication processes involve the same general, multistep process:

The user is prompted to log in by the application or the website. The user enters what they know -- usually, username and password. Then, the site"s server finds a match and recognizes the user. For processes that don"t require passwords, the website generates a unique security key for the user. The authentication tool processes the key, and the site"s server validates it. The site then prompts the user to initiate the second login step. Although this step can take a number of forms, the user has to prove that they have something only they would have, such as biometrics, a security token, an ID card, a smartphone or other mobile device. This is the inherence or possession factor. Then, the user may have to enter a one-time code that was generated during step four. After providing both factors, the user is authenticated and granted access to the application or website.

Elements of two-factor authentication

Two-factor authentication is a form of MFA. Technically, it is in use any time two authentication factors are required to gain access to a system or service. However, using two factors from the same category doesn"t constitute 2FA. For example, requiring a password and a shared secret is still considered SFA as they both belong to the knowledge authentication factor type.

2FA involves two of three potential authentication factors.

As far as SFA services go, usernames and passwords are not the most secure. One problem with password-based authentication is it requires knowledge and diligence to create and remember strong passwords. Passwords require protection from many insider threats, such as carelessly stored sticky notes with login credentials, old hard drives and social engineering exploits. Passwords are also prey to external threats, such as hackers using brute-force, dictionary or rainbow table attacks.

Given enough time and resources, an attacker can usually breach password-based security systems and steal corporate data. Passwords have remained the most common form of SFA because of their low cost, ease of implementation and familiarity.

Multiple challenge-response questions can provide more security, depending on how they are implemented, and standalone biometric verification methods can also provide a more secure method of SFA.

Types of two-factor authentication products

There are many different devices and services for implementing 2FA -- from tokens to radio frequency identification (RFID) cards to smartphone apps.

Two-factor authentication products can be divided into two categories:

tokens that are given to users to use when logging in; and infrastructure or software that recognizes and authenticates access for users who are using their tokens correctly.

Authentication tokens may be physical devices, such as key fobs or smart cards, or they may exist in software as mobile or desktop apps that generate PIN codes for authentication. These authentication codes, also known as one-time passwords (OTPs), are usually generated by a server and can be recognized as authentic by an authentication device or app. The authentication code is a short sequence linked to a particular device, user or account and can be used only once as part of an authentication process.

Organizations need to deploy a system to accept, process and allow or deny access to users authenticating with their tokens. This may be deployed in the form of server software or a dedicated hardware server, as well as provided as a service by a third-party vendor.

An important aspect of 2FA is ensuring the authenticated user is given access to all resources the user is approved for and only those resources. As a result, one key function of 2FA is linking the authentication system with an organization"s authentication data. Microsoft provides some of the infrastructure necessary for organizations to support 2FA in Windows 10 through Windows Hello, which can operate with Microsoft accounts, as well as authenticate users through Microsoft Active Directory, Azure AD or Fast IDentity Online (FIDO).

How 2FA hardware tokens work

Hardware tokens for 2FA are available supporting different approaches to authentication. One popular hardware token is the YubiKey, a small Universal Serial Bus (USB) device that supports OTPs, public key encryption and authentication, and the Universal 2nd Factor protocol developed by the FIDO Alliance. YubiKey tokens are sold by Yubico Inc., based in Palo Alto, Calif.

When users with a YubiKey log in to an online service that supports OTPs -- such as Gmail, GitHub or WordPress -- they insert their YubiKey into the USB port of their device, enter their password, click in the YubiKey field and touch the YubiKey button. The YubiKey generates an OTP and enters it in the field.

The OTP is a 44-character, single-use password; the first 12 characters are a unique ID that represents the security key registered with the account. The remaining 32 characters contain information that is encrypted using a key known only to the device and Yubico"s servers, established during the initial account registration.

The OTP is sent from the online service to Yubico for authentication checking. Once the OTP is validated, the Yubico authentication server sends back a message confirming this is the right token for this user. 2FA is complete. The user has provided two factors of authentication: The password is the knowledge factor, and the YubiKey is the possession factor.

Two-factor authentication for mobile devices

Smartphones offer a variety of 2FA capabilities, enabling companies to use what works best for them. Some devices can recognize fingerprints, use the built-in camera for facial recognition or iris scanning, and use the microphone for voice recognition. Smartphones equipped with GPS can verify location as an additional factor. Voice or Short Message Service (SMS) may also be used as a channel for out-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 mobile 2FA.

Apple iOS, Google Android and Windows 10 all have apps that support 2FA, enabling the phone itself to serve as the physical device to satisfy the possession factor. Duo Security, based in Ann Arbor, Mich., and purchased by Cisco in 2018 for $2.35 billion, has a platform that enables customers to use their trusted devices for 2FA. Duo"s platform first establishes that a user is trusted before verifying the mobile device can also be trusted as an authentication factor.

Authenticator apps 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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These and other 2FA products offer information on the minimum system requirements necessary to implement 2FA.

Biometric authentication has become an increasingly popular option on mobile devices.