A Guide to Common Types of Two-Factor Authentication on the Web

Two-factor authentication (or 2FA) is one of the biggest-bang-for-your-buck ways to improve the security of your online accounts. Luckily, it's becoming much more common across the web. With often just a few clicks in a given account's settings, 2FA adds an extra layer of security to your online accounts on top of your password.

In addition to requesting something you know to log in (in this case, your password), an account protected with 2FA will also request information from something you have (usually your phone or a special USB security key). Once you put in your password, you'll grab a code from a text or app on your phone or plug in your security key before you are allowed to log in. Some platforms call 2FA different things—Multi-Factor Authentication (MFA), Two Step Verification (2SV), or Login Approvals—but no matter the name, the idea is the same: Even if someone gets your password, they won't be able to access your accounts unless they also have your phone or security key.

There are four main types of 2FA in common use by consumer websites, and it's useful to know the differences. Some sites offer only one option; other sites offer a few different options. We recommend checking twofactorauth.org to find out which sites support 2FA and how, and turning on 2FA for as many of your online accounts as possible. For more visual learners, this infographic from Access Now offers additional information.

Finally, the extra layer of protection from 2FA doesn't mean you should use a weak password. Always make unique, strong passwords for each of your accounts, and then put 2FA on top of those for even better log-in security.

SMS 2FA

When you enable a site's SMS 2FA option, you'll often be asked to provide a phone number. Next time you log in with your username and password, you'll also be asked to enter a short code (typically 5-6 digits) that gets texted to your phone. This is a very popular option for sites to implement, since many people have an SMS-capable phone number and it doesn't require installing an app. It provides a significant step up in account security relative to just a username and password.

There are some disadvantages, however. Some people may not be comfortable giving their phone number—a piece of potentially identifying information—to a given website or platform. Even worse, some websites, once they have your phone number for 2FA purposes, will use it for other purposes, like targeted advertising, conversion tracking, and password resets. Allowing password resets based on a phone number provided for 2FA is an especially egregious problem, because it means attackers using phone number takeovers could get access to your account without even knowing your password.

Further, you can't log in with SMS 2FA if your phone is dead or can't connect to a mobile network. This can especially be a problem when travelling abroad. Also, it's often possible for an attacker to trick your phone company into assigning your phone number to a different SIM card, allowing them to receive your 2FA codes. Flaws in the SS7 telephony protocol can allow the same thing. Note that both of these attacks only reduce the security of your account to the security of your password.

Authenticator App / TOTP 2FA

Another phone-based option for 2FA is to use an application that generates codes locally based on a secret key. Google Authenticator is a very popular application for this; FreeOTP is a free software alternative. The underlying technology for this style of 2FA is called Time-Based One Time Password (TOTP), and is part of the Open Authentication (OATH) architecture (not to be confused with OAuth, the technology behind "Log in with Facebook" and "Log in with Twitter" buttons).

If a site offers this style of 2FA, it will show you a QR code containing the secret key. You can scan that QR code into your application. If you have multiple phones you can scan it multiple times; you can also save the image to a safe place or print it out if you need a backup. Once you've scanned such a QR code, your application will produce a new 6-digit code ev