What Is Two-Factor Authentication (2FA)?

About 2 min read

Two-factor authentication (2FA) is a security method that prevents unauthorized access to an account by combining password authentication with another, different authentication factor. Even if a password is leaked, no one can log in without the second authentication factor, which greatly improves account security. According to a 2024 study by Microsoft, accounts with two-factor authentication enabled can block more than 99.9% of unauthorized access attempts.

Historical Background

The concept of two-factor authentication dates back to bank ATMs in the 1980s. The combination of a cash card (something you have) and a PIN (something you know) is one of the earliest examples of two-factor authentication in practice. With the spread of the internet, one-time password tokens were introduced for online banking in the 2000s. In the 2010s, Google introduced two-factor authentication for Gmail, accelerating its adoption among general users. Today, passkey technology promoted by the FIDO Alliance is driving the shift toward safer and more user-friendly authentication methods.

The Three Authentication Factors

Authentication factors are broadly classified into three types. "Something you know" is information that only the person knows, such as a password or PIN code. "Something you have" refers to a physical device that only the person possesses, such as a smartphone or security key. "Something you are" uses the person's physical characteristics, such as fingerprints or facial recognition. Two-factor authentication verifies identity by combining two different factors from among these.

For those who want to systematically learn the fundamentals of authentication technology, security authentication books on Amazon are also a useful reference.

The Difference Between Two-Factor and Multi-Factor Authentication

Two-factor authentication (2FA) and multi-factor authentication (MFA) are often confused, but strictly speaking they differ. Two-factor authentication means there are two authentication steps, and it can be satisfied even with factors of the same type (e.g., a password plus a security question). Multi-factor authentication, on the other hand, requires combining factors of different types (knowledge plus possession, knowledge plus biometrics, and so on). From a security standpoint, multi-factor authentication, which combines factors of different types, is more robust. In practice, even when something is called "two-factor authentication," it is usually implemented as multi-factor authentication.

Main Authentication Methods

The most widely used method is the one-time password (OTP) sent via SMS or email. However, because SMS carries the risk of SIM swap attacks, the TOTP (time-based one-time password) method, which uses authentication apps such as Google Authenticator or Authy, is considered more secure. For even higher security, the FIDO2/WebAuthn method, which uses hardware security keys such as YubiKey, is recommended.

Real-World Use Cases

"After making two-factor authentication mandatory company-wide, unauthorized logins via phishing decreased by 95% year over year. In particular, departments that introduced FIDO2 security keys have had zero incidents."

Authentication Flow

Practical Pitfalls

A common misconception when introducing two-factor authentication is the idea that "SMS authentication is enough." The SMS communication path is not encrypted, and there is a risk of SIM swap attacks or exploitation of vulnerabilities in the SS7 protocol. Whenever possible, choose an authentication app or a hardware key. Managing recovery codes is also important. If you do not store your recovery codes in a safe place in preparation for losing or breaking your device, you may end up locked out of your own account. By combining a strong password generated with passtsuku.com and two-factor authentication, you can strengthen your account security in multiple layers.

It is also worth making use of guidance on how to set up two-factor authentication and multi-factor authentication guides (Amazon) to achieve a safer online life.