What Are Time-Based One-Time Passwords (TOTP)?

Keeping accounts secure requires more than just a strong password. Even the most complex phrases can be stolen through phishing attacks or data breaches. This challenge led to the creation of two-factor authentication (2FA), and one of the most reliable forms of 2FA is the Time-based One-time Password (TOTP). TOTP provides a frequently changing, temporary code that acts as a second lock on your sensitive accounts, making it vastly harder for unauthorized people to gain access.

What is a Time-based One-time Password?

A TOTP is a temporary numerical code generated using a secret key and the current time. These codes are valid for a short period, typically 30 seconds. After that window closes, the code instantly expires, and a new one takes its place. This short lifespan is what makes TOTP a powerful defense against many common cyber threats. Since the code is only valid for a few moments, even if an attacker steals the code, it becomes useless before they can use it to log into the protected account.

The Simple Math Behind the Code

The generation of a TOTP code relies on a straightforward, standardized mathematical process. Two primary inputs are used:

1. A Shared Secret Key: This is a long, unique string of characters created when you first set up 2FA for an account. The key is securely stored on the service provider's server and is copied to your authenticator application (like an app on your phone) during the setup process, usually by scanning a QR code. This key never changes.
2. The Current Time: The TOTP algorithm does not use the absolute time but rather a counter based on time intervals, usually 30 seconds. The current time is divided by the interval length to create a simple, sequential time-step value.

The algorithm, formalized in a widely accepted standard, uses these two inputs to perform a Cryptographic Hash operation. This operation combines the secret key and the time-step value to produce a long, scrambled numerical output. A portion of this output is then extracted and converted into the short, six- or eight-digit code you see on your screen.

How the Server Verifies Your Login

When you attempt to log in using your username, password, and the TOTP code from your phone, the server performs the exact same calculation.

1. The server retrieves your unique secret key from its database.
2. It calculates the current time-step value.
3. It runs the TOTP algorithm using your secret key and the time-step. This generates the expected code.
4. It compares its calculated code with the code you submitted.

If the two codes match, you are granted access. Due to potential clock differences between your device and the server, a system will often check not only the current time-step's code but also the codes from the time-step immediately before and the one immediately after. This slight tolerance prevents minor clock drift from locking you out of your account.

Why is TOTP Better Than SMS?

Older forms of 2FA often relied on sending a code via SMS (text message). While better than nothing, SMS is vulnerable to attacks like SIM-swapping, where a fraudster convinces a phone carrier to transfer your number to their device.

TOTP eliminates this vulnerability entirely because the code generation happens locally on your device, within your authenticator app, and does not rely on a telephone network. The secret key is static and the codes change every 30 seconds, providing superior security and reliability compared to codes delivered over insecure text messaging systems. This system gives users strong protection without complicating the login process for legitimate owners.