![]() This means the user has ample time to enter the OTP. HOTP tends to be user friendly since it doesn’t increment until the user requests a new OTP. Fig: HOTP uses a counter as the moving factor The counter on the server increments by one when an OTP is successfully validated. The counter on the token increments by one when a new OTP is requested. This counter is stored on both the token and the server. In HOTP, the moving factor is a counter that’s incremented every time a new OTP is requested. The main difference between HOTP and TOTP is how the moving factor is calculated. Thus, HOTP stands for HMAC-based One-time Password. The “H” in HOTP stands for Hash-based Message Authentication Code (HMAC). HOTP is the original standard that TOTP was based on. Step 4: If the two codes match, the user is successfully logged in.Īlso read: Passwordless Authentication 101 TOTP vs HOTP The server then compares the two TOTP codes – the one it generated independently and the one it got from the client – to check if they match. Step 3: The server independently generates a TOTP code using the same seed and moving factor. The client sends the TOTP code to the server. The seed used is the one created during the registration process. ![]() Step 2: The client generates a TOTP code using the seed and moving factor (Unix time). Step 1: The user begins the login process and successfully presents the first factor of authentication. This process happens every time TOTP is used to authenticate a user: Here’s a simplified flow showing TOTP validation. Note: Some applications require two TOTP validations to complete registration. Step 4: To complete the registration, a process of TOTP validation occurs (see the section below). The TOTP authenticator stores the seed in the client device in a secure manner. Step 3: When the user clicks the URL or scans the QR code, TOTP registration is complete. The server also stores the seed in a database (secret manager) for future retrieval. The seed is embedded in a URL / QR code and passed on to the client. Step 2: The server generates a shared secret key (the seed). They then choose authenticator apps as their preferred second factor while setting up 2FA. Step 1: The user enters their username and presents the first factor of authentication. Here’s a simplified flow when TOTP authenticator apps are registered: Fig: How TOTP registration works Validation happens every time a user tries to authenticate using TOTP. Validation, where the client generates a TOTP code using the seed and moving factor and passes it on to the server for validation. Registration happens once, when the user chooses TOTP as their preferred 2FA factor for an app. Registration, where the server generates the seed and communicates it to the client. ![]() Fig: TOTP uses time as the moving factorĪuthentication using TOTP consists of two stages: This algorithm uses a form of symmetric key cryptography since the same key is used by both the client and the server to independently generate the OTP. The moving factor used by the TOTP algorithm is Unix time. In TOTP, the seed is a secret key that is shared between the authentication server and the token during first-time use. This is a component that changes every time a new OTP is requested or at set periods of time. It is created when a new account is established on the authentication server.Ī moving factor. This is a static secret key that is shared between the token and the server. Two inputs are used to generate OTP codes:Ī seed. Fig: Screenshots of Google Authenticator with TOTP codes (Source: Vox) How TOTP worksīefore going into specifics, it’s important to understand how OTP generation algorithms work in general. TOTP was published as RFC 6238 by the Internet Engineering Task Force (IETF) in 2011. This makes TOTP authentication a strong second factor in a multi-factor authentication (MFA) or two factor authentication (2FA) flow. Unlike passwords – which are static and can be easily stolen – a TOTP code changes at set time intervals (usually 30 to 90 seconds) and is very difficult for attackers to compromise. TOTP can be implemented in both hardware and software tokens:Ī TOTP hardware token is generally a physical fob or security key that displays the current code on a screen built into the device.Ī TOTP software token is generally an authenticator application on a mobile device (like Authy or Google Authenticator) that displays the current code on the phone screen. This code is meant to grant users one-time access to an application. ![]() A TOTP code is generated with an algorithm that uses a shared secret and the current time as inputs. TOTP stands for time-based one-time password (or passcode). ![]()
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