This blog post explains how to use the Authy API to implement Time-based One-time Passwords (TOTP) in an application, allowing users to choose their preferred authenticator app for two-factor authentication. The TOTP algorithm is defined in RFC 6238 and can be implemented in multiple applications, including Google Authenticator and Microsoft Authenticator. The post shows how to register a user with the Authy API, generate a unique Authy ID, create a QR code for the user to onboard with their chosen authenticator app, and verify TOTP codes using the Authy API. The Authy API provides a flexible solution for implementing 2FA, allowing users to choose their preferred method of authentication, including SMS, voice, or email channels.