Hacker Public Radio

The Hacker Public Radio Old Microphone Logo



HPR3784: Two factor authentication without a phone number

Many services implement 2FA (Two factor authentication) by sending you a OTP (One Time Password) using an SMS with a random code, but this forces you to give them your valuable phone number. What alternatives do exist?

Let's dive into the HOTP, used by some banks years ago through a physical token and the recent TOTP, which both let you generate completely offline codes without using any phone number or any other personal detail. They use the HMAC technique usually with a SHA-1 one-way hashing function, but other hashing functions can be used too.

Useful links:

  • a little visual explanation I found here
  • Aegis android OTP generator
  • use TOTP in KeepassXC for a desktop generator guide

Let's keep Webauthn maybe for a future episode, I'm still exploring it and have to do more research.


 2023-02-02  19m