> PS - I am ideating on a 2FA method using cryptography where the user will not need internet on his phone.
If the user needs to be loggedin somewhere, they must be online. So it's safe to assume their phone has a connection too.
I will be using JWTs, with asymmetric signatures. An internet connection will only be needed while adding the device for 2FA. The JWT will be generated from the private key already on the phone so internet is not necessary.
However from the responses it looks like this is not an issue and I'll probably not work further on this idea.
A very well known issue with old SMS based OTP, is that they can stop working when the user is overseas.