How do 6-digit codes work for Two-Factor Authentication?

Question

So, I get that broadly the answer is “It’s an algorithm”, but I’d like to know more specifics if there are any.

For some websites/services, Two-Factor Authentication requires a (usually 6-digit) code to authenticate the user. This code changes frequently, usually every 30 seconds or so. The code generators for doing this are sometimes connected to internet/cell service, but they don’t have to be for a lot of authentication services.

I guess what I’m wondering is how a server is able to verify that my code of (for example, of course) 554-048 is now the code to authenticate my account, which I just obtained from a device that is not connected to any way of transmitting data, and that the previous code, 685-392 is no longer valid.

In: 4

Answer 1

uh, because THEY sent the code to a second device, and are waiting for the SAME code they sent you?

Any randomly generated number works fine, they are just waiting for the SAME one they sent you.

A check-sum, on the other hand, is completely different. And rolling-code transmitters for apartment garage doors are different too.