[zimpenfish]

Or use a TOTP with a long period (10 minutes?) and use that value mod, say, 10k with a base of something like 9000. Easy to calculate the port in your head, impossible to guess without knowing the TOTP secret (I think) and can be extended with other fun* rules like "But subtract 10* the first digit" or "Add the first and second digits multiplied".

[gen220]

Ah, this is a great twist! Thank you for this great contribution to the "locked myself out" idea bank.

[zimpenfish]

That's a fair point - e.g. Authy is currently broken on iOS 14 which means I'd be locked out if I was doing this and using Authy (although I have a VPN to one server which can then get to my others.)