Depends on D19890. Ref T13222. See PHI873. Currently, we only validate TOTP responses against the current (realtime) timestep. Instead, also validate them against a specific challenge.
This mostly just moves us toward more specifically preventing responses from being reused, and supporting flows which must look more like this (SMS/push).
One rough edge here is that during the T+3 and T+4 windows (you request a prompt, then wait 60-120 seconds to respond) only past responses actually work (the current code on your device won't). For example:
- At T+0, you request MFA. We issue a T+0 challenge that accepts codes T-2, T-1, T+0, T+1, and T+2. The challenge locks out T+3 and T+4 to prevent the window from overlapping with the next challenge we may issue (see D19890).
- If you wait 60 seconds until T+3 to actually submit a code, the realtime valid responses are T+1, T+2, T+3, T+4, T+5. The challenge valid responses are T-2, T-1, T+0, T+1, and T+2. Only T+1 and T+2 are in the intersection. Your device is showing T+3 if the clock is right, so if you type in what's shown on your device it won't be accepted.
- This may get refined in future changes, but, in the worst case, it's probably fine if it doesn't. Beyond 120s you'll get a new challenge and a full [-2, ..., +2] window to respond, so this lockout is temporary even if you manage to hit it.
- If this doesn't get refined, I'll change the UI to say "This factor recently issued a challenge which has expired, wait N seconds." to smooth this over a bit.