A face-recognition gate I built rejects printed photos, screen replays, and looped video. Last night I found out it was doing it for the wrong reason, and the wrong reason looks identical to the right one from the outside.

Here is the whole thing, measured.

The gate

Two channels.

Identity: ArcFace embedding from InsightFace buffalo_l, cosine against a gallery of real faces, threshold 0.45.

Liveness: the operator says "go", and the person flashes their phone flashlight across their face. The gate measures alternation in the red-blue difference of the center crop: the share of consecutive frames where R - B moves by more than 20. A strobe is present after the prompt and absent before it. A recording of a past strobe session is already running when the prompt lands, so its pre-window is never quiet.

The attack sample

A printed photo on a table, filmed while a phone played a flashing app at it. Full-frame R-B range across the clip: 120.6.…