Residue

A four-digit code was entered on this keypad and the person who entered it has gone. How long do you have? And how long do you have for the harder question, which is what order they pressed them in?

What the finger left

Contact warms the key by roughly three degrees above ambient. The key cools back through its own mass and the air, on a time constant set by the material and by nothing the device is running.

Every other channel in this course stops when the computation stops. This one starts then.

Four keys, cooling

Four decay curves, so close together they read as one band, falling from about three degrees at release to below a quarter of a degree by fifty-six seconds

01.53.005090seconds after releaseK above ambientfloor56 s

The first window is the obvious one

Every curve falls through the segmentation floor at about the same moment, because they differ by four seconds out of ninety.

Fifty-six seconds to recover which four keys were pressed. That is the number a defender who has read the abstract will quote.

The order is not in any one curve

The smallest gap between two adjacent keys, starting near sixteen hundredths of a degree and crossing the camera’s noise floor at twenty-eight seconds

00.100.2005090seconds after releasegap between adjacent keys, KNETD28 s

It is carried by the differences, and the differences decay on the same constant from a much smaller start.

Fifty-six and twenty-eight

The set of keys survives twice as long as their order.

A defender working from one number is working from the wrong one, and it is the generous one.

Nobody was there to defend

Shield the room before the monitor is on. Rewrite the comparison before it runs. Disable deduplication before the tenant arrives.

There is no equivalent here. The leak begins after the interaction ends and after an attacker’s presence would have been noticed. No detection, no interruption, no incident.

One lever, and it is not software

The time constant is a material property. Change the keypad, or change nothing.

KeypadFitted constantSet recoverableOrder recoverable
ABS plastic24 s56 s28 s
Brushed aluminium4 s9 s5 s

Aluminium does not close the channel. It moves the shorter window inside the time it takes to walk to the keypad, which is the same thing to a defender and a different thing to an argument.

Before the Bench

Bring something that reads per-pixel intensity out of an image and plots a series. The Bench supplies ninety seconds of frames after a known four-digit entry.

Fit the decay, then plot the pairwise differences beside the raw series and find both windows. Keep the constant: week 10 puts this channel on a common rubric, and your fitted number is what it wants.