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Inspired by A double-agent with a conundrum. DISCLAIMER: The people and events described here are entirely fictional. While the choice of countries is important for the puzzle to have a reasonable solution, any connection nor resemblance to actual people or events is purely coincidental.

Tension between the US and China is rising. You are a senior officer working for the CIA, in charge of protecting secret technology from the hands of Chinese spies. You also appear in public often, making you the target of many hostile groups.

One day, there appeared a video, showing you conversing and divulging classified information to a Chinese intelligence counterpart. Espionage, if charged, means maximum security prison for life. The problem? That was not you. You believed that the Chinese government hired a body double and faked the video from mainland China to frame you for espionage. You have to prove your innocence before it's too late, because the system will err on the side of guilt unless there exists "smoking gun" evidence.

You scrutinized the video. It started with the title "CIA senior officer at a secret meeting in Denver, August 2nd 2019." The video then cut to a likeness of you, sitting opposite a desk from two other Chinese senior ranking officers. The video seems to be captured by a hidden high quality camera. The faces were blurred in the video, but the voice sounded exactly like yours. The room was white, illuminated by bright fluorescent light. Besides the desk, chairs, and a clock on the wall, the room was bare. Nothing about the furniture could have told you anything about its origin. Your likeness was able to capture you quite well, and copy your very mannerisms and accent. He must have watched a lot of your interviews. He even wore everything that you have, down to the wrist watch. You know that everything talked about in the video is true, but the Chinese could have known it already from previous espionage attempts. The metadata was stripped bare, and no one knows who tipped the video in the first place. You also cannot unblur the face to show that it is actually not you.

Suddenly, you had an idea. You went back to your computer and used one last forensics trick...Bingo! The Chinese might have tried very hard to hide anything that revealed the video's actual location, but this showed that the video is recorded in China and not in America! Coupled with the fact that you never left America, you have found the smoking gun to prove your innocence!

What is your evidence?

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    $\begingroup$ There's the rather obvious, "I was nowhere near Denver on 2nd August 2019, and I have a dozen witnesses to prove it". $\endgroup$
    – nzaman
    Commented Aug 24, 2019 at 9:50

5 Answers 5

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I don't think there's anything in the story that shows that the (fictional) Chinese didn't fake this right, but here's a thing that seems like it might be easy to get wrong and possible to detect:

The US uses 60 Hz electricity and China uses 50 Hz electricity, which will produce different speeds of lighting flicker from those fluorescent lights (a kind of lighting that is consistently very flickery). If the lighting flicker rate matches the video's frame rate, then there will be either no visible artefacts or there will be more or less static variation in brightness across the image as a result. If it mismatches substantially, then successive frames will either vary in overall brightness or vary in spatial brightness variation, depending on the details of the camera. Either way, if we know the video's frame rate then we should be able to tell whether the lighting is powered by 50 Hz or 60 Hz electricity.

Some caveats:

It is possible to reprocess video to change its frame rate. It is possible to rig up your own electricity generation so as to produce electricity at a frequency different from what the mains gives you.

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    $\begingroup$ Nice idea! I was thinking about what to make of the clock, but anything I could come up with was boring. $\endgroup$
    – Arnaud Mortier
    Commented Aug 22, 2019 at 17:58
  • $\begingroup$ FYI, you cannot reprocess in the way you described without leaving evidence. Doing a perfect reprocessing would require information that simply doesn't exist in the video. $\endgroup$
    – piojo
    Commented Aug 23, 2019 at 7:01
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    $\begingroup$ I think you could (in principle) capture the video at a much higher frame rate (say 150fps), correct for lighting flicker using the higher-temporal-resolution data, and then select every 5th or 6th frame of the corrected result, and get something good enough that (especially after lossy video compression) it would not be easy to prove anything. $\endgroup$
    – Gareth McCaughan ♦
    Commented Aug 23, 2019 at 10:23
  • $\begingroup$ (High-frame-rate cameras do exist, and 150fps isn't so high that getting enough light would be infeasible. I think.) $\endgroup$
    – Gareth McCaughan ♦
    Commented Aug 23, 2019 at 10:23
  • $\begingroup$ -af scaletempo: you don't know the video's framerate unless somebody drops something. $\endgroup$
    – Joshua
    Commented Aug 23, 2019 at 16:26
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Right, I did not read Gareth's answer, but I have my own guess.

The sound from the fluorescent light was 50 Hz, not 60Hz, as it is in the US. Fluorescent lights have a very notorious 'buzz' when they're plugged in.

This small, but trying detail would reveal that the video could not have been taken in the US.

Electricians and other electrical engineers can often tell this sound by heart.

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My solution is a very simple one.

If the Chinese officers are identified, the agent can simply look up their arrival and departure date in America, if they ever visited America. If the video is really taken in China, the dates will not coincide.

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    $\begingroup$ The faces are blurred, so the first "if" part of your answer (positive identification) is nontrivial. $\endgroup$
    – Lawrence
    Commented Aug 24, 2019 at 8:07
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The clock on the wall, and the wristwatch on the person in the video, show different times?

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An attempt:

Since it's a high quality camera, we might zoom in and compare the hair on his hands, or the palm prints, or the finger prints or anything in the vicinity of his hands that could demonstrate it's not the CIA agent.

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