> Being in a location at a certain place and time could be trivial to determine, no matter how careful you are.

Well, that would determine that your DNA was there, but as we saw in that case discussed here earlier this week [1] is it a lot easier than most people realize for your DNA to get transferred to things or people who then transfer it to a crime scene, where it can be discovered and thought to be from the criminal.

I spent a while thinking about what one could do to defend against the risk that one's DNA might be falsely found at a crime scene for a crime that occurred when you are home alone. I couldn't think of a good way to prove you were home, but you don't need that. You just need evidence sufficient to raise reasonable doubt.

I came up with two ideas, one that you can do yourself, and one that would involve a startup that provides an alibi service. Not sure if anyone actually needs anything like this yet, but it is an interesting problem to thing about.

(I live alone, and my office switched to work at home a few months ago. I can now easily go several days now where the only people who see me are fast food workers, gas station attendants, and others in retail. Even at the fast food place I visit several times a week, where they remember me sufficiently to say "the usual?" when I walk in, and make the right thing if a say "yes", I doubt that if they were asked if I was there on a particular morning they would not be able to remember. All they could say is a come in 3 or 4 times a week but they wouldn't know which were the days I did not come in...so if somehow I was tied to a crime, I would probably have a very hard time establishing that I was not there).

First the self-hosted version. This is based on the old classic method that people, such as kidnappers, would sometimes use to prove that a hostage was still alive as of a particular date: send a photograph of the hostage holding a major newspaper from that date.

Set up a camera in your home that periodically takes a photo (or maybe a short video) of you standing next to a couple of monitors. One of the monitors displays current prices for several major stocks on exchanges around the world, and exchange rates on several currency exchanges around the world. The other shows several major news sites. After the photo is taken it is hashed and timestamped using a trusted timestamp service [2] (or if you want to do it simpler, just Tweet the hash).

If you ever need to raise doubt about a claim that you were somewhere at a time you claim you were at home, you offer the photo or videos taken near that time. The timestamp for the hash establishes an upper limit for the time that the photo was made. The prices on the stocks and currencies and the headlines on the news sites should allow establishing a lower limit on when it was made. That gives you evidence that you were home between those times.

Could this be faked? Yes. You could take a photo made earlier in next to blank monitors, and then around the time you are committing the crime ssh home and make the system take a photo of the monitors displaying the price and news data, and then Photoshop that into the earlier photo, and upload it.

But if your system is set up to work honestly, I think it would be good enough for reasonable doubt, unless the prosecutor can offer actual evidence that you DID hack it rather than just speculating that you MIGHT have done so.

The startup version, offering an alibi as a service, would just need you to have a webcam and microphone at home. With this service, you periodically initiate a video chat with the service, which it records. You should do the chat from a place where the background is easily recognized and unique. During the chat, the service gives you a random number of 6 digits. You speak the digits, while simultaneously holding up a number of fingers corresponding to those digits. The service saves the video and the random number. Maybe also hold up your phone with it using a GPS or mapping app showing your location.

In fact, the alibi service could probably make the whole client side a smartphone app. Use the smartphone front facing camera instead of a webcam. The app can use the phone GPS to record the location.

If you are worried about privacy issues over the videos of you being uploaded to the alibi service, there could be an option to store everything (video, GPS location) locally or on whatever secure cloud storage you use for your own videos and photos, with just a hash uploaded to the alibi server.

[1] https://news.ycombinator.com/item?id=16903330

[2] https://en.wikipedia.org/wiki/Trusted_timestamping