This post is part of a series called "Route Around ISPs - Altnet". Below is a list of previous posts.

What is a website? In a post on SN titled Explain NOSTR Like I'm 5 I explain the concept of websites as bulletin boards that people post to and visit to see what other people have posted. I think its important to take this metaphor as not being very imaginary. An IP address (although not always) refers to a real location in the real world that can be traveled to in very literally the same way that post office mailing address refers to a real world location that can be traveled to. Its just that we're doing these things in an automated fashion.

In my dronenet post, I mentioned that "I don't expect a dronenet to deliver information directly to people's windows". A large reason for this, is because if you deliver to people's windows, you risk informing the authoritarian regime you're trying to penetrate information into, of where people live. Besides, its less efficient anyway in the context of a city.

You could hold secret meetings as well of course and risk a member of the CCP or KGB or what have you taking a visit in plainclothes. Alternatively, you could hide in plain sight.

This is one of Darthcoin's favorite pictures to post around. I think it makes him a little giddy inside knowing that you can see the picture and not the data hidden inside it. This does not work very well for printed pictures however. The colors that the printer uses won't be the same nor does the pixel number match exactly the number of dots the printer uses and the smudging or fading that may occur does not help. So instead, lets redefine what solution we're looking for.

"Can I hide data in an image where the fact that data is hidden within that image is not immediately obvious to the human eye, and be able to digitally retrieve that hidden information in a physical copy using a digital medium"

The reason for doing this would be so that we can post up our printed copy to replace an original copy that is supposed to be there like a picture of a missing cat on a telephone pole, or have custom distribution points where people know to pick up their data from.

As it turns out, there's a few ways of doing this.

Thermochromic ink is a type of dye that changes color when its temperature increases or decreases. You can see the prices of this method here: https://www.sfxc.co.uk/collections/products-for-paper

Here's how it would look:

Invisible ink is a type of writing material that is revealed when a certain process is applied to it like a chemical reaction, ultraviolet light, heat, water, etc. This site served as a good source: https://sciencenotes.org/how-to-make-invisible-ink-and-reveal-secret-messages/

A user created a github in 2014 which alters an image to encode information within it. It does not hide information in colors, but rather changes the image itself in details that human eyes may not pick up on.

If you look very closely at this image you can see these white dots within this black background that resemble the shape of the castle. That's the data. Unfortunately, the project looks like it never finished, so ask SN, should we revive this project by creating a funding page for it or something?

You can view the full project here https://github.com/leonardean/StegoQR

So effectively this allows data recipients to take a picture of the picture and retrieve their data that way all while everyone else has no idea that the image in question is anything other than a lost cat ad. 

Last but not least, this was a fun thought experiment, but ultimately, you probably just want to host the data on a raspberry pi and have receivers simply lay their phone down on the armchair of a couch to retrieve the data through NFC or something. In spite of much simpler solutions being out there, I hope I managed to get your imagination running wild. When one way doesn't work, there's a thousand other ways to get something done.