Unenforceable license terms

[kategray]

When you state "The authors DO NOT ALLOW any users to sell keys created with the help of this app.", that's unenforceable.

For one thing, the output put is physical data (measurements) that can't be copyrighted. It possibly gets turned into bitting codes that again can't be copyrighted.

When you say "the authors DO NOT ALLOW", it's irrelevant whether or not you do, because you have no legal means to stop people from doing so. With copyright law, it's copyright that takes people's ability to copy, and licenses that give them more rights than they had. You can't take more privileges than what your technical ability and the law give you.

You say "fair use is allowed if used by security enthusiasts". The whole point of "fair use" is that it doesn't require a license, because fair use is a limitations on the privileges of the copyright holder.. In other words, fair use stops you from limiting anyone who copies your media, it's not a limitation on them.

In the US, a relevant case would be "Design Data Corporation v. Unigate Enterprise, Inc.", in which the courts ruled that the output of a computer program was essentially not copyrightable by the authors of the computer program. If you make a word document (for example), the output belongs to you, and microsoft has no IP in the final document. With this software, the "output" would be the key. There are similar cases worldwide.

If this weren't an android app, you could try to get users to agree to terms before you distribute it to them. Since this is an android app, it's distributed through the Google Play Store, and is distributed by Google users under the Google Play terms.

You are trying to amend the terms after the sale essentially (which you can't do), and don't provide the consideration necessary to have some sort of contract. They already have the right to have the software and use it from google, so your "Education and Consulting Use Only" disclaimer is meaningless.

In general, the Google Play terms don't allow redistribution, so your distribution license itself accomplishes the goal of limiting who can distribute the app and under what purposes. It keeps companies from rebranding and charging for it. That's all the control you get.

If you want another bite at the apple (a chance to renegotiate terms), then you will need to do something like add a web service into the app. If you do that, then you can offer them something (access to the online service) in exchange for something (the agreement from them not to do certain things).