Soon it may not be.
To explore this question you and I must differentiate between the legality of something and the ethics of it (or what is inherently “right”).
For example, in your state it may be perfectly legal to stand your ground during a home invasion. In Texas you can stand and fight and even use deadly force to protect yourself, your family and your property. In New York you must flee if you can. These are legal issues, not ethical ones. It’s not right to abandon your family and property during an attack. It’s unethical for the law to demand you acquiesce to an aggressor. In New York this is perfectly legal and totally unethical. For those who slavishly adhere to the law, this may be a hot button. Let’s examine a few more hot buttons.
Consider the ethics of protecting the defenseless, such as unborn children.
The issue is neither about “pro-life” nor “pro-choice”—it’s about due process. Under The Constitution, vile citizens—rapists, murderers and those who prey on children and the elderly—are granted due process of law (trial by jury, appeals, etc.). In contrast, the most innocent of us (babies in the womb) may have their lives taken on a whim without so much as a court hearing. There’s legal, and then there’s ethical. It’s ethical to protect the defenseless. It’s the right thing to do, even to the point of disregarding the law.
How about self-protection and gun control?
When we criminalize gun ownership we can be sure at the end of the day criminals will have much more guns than those who follow the law. The law will never deter lawbreakers, but it can easily make victims of those who confuse ethics for legality. The law can be used as a weapon to deprive law-abiding citizens of rights they never should have considered surrendering in the first place. Rights like self-protection. And privacy.
Nothing protects online digital privacy like encryption.
When we make it illegal, only criminals will use it and benefit from it. By definition. The issue isn’t about having something to hide. It’s about not being compelled to reveal something you don’t want anyone to see. It’s about privacy. Legislation cannot secure privacy, it can only be guaranteed by encryption. While it may be momentarily legal to design and use encryption, there are plenty of rumblings to make it illegal. Or at least to make it so weak it’s useless.
So, is it legal to use PrismCipher? This is the wrong question. (When you ask the wrong question you always get the wrong answer, even if it’s correct.) Better to ask: is it ethical to use PrismCipher? Is it right to assert your right to privacy?
Governments make encryption legal or illegal as they please, but they cannot change the ethics of using it.