IANAL but a proud American haha, and we're very specific on not allowing ex post facto laws per our constitution. It would be a huge avenue for abuse of power by the government against the people.

The caveat I added to my initial comment that you also mentioned was that they could try to find a relevant existing law and retrofit it, e.g. general securities laws, and say that this is a securities market and so this has always been illegal — but it's very unlikely and I doubt it would work. Far, far more likely that we pass explicit laws about this.

The executive branch executes laws (prosecutes crime) under its own interpretation of the law. It's the supreme court that determines if that interpretation is acceptable or fair. I.e. you could be prosecuted under an executive's interpretation of an existing law when the crime was committed and if the court deems it acceptable you could be found guilty by a jury. (In the USA)

They absolutely cannot make it a retroactive crime. This isn't so much con law as high school civics in the US.

“Retroactive” crime is as you say, ex post facto, and specifically disallowed by the US Constitution.

No, you cannot retroactively make something a crime as you have found.

That way lies madness.

If the govt could do that, they could arrest anyone, anytime simply by making whatever they did yesterday retroactively illegal.