In the US, insider trading requires material nonpublic information, but it officially also requires that somebody break a fiduciary duty to the company involved

In the US, insider trading requires material nonpublic information, but it officially also requires that somebody break a fiduciary duty to the company involved. So, in your hypothetical, the person breaking cryptography algorithms knows something nobody else does (i.e., isn’t public), and that secret is definitely material. But the researcher didn’t get that information through a breach of fiduciary duty, so it’s legal to trade.