If you look up the definitions of "security", "financial instrument", "commodity", and "currency", it is pretty clear that a plain vanilla crypto-currency like Bitcoin is no ordinary security nor a financial instrument. It is not a claim on something else, even less so than a traditional bank note. It is more like a commodity. Since when is "aluminum" a security?

A crypto currency business however may be engaged in trading of crypto currencies which are negotiable financial instruments, and a stable coin that is a claim on a traditional currency is an example of that. A stable coin bears a direct relationship to a bank note. It is a claim on something else. It is not really an investment though, so not much of a security.

And then of course there are more creative negotiable financial instruments which are definitely securities. Options, derivatives, swaps, futures, and the like.

The Supreme Court has a broader definition of securities, that involves a promoted investment in a "common enterprise", but it is unclear that a commodity currency really is a common enterprise like a corporation is for example. It seems unlikely to me that Bitcoin as such would even meet that definition of a security. But other products offered by crypto currency exchanges might.