The replies to this make it pretty clear that no “single” hard skill or really even set of hard skills will remain defensible indefinitely
Adding my own anecdote as an employer, I mainly hire engineers based on their demonstrated ability to efficiently identify practical solutions to problems, and to self manage the translation of those solutions into maintainable, production systems (tldr, ship shit that works and that others can work on with you, all in a reasonable timeframe)
That being said, I can tell you it is extremely hard to find an experienced cryptographer, and I expect that need to grow over the next several years (though it’s very niche and your options will certainly be limited, if lucrative)
If not, I strongly encourage you to discuss your area of expertise with it and rate based on that
It is incredibly competent