Joseph Silverman
Brown University number theorist who wrote the standard text on elliptic curves, then helped invent a cryptosystem that doesn't use them.
Silverman's The Arithmetic of Elliptic Curves (1986) is the book elliptic-curve cryptographers actually reach for, and an ECC thesis that doesn't cite it is hard to find. Ten years later he presented NTRU with Jeffrey Hoffstein and Jill Pipher – lattice-based cryptography that now counts as one of the main fallbacks for when quantum computers break elliptic curves and RSA.
He also co-wrote the textbook An Introduction to Mathematical Cryptography (2008) and a long list of papers on arithmetic dynamics, his other main interest.