Alfred Menezes
Waterloo mathematician behind the MOV attack on elliptic curves, co-author of the Handbook of Applied Cryptography and one of the field's more vocal sceptics.
In 1993 Menezes, Tatsuaki Okamoto and Scott Vanstone showed that discrete logarithms on supersingular elliptic curves can be carried over to a finite field via the Weil pairing, where they are much easier to solve. The MOV attack ruled out a whole class of curves — and, with some irony, laid the groundwork for the pairing-based cryptography that later built on exactly that property.
The Handbook of Applied Cryptography (1996, with Paul van Oorschot and Vanstone) became the reference for a generation of implementers, not least because the publisher allowed the chapters to be posted for free. Since 2004 he and Neal Koblitz have written the "Another look at…" series, examining how much cryptographic security proofs actually prove. The answer is often: less than the title.