Cas Cremers
Dutch researcher whose formal protocol-analysis tools find flaws in security protocols before attackers do.
Cremers built the Scyther tool during his PhD in Eindhoven and went on to co-develop the Tamarin prover at ETH Zurich. Both let you describe a protocol and an attacker who controls the network, then mechanically search for runs where secrets leak or parties are fooled about who they are talking to.
It turned out to matter outside academia. His group modelled the TLS 1.3 drafts during standardisation and found an authentication flaw that was fixed before release, and co-authored the first formal analysis of the Signal protocol. After a spell at Oxford he now leads a group at CISPA in Saarbrücken. On his watch, formal verification of protocols went from curiosity to something standards bodies actually wait for.