Ivan Damgård
Danish cryptographer at Aarhus University whose name is on the construction behind MD5, SHA-1 and SHA-2 — and who has mostly spent the time since computing on secrets.
In 1989 Damgård presented his design principle for hash functions: build a collision-resistant compression function and iterate it, and the whole inherits the property. Ralph Merkle reached the same result independently at the same time, hence Merkle–Damgård. That the construction also invites length-extension attacks was understood later, and is why HMAC looks the way it does.
Most of his career has gone into secure multiparty computation. The Aarhus group was behind the 2008 Danish sugar beet auction, in which farmers bid without anyone — the auctioneer included — seeing the bids, often cited as the first large-scale MPC deployment. The SPDZ protocol (2012, with Pastro, Smart and Zakarias) became a de facto basis for practical MPC. He also co-founded Cryptomathic.