Bert den Boer
Dutch cryptographer who exposed the first crack in MD5 in 1993 — eleven years before anyone produced a real collision.
At Eurocrypt 1993, den Boer and Antoon Bosselaers presented collisions in MD5's compression function: two different initial values that produced the same output for the same block. That did not break MD5 in practice, but it did break the assumption the Merkle–Damgård construction leans on. The warning was widely ignored until Xiaoyun Wang delivered full collisions in 2004.
Back in 1988 he had shown that the Diffie-Hellman problem is as hard as the discrete logarithm for primes with a particular structure — an early result on whether the two problems are equivalent. He also took part in the EU's RIPE project, whose RIPEMD hash preceded RIPEMD-160.