Nicolas Courtois
French cryptanalyst at University College London, best known for claiming in 2002 that AES could be solved as a system of equations.
In 2002 Courtois and Josef Pieprzyk presented the XSL attack, which rewrites AES as a large system of quadratic equations over small fields. If that system could be solved efficiently, the cipher's structure rather than its key length would decide its strength. Bruce Schneier wrote some rattled commentary; later analysis suggested the method probably does not work as claimed. AES is still standing.
The algebraic school was vindicated elsewhere, though. In 2003 Courtois and Willi Meier broke several LFSR-based stream ciphers, and he later went after the KeeLoq car-key cipher and the MIFARE Classic card — systems whose designers had not asked for his opinion.