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Nicolas Tadeusz Courtois (born 14 November 1971) is a cryptographer, a senior lecturer in computer science at University College London. 
Courtois was one of the co-authors of both the
XSL attack against block ciphers such as the Advanced Encryption Standard  and the XL system for solving systems of algebraic equations,  used in the attack. Other cryptographic results of Courtois include algebraic attacks on  stream ciphers, attacks on the  KeeLoq and Hitag 2 systems used for remote keyless automobile entry systems, and an analysis of cryptographic weaknesses in  public transit smart cards including the London Underground Oyster card and the  Dutch OV-chipkaart.  More recently, he has written about  cryptocurrency. 
Courtois graduated from
University of Paris VI: Pierre et Marie Curie, where he got his doctoral degree in cryptography. 
^ a b Nicolas T. Courtois at the Mathematics Genealogy Project
^ "Staff profile". UCL . Retrieved . 2 June 2014
^ Courtois, Nicolas T.; Pieprzyk, Josef (2002), "Cryptanalysis of block ciphers with overdefined systems of equations", Advances in Cryptology – AsiaCrypt 2002, Lecture Notes in Computer Science, 2501, Springer-Verlag, pp. 267–287, doi: 10.1007/3-540-36178-2_17 .
^ Greene, Thomas C. (September 16, 2002), , New AES crypto standard broken already? Depends who you ask The Register .
^ Courtois, Nicolas; Klimov, Alexander; Patarin, Jacques; Shamir, Adi (2000), "Efficient algorithms for solving overdefined systems of multivariate polynomial equations", Advances in Cryptology – EuroCrypt 2000, Lecture Notes in Computer Science, 1807, Springer-Verlag, pp. 392–407, doi: 10.1007/3-540-45539-6_27 .
^ Courtois, Nicolas T. (2003), "Fast algebraic attacks on stream ciphers with linear feedback", Advances in Cryptology – Crypto 2003, Lecture Notes in Computer Science, 2729, Springer-Verlag, pp. 176–194, doi: 10.1007/978-3-540-45146-4_11 .
^ a b Blincoe, Robert (July 28, 2008), , v3.co.uk Oyster cracker vows to clone cards: Cloning kit could sell for just £200, says researcher .
^ Charlesworth, Andrew (June 23, 2008), , v3.co.uk Researchers crack Oyster card security .
^ de Winter, Brenno (April 18, 2008), "Weer nieuwe kraak chip OV-chipkaart", WebWereld (in Dutch) .
^ "Emerging Technology From the arXiv". MIT Technology Review. 17 May 2014 . Retrieved . 2 June 2014 On The Longest Chain Rule and Programmed Self-Destruction of Crypto Currencies