Baudoin Collard François-Xavier Standaert Jean-Jacques Quisquater Improved and Multiple Linear Cryptanalysis of Reduced Round Serpent. 51-65 2007 Inscrypt http://dx.doi.org/10.1007/978-3-540-79499-8_6 conf/cisc/2008 db/conf/cisc/inscrypt2008.html#CollardSQ08