Liam Keliher Henk Meijer Stafford E. Tavares Modeling Linear Characteristics of Substitution-Permutation Networks. 78-91 http://link.springer.de/link/service/series/0558/bibs/1758/17580078.htm 1999 conf/sacrypt/1999 Selected Areas in Cryptography db/conf/sacrypt/sacrypt1999.html#KeliherMT99