spelling check
set an introduction to the section where the operations in the Rijndael are described in a generalized way.
add some questions to be answered in the subsection of the 'number of rounds'.
fix an error on the sub(sub)section about the improve on the polynomial ring product.
fix half a sentense that has crept (in catalan) in the middle of another
change the png file of the rijndael diagram to the pgf
direct improving of the rijndael diagram in pgf (it was a dia output)