KEY SCHEDULE
Round Key Selection (for Nb = 6, Nk = 4)
KeyExpansion(CipherKey,W) {
for( i=0 ; i<Nk ; i++ ) W[i] = CipherKey[i] ;
for( j=Nk ; j<Nb*(Nr+1) ; j += Nk )
W[j] = W[j-Nk] ^ SubByte(Rotl(W[j-1])) ^ Rcon[j/Nk] ;
for( i=1 ; i<Nk && i+j < Nb*(Nr+1); i++ )
W[i+j] = W[i+j-Nk] ^ W[i+j-1] ;