/**

* Sbox class calculates Sbox for AES.

*

* @author Manu Chandel

* @version 1.0

* @since 2016-09-16

*/

public class Sbox{

// data structure to store sbox

private static int sbox[][]=new int[16][16];

private static int inverseSbox[][]=new int[16][16];

static {

for(int i=0;i<16;i++){

for(int j=0;j<16;j++){

int entry=Sbox.compute(i,j);

Sbox.sbox[i][j]=entry;

// calculate inverse entry

int invi=(entry&(15<<4))>>4;

int invj=entry&(15);

Sbox.inverseSbox[invi][invj]=(i<<4)|j;

}

}

}

// computes sbox entry and inverse sbox entry

private static int compute(int i,int j){

// construct entry

int a=(i<<4)|j;

// constant

int c=0x63;

// get inverse

int inverse;

if(i==0 && j==0)

inverse=0;

else inverse=Polynomial.inverse(new Polynomial(a)).get();

// calculate bits

int b[]=new int[8];

// calculate bits

for(int k=0;k<8;k++){

b[k]=Math.min(1,inverse & (1<<k));

}

// new bits

int bprime[]=new int[8];

for(int k=0;k<8;k++){

bprime[k]=0;

bprime[k]=b[k]^b[(k+4)%8]^b[(k+5)%8]^b[(k+6)%8]^b[(k+7)%8];

bprime[k]=bprime[k]^(Math.min(1,c&(1<<k)));

}

int aprime=0;

// calculate entry

for(int k=0;k<8;k++){

aprime=aprime|(bprime[k]<<k);

}

return aprime;

}

// returns entry at i,j of Sbox

public static int getSbox(int i,int j){

return Sbox.sbox[i][j];

}

// returns entry at i,j of Inverse Sbox

public static int getInvSbox(int i,int j){

return Sbox.inverseSbox[i][j];

}

}