java - What is wrong with this implementation of bitwise multiplication? -

i attempting implement method bitwise multiplication in galois field 256 sake of building implementation of aes. current multiplication method follows:

public static int multiplypolynomials(int n, int m) { int result = 0x00000000; string ns = tobitstring(n); string ms = tobitstring(m);  (int = 0; < ns.length(); i++) {     if (ns.charat(i) == '1')     {         /*          * if there's 1 @ place i, add value of m left-shifted places result.          */         int temp = m;         (int j = 0; j < i; j++) { temp = temp << 1; }          result += temp;     } } return result; } 

the tobitstring(int n) method purely shortcut integer.tobinarystring(int n).

given input of (0x0000ef00, 2), output of function 494 (should 478). printing direct call tobitstring(0x0000ef00) confirms output of function expected (in case, 1110111100000000). if first input shifted 1 byte right (0x000000ef) output still 494.

with above inputs, value of ns 1110111100000000 , bit-string equivalent of result 111101110. ns correct.

what error in method above?

you reading binary string wrong way round.

try this...

public static int multiplypolynomials(int n, int m) {     int result = 0x00000000;     string ns = integer.tobinarystring(n);      (int = 0; < ns.length(); i++) {         // read string other way round...         int bitindex = (ns.length() - i) - 1;         if (ns.charat(bitindex) == '1') {             /*              * if there's 1 @ place i, add value of m left-shifted              * places result.              */             int temp = m;             // don't need loop here, shift "i" places             temp = temp << i;             result += temp;         }     }     return result; } 

instead of turning number binary string, use instead...

public static int multiplypolynomials(int n, int m) {     int result = 0x00000000;      (int = 0; < 32; i++) {         int mask = 1 << i;          if ((n & mask) == mask) {             result += m << i;         }     }     return result; } 

you might need store answer long prevent overflows , won't work negative numbers...