galois field galois_mul2 convertion to javascript?

Question

I am very hard time to convert galois_mul2 function into javascript.

I am having following function in c

 unsigned char galois_mul2(unsigned char value)
{
    if (value >> 7)
    {
        return ((value << 1) ^ 0x1b);
    }
    else
        return (value << 1);
}

Javascript Code

  galois_mul2( value){


      if (value >> 7)
      {
        return ((value << 1) ^ 0x1b);
      }
      else
          return (value << 1);
    }

If i give input 222 its return 167 in c code whereas in my code its return 423.

What's Problem?

Answer 1

As you probably know, numbers in JavaScript are 64-bit floats. But even if you have a UInt8Array object, and index into it, JavaScript will convert the internal 8-bit number into a double before giving it to you, so value << 1 can become a 9-bit number: try var arr = new Uint8Array([222]); (arr[0] << 1).toString(2) and notice the output is nine long.

To fix this, just mask with 0xFF to keep the lowest eight bits:

function galois_mul2(value) {
  if (value >> 7) {
    return ((0xff & (value << 1)) ^ 0x1b);
  }
  return 0xff & (value << 1);
}

Then, galois_mul2(222) returns 167.