elements of GF(2¹²⁸) and returns the sum. func ghashAdd(x, y *gcmFieldElement) gcmFieldElement { // Addition in a characteristic 2 field is just XOR. return gcmFieldElement{x.low ^ y.low, x.high ^ y.high} } // ghashDouble returns the result of doubling an element of GF(2¹²⁸). func ghashDouble(x *gcmFieldElement) (double gcmFieldElement) { msbSet := x.high&1 == 1 // Because of the bit-ordering, doubling is actually a right shift. double.high = x.high >> 1 double.high |= x.low << 63 double.low = x.low...