not(c choice) choice { return 1 ^ c } const yes = choice(1) const no = choice(0) // ctMask is all 1s if on is yes, and all 0s otherwise. func ctMask(on choice) uint { return -uint(on) } // ctEq returns 1 if x == y, and 0 otherwise. The execution time of this // function does not depend on its inputs. func ctEq(x, y uint) choice { // If x != y, then either x - y or y - x will generate a carry. _, c1 := bits.Sub(x, y, 0) _, c2 := bits.Sub(y, x, 0) return not(choice(c1 | c2)) } // Nat represents an...