def sxor(s1, s2)
      raise ArgumentError, "strings must have equal size" unless s1.size == s2.size

      s1.bytes.zip(s2.bytes).map {|a, b| (a ^ b).chr }.join
    end
def sxor(s1, s2)
      raise ArgumentError, "strings must have equal size" unless s1.size == s2.size

      s1.bytes.zip(s2.bytes).map {|a, b| (a ^ b).chr }.join
    end
String xor.