def self.uuid
        ary = self.random_bytes(16).unpack("NnnnnN")
        ary[2] = (ary[2] & 0x0fff) | 0x4000
        ary[3] = (ary[3] & 0x3fff) | 0x8000
        "%08x-%04x-%04x-%04x-%04x%08x" % ary