def setnonce(self, text=None):
        """
        Set I{nonce} which is arbitraty set of bytes to prevent
        reply attacks.
        @param text: The nonce text value.
            Generated when I{None}.
        @type text: str
        """
        if text is None:
            s = []
            s.append(self.username)
            s.append(self.password)
            s.append(Token.sysdate())
            m = md5()
            m.update(':'.join(s).encode("utf-8"))
            self.nonce = m.hexdigest()
        else:
            self.nonce = text
Set I{nonce} which is arbitraty set of bytes to prevent
        reply attacks.
        @param text: The nonce text value.
            Generated when I{None}.
        @type text: str