nonce
     A server-specified data string which should be uniquely generated
     each time a 401 response is made. It is recommended that this
     string be base64 or hexadecimal data. Specifically, since the
     string is passed in the header lines as a quoted string, the
     double-quote character is not allowed.

     The contents of the nonce are implementation dependent. The quality
     of the implementation depends on a good choice. (...)