extended-challenge = otp-challenge 1*LWSP-char capability-list
                        (NL / *LWSP-char)
   otp-challenge     = &lt;a standard OTP challenge&gt;
   capability-list   = "ext" *("," extension-set-id)
   extension-set-id  = *&lt;any CHAR except LWSP, CTLs, or ","&gt;
   extended-response = type 1*(":" argument) NL
   type              = token
   argument          = token
   token             = 1*&lt;any CHAR except ":" and CTLs&gt;
   NL                = &lt;new line sequence appropriate for th