code: def generateOneTimePad(self, userStore):
          
          pad = secureRandom(16).encode('hex')
          self._oneTimePads[pad] = userStore.idInParent
          def expirePad():
              self._oneTimePads.pop(pad, None)
          self.callLater(self.ONE_TIME_PAD_DURATION, expirePad)
          return pad
docs: Generate a pad which can be used to authenticate via AMP.  This pad
          will expire in L{ONE_TIME_PAD_DURATION} seconds.