code: func (hc *halfConn) changeCipherSpec() error {
  	if hc.nextCipher == nil {
  		return alertInternalError
  	}
  	hc.cipher = hc.nextCipher
  	hc.mac = hc.nextMac
  	hc.nextCipher = nil
  	hc.nextMac = nil
  	for i := range hc.seq {
  		hc.seq[i] = 0
  	}
  	return nil
  }
docs: // changeCipherSpec changes the encryption and MAC states
  // to the ones previously passed to prepareCipherSpec.