A printer is communicatively connected to a terminal and a slave. The printer creates a secure printing group, and allows the terminal and slave to join the secure printing group. The terminal generates a print job, and sends the print job to the printer. The printer generates a first inspection message according to the print job, and sends the first inspection message to the terminal. The terminal selects the slave as a printing delegate, and sends a message that is derived at least in part from the first inspection message to the slave. The slave sends a second inspection message to the printer, wherein the second inspection message is derived at least in part from the message sent from to the terminal to the slave, and requests the printer to execute the print job. The printer executes the print job when the second inspection message matches the first inspection message.