lost 2003 buick century ignition key

You need a locksmith who is familiar with GMs VATS security system or your local GM Dealer. Your ignition key has a visible chip on it. This is a resistance block that tells the computer that the correct key is being used to start it. There were 15 different resistance value keys available. See the chart below. Your locksmith or GM Dealer will need a VATS Interrogator tool to determine which resistance value key you need to be able to start the engine after the key code is determined and new keys are cut. Hope that helps! Jim