THE KEY WILL NOT ALWAYS WORK IN THE IGNITION. And sometimes when it does work the SECURITY light stays on.
key problem on 2003 Buick Century
Possibly your ignition switch is beginning to fail, some of these models have a chip on the ignition key and as they age the vehicle will not recognize the key, call the buick parts department and asked the counter person if this is common and what do they recommend sometimes you can just buy a new key.
You need to have it renewed..it has a sensor built into it. bring it to a GM dealership, have a credit card handy!