A vehicle telematics unit or other electronic hardware in a vehicle authenticates an incoming SMS message and provides additional security features above and beyond those inherent to the SMS protocol. A call center uses a mathematical function to derive a security code, and then sends an SMS message to the vehicle. The telematics unit can utilize the transmitted information to authenticate the sender of the SMS message and/or its contents. One group of suitable mathematical functions utilizes keyed-hash message authentication codes (HMACs).