A memory stores a program to be executed by a microprocessor. The program includes a first program part and a second program part. An authenticator is configured to authenticate the program and includes a module that is external to the microprocessor and configured to authenticate said first program part when the microprocessor is inactive. The authenticator further activates the microprocessor to execute the first program part and authenticate said second program part using instructions of the first program part if the module has authenticated the first program part. The microprocessor then executes the second program part if the microprocessor has authenticated said second program part.