Abstract: The present invention relates to systems (1) and a method for executing code. According to the method a non-critical code portion is executed on a computer (3). When an application (5) on the computer detects a critical code portion to be executed, the application sends a request to a secure execution unit (4) connected to the computer to execute the critical code portion. The secure execution unit (4) executes the critical code portion in response to the request. Thereafter the secure execution unit authenticates the result of the execution of the critical code portion using a secret key (7). The authentication allows for another party (2) to verify that the execution was carried out in a trusted way. An advantage of the present invention is that it provides a reliable execution environment that can be trusted to execute critical code.