Personal servers are an attractive concept: People carry around a device that takes care of computing, storage and communication on their behalf in a pervasive computing environment. So far personal servers have mainly been considered for accessing personal information. In this paper, we consider personal servers in the context of a digital key system. Digital keys are an interesting alternative to physical keys for mail or good delivery companies whose employees access tens of private buildings every day. In this paper, we present a digital key system tailored for the current incarnation of personal servers, i.e., a Bluetooth-enabled mobile phone. We describe how to use Bluetooth for this application, we present a simple authentication protocol and we provide a detailled analysis of response time and energy consumption on the mobile phone.