One of the huge problems for security in sensor networks is the lack of resources. Based on microcontroller architectures with severe limited computing abilities, strong public-key cryptography is commonly seen as infeasible on sensor devices. In contrast to this prejudice this paper presents an efficient and lightweight implementation of public-key cryptography algorithms relying on elliptic curves. The code is running on Atmels popular 8Bit ATMEGA128 microcontroller, the heart of the MICA2 platform.