Abstract. In this article we present the fastest known implementation of a modular multiplication for a 160-bit standard compliant elliptic curve (secp160r1) for 8-bit micro controller which are typically used in WSNs. The major part (77%) of the processing time for an elliptic curve operation such as ECDSA or EC Diffie-Hellman is spent on modular multiplication. We present an optimized arithmetic algorithm which significantly speed up ECC schemes. The reduced processing time also yields a significantly lower energy consumption of ECC schemes. With our implementation results we can show that a 160-bit modular multiplication can be performed in 0.39 ms on an 8-bit AVR processor clocked at 7.37 MHz. This brings the vision of asymmetric cryptography in the field of WSNs with all its benefits for key-distribution and authentication a step closer to reality.