This work describes a low-cost Public-Key Cryptography (PKC) based solution for security services such as key-distribution and authentication as required for wireless sensor networks. We propose a custom hardware assisted approach to implement Elliptic Curve Cryptography (ECC) in order to obtain stronger cryptography as well as to minimize the power. Our compact and low-power ECC processor contains a Modular Arithmetic Logic Unit (MALU) for ECC field arithmetic. The best solution features 6718 gates for the MALU and control unit (data memory not included) in 0.13 μm CMOS technology over the field \mathbbF_2¹³¹{\mathbb{F}_{2^{131}}}, which provides a reasonable level of security for the time being. In this case the consumed power is less than 30 μW when operating frequency is 500 kHz.