Access control in sensor networks is used to authorize and grant users the right to access the network and data collected by sensors .Different users have different access right due to the access restriction implicated by the data security and confidentiality .Even though symmetric-key scheme, which has been investigated extensively for sensor networks, can fulfill the requirement, public-key cryptography is more flexible and simple rendering a clean interface for the Security component .We quantify the energy cost of authentication and key exchange based on public-key cryptography on an 8-bit microcontroller platform .We present a comparison of two public-key algorithms, RSA and Elliptic Curve Cryptography (ECC), and consider mutual authentication and key exchange between two untrusted parties such as two nodes in a wireless sensor network .Our measurements on an Atmel ATmega128L low-power microcontroller indicate that public -key cryptography is very viable on 8-bit energy constrained platforms even if implemented in software.