## 2 Related Work  
Several research projects have been developing prototype medical sensor networks, but to provide a comprehensive security solution for medical sensor networks remains an open problem. The CodeBlue [4] project at Harvard has proposed a more-based sensor network platform and developed an operational prototype for use in hospitals. The CodeBlue designers acknowledge the need for security in a medical environment, but addressing security requirements is not a main focus of their study. They did develop an elliptic curve cryptography (ECC) public-key implementation on Crossbow motes [9], which is much less efficient than later implementations. The I-Living project [10] and PAS [11] propose an architecture to enable assisted living at home for elderly citizens. Although they propose that the medical sensors use IEEE 802.11 or Bluetooth for wireless communication, in contrast to the IEEE 802.15.4 radios often found on motes, the aims of the project are very similar. The authors realize the need for privacy when dealing with patient data and propose a symmetric security scheme, in which security information such as keys is stored in USB sticks that are automatically recognized once plugged into a device. However, the scheme seems to require that each device be individually configured, reducing its scalability. ALARM-NET [6] shares many similarities with the above work, aiming to develop an architecture for wireless monitoring of residents in assisted-living facilities. ALARM-NET combines wearable medical sensors with static environmental sensors for measuring quantities such as temperature and light. Here, the authors propose that security be provided via the symmetric Advanced Encryption Standard (AES) cipher, but they are not specific about key management. Much of the aforementioned work has made impressive progress from a hardware perspective, including development of custom medical sensors or integration of sensors with motes and other communication devices. Security has generally been covered separately and not as an integrated component of the architecture. We feel our work is complementary to these efforts, as we focus on addressing security concerns from an architectural standpoint. There has been extensive research on wireless sensor security in general. Instead of enumerating all the prior work in this area, we highlight several classes of approaches and explain why they cannot be directly applied to long-term patient monitoring. Efficient public-key schemes have been proposed for mutual authentication between motes and base stations as well as the establishment of shared keys, e.g., [12; 13]. However, these schemes often require each mote to have its own public key, which is vulnerable to the _physical compromise of motes_ (i.e., an attacker obtaining physical access to a mote) as we explain later. Another efficient approach to authentication is to use symmetric cryptography but delay the disclosure of the symmetric keys, e.g., \(\mu\)TESLA [14]. This approach is not suitable for our application because message authentication must be instantaneous to ensure timely reaction to patient emergencies. Thus, we developed our own two-tier authentication scheme (detailed in Section 6) to account for the unique characteristics of medical sensor networks. A large body of work has been done on key pre-distribution using symmetric cryptography (e.g., [15]). These schemes focus mainly on generating shared keys between sensor nodes. Often a pre-configured secret key is used between the base station and each mote, which is not resilient against physical compromise. In this paper, we choose ECC and asymmetric cryptography as the basis of our key agreement protocol because recent work [9; 16; 17; 18] has shown that performing ECC public-key computations on resource-constrained devices is viable. Several ECC-based key agreement protocols have been proposed, including ECDH [19; 20] and ECMQV [20].