Asymmetric key cryptosystems are unsuitable for use in the resource constrained sensor nodes and typically the biggest challenge in all symmetric security systems is how to exchange the initial encryption key safely. In many communication protocols, this is carried out without any security procedure or the challenge has been left to the application developer to solve. If an adversary receives this first insecure message, the security of the whole network is threatened. This is a very serious security problem especially in wireless and dynamic systems such as KILAVI where new off-the-shelf devices are attached to the system. KILAVI contains special security procedures to authenticate new devices safely into the system with easy and fast registration. For encryption, KILAVI uses the RC5 algorithm in a counter (CTR) mode. The length of encrypted and plain messages is the same due to the counter mode, an important quality when minimizing the power consumption in wireless data transfer. 1

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