Reprogramming is an essential service for wireless
sensor networks. Authenticating reprogramming process is important as
sensors need to verify that the code image is truly from a trusted
source. There are two ways to achieve authentication: public key based
and symmetric key based. Although previous work has shown that public
key authentication is feasible on sensor nodes if used sparingly, it is
still quite expensive compared to symmetric key based approach. In this
paper, we propose a symmetric key based protocol for authenticating
reprogramming process. Our protocol is based on the secret
instantiation algorithm from \cite{gke06,kg06}, which requires only
$O(\log n)$ keys to be maintained at each sensor. We integrate this
algorithm with the existing reprogramming protocol. Through simulation,
we show that it is able to authenticate reprogramming process at very
low communication cost, and has very short delay.
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