  A smart-card-based user authentication scheme for wireless sensor networks
(hereafter referred to as a SCA-WSN scheme) is designed to ensure that only
users who possess both a smart card and the corresponding password are allowed
to gain access to sensor data and their transmissions. Despite many research
efforts in recent years, it remains a challenging task to design an efficient
SCA-WSN scheme that achieves user anonymity. The majority of published SCA-WSN
schemes use only lightweight cryptographic techniques (rather than public-key
cryptographic techniques) for the sake of efficiency, and have been
demonstrated to suffer from the inability to provide user anonymity. Some
schemes employ elliptic curve cryptography for better security but require
sensors with strict resource constraints to perform computationally expensive
scalar-point multiplications; despite the increased computational requirements,
these schemes do not provide user anonymity. In this paper, we present a new
SCA-WSN scheme that not only achieves user anonymity but also is efficient in
terms of the computation loads for sensors. Our scheme employs elliptic curve
cryptography but restricts its use only to anonymous user-to-gateway
authentication, thereby allowing sensors to perform only lightweight
cryptographic operations. Our scheme also enjoys provable security in a formal
model extended from the widely accepted Bellare-Pointcheval-Rogaway (2000)
model to capture the user anonymity property and various SCA-WSN specific
attacks (e.g., stolen smart card attacks, node capture attacks, privileged
insider attacks, and stolen verifier attacks).
