"Wireless sensor networks (WSN) are typically deployed in an unattended environment, where the legitimate users
can login to the network and access data as and when demanded.
Consequently, user authentication is a primary concern in this
resource-constrained environment before accessing data from the
sensor/gateway nodes. In this letter, we present a two-factor
user authentication protocol for WSN, which provides strong
authentication, session key establishment, and achieves efﬁciency"