With the recent advances in communication technologies, wireless sensor networks (WSN) have emerged as a very active research avenue. WSNs have many common features with wireless ad hoc networks, and in several cases they are considered as a special case of them [1]. A WSN usually consists of a large number of autonomous sensor nodes, which are generally deployed in unattended environments. Each sensor node has some level of computing power, limited storage, and a small communication module to communicate with the outside world over an ad hoc wireless network [2]. WSNs are widely used, including in areas such as military, battlefield, homeland security, healthcare, environment monitoring, agriculture and cropping, manufacturing, etc.

Since the sensor network may operate in a hostile environment such as a military battlefield, security is critical. Robust techniques are needed to provide low-latency, survivable, and secure networks during the deployment of WSN. In addition, the network should be protected against intrusions and spoofing attacks [3]. Access control is an indispensable cryptographic primitive upon which other security primitives are built. A WSN should be smart enough to distinguish legitimate users from illegitimate users, resulting in the problem of user authentication [3]. If a WSN is deployed for a highly secure application, then the data collected within the sensor work is valuable and should only be given access to the registered or legitimate users. Benenson et al. first sketched the security issues of user authentication in WSN and introduced the notion of n-authentication [4]. Later on, Watro et al. proposed a TinyPK authentication protocol with public key cryptography that uses RSA and Diffie-Hellman algorithms [5], however, this protocol suffers from masquerade sensor node attack, in which an adversary can spoof the user.

In 2006, Wong et al. [6] proposed a light-weight dynamic user authentication scheme in WSN environment. They justified their scheme through security and cost analysis and discussed the implementation issues with the recommendations of using the security features of IEEE 802.15.4 MAC sublayer. Later, Tseng et al. [7] identified some security weaknesses in the scheme of Wong et al., which prevent it from being implemented in real-life environments. They showed that Wong et al.’s scheme is not protected from replay and forgery attacks, passwords can easily be revealed by any of the sensor nodes, and users cannot freely change their passwords. To overcome these discrepancies, Tseng et al. proposed an enhanced scheme and claimed that their scheme not only retains the advantages of Wong et al.’s scheme, but provides: resistance to replay and forgery attacks, reduction of password leakage risk, and capability of changeable password with better efficiency [7]. Lately, T.H. Lee [8] also analyzed Wong et al.’s scheme and proposed two simple dynamic user authentication protocols that are variations of Wong et al.’s scheme. In his first protocol, T.H. Lee simplified the authentication process by reducing the computational load of sensor nodes while preserving the same security level of Wong et al.’s scheme. On the other hand, in his second protocol, T.H. Lee proposed a scheme in which an intruder cannot impersonate the gateway node to grant access to illegitimate users.

L.C. Ko [9] proved that while Tseng et al.’s scheme achieves several security measures above Wong et al.’s scheme, it is still insecure under a reasonable attack model [9]. L.C. Ko discussed that Tseng et al.’s scheme does not achieve mutual authentication between the Gateway node (GW) and the Sensor node (SN), and between the User (U) and the SN. Furthermore, L.C. Ko identified that an adversary can forge the communication message which is sent from sensor node to the gateway node. Consequently, L.C. Ko proposed a modified scheme which attempts to overcome the aforementioned security pitfalls of Tseng et al.’s protocol and proved that his scheme has better security features than Tseng et al.’s scheme. [7]

Binod et al. [10] cryptanalyzed the authentication schemes of Wong et al. and Tseng et al. and proposed their improved scheme. Binod et al. showed that their scheme is more robust than previously published schemes and can withstand replay attack, forgery attack, man-in-the-middle attack and provides mutual authentication between login node and gateway node.

Recently, M.L. Das [11] proposed a two-factor user authentication scheme in WSNs. M.L. Das also identified that Wong et al.’s protocol is vulnerable to many logged-in users with the same login-id threat, that is, who has a valid user’s password can easily login to the sensor network [11]. He also identified that Wong et al.’s protocol is susceptible to stolen-verifier attack, because the GW-node and login-node maintain the lookup table of all the registered users’ credentials. Consequently, M.L. Das proposed his protocol to overcome the security flaws of Wong et al.’s scheme. His protocol uses the two factor authentication concept based on password and smart card and resists many logged-in users with the same login identity, stolen-verifier, guessing, replay, and impersonation attacks.

More recently, Nyang and Lee pointed out that the protocol of M.L. Das is vulnerable to offline password guessing attack, sensor node compromising attack, and does not protect query response messages by establishing a unique secure channel from sensor node to a user, which is an important way of serving a registered user in a secure and legitimate way [17]. Consequently, Nyang and Lee proposed their improved two-factor authentication protocol for WSNs, which attempts to overcome their identified discrepancies in the M.L. Das scheme.

However, in this paper, we identify that the M.L. Das-scheme is still not secure and vulnerable to several critical security attacks. In addition to the problems identified by Nyang and Lee, we show that the M.L. Das-scheme is defenseless against GW-node by-passing attack, does not provide mutual authentication between GW-node and sensor nodes, has the security threat of insider attack, and does not have provision for changing or updating passwords of registered users. To fix the aforementioned weaknesses of the M.L. Das-scheme, we propose security improvements in our paper. Our enhanced security patch contains secure features of changing or updating passwords of users, provides protection against insider attack, overcomes the GW-node bypassing attack, and provides mutual authentication between GW-node and sensor node. The proposed security improvements can easily be incorporated into the M.L. Das-scheme to take the benefit of more secure and robust two-factor user authentication in WSNs.

The rest of the paper is organized as follows; Section 2 briefly reviews the M.L. Das-scheme, Section 3 elaborates on the weaknesses and security pitfalls of his scheme, Section 4 presents our proposed security patch, improvements and analysis over the M.L. Das-scheme, Section 5 reveals the performance analysis of the presented scheme, and finally, Section 6 concludes this paper.

In this section, we briefly review user the authentication scheme of M.L. Das, which is divided into two phases, namely the registration phase and the authentication phase.

In this section, we propose security improvements over the scheme of M.L. Das and perform analysis of our security patches as follows:

In this section, we summarize security features and performance analysis of our proposed scheme and compare its security and robustness with the schemes of M.L. Das [11], and Nyang and Lee [17]. Table 1 demonstrates that our scheme is more secure and robust than the schemes of [11] and [17], and achieves more security features, which were not considered in the aforementioned schemes and are essentially required to implement a practical and universal two-factor user authentication protocol in WSNs.

Furthermore, it can be seen from Table 1 that our scheme needs only 13 hashing operations, in contrast to the protocols of M.L. Das and Nyang-Lee, which require 10 and 17 hash computations, respectively. Our scheme provides protection against insider attack, gateway node bypassing attack, password change/update option, and achieves mutual authentication between gateway and sensor nodes, which require few more hashing operations than [11] to enhance the security of overall authentication system. Hence, the computational overhead of the proposed scheme are not too high, but the scheme contains several enhanced security features, which are indispensable for implementing a reliable and trustworthy remote user authentication scheme in the WSN environment.

In this paper, we have shown that a recently proposed two-factor user authentication scheme in WSN environment is insecure against different kinds of attack and should not be implemented in real-applications. We have demonstrated that in the M.L. Das-scheme, there is no provision for users to change or update their passwords, the GW-node bypassing attack is possible, it does not provide mutual authentication between GW-node and sensor node, and it is susceptible to privileged-insider attack. To remedy the aforementioned flaws, we have proposed security patches and improvements, which overcome the weak features of the M.L. Das-scheme. The presented security improvements can easily be incorporated in the M.L. Das-scheme for a more secure and robust two-factor user authentication in WSNs.