Security in Mobile Wireless Sensor Networks – A Survey <s> Issues Publications <s> We describe a digital signature scheme in which the public key is fixed but the secret signing key is updated at regular intervals so as to provide a forward security property: compromise of the current sccret key does not enable an adversary to forge signatures pertaining to the past. This can be useful to mitigate the damage caused by key exposure without requiring distribution of keys Our construction uses ideas from the Fiat-Shamir and Ong-Schnorr identification and signature schemes, and is proven to be forward secure based on the hardness of factoring, in the random oracle model The construction is also quite efficient. <s> BIB001 </s> Security in Mobile Wireless Sensor Networks – A Survey <s> Issues Publications <s> We propose the first forward-secure signature scheme for which both signing and verifying are as efficient as for one of the most efficient ordinary signature schemes (Guillou-Quisquater [GQ88]), each requiring just two modular exponentiations with a short exponent. All previously proposed forward-secure signature schemes took significantly longer to sign and verify than ordinary signature schemes. ::: ::: Our scheme requires only fractional increases to the sizes of keys and signatures, and no additional public storage. Like the underlying [GQ88] scheme, our scheme is provably secure in the random oracle model. <s> BIB002 </s> Security in Mobile Wireless Sensor Networks – A Survey <s> Issues Publications <s> In an intrusion-resilient cryptosystem [10], two entities (a user and a base) jointly evolve a secret decryption key; this provides very strong protection against an active attacker who can break into the user and base repeatedly and even simultaneously. Recently, a construction of an intrusion-resilient public-key encryption scheme based on specific algebraic assumptions has been shown [6]. We generalize this previous work and present a more generic construction for intrusion-resilient public-key encryption from any forward-secure public-key encryption scheme satisfying a certain homomorphic property. <s> BIB003 </s> Security in Mobile Wireless Sensor Networks – A Survey <s> Issues Publications <s> One of the most notable challenges threatening the successful deployment of sensor systems is privacy. Although many privacy-related issues can be addressed by security mechanisms, one sensor network privacy issue that cannot be adequately addressed by network security is source-location privacy. Adversaries may use RF localization techniques to perform hop-by-hop traceback to the source sensor's location. This paper provides a formal model for the source-location privacy problem in sensor networks and examines the privacy characteristics of different sensor routing protocols. We examine two popular classes of routing protocols: the class of flooding protocols, and the class of routing protocols involving only a single path from the source to the sink. While investigating the privacy performance of routing protocols, we considered the tradeoffs between location-privacy and energy consumption. We found that most of the current protocols cannot provide efficient source-location privacy while maintaining desirable system performance. In order to provide efficient and private sensor communications, we devised new techniques to enhance source-location privacy that augment these routing protocols. One of our strategies, a technique we have called phantom routing, has proven flexible and capable of protecting the source's location, while not incurring a noticeable increase in energy overhead. Further, we examined the effect of source mobility on location privacy. We showed that, even with the natural privacy amplification resulting from source mobility, our phantom routing techniques yield improved source-location privacy relative to other routing methods <s> BIB004 </s> Security in Mobile Wireless Sensor Networks – A Survey <s> Issues Publications <s> Typical packet traffic in a sensor network reveals pronounced patterns that allow an adversary analyzing packet traffic to deduce the location of a base station. Once discovered, the base station can be destroyed, rendering the entire sensor network inoperative, since a base station is a central point of data collection and hence failure. This paper investigates a suite of decorrelation countermeasures aimed at disguising the location of a base station against traffic analysis attacks. A set of basic countermeasures is described, including hop-by-hop reencryption of the packet to change its appearance, imposition of a uniform packet sending rate, and removal of correlation between a packet's receipt time and its forwarding time. More sophisticated countermeasures are described that introduce randomness into the path taken by a packet. Packets may also fork into multiple fake paths to further confuse an adversary. A technique is introduced to create multiple random areas of high communication activity called hot spots to deceive an adversary as to the true location of the base station. The effectiveness of these countermeasures against traffic analysis attacks is demonstrated analytically and via simulation using three evaluation criteria: total entropy of the network, total overhead/energy consumed, and the ability to frustrate heuristic-based search techniques to locate a base station. <s> BIB005 </s> Security in Mobile Wireless Sensor Networks – A Survey <s> Issues Publications <s> Wireless sensors are employed in a wide range of applications. One common feature of many sensor settings is the need to communicate sensed data to some collection point or sink. This communication can be direct (to a mobile collector) or indirect-via other sensors towards a remote sink. In either case, a sensor might not be able to communicate to a sink at will. Instead it might collect data and wait (for a potentially long time) for a signal to upload accumulated data directly. In a hostile setting, a sensor may be compromised and its post-compromise data can be manipulated. One important issue is forward security - how to ensure that pre- compromise data cannot be manipulated? Since a typical sensor is limited in storage and communication facilities, another issue is how to minimize resource consumption by accumulated data. It turns out that current techniques are insufficient to address both challenges. To this end, we explore the notion of forward-secure sequential aggregate (FssAgg) authentication schemes. We consider FssAgg authentication schemes in the contexts of both conventional and public key cryptography and construct a FssAgg MAC scheme and a FssAgg signature scheme, each suitable under different assumptions. This work represents the initial investigation of forward-secure aggregation and, although the proposed schemes are not optimal, we believe it opens a new direction for follow-on research. <s> BIB006 </s> Security in Mobile Wireless Sensor Networks – A Survey <s> Issues Publications <s> Sensor networks have been an attractive platform for pervasive computing and communication. Due to the lack of physical protection, however, sensor networks are vulnerable to attacks if deployed in hostile environments. When a sensor network is under attack, the most fundamental concern is that information communicated or stored in the network remains safe. The past research has focused on securing information in communication, but how to secure information in storage has been generally overlooked. Meanwhile, distributed data storage and retrieval have become popular for efficient data management in sensor networks, which renders the absence of schemes for securing stored information to be a more severe problem. Hence, we propose in this paper three schemes to deal with the problem. All the schemes have the following properties: (i) only authorized entities can access data stored in the sensor network; (ii) the schemes are resilient to a large number of sensor node compromises. The second and the third schemes do not involve any centralized entity except for a few initialization or renewal operations, and thus support secure, distributed data storage and retrieval. The third scheme further provides high scalability and flexibility, and hence is most suitable in real applications. The effectiveness and efficiency of the proposed schemes have also been verified through extensive analysis and TOSSIM-based simulations <s> BIB007 </s> Security in Mobile Wireless Sensor Networks – A Survey <s> Issues Publications <s> While many protocols for sensor network security provide confidentiality for the content of messages, contextual information usually remains exposed. Such information can be critical to the mission of the sensor network, such as the location of a target object in a monitoring application, and it is often important to protect this information as well as message content. There have been several recent studies on providing location privacy in sensor networks. However, these existing approaches assume a weak adversary model where the adversary sees only local network traffic. We first argue that a strong adversary model, the global eavesdropper, is often realistic in practice and can defeat existing techniques. We then formalize the location privacy issues under this strong adversary model and show how much communication overhead is needed for achieving a given level of privacy. We also propose two techniques that prevent the leakage of location information: periodic collection and source simulation. Periodic collection provides a high level of location privacy, while source simulation provides trade-offs between privacy, communication cost, and latency. Through analysis and simulation, we demonstrate that the proposed techniques are efficient and effective in protecting location information from the attacker. <s> BIB008 </s> Security in Mobile Wireless Sensor Networks – A Survey <s> Issues Publications <s> Due to the open nature of a sensor network, it is relatively easy for an adversary to eavesdrop and trace packet movement in the network in order to capture the receiver physically. After studying the adversary's behavior patterns, we present countermeasures to this problem. We propose a location-privacy routing protocol (LPR) that is easy to implement and provides path diversity. Combining with fake packet injection, LPR is able to minimize the traffic direction information that an adversary can retrieve from eavesdropping. By making the directions of both incoming and outgoing traffic at a sensor node uniformly distributed, the new defense system makes it very hard for an adversary to perform analysis on locally gathered information and infer the direction to which the receiver locates. We evaluate our defense system based on three criteria: delivery time, privacy protection strength, and energy cost. The simulation results show that LPR with fake packet injection is capable of providing strong protection for the receiver's location privacy. Under similar energy cost, the safe time of the receiver provided by LPR is much longer than other methods, including Phantom routing (Kamat et al., 2005) and DEFP (Deng et al., 2005). The performance of our system can be tuned through a couple of parameters that determine the tradeoff between energy cost and the strength of location-privacy protection. <s> BIB009 </s> Security in Mobile Wireless Sensor Networks – A Survey <s> Issues Publications <s> Unattended sensor networks operating in hostile environments might collect data that represents a high-value target for the adversary. The unattended sensor's inability to off-load - in real time - sensitive data to a safe external entity makes it easy for the adversary to mount a focused attack aimed at eliminating certain target data. In order to facilitate survival of this data, sensors can collectively attempt to confuse the adversary by changing its location and content, i.e., by periodically moving the data around the network and encrypting it. In this paper, we focus on data survival in unattended sensor networks faced with an adversary intent on surgically destroying data which it considers to be of high value. After motivating the problem and considering several attack flavors, we propose several simple techniques and provide their detailed evaluation. <s> BIB010 </s> Security in Mobile Wireless Sensor Networks – A Survey <s> Issues Publications <s> Mobile sinks are needed in many sensor network applications for efficient data collection, data querying, localized sensor reprogramming, identifying and revoking compromised sensors, and other network maintenance. Employing mobile sinks however raises a new security challenge: if a mobile sink is given too many privileges, it will become very attractive for attack and compromise. Using a compromised mobile sink, an adversary may easily bring down or even take over the sensor network. Thus, security mechanisms that can tolerate mobile sink compromises are essential. In this paper, based on the principle of least privilege, we first propose several efficient schemes to restrict the privilege of a mobile sink without impeding its capability of carrying out any authorized operations for an assigned task. To further reduce the possible damages caused by a compromised mobile sink, we then propose efficient message forwarding schemes for depriving the privilege assigned to a compromised mobile sink immediately after its compromise has been detected. Through detailed analysis and simulations, we show that our schemes are secure and efficient, and are highly practical for sensor networks consisting of the current generation of sensors. <s> BIB011 </s> Security in Mobile Wireless Sensor Networks – A Survey <s> Issues Publications <s> One of the most vexing problems in wireless sensor network security is the node capture attack. An adversary can capture a node from the network as the first step for further different types of attacks. For example, the adversary can collect all the cryptographic material stored in the node. Also, the node can be reprogrammed and re-deployed in the network in order to perform malicious activities. To the best of our knowledge no distributed solution has been proposed to detect a node capture in a mobile wireless sensor network. In this paper we propose an efficient and distributed solution to this problem leveraging emergent properties of mobile wireless sensor networks. In particular, we introduce two solutions: SDD, that does not require explicit information exchange between the nodes during the local detection, and CCD, a more sophisticated protocol that uses local node cooperation in addition to mobility to greatly improve performance. We also introduce a benchmark to compare these solutions with. Experimental results demonstrate the feasibility of our proposal. For instance, while the benchmark requires about 9,000 seconds to detect node captures, CDD requires less than 2,000 seconds. These results support our intuition that node mobility, in conjunction with a limited amount of local cooperation, can be used to detect emergent global properties. <s> BIB012 </s> Security in Mobile Wireless Sensor Networks – A Survey <s> Issues Publications <s> Unattended Wireless Sensor Networks (UWSNs) are composed of many small resource-constrained devices and operate autonomously, gathering data which is periodically collected by a visiting sink. Unattended mode of operation, deployment in hostile environments and value (or criticality) of collected data are some of the factors that complicate UWSN security. This paper makes two contributions. First, it explores a new threat model involving a mobile adversary who periodically compromises and releases sensors aiming to maximize its advantage and overall knowledge of collected data. Second, it constructs a self-healing protocol that allows sensors to continuously and collectively recover from compromise. The proposed protocol is both effective and efficient, as supported by analytical and simulation results. <s> BIB013 </s> Security in Mobile Wireless Sensor Networks – A Survey <s> Issues Publications <s> In recent years, wireless sensor networks (WSNs) have been a very popular research topic, offering a treasure trove of systems, networking, hardware, security, and application-related problems. Much of prior research assumes that the WSN is supervised by a constantly present sink and sensors can quickly offload collected data. In this paper, we focus on unattended WSNs (UWSNs) characterized by intermittent sink presence and operation in hostile settings. Potentially lengthy intervals of sink absence offer greatly increased opportunities for attacks resulting in erasure, modification, or disclosure of sensor-collected data. This paper presents an in-depth investigation of security problems unique to UWSNs (including a new adversarial model) and proposes some simple and effective countermeasures for a certain class of attacks. <s> BIB014 </s> Security in Mobile Wireless Sensor Networks – A Survey <s> Issues Publications <s> Some sensor network settings involve disconnected or unattended operation with periodic visits by a mobile sink. An unattended sensor network operating in a hostile environment can collect data that represents a high-value target for the adversary. Since an unattended sensor can not immediately off-load sensed data to a safe external entity (such as a sink), the adversary can easily mount a focused attack aiming to erase or modify target data. To maximize chances of data survival, sensors must collaboratively attempt to mislead the adversary and hide the location, the origin, and the contents of collected data. In this paper, we focus on applications of well-known security techniques to maximize chances of data survival in unattended sensor networks, where sensed data can not be off-loaded to a sink in real time. Our investigation yields some interesting insights and surprising results. The highlights of our work are: (1) thorough exploration of the data survival challenge, (2) exploration of the design space for possible solutions, (3) construction of several practical and effective techniques, and (4) their evaluation. <s> BIB015 </s> Security in Mobile Wireless Sensor Networks – A Survey <s> Issues Publications <s> Unattended Wireless Sensor Networks (UWSNs) operating in hostile environments face great security and performance challenges due to the lack of continuous real-time communication between senders (sensors) and receivers (e.g., mobile data collectors, static sinks). The lack of real-time communication forces sensors to accumulate the sensed data possibly for long time periods, along with the corresponding signatures for authentication purposes. Moreover, non-real-time characteristic of UWSNs makes sensors vulnerable especially to active adversaries, which compromise sensors and extract all data stored in them. Hence, it is critical to have forward security property such that even if the adversary can compromise the current keying materials, she cannot modify or forge authenticated data generated before the node compromise. Forward secure and aggregate signatures are cryptographic primitives developed to address these issues. Unfortunately, existing forward secure and aggregate signature schemes either impose substantial computation and storage overhead, or do not allow public verifiability, thereby impractical for resource-constrained UWSNs. In order to address these problems, we propose a new class of signature schemes, which we refer to as Hash-Based Sequential Aggregate and Forward Secure Signature (HaSAFSS). Such a scheme allows a signer to sequentially generate a compact, fixed-size, and publicly verifiable signature at a nearly optimal computational cost. We propose two HaSAFSS schemes, Symmetric HaSAFSS (Sym-HaSAFSS) and Elliptic Curve Cryptography (ECC) based HaSAFSS (ECC-HaSAFSS). Both schemes integrate the efficiency of MAC-based aggregate signatures and the public verifiability of bilinear map based signatures by preserving forward security via Timed-Release Encryption (TRE). We demonstrate that our schemes are secure under appropriate computational assumptions. We also show that our schemes are significantly more efficient in terms of both computational and storage overheads than previous schemes, and therefore quite practical for even highly resource-constrained UWSN applications. <s> BIB016 </s> Security in Mobile Wireless Sensor Networks – A Survey <s> Issues Publications <s> The owner and users of a sensor network may be different, which necessitates privacy-preserving access control. On the one hand, the network owner need enforce strict access control so that the sensed data are only accessible to users willing to pay. On the other hand, users wish to protect their respective data access patterns whose disclosure may be used against their interests. This paper presents DP 2 AC, a Distributed Privacy- Preserving Access Control scheme for sensor networks, which is the first work of its kind. Users in DP 2 AC purchase tokens from the network owner whereby to query data from sensor nodes which will reply only after validating the tokens. The use of blind signatures in token generation ensures that tokens are publicly verifiable yet unlinkable to user identities, so privacy- preserving access control is achieved. A central component in DP 2 AC is to prevent malicious users from reusing tokens. We propose a suite of distributed techniques for token-reuse detection (TRD) and thoroughly compare their performance with regard to TRD capability, communication overhead, storage overhead, and attack resilience. The efficacy and efficiency of DP 2 AC are confirmed by detailed performance evaluations. <s> BIB017 </s> Security in Mobile Wireless Sensor Networks – A Survey <s> Issues Publications <s> Unattended Wireless Sensor Networks (UWSNs) operated in hostile environments face a risk on data security due to the absence of real-time communication between sensors and sinks, which imposes sensors to accumulate data till the next visit of a mobile sink to off-load the data. Thus, how to ensure forward secrecy, backward secrecy and reliability of the accumulated data is a great challenge. For example, if a sensor is compromised, pre-compromise data accumulated in the sensor is exposed to access. In addition, by holding key secrecy of the compromised sensor, attackers also can learn post-compromise data in the sensor. Furthermore, in practical UWSNs, once sensors stop working for accidents due to node crash or battery depletion, all the data accumulated will be lost. To address the challenges, we propose a secure and reliable data distribution scheme in this paper. Detailed analysis shows that our scheme can provide forward secrecy, probabilistic backward secrecy and data reliability. To further improve probabilistic backward secrecy and data reliability, a constrained optimization data distribution scheme is proposed. Detailed analysis and simulation results show the superiority of the proposed scheme in comparison with several previous approaches developed for UWSNs. <s> BIB018 </s> Security in Mobile Wireless Sensor Networks – A Survey <s> Issues Publications <s> Mobile Ad Hoc networks, due to the unattended nature of the network itself and the dispersed location of nodes, are subject to several unique security issues. One of the most vexed security threat is node capture. A few solutions have already been proposed to address this problem; however, those solutions are either centralized or focused on theoretical mobility models alone. In the former case the solution does not fit well the distributed nature of the network while, in the latter case, the quality of the solutions obtained for realistic mobility models severely differs from the results obtained for theoretical models. The rationale of this paper is inspired by the observation that re-encounters of mobile nodes do elicit a form of social ties. Leveraging these ties, it is possible to design efficient and distributed algorithms that, with a moderated degree of node cooperation, enforce the emergent property of node capture detection. In particular, in this paper we provide a proof of concept proposing a set of algorithms that leverage, to different extent, node mobility and node cooperation - that is, identifying social ties - to thwart node capture attack. In particular, we test these algorithms on a realistic mobility scenario. Extensive simulations show the quality of the proposed solutions and, more important, the viability of the proposed approach. <s> BIB019 </s> Security in Mobile Wireless Sensor Networks – A Survey <s> Issues Publications <s> Wireless Sensor Networks (WSNs) are susceptible to a wide range of attacks due to their distributed nature, limited sensor resources and lack of tamper-resistance. Once a sensor is corrupted, the adversary learns all secrets and (even if the sensor is later released) it is very difficult for the sensor to regain security, i.e., to obtain intrusion-resilience. Existing solutions rely on the presence of an on-line trusted third party, such as a sink, or on the availability of secure hardware on sensors. Neither assumption is realistic in large-scale Unattended WSNs (UWSNs), characterized by long periods of disconnected operation and periodic visits by the sink. In such settings, a mobile adversary can gradually corrupt the entire network during the intervals between sink visits. As shown in some recent work, intrusionresilience in UWSNs can be attained (to a degree) via cooperative self-healing techniques. In this paper, we focus on intrusion-resilience in Mobile Unattended Wireless Sensor Networks (μUWSNs) where sensors move according to some mobility model. We argue that sensor mobility motivates a specific type of adversary and defending against it requires new security techniques. Concretely, we propose a cooperative protocol that - by leveraging sensor mobility - allows compromised sensors to recover secure state after compromise. This is obtained with very low overhead and in a fully distributed fashion. We provide a thorough analysis of the proposed protocol and support it by extensive simulation results. <s> BIB020 </s> Security in Mobile Wireless Sensor Networks – A Survey <s> Issues Publications <s> A color image quality assessment (C-IQA) method was proposed based on the primary and fundamental image quality parameters perceived by human vision system. The methods for measuring these parameters were given. A comprehensive image quality assessment model, called as the best quality assessment function for the color image, was established through trials by means of these parameters. The maximum of the function corresponds to the color image with the best quality. The function could be used either in FR-IQA or in NR-IQA. Not only it does not relied to any reference image but also could be used to assess the reference image quality. The method could also be used to acquire the color image with the best quality by means of a reference image through a gray transformation our called Zadeh-X transformation if the reference image quality is not good enough. <s> BIB021
Secrecy  - BIB018 Data Survival BIB010 , BIB014 , BIB015 Authentication BIB006 , BIB016 Access Control BIB007 - BIB017 Access Privacy BIB017 Data Source Location Privacy BIB004 - BIB008 Sink Location Privacy BIB009 , BIB005 Key Management BIB011 , BIB011 Intrusion Detection BIB012 , BIB019 Intrusion Resilience BIB020 , BIB021 A. Secrecy (Countermeasures against Read-only ADV) To solve the security issues arising due to read-only ADV, we have to guarantee both forward secrecy and backward secrecy. As shown in Fig. 2 , let us assume that an ADV compromises a sensor node s i at round r 1 , and releases the s i at round r 2 (r 1 < r 2 ). Between round r 1 and r 2 , the ADV is residing in s i , and we define this time interval as reside period T rp . Thus, the forward secrecy is secrecy of data generated before round r 1 . The forward secrecy of a sensor s i is compromised if the data generated and encrypted before round r 1 can be decrypted by an ADV which holds the secret obtained during reside period T rp . In contrast, backward secrecy is about secrecy of data generated after round r 2 . The backward secrecy of a sensor s i is compromised if the data generated and encrypted after round r 2 can be decrypted by an ADV which holds a secret obtained during reside period T rp . Existing schemes in cryptography were proposed to achieve either forward secrecy BIB001 , BIB002 only or both forward secrecy and backward secrecy  - BIB003 . Key evolution is a common approach in all of these schemes to provide forward secrecy. Its basic idea is that secret key K i is updated by applying hash function at each round, e.g., of the one-way property of hash function, deriving the previous rounds key (pre-compromised) based on current round key is impossible. However, the aforementioned public cryptography approaches  - BIB003 are not suitable for MWSNs because sensor nodes use only the public key of the sink to encrypt data. The private key evolved at each round is kept in the mobile sink which is assumed to be a trusted party, whereas sensors are usually considered as source constrained devices that cannot afford for public key cryptography. Therefore, symmetric key-based schemes are needed. In the rest of this subsection, we summarize two popular schemes designed specifically for MWSNs. DIstributed Self-Healing (DISH)  . In DISH, the authors first define a mobile adversary that has compromising capability k, meaning that the mobile adversary can compromise up to k nodes during the time interval T while the mobile sink is not available. Three states of sensors are defined (called healthy, sick, and occupied) in DISH. At the initial stage, all sensors are healthy, in the sense that ADV has no information about their data and secret keys. As soon as the mobile sink collects data and leaves the network, the ADV starts to compromise sensors. A sensor is occupied if it is currently compromised. Once the sensor is compromised, the ADV can access the local memory and learn the secret key. The sensors are sick if the sensors have been previously compromised, and the ADV can still compute the current secret keys based on the keys acquired when it occupied them, even if it no longer does so. The basic requirement of DISH is that unattended sensors attempt to recover from sick states and maintain both forward and backward secrecy of the collected data. In DISH, forward secrecy is provided through key evolution. However, DISH does not guarantee absolutely backward secrecy. Instead, it provides probabilistic backward secrecy, a state which depends on conditions such as: compromising capability of the mobile adversary (number of nodes it can compromise at a given time interval T ), and for how long time the mobile sink successively visits the network. The main idea of DISH is to let healthy sensors heal sick sensors. As shown in Fig. 3 , sensor A receives contribution r from its neighbors B, C and D, which is a random number generated by its corresponding neighbors, and then uses the contributions along with its current key as input to a one-way hash function to compute its next round key. If at least one of the sensors B, C, D is healthy (in the sense that at least one of the contributions r B , r C , r D is unknown to the ADV), the ADV is unable to learn the newly generated key. Here, the contribution is a pseudo-random value. As a consequence, sensor A is healed, and its state transfer from sick to healthy. We claim that DISH can provide probabilistic backward secrecy because if all its neighbors are sick, sensor A cannot become healthy. Fig. 4 further shows the states transition of sensors. With the sensor cooperation approach, healthy sensors always remain healthy as long as they are not directly compromised, whereas sick sensors can become healthy if at least one of their received contributions is from healthy peers. Proactive co-Operative Self-Healing (POSH) BIB013 . Compared with DISH  , the basic idea of self-healing in POSH is the same, which causes sick sensors receiving random contributions from healthy peers to become healthy again. The difference is that the DISH scheme is referred to as a pull model because sensors request contributions from peers; on the contrary, the POSH scheme is referred to as a push model that involves sponsors volunteering their contributions. In other words, in the push model, sensors send their contributions to t randomly selected neighbors without receiving any requests. POSH improves DISH in the following significant manner: the POSH scheme can guarantee better probabilistic backward secrecy than the DISH scheme. In the DISH scheme, ADV knows the peers from which each sick sensor has asked for a contribution. For example, as shown in Fig. 3 , sensor A requests contributions from its neighbors B, C, and D. If sensor A is sick, meaning that it was compromised before, the ADV knows its previous key K r1 and the set of sensor A's future sponsors. The ADV can then compromise all the sponsors in the set and acquire their contributions to sensor A, which allows ADV to compute the new key. However, in the POSH scheme, the sponsors randomly select t neighbors to send contributions. As a consequence, if a sponsor is healthy, ADV cannot determine for certain the set of sensors it might contribute to, meaning that the ADV cannot know all the sponsors who sponsor the contributions to sensor A's new key. Moreover, the POSH scheme requires only half of the message required in the DISH scheme because DISH needs two messages -a request and a reply -for each contribution. More recent work BIB018 utilizes (m, n) Reed-Solomon (RS) coding to improve data reliability and backward secrecy. Sensors take advantage of (m, n) RS codes to divide data into n parts and then send parts of their data to their neighbors, where m of n (m < n) data parts are required to reconstruct data, adding data redundancy to provide resilience to node invalidation and Byzantine failure. Data can be recovered if less than n − m data parts are compromised. Since data parts are distributed among sensor's neighbors, backward secrecy of s i can be compromised if and only if 1) s i is compromised by the mobile adversary. 2) The mobile adversary's compromising ability is θ > m. 3) The mobile adversary has compromised at least m neighbor nodes of s i that store the corresponding data parts. Our scheme in BIB018 makes an obstacle that forces the mobile adversary to compromise more sensors to compromise s i 's backward secrecy. Assuming that P i is the probability that s i can be compromised by the mobile adversary, and that P i,j is the probability that s i,j , a neighbor node of s i , is compromised, it is easy to understand that the backward secrecy of s i is improved due to the fact P i,j is usually different for different nodes and could be evaluated from the feedback of certain security monitoring software and/or assigned manually by the mobile sink based on information such as the physical protection, the location, or the role of the nodes. We further improved the scheme BIB018 by selecting the top m security level nodes, e.g., m pairs with the lowest P i,j to decrease P i Π m j=1 P i,j .