## 5 Related Work  
The systematic definition of security metrics and security level estimation are fields that are relatively new. To date, very few security estimators have been proposed for both traditional networks (e.g., [1, 2, 3]) and wireless sensor networks (e.g., [5, 6, 7, 8]).  
In traditional networks, to quantify security, most of the proposals are based on vulnerability and risk analysis (e.g., [1,44]) or, also, on the attack graph technique, used to model the relationship between multiple vulnerabilities [3]. These proposals use automated tools to identify vulnerabilities in the network and to quantify their severity based on the Common Vulnerability Scoring System [4]. [PERSON] _et al._[2] present a stochastic model to quantify the security level that, in addition to using CVSS and attack graph, takes into account both the effectiveness and vulnerabilities of the security mechanisms deployed in the network.  
For sensor networks, [PERSON] _et al._[5] define the metric, security leakage factor, for quantifying security in WSNs based on a key management mechanism proposed by the authors themselves, called cluster key grouping. On the basis of the proposed mechanism, the authors define a value called the compromise factor, given by the number of keys compromised if a single node is compromised divided by the key pool size. Finally, they compute the security leakage factor by multiplying the compromise factor by a subjective value called the security weight, which can be any natural number that represents the importance that a security compromise has for the network.  
[PERSON] _et al._[6] evaluate the security of a sensor network based on a metric for assessing vulnerability to key exposure of the key management mechanism, the link key security metric (LKSM). This metric allows one to analyze and compare the resilience of the key management and is defined as the time required for all of the keys comprising a secure connection to be captured, _i.e._, the time required to fully compromise a connection. This value is obtained by uniformly capturing nodes at random, adding their cryptographic keys to a list of captured keys and then repeating this process, until all of the keys securing a given connection between two nodes have been captured. This metric is able to quantify the security of keys used for each connection, because if one key is used by a relatively small number of nodes (and is therefore more secure), a larger amount of nodes, on average, need to be captured until a node holding this key is captured.  
[PERSON] and [PERSON] [7] present a security model that uses several WSN protocol parameters to estimate security level. In the case that this level is below a certain threshold, the system security is dynamically optimized. In order to accomplish this, the authors assume a sensor network with high bandwidth connections, nodes that can detect incidents wherever in the network, and they propose equations that depend on several intuitively interpreted factors.  
[PERSON] _et al._[8] describe a WSN security framework comprised of key management, trust scheme, among others, and propose the intelligent security agent (ISA), a component to evaluate the security level based on node information, such as available memory, available energy, trust level and predefined policies, as well as recommendation. However, the authors do not provide any further details about how the proposed component uses this information to compute the security level.