## 6 Evaluation of Existing SRE Approaches
### Evaluation Iteration 1--Use Case Scenario 1
#### 6.1.2 Scenario Implementation Using STS  
STS (Secure Socio-technical Systems) [16] implements the agent-oriented approach which mainly focuses on early elicitation of security requirements based on the social dependency interaction relationships. Similar to KAOS, the STS framework offers the possibility to create composite goals via the AND/OR constructs. However, the respective goals and sub-goals are determined in the scope of each actor. An actor can be either a role or an agent. This methodology mainly defines three views: social view, authorization view and information view. The social relationships between actors are manifested by  
Figure 6: Secure KAOS goal specification (sample). the relationships such as goal delegation and resource provision. This is modelled in the social view. Delegated goal implies the dependency of the interaction. An actor depends on another actor to achieve a goal. In addition, the social relationships between actors may also include the exchange of documents (informational resources) that contain necessary information for the achievement of a goal. STS captures this exchange via the relationship: document transmission (resource provision). The ownerships of the resources are modelled in the authorization view. This view also permits to explicitly define the authorization rules over the access permissions on the documents (resources). Another view called the information view allows to define the primitives and relationships that permit the differentiation between the information (content) and the representation of the information (document). STS is supported by a tool of the same name. The tool is not open source but it is freely available to public use (R\({}^{\text{M}}\)2.1.2). The usage of a simple terminology as well as the user-friendly tool took less effort to become used to the overall concepts and terminology (R\({}^{\text{M}}\)2.1.1 and R\({}^{\text{M}}\)2.1.3). The security experts provided a positive feedback concerning the comprehensibility aspects during the first intermediary meeting. Unlike KAOS, STS does not support full-fledged traceability of the security goals being refined into security requirements (R\({}^{\text{M}}\)4.1). From the STS point of view, the security needs are implicitly expressed either in the form of security constraints attributes (e.g., integrity/availability/non-repudiation, etc.) over the interaction dependencies in the social view; or in the form of constrained permissions (e.g., read/modify/produce/transmit) upon the provision of the resources in the authorization view. Figure 7 highlights the security constraints over delegated goals in blue dashed rectangles, which cannot be refined further into network security requirements (R\({}^{\text{M}}\)3.1). On the other hand, STS provides a good support to express network agents and interaction dependencies. However, we had an issue in handling the number of dependencies between multiple network agents as the social view grows. Furthermore, from the STS modelling perspective, the abstractions are expressed using the three modeling views (i.e., social view, authorization view, information view). This abstraction perspective respects separation of concerns specific to authorization security problem context only. Similarly to  
Figure 7: STS social view specification (sample). KAOS, STS does not guide the security requirement engineers in asking the right questions at the right time to structure the security engineering process in order to build a security architecture (R\({}^{\text{M}}\)3.2). In STS, the threat analysis can show the effects of a threatening event over goal trees and goal/resource relationships. In addition, it defines attributes (implicit) to link countermeasures to threat events (R\({}^{\text{M}}\)6.2). However, the threat analysis is limited to threat event propagation. Thereby it does not facilitate the expression of security needs upon the threats related to environmental risks.