## 1 Introduction
### Problem Statement  
The on/off attack is a widely encountered threat in cloud environments. Here, a few entities show proper behavior for some time to create a positive reputation. Once they have generated trust, they start their deception. Malicious bodies win the trust of the trust management system (TMS) by exhibiting good behavior during interactions that have small impacts. However, when they come across a suitable opportunity for major interactions, their malicious intentions become evident. For example, sellers on eBay carry out various small transactions to generate a positive reputation, and then after gaining trust, cheat buyers on a more expensive product. Since these entities suddenly change their transactional behavior, it becomes difficult for other entities to sufficiently diminish the attacker's reputation. This also consists of oscillatory transaction behavior, where an entity keeps shifting from honest to dishonest behavior so that the attacker's reputation cannot be updated in a timely manner.  
Collusion attacks are threats that emerge when a group or groups of malicious entities make attempts to destabilize the system. In most cases, when different malicious entities act together, there is a greater danger to the reliability of a reputation system than when every entity exhibits malicious behavior separately. Some specific instances of collusion attacks are described below.  
Collusive slandering attack: also known as a collusive badmouthing attack, malicious users work together to propagate negative reviews about an honest user to significantly decrease their reputation. They also try to enhance their own reputations by giving positive reviews of each other.  
Collusive reducing reputation: the reputation of honest users can be reduced through coordinated malicious nodes that criticize only some sections of the entities they deal with. They do so to generate conflicting views regarding the transactional behavior of the victims and the reputation of honest entities recommending the victims. There is very little effect on their own reputation, since they give honest feedback about other entities.  
Collusive self-promoting attack: this kind of attack is also referred to as a ballot stuffing attack, in which negative behavior is exhibited by all entities of a group of malicious entities, but they give positive feedback about each other. This attack is carried out in different ways. In one instance, dishonest behavior is exhibited by just one entity of the collusive group, while positive reviews are given by other entities. In another instance, unfavorable behavior is exhibited by a single entity some of the time to avoid being identified, whereas other members of this group give positive reviews for it. Often, the term \"ballot stuffing\" is used to refer to an attack of this category. In this kind of attack, ratings are given for fake transactions. For instance, a seller can collude in an online auction system with a group of buyers to achieve high ratings that are not reflective of actual transactions. This will increase their reputation, leading to a greater number of orders for that seller from other buyers and allowing them to sell at a greater price than justified.