Dynamical Analysis of a Computer Virus Propagation Model with Delay and Infectivity in Latent Period

A delayed SLB computer virus propagation model with infectivity in latent period is proposed in this paper. We establish sufficient conditions for local stability of the positive equilibrium and existence of Hopf bifurcation by analyzing distribution of the roots of the associated characteristic equation and applying the Hopf bifurcation theorem. Furthermore, properties of the Hopf bifurcation are determined by using the normal form theory and the center manifold theorem. Finally, numerical simulations supporting the theoretical analysis are carried out.


Introduction
Due to their striking features such as destruction, polymorphism, and unpredictability [1][2][3], computer viruses, ranging from conventional viruses to network worms and other malicious codes such as Trojans and spyware, have addressed a serious threat to confidentiality, integrity, and availability of computer resources on the Internet.They have also come to be one major threat to our work and daily life.
In recent years, people have paid attention to the necessity of monitoring computer viruses in the Internet and proposed many mathematical models to depict the spread of computer viruses in the Internet based on the classic epidemic models [4][5][6][7][8][9].In [4], Li et al. proposed a worm propagation model with removable storage device and studied stability of the model.Mishra and Keshri proposed a SEIRS-V model to describe the dynamics of worm propagation in wireless sensor networks [6].However, most of the above models assume that only breaking-out computers have infectivity.This is not consistent with reality.Because computer viruses have a feature of latency, an infected computer which is in latency can also infect other computers through file copying or file downloading.Based on this consideration, in [8], Yang where () is the percentage of uninfected computers in the Internet, at time .() and () are the percentages of latent and breaking-out computers in the Internet, at time , respectively. is the connected rate of external computers to the Internet and it is also the disconnected rate of internal computers from the Internet. is the infected rate of the uninfected computers. is the breaking-out rate of the latent computers. 1 and  2 are the cured rates of the latent and breakingout computers, respectively.Obviously, it is assumed that both seizing and latent computers have infectivity.
However, system (1) ignores time delay due to the period that a computer user uses to cure the latent and the breakingout computers by using antivirus software or reinstalling system.As is known, time delay can make a dynamical system lose its stability and then lead to the occurrence of a Hopf bifurcation.Hopf bifurcation of dynamical systems with delay has been investigated by many authors [10][11][12][13][14][15][16].In [10], Zhuang and Zhu analyzed the existence of Hopf bifurcation for an improved HIV model with time delay and cure rate by regarding the time lag from infection of cells to the cells becoming actively infected as a bifurcation parameter.In [11], Liu investigated the existence and properties of the Hopf bifurcation in a delayed SEIQRS model for the transmission of malicious objects in computer network.In [14], Bianca et al. studied the bifurcation of a delayed-energy-based model of capital accumulation.Motivated by the work above, we introduce the time delay due to the period that a computer user uses to cure the latent and breaking-out computers into system (1) and get the following delayed computer virus model: where  is the delay due to the period that a computer user uses to cure the latent and breaking-out computers by using antivirus software or reinstalling system.The remainder of this paper is organized as follows.Local stability and existence of Hopf bifurcation for system (2) are analyzed in Section 2. Properties of the Hopf bifurcation are investigated in Section 3 by means of the center manifold theorem and the normal form theory.A numerical example is presented in Section 4 to illustrate the theoretical results.This work is summarized in Section 5.
For  > 0, let  =  ( > 0) be the root of (4); then from which we can get the following sixth-degree equation for : where Let  2 = V; then (9) becomes From the analysis above, we know that if the coefficients of system ( 2) are given, one can easily get the coefficients of ( 11) by MATLAB software package.Correspondingly, the roots of ( 11) can be obtained.Thus, we make the following assumption in order to give the main results in this paper.
If condition ( 2 ) holds, then there exists one positive root V 0 of (11) such that (4) has a pair of purely imaginary roots ± 0 = ± √ V 0 .For  0 , the corresponding critical value of delay is And then, we will verify the transversality condition.From (4), we can obtain This gives Re where According to the Hopf bifurcation theorem in [15], we have the following results for system (2).2) is locally asymptotically stable for  ∈ [0,  0 ) and system (2) undergoes a Hopf bifurcation at the positive equilibrium  + ( * ,  * ,  * ) when  =  0 and a family of periodic solutions bifurcate from the positive equilibrium  + ( * ,  * ,  * ) near  =  0 .

Properties of the Hopf Bifurcation
In the previous section, we have obtained the conditions under which a Hopf bifurcation occurs and a family of periodic solutions bifurcate from the positive equilibrium  + ( * ,  * ,  * ) of system (2) when the delay passes through the critical value  0 .In this section, we will derive the explicit formulae determining the direction of the Hopf bifurcation and the stability of bifurcating periodic solution of system (2) at  =  0 .The approach employed here is the normal form method and center manifold theorem introduced by Hassard et al. in [17].
In what follows, we will find out  1 and  2 .It follows from the definition of (0) and ( 55) and (56) that where () = (0, ).From ( 52) and ( 53), we get Notice that and substituting (58) and ( 62) into (60), we obtain That is, (72) Thus, we can compute the following coefficients: which determine the properties of the bifurcating periodic solutions in the center manifold at the critical value  0 and we obtain the following results.

Numerical Simulation
In this section, we try to present some numerical simulations for system (2) to validate the previous main results.By extracting some values from [8] and considering the conditions for the existence of the Hopf bifurcation, we choose a set of parameters as follows:  = 0.3,  = 0.35,  1 = 0.1,  2 = 0.3, and  = 0.05.Then, we can get a specific case of system (2).By a simple computation, we get  0 = 1.4444 > 1 and the unique positive equilibrium  + (0.6924, 0.1657, 0.1420) of the system.Further, from (11) and (12), we obtain  0 = 0.2766 and  0 = 22.9208.According to Theorem 1, we can conclude that if  ∈ [0, 6.0897), then the positive equilibrium  + (0.6924, 0.1657, 0.1420) is locally asymptotically stable.As can be seen from Figures 1 and 2, when we choose  = 18.3656 <  0 , the positive equilibrium  + (0.6924, 0.1657, 0.1420) is locally asymptotically stable.Once the value of the delay  passes through the critical value  0 , as is shown in Figures 3 and 4, when we choose  = 32.8965>  0 , a Hopf bifurcation occurs and a family of periodic solutions bifurcate from the positive equilibrium  + (0.6924, 0.1657, 0.1420).
In addition, we also find that onset of the Hopf bifurcation can be delayed if the values of the parameters  1 and  2 increase from the numerical simulation.And this can be realized by means of strengthening the immunization of the new computers connected to the network.Thus, we can conclude that the managers of the network should strengthen the immunization of the new computers connected to the network so as to predict and control the propagation of the computer virus in the network easily.

Conclusion
A new epidemic model of computer virus with time delay has been proposed in this paper.Compared with the model considered in the literature [8], we mainly investigate the effect of the delay due to the period that a computer user uses to cure the latent and breaking-out computers by using antivirus software or reinstalling system on system (2) by analyzing the corresponding characteristic equation and regarding the time delay as a bifurcation parameter.It is found that when the value of the time delay is below the critical value  0 the model is asymptotically stable.In this case, the prevalence of the viruses in the Internet can be easily controlled.However, when the value of the time delay is above  0 , the model will lose its stability and a Hopf bifurcation occurs, which indicates that computers of the three classes in system (2) may coexist in an oscillatory mode under some conditions.This phenomenon is not welcome in the Internet and it is difficult to control and prevent the viruses in the Internet in this case.Hence, we should control the occurrence of the Hopf bifurcation by combining some bifurcation control strategies and other relative features of virus prevalence, in order to make the propagation of computer viruses possible to be predicted and controlled easily.Specially, with the help of the normal form theory and the center manifold theorem, explicit formulae determining the properties of the Hopf bifurcation have been derived.Finally, numerical examples are given to illustrate the main results obtained in the paper.