## 4 Performance Evaluation and Discussion
### Performance Using Three RATs for Communication  
Figure 12 shows the characteristics of the mutual information with respect to the shortened information length when the sender and the receiver communicate using three RATs. The circle marks and the square marks indicate the mutual information between the sender and receiver when two and one RAT(s) cannot be used for communication due to an attack, respectively. The triangle marks and the cross marks indicate the mutual information between the sender and the cyberterrorist when two and one RAT(s) are/is wiretapped, respectively. By communicating using three RATs, even if one RAT is attacked, communication is possible without shortening. In addition, information cannot be wiretapped simply by wiretapping on one RAT. Simultaneous use of multiple RATs is an effective approach to achieve both countermeasures against wiretapping and attacks. When the cyberterrorist attacks two RATs, the mutual information increases slightly up to the shortened information length of about 300 bits, but increases exponentially beyond that, and the mutual information becomes one when the shortened information length is 373 bits or more. On the other hand, when two RATs are wiretapped, the mutual information is one up to the shortened information length of 152 bits, but rapidly decreases as the shortened information length becomes 153 bits or more.  
Figure 13 shows the characteristics of the secrecy capacity. The circle marks, the square marks, the triangle marks, and the cross marks indicate the secrecy capacity of the cyberterrorist attacking on three RATs, attacking on two RATs and wiretapping on one RAT, attacking on one RAT and wiretapping on two RATs, respectively. When the cyberterrorist attacks or wiretaps on three RATs, the secrecy capacity does not improve and is zero. If the cyberterrorist attacks on one RAT and wiretaps on two RATs simultaneously, the secrecy capacity increases to approximately 0.6 by increasing the shortened information length. If the cyberterrorist attacks on two RATs and wiretaps on one RAT simultaneously, the secrecy capacity gradually increases by increasing the shortened information length, and when the shortened information length exceeds 373 bits, the secrecy capacity becomes about 0.9. As a result, it is confirmed that information-theoretic security is further improved even when the sender and the receiver communicate using three RATs. Since the degree of freedom of countermeasures increases as the number of RATs used for communication increases, various countermeasures can be implemented depending on the assumed cyberterrorist by changing the shortened information length. However, since it is ineffective if all the RATs are wiretapped or attacked, it is desirable to implement countermeasures at the upper layers as well.  
Figure 11: Characteristics of the secrecy capacity versus the shortened information length when the sender and the receiver communicate using two RATs. The square marks, the circle marks and the cross marks indicate the secrecy capacity of the cyberterrorist attacking on two RATs, attacking on one RAT and wiretapping on one RAT, and wiretapping on two RATs, respectively.