communication between Alice and Bob. Reading other people's e-mail is an example of Eve (Intruder) Alice Figure 6.3 Eve has unauthorized access to the communication between Alice and Bob Security a passive intruder. Depending on what is written in the e-mail, the harm could be minor or major for either Alice or Bob or both. Ethernet has the weakness that all messages are broadcast on the LAN to all connected stations. It is thus fairly easy for an intruder to pick up the information and record the traffic. Figure 6.4 shows an example of monitoring on a LAN. The simplest case is of course when A and B communicate without any encryption. This makes life very easy for the intruder. Note that the monitoring can lead to a second attack by the intruder: he may monitor user identities and passwords and then access the system with the stolen user identity. 6.3.3 AUTHENTICATION PROBLEMS Figure 6.5 shows a simple example where Alice thinks she is communicating with Bob but she is not. An intruder (Eve) is acting like Bob without Alice knowing that. This means that Eve is taking a rather active part as she is acting like another person or company. The problem of authentication also exists in real life. Try to empty your bank account without proper identification papers or via a telephone line. You will not succeed as the bank has some rules regarding authentication. The simplest way of getting around an authentication system is to steal someone's user identity and password. The way to do this can of course differ but monitoring, as described above, is one way. Figure 6.6 shows how it can be done. Once the intruder has the false identity and the password, it is almost impossible for the system to detect this. One single access with the stolen user identity might never be detected. Intruder Ethernet Figure 6.4 An intruder using monitoring Alice Eve (Intruder) Figure 6.5 You never know who you are communicating with Understanding Changing Telecommunications Building a Successful Telecom Business User D:Bob. Password: Delta Int