nly listen to a communication between two other parties, he or she may also alter information sent between A and B. Imagine an economical transaction between a person and a bank. The intruder may alter the message sent to the bank SO money is transferred to his bank account instead. This can be done without the knowledge of the bank or the person requesting the change. Figure 6.7 shows an example where the intruder (Eve) alters some information in the message sent from Alice to Bob. The problem with integrity is that Bob probably trusts the information coming from Alice as some type of authentication has been done. The intruder can in this case cause great damage to Alice and Bob. A good security system prevents this from happening. When you receive an e-mail from someone, you think that what you see on the screen is what the sender wrote. Unfortunately, that might not be the case if an active intruder has changed the mail. Without encryption, it is very simple and can be done in any editor. Figure 6.8 shows the case when an active intruder modifies an e-mail. Exactly how the intruder gets the mail is not interesting, the main point is that it is possible in a number of ways and that you should not trust e-mail for critical information (business or private). Eve (Intruder) Alice Figure 6.7 Integrity problems Security Hi Bill! Hi Bill! My account number My account number Is 1234-1234 Is 9898-9890 Please pay me Please pay me $ 500 $ 500 Ethernet Intruder Figure 6.8 Modification of a mail Non-Repudiation Non-repudiation means that you have sent some information or document to another party, and a number of weeks later you deny having sent it. It could, for example, be some legal document regulating a deal between you and the other person. You say that it must have been some other person that sent the document. To say who is right could be very difficult and time consuming. Please note that the function may be affected by laws and regulations in a country. For example, a digitally signed document may be valid as a pr