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Pao-Chi and Gwen now share a key. This scheme will work; if attackers try to intercept their messages encrypted using that key, the attackers will not be able to recover the information. But this solution does have its problems. Suppose the parties want to share keys with more than one person. Pao-Chi is not the company s only sales rep, and he may want to securely send information to his sales colleagues as well as people in the engineering, accounting, and shipping departments. To communicate securely with all these people, Pao-Chi will have to visit their offices and perform the key exchange. What s more, Gwen will have to make similar visits (or her colleagues will have to visit her after all, she is the VP). Everyone will have to exchange keys in person with everyone with whom they share confidential information. The logistics quickly become burdensome. Some colleagues may have offices in other parts of the country or even in other countries. The company can t send everyone on all the trips required to exchange keys. Maybe the solution would be to gather all the employees at one location and have a giant key exchange party. But what happens when the company hires someone new Does it have yet another key exchange party Send the new employee on a worldwide trip to exchange keys Furthermore, as more people need to share keys, the number of required meetings grows dramatically. When two people share a key, there s one meeting. When three people share keys, there are two meetings; with four people, six meetings, and so on. In general, n people, must n) key exchanges. If your company has 10 employees make 1/2(n2 involved in secure data sharing, that s 1/2(100 10) key exchanges, or 1/2(90) 45. For 20 employees, it s 190 meetings. A company with 1,000 employees would need to perform 499,500 key exchanges. One solution is for everyone in the company to share the same key. You could have a key master who gives the key to all employees. The drawback is what happens when someone leaves the company. If the company does not change the key, an unauthorized individual can now decrypt sensitive materials. If, on the other hand, the company changes keys, the key master will have to revisit everyone in the company. A second problem with the shared secret key is that if attackers crack one message, they crack them all. Because all messages between two people are encrypted with the same key, finding the key for one message means finding the key for all messages. It s not likely that attackers will
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find the key if the correspondents use a 128-bit key and an algorithm with no weaknesses. On the other hand, if it is possible to easily use a separate key for each message, why not take that extra measure of precaution Although this is a drawback of the shared key approach, it s trivial compared with the pitfalls of trying to exchange keys in person.
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If sharing keys in advance is not an option, Pao-Chi and Gwen can try using a trusted third party (TTP). This is a variation on the key master solution. In this scheme, the trusted third party let s call her Michelle shares a key with each individual in the company. Actually, the keys are key-encrypting keys, or KEKs. Pao-Chi visits Michelle and asks for a KEK. She generates one, stores it securely, and gives a copy to Pao-Chi. The two of them now share a KEK. Gwen also visits Michelle, and the two of them share a different KEK (see Figure 4-2). When Pao-Chi wants to communicate with Gwen, he sends a message to Michelle, requesting a session key he can use in his messages with Gwen. To fulfill the request, Michelle generates a new session key and sends it to Pao-Chi. She encrypts the new session key using the KEK she shares with him, so anyone intercepting that message cannot identify this new key. Michelle also sends this same new session key to Gwen, encrypting it using the KEK those two share (see Figure 4-2). Pao-Chi and Gwen now share a key, and neither had to make a trip to the other s office. Anyone else wanting to share a key with any other employee simply establishes a KEK with Michelle, who distributes the key. In a trusted third party scheme, the correspondents are the first two parties. In our example, Michelle is the third party. Just as important, Michelle must be trusted because she has everyone s keys. When Pao-Chi and Gwen exchange encrypted messages, normally they are the only people who can decrypt them. But now Michelle also has their session key, so she can decrypt their messages. Pao-Chi and Gwen must trust Michelle not to read their sensitive material or release their key to anyone else. The trusted third party still has to exchange keys with all the employees in person. As you saw in the preceding section, that s a daunting task. To make things easier, you can create a hierarchy of trusted third parties. Everyone goes to a local TTP, each of whom has established a key with every other TTP. For all the TTPs to exchange keys is still a formidable
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