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applied cryptography - protocols, algorithms, and source code in c

While the user can use the key he does not know it

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Unformatted text preview: e also gets no known-plaintext values to try out, and even if she manages to choose our random value for us, she never gets a chosen-plaintext out of us, since it goes through the hash function before she sees it. 8.5 Using Keys Software encryption is scary. Gone are the days of simple microcomputers under the control of single programs. Now there’s Macintosh System 7, Windows NT, and UNIX. You can’t tell when the operating system will suspend the encryption application in progress, write everything to disk, and take care of some pressing task. When the operating system finally gets back to encrypting whatever is being encrypted, everything will look just fine. No one will ever realize that the operating system wrote the encryption application to disk, and that it wrote the key along with it. The key will sit on the disk, unencrypted, until the computer writes over that area of memory again. It could be minutes or it could be months. It could even be never; the key could still be sitting there when an adversary goes over the hard drive with a fine-tooth comb. In a preemptive, multitasking environment, you can set your encryption operation to a high enough priority so it will not be interrupted. This would mitigate the risk. Even so, the whole thing is dicey at best. Hardware implementations are safer. Many encryption devices are designed to erase the key if tampered with. For example, the IBM PS/2 encryption card has an epoxy unit containing the DES chip, battery, and memory. Of course, you have to trust the hardware manufacturer to implement the feature properly. Some communications applications, such as telephone encryptors, can use session keys. A session key is a key that is just used for one communications session—a single telephone conversation—and then discarded. There is no reason to store the key after it has been used. And if you use some key-exchange protocol to transfer the key from one conversant to the other, the key doesn’t have to be stored before it is used either. This makes it far less likely t...
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