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Brief Full Advanced Search Search Tips (Publisher: John Wiley & Sons, Inc.) Author(s): Bruce Schneier ISBN: 0471128457 Publication Date: 01/01/96 Search this book:
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----------- Some messages have a common header: a letterhead, or a “From” line, or whatever. While block replay would still be impossible, this identical beginning might give a cryptanalyst some useful information. Prevent this by encrypting random data as the first block. This block of random data is called the initialization vector (IV), initializing variable, or initial chaining value. The IV has no meaning; it’s just there to make each message unique. When the receiver decrypts this block, he just uses it to fill the feedback register and otherwise ignores it. A timestamp often makes a good IV. Otherwise, use some random bits from someplace. With the addition of IVs, identical plaintext messages encrypt to different ciphertext messages. Thus, it is impossible for an eavesdropper to attempt block replay, and more difficult for him to build a code book. While the IV should be unique for each message encrypted with the same key, it is not an absolute requirement. The IV need not be secret; it can be transmitted in the clear with the ciphertext. If this seems wrong, consider the following argument. Assume that we have a message of several blocks: B1, B2,..., Bi. B1 is encrypted with the IV. B2 is encrypted using the ciphertext of B1 as the IV. B3 is encrypted using the ciphertext of B2 as the IV, and so on. So, if there are n blocks, there are n-1 exposed “IVs,” even if the original IV is kept secret. So there’s no reason to keep the IV secret; the IV is just a dummy ciphertext block—you can think of it as B0 to start the chaining. Padding
Padding works just like ECB mode, but in some applications the ciphertext has to be exactly the same size as the plaintext. Perhaps a plaintext file has to be encrypted and then replaced in the exact same memory location. In this case, you have to encrypt the last short block differen...
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