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

Figure 134 is a block diagram of function f the two

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Unformatted text preview: he outer cycle repeats eight times (although this could be increased if security warrants) and consists of an application of the inner cycle to the plaintext. The inner cycle transforms plaintext to ciphertext and repeats once for each 8-bit block (byte) of the plaintext. Thus, the algorithm passes through the entire plaintext eight successive times. An iteration of the inner cycle operates on a 3-byte window of data, called the working frame (see Figure 13.1). This window advances 1 byte for each iteration. (The data are considered circular when dealing with the last 2 bytes.) The first 2 bytes of the working frame are together rotated a variable number of positions, while the last byte is XORed with some key bits. As the working frame advances, all bytes are successively rotated and XORed with key material. Successive rotations overlap the results of a previous XOR and rotation, and data from the XOR is used to influence the rotation. This makes the entire process reversible. Because every byte of data influences the 2 bytes to its left and the 1 byte to its right, after eight passes every byte of the ciphertext is dependent on 16 bytes to the left and 8 bytes to the right. When encrypting, each iteration of the inner cycle starts the working frame at the next-to-last byte of the plaintext and advances circularly through to the third-to-last byte of the plaintext. First, the entire key is XORed with a random constant and then rotated to the left 3 bits. The low-order 3 bits of the low-order byte of the working frame are saved; they will control the rotation of the other 2 bytes. Then, the low-order byte of the working frame is XORed with the low-order byte of the key. Next, the concatenation of the 2 high-order bytes are rotated to the left the variable number of bits (0 to 7). Finally, the working frame is shifted to the right 1 byte and the whole process repeats. Figure 13.1 One iteration of Madryga. The point of the random constant is to turn the key into a pseudo-random sequence. The length of this cons...
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This note was uploaded on 10/18/2010 for the course MATH CS 301 taught by Professor Aliulger during the Fall '10 term at Koç University.

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