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Unformatted text preview: ennyi Standard Soyuza SSR, or Government Standard of the Union of Soviet Sot Republics.) This standard is number 2814789. The Government Committee for Standards of the USSR authorized the standard, whoever they were. I don’t know whether GOST 2814789 was used for classified traffic or just for civilian encryption. A remark at its beginning states that the algorithm “satisfies all cryptographic requirements and not limits the grade of information to be protected.” I have heard claims that it was initially used for very highgrade communications, including classified military communications, but I have no confirmation. Description of GOST
GOST is a 64bit block algorithm with a 256bit key. GOST also has some additional key material that will be discussed later. The algorithm iterates a simple encryption algorithm for 32 rounds. To encrypt, first break the text up into a left half, L. and a right half, R. The subkey for round i is Ki. A round, i, of GOST is: Li = Ri  1 Ri = Li 1 • f(Ri  1, Ki) Figure 14.1 is a single round of GOST. Function f is straightforward. First, the right half and the ith subkey are added modulo 232. The result is broken into eight 4bit chunks, and each chunk becomes the input to a different Sbox. There are eight different Sboxes in GOST; the first 4 bits go into the first Sbox, the second 4 bits go into the second Sbox, and so on. Each Sbox is a permutation of the numbers 0 through 15. For example, an Sbox might be: Figure 14.1 One round of GOST. 7, 10, 2, 4, 15, 9, 0, 3, 6, 12, 5, 13, 1, 8, 11 In this case, if the input to the Sbox is 0, the output is 7. If the input is 1, the output is 10, and so on. All eight Sboxes are different; these are considered additional key material. The Sboxes are to be kept secret. The outputs of the eight Sboxes are recombined into a 32bit word, then the entire word undergoes an 11bit left circular shift. Finally, the result XORed to the left half to become the new right half, and the right half becomes the new left half. Do this 32 times and you’re done. The subke...
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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.
 Fall '10
 ALIULGER
 Cryptography

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