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1 Overview of the DES A block cipher: –encrypts blocks of 64 bits using a 64 bit key –outputs 64 bits of ciphertext A product cipher –basic unit is the bit –performs both substitution and transposition (permutation) on the bits Cipher consists of 16 rounds (iterations), each with a 48-bit round key generated from the 64-bit key

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2 Generation of Round Keys Round keys are 48 bits each

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3 Encipherment

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4 The f Function

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5 S-Box –There are eight S-Box, each maps 6-bit input to 4-bit output –Each S-Box is a look-up table –This is the only non-linear step in DES and contributes the most to its safety P-Box –A permutation

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6 Controversy Considered too weak –Diffie, Hellman said “in a few years technology would allow DES to be broken in days” DES Challenge organized by RSA In 1997, solved in 96 days; 41 days in early 1998; 56 hours in late 1998; 22 hours in Jan 1999 http://w2.eff.org/Privacy/Crypto/Crypto_misc/DESC racker/HTML/19990119_deschallenge3.htmlhttp://w2.eff.org/Privacy/Crypto/Crypto_misc/DESC racker/HTML/19990119_deschallenge3.html –Design decisions not public S-boxes may have backdoors

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7 Undesirable Properties 4 weak keys –They are their own inverses 12 semi-weak keys –Each has another semi-weak key as inverse Complementation property –DES k (m) = c DES k (m) = c S-boxes exhibit irregular properties –Distribution of odd, even numbers non-random –Outputs of fourth box depends on input to third box

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8 Number of rounds –After 5 rounds, every cipher bit is impacted by every plaintext bit and key bit –After 8 rounds, cipher text is already a random function –When the number of rounds is 16 or more, brute force attack will be the most efficient attack for known plaintext attack –So NSA knows a lot when it fixes the DES

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9 Differential Cryptanalysis A chosen ciphertext attack –Requires 2 47 (plaintext, ciphertext) pairs Revealed several properties –Small changes in S-boxes reduce the number of (plaintext, ciphertext) pairs needed –Making every bit of the round keys independent does not impede attack Linear cryptanalysis improves result –Requires 2 43 (plaintext, ciphertext) pairs

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10 Multiple encryption of DES –Before we study multiple DES, a question must be answered. Is DES a group? E K2 ( E K1 (P)) = E K3 (P) –It is proven that DES is not a group in 1993

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11 Double encryption –Encrypt the plaintext twice with different keys C = E K2 (E K1 (P)), P = D K1 (D K2 (C)) –If DES uses 56 bit key, can we get 112 bit key security? –Meet-in-the-middle attack makes the safety to 57 bits instead of 112 bit –Tradeoff storage and search for computation –Double encryption will not achieve your goal

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12 DES Modes Electronic Code Book Mode (ECB) –Encipher each block independently Cipher Block Chaining Mode (CBC) –Xor each plaintext block with previous ciphertext block –Requires an initialization vector for the first one –The initialization vector can be made public

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13 CBC Mode Encryption init. vector m1m1 DES c1c1 m2m2 c2c2 sent … … …

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14 CBC Mode Decryption init. vector c1c1 DES m1m1 … … … c2c2 m2m2

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15 Self-Healing Property What will happen if a bit gets lost during transmission? –All blocks will not be aligned When one bit in a block flipped, only the next two blocks will be impacted. –Plaintext “heals” after 2 blocks

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16 Current Status of DES Design for computer system, associated software that could break any DES-enciphered message in a few days published in 1998 Several challenges to break DES messages solved using distributed computing NIST selected Rijndael as Advanced Encryption Standard, successor to DES –Designed to withstand attacks that were successful on DES

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