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Symmetric Encryption Example: DES

Symmetric Encryption Example: DES. Weichao Wang. 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

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Symmetric Encryption Example: DES

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  1. Symmetric Encryption Example: DES Weichao Wang

  2. 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

  3. Generation of Round Keys • Round keys are 48 bits each

  4. Encipherment

  5. The f Function

  6. 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

  7. 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/DESCracker/HTML/19990119_deschallenge3.html • Design decisions not public • S-boxes may have backdoors

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

  9. 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

  10. Differential Cryptanalysis • A chosen ciphertext attack • Requires 247 (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 243 (plaintext, ciphertext) pairs

  11. 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 • Encrypt-Decrypt-Encrypt Mode (2 keys: k, k) • Encrypt-Encrypt-Encrypt Mode (3 keys: k, k, k)

  12. init. vector m1 m2 …   DES DES … c1 c2 … sent sent CBC Mode Encryption

  13. CBC Mode Decryption init. vector c1 c2 … DES DES …   m1 m2 …

  14. 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

  15. 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 • 128 bit block size; 128, 192, or 256 bit key • Encryption speed can be 700MB/sec on an i7 CPU

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