Introduction to Information Security


Controversy Surrounding DES



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Controversy Surrounding DES

  • Development process was hidden from public
    • Suspicions that the government had put in a “backdoor”
  • Government attempted to shut down research in cryptography

Breaking DES

  • The key length of DES was too short
    • If a key is 56 bits long, that means there are 256 possible keys
    • “DES Cracker” machines were designed to simply brute force all possible keys
  • People began encrypting the plaintext multiple times with different keys in order to increase the number of keys that need to be checked

Breaking DES cont.

  • DES was further weakened by the discovery of differential cryptanalysis
    • Biham and Shamir in 1990
    • The most significant advance in cryptanalysis since frequency analysis
  • Ideally a ciphertext should be completely random, there should be no connection to its matching plaintext
    • Differential analysis exploits the fact that this is never actually the case
    • Uses patterns between plaintext and ciphertext to discover the key
  • There is evidence that IBM knew about differential cryptanalysis back when they were designing DES in 1976

Developing the AES

  • With DES effectively broken, a new standard was needed
  • U.S. Government made it an open application/review process this time, and received many submissions
  • In 2001, after five years, the Rijndael cipher was selected to become the Advanced Encryption Standard

AES (Rijndael)

  • Developed by Vincent Rijmen and Joan Daemen
  • SPN structure
  • Block size of 128 bits
  • Key size of 128, 192, or 256 bits
  • 10, 12, or 14 rounds depending on the key size

Current attacks against AES

  • On AES with 128-bit keys, a brute force attack would require 2128 work
    • Any technique that can decrypt a ciphertext with less than 2128 work is considered an attack
  • Currently the best attacks on AES use variations of differential cryptanalysis
    • None of them could actually be completed before the sun burns out
    • None of them work on the full number of rounds

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