Introduction to Information Security

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