Tuesday, September 10, 2013

Notes - Secure Communications

The following are notes from Introduction to Cryptography with Coding Theory.
  • Secure Communications
    • Actual message is called plaintext
    • Encryption methods are called keys
    • Ciphertext is the encrypted message
    • Goals of an attacker or eavesdropper
      • Read the message
      • Find the key and read all messages encrypted with that key
      • Corrupt initial message
      • Communicated with receiver of message masquerading as sender
  • Possible Attacks
    • Attacker has ciphertext only
    • Attacker has copy of ciphertext and plaintext
    • Attacker gains temporary access to encryption machine to create large quantities of ciphertext with known plaintext
    • Attacker gains temporary access to decryption machine to decrypt several strings of symbols
    • Kerckhoff's Principle
      • In assessing the security of a cryptosystem one should always assume the enemy knows the method being used
  • Symmetric and Public Key Algorithms
    • symmetric key
      • encryption and decryption keys are known to both sender and receiver
      • many cases encryption and decryption key is same
      • DES Data Encryption Standard AES Advanced Encryption Standard
      • Two types of ciphers
        • stream ciphers
          • data is fed in small bits, output in small bits
        • block ciphers
          • data is fed in blocks and fed into algorithm at once outputed at once
    • public key
      • introduced in 1970s
      • RSA encryption
      • Non-mathematical way to do public key encryption Receiver is Alice Sender is Bob
        • Bob sends Alice a box and an unlocked padlock
        • Alice puts message in box locks Bob's lock on it then sends the box back to Bob
          • authorization issues if first transmission is intercepted and lock is substituted
      • Computation of public keys are several orders of magnitude higher than symmetric keys
    • Codes - one to one usages Ciphers - encrypts every string of characters
  • Key Length
    • Brute Force attack -Try every single possible key to see which one yields meaningful decryptions
    • DES Algorithm has 56 bit key and thus 256 ~ 7.2x1016 keys
      • if you have a computer that can do 109 calculations a second would take about 3x1013 years to complete
      • Longer keys are advantageous but not necessarily more difficult to break
    • Other methods of attack
      • Frequency Analysis
      • BirthdayAttacks
    • One time pad is an unbreakable code
      • however requires a key as long as plaintext and the key can only be used once so is not practical

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