CSCI 587 - Lecture 5 Text Encryption

• Text Compaction reviewed
  • Creating a Huffman code
  • Variations of Ziv-Lempel
  • Entropy examples and theorems
    • H(M) =< length(M) < H(M)+1
    • H(M) =< log2(n) and equal if and only if uniform probabilities
• Text Encryption
• Terminology
  • Plaintext, ciphertext, encoded text
  • Convention: Alice -> Bob (Oscar listens in)
  • key
• Substitution ciphers- encryption algorithm based on substitutions
  • Simple substitution ciphers
    • ROT3, ROT13
  • Homophonic - map a character to one of a set of values
    • A->13, 38, 53 or 57
  • Polyalphabetic - use different rules for different positions within the plaintext
    • Example of Period 2: for 1rst, 3rd ... use ROT3, for 2nd, 4th ... use ROT13
  • Polygram - encode longer strings
    • Playfair ciphers used by British in WW I encoded pairs
• Transposition ciphers: characters remain the same, the order is shuffled
  • write plaintext in rows; read off ciphertext by columns
  • German WWI ADFGVX cipher used transposition and simple substitution
• Cryptographic machines
  • The Enigma used by the Germans in WW II
  • rotors, keyboard, plugboard
  • polyalphabetic with period
  • Alan Turing
• DES
  • Secret-key or symmetric-key cryptography
  • Adopted as Federal standard in 1976 by NBS
  • Modes
    • Electronic Code Book (ECB) - encode 64 bit block with 56 bit key
    • Cipher Block Chaining (CBC) - XOR block with previous block before encoding
  • DES algorithm
    • NSA hand in specifications
    • key space too small
    • DES Controversy
  • RSA
    • Rivest, Shamir, Alderman 1978
    • public key encryption
    • Finding a key pair
    • encryption
    • decryption
    • 1000 times slower than DES
    • RSA and DES together
  • Digital signatures
  • Unix: passwords, crypt, vi -x, crack
  • PGP: Phil Zimmerman's Pretty Good Protection
  • Clipper Chip: trapdoor
  • Digital cash:
  Readings
  digital money by Daniel Lynch and Leslie Lundquist, John Wiley, 1996.
  Cryptography: Theory and Practice by Douglas R. Stinson, CRC Press, 1995.
  Alan Turing: the enigma By Andrew Hodges, Simon and Schuster, 1983.

  URL = http://sourgum.cs.sc.edu/~matthews/Courses/587/Lectures/lecture5.html