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Information Assurance Management. Cryptography Foundations Week 10-2. Substitution and Transposition Ciphers. Substitution cipher - one character in plaintext is substituted for another Four types: Monoalphabetic cipher - one plaintext character is substituted with a cipher test character

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information assurance management

Information Assurance Management

Cryptography

Foundations

Week 10-2

substitution and transposition ciphers
Substitution and Transposition Ciphers
  • Substitution cipher - one character in plaintext is substituted for another
  • Four types:
    • Monoalphabetic cipher - one plaintext character is substituted with a cipher test character
    • Homophonic substitution cipher - single character of plaintext can map to one of several ciphertext characters
substitution and transposition ciphers1
Substitution and Transposition Ciphers
  • Polygram substitution cipher - blocks of characters are encrypted as a group
  • Polylalphabetic substitution cipher - multiple simple substitution ciphers
    • changes with the position of each plaintext character
substitution and transposition ciphers2
Substitution and Transposition Ciphers
  • Transposition Ciphers - Plaintext remains the same, but the order of characters is changed
  • SCOAO OHNMN METI EAHN OTEA NEET EDXI
substitution and transposition ciphers3
Substitution and Transposition Ciphers
  • Rotor Machines - Mechanical device
    • Uses rotors
    • Arbitrary permutation of the alphabet
    • Simple substitution
    • Rotors move at different rates
    • Enigma
one time pads
One-Time Pads
  • The ONLY perfect encryption scheme
  • large nonrepeating set of truly random key letters
  • Each key letter is used just once to encrypt one plaintext character (addition modulo 26)
  • Used then destroyed - new message, new key letters
one time pads1
One-Time Pads
  • But...the key letters must be generated randomly! NEVER use that key again, ever
  • Limitation on key length...
  • Ultra secure low bandwidth messages
random numbers
Random Numbers
  • RAND Tables - One million random numbers in 5 digit groups
  • Random noise...events that happen regularly but randomly, measure time between event
  • Distill randomness
    • Actual Mouse positions
    • Arrival times of network packets
large numbers
Large Numbers
  • A measure of “Computational Security”
  • 56-bit DES key - 8,000 MIPS-years
  • 4.7 months with 2 purpose built computers, 3000+ PC’s working a brute force process
algorithms
Algorithms
  • Many kinds...here are three of the most popular:
    • DES - Data Encryption Standard
      • Most popular
      • US and International standard
      • Symmetric algorithm
    • RSA - Public-key algorithm
    • DSA - Digital Signature Algorithm...used only for digital signatures