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Information Assurance Management

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

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  1. Information Assurance Management Cryptography Foundations Week 10-2

  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 • Homophonic substitution cipher - single character of plaintext can map to one of several ciphertext characters

  3. 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

  4. Substitution and Transposition Ciphers • Transposition Ciphers - Plaintext remains the same, but the order of characters is changed • SCOAO OHNMN METI EAHN OTEA NEET EDXI

  5. Substitution and Transposition Ciphers • Rotor Machines - Mechanical device • Uses rotors • Arbitrary permutation of the alphabet • Simple substitution • Rotors move at different rates • Enigma

  6. 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

  7. 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

  8. 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

  9. 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

  10. 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

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