CPSC 463

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CPSC 463

• Week 1

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Historical Interlude 1

• Who is the largest employer of mathematicians in
  the US?

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Answer

• The National Security Agency
• Established in 1952 by Presidential Directive
• Assignment 1 (due 1/22)

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Terms

• Cryptology
• Study of communication over insecure channels
• Cryptography designing secure systems
• Cryptanalysis breaking such systems

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Cryptology is Part of Larger Field

• Information Assurance
• Includes
• Cryptology
• Securing systems from unauthorized entry
• Study of malicious software

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The Three Stages of Cryptograyphy(The Seven Ages
of Man)

• The time of the Greeks to the Kerchoff Principle
  (1883)
• Both method and key are secret
• Kerchoff Principle One should assume that the
  enemy knows the cryptographic method
• Kerchoff Principle to the 1970s
• Security resides in the key
• 1970s to the present
• Both the method and the key are public

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Key?

• Three Actors (All the worlds a stage)
• Bob
• Alice
• The evil Eve
• Alice wants to send a message to Bobcalled
  plaintext
• She encrypts it using a keyoutput is called
  ciphertext
• Bob decrypts the message using the same (or
  derived key)

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The Sequence

• Alice Encrypt Ciphertext
• plaintext encryption Key Eve Listens
• Decrypt Bob
• decryption key

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Another Historical Division

• Symmetric Key
• Monoalphabetic shift ciphers (100 B.C.E.)
• Polyalphabetic shift ciphers (1586) (Vignere
  Cipher)
• Public Key
• Diffie-Hellman-Merkle Key Exchange Protocol (1976)

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Some Preliminary Issues Keys

• Key Exchange
• The key can be stolen
• How about encrypting the key
• Key Length
• Long keys are better than short keys
• But the algorithm is important too
• Simple monoalphabetic substitution cipher (msc)
• 26! 4 1026 different alphabets (and so that
  many keys)
• DES has only 256 7 1016 different keys
• But harder to break since msc preserves the
  structure of the message

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Some Preliminary Issues Attacks

• Cipher text only Eve has copy of the cipher text
• Known plaintext
• Eve has cipher text and known plaintext
• Turings deduction that Enigman-encoded reports
  at 6 A.M. would contain the word wetter.
• Chosen plaintext
• Eve gains access to the encryption machine
• Can encrypt a plaintext message
• Chosen ciphertext
• Even gets temporary access to the decryption
  machine and so can decrypt a string of symbols

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Historical Interlude 2

• Ciphers can be classified into two types
• Substitution ciphers each letter in the alphabet
  is replaced by another (Caesar)
• Transposition ciphers rearranging the letters in
  a message to create an anagram
• Cipher?hericp
• Problem must be a systematic way to decipher the
  anagram

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Rail Fence Cipher

• From Sonnet 29
• Like to the lark at break of day arising
• Drop every other letter down a row
• L K T T E A K T R A O D Y
• I E O H L R A B E K F A
• Append the bottom row to the tip
• LKTTEAKTRAODYIEOHLRABEKFA
• Leads to the first recorded military
  cryptographic device Scytale

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scytale

• Sparta 5th Century BCE