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CPSC 463

CPSC 463. Week 1. Historical Interlude 1. Who is the largest employer of mathematicians in the US?. Answer. The National Security Agency Established in 1952 by Presidential Directive Assignment 1 (due 1/22). Terms. Cryptology Study of communication over insecure channels

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CPSC 463

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  1. CPSC 463 Week 1

  2. Historical Interlude 1 • Who is the largest employer of mathematicians in the US?

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

  4. Terms • Cryptology • Study of communication over insecure channels • Cryptography: designing secure systems • Cryptanalysis: breaking such systems

  5. Cryptology is Part of Larger Field • Information Assurance • Includes • Cryptology • Securing systems from unauthorized entry • Study of malicious software

  6. 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 1970’s • Security resides in the key • 1970’s to the present • Both the method and the key are public

  7. Key? • Three Actors (All the world’s a stage) • Bob • Alice • The evil Eve • Alice wants to send a message to Bob—called plaintext • She encrypts it using a key—output is called ciphertext • Bob decrypts the message using the same (or derived key)

  8. The Sequence Alice Encrypt Ciphertext plaintext encryption Key Eve Listens Decrypt Bob decryption key

  9. 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)

  10. 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

  11. Some Preliminary Issues: Attacks • Cipher text only: Eve has copy of the cipher text • Known plaintext • Eve has cipher text and known plaintext • Turing’s 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

  12. 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

  13. 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

  14. scytale • Sparta 5th Century BCE

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