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1. FNNC LNQMHMF ! Sghr kdbstqd hr zants dmbqxoshnm Harvard Bits

2. The Caesar Cipher (Suetonius) • “If Caesar had anything confidential to say, he wrote it in cipher, that is, by so changing the order of the letters of the alphabet, that not a word could be made out. If anyone wishes to decipher these, and get at their meaning, he must substitute the fourth letter of the alphabet, namely D, for A, and so with the others.” Harvard Bits

3. Caesar cipher • Replace each letter by the letter that comes some fixed distance before or after it in the alphabet. Shift = 3 Gallia est omnis divisa in partes tres JDOOLD HVW RPQLV GLYLVD LQ SDUWHV WUHV decryption encryption Harvard Bits

4. Cryptography and National Security Harvard Bits

5. Unless the issue of encryption is resolved soon, criminal conversations over the telephone … will become indecipherable by law enforcement. This, as much as any issue, jeopardizes the public safety and national security of this country. FBI Director Louis Freeh, March 30, 1995 Harvard Bits

6. The Stakes Rise After 9/11 • Sept. 13, 2001: Sen. Judd Gregg (NH) calls for encryption regulations, saying encryption makers should be required to include decryption methods for government agents. • US market force would be used to constrain foreign makers of encryption products Harvard Bits

7. A month later, encryption is OK! • October 24, 2001: USA PATRIOT Act passes • Vastly enhanced authorization for government surveillance in the interest of national security • Not one word about encryption! • Why did US Congress drop its efforts to control encryption, barely a month after the attack on the US? Harvard Bits

8. Electronic Commerce! Harvard Bits

9. Treatise on the Astrolabe, 1391 (once attributed to Chaucer) Harvard Bits

10. Letter Frequencies Source: Wikipedia Harvard Bits

11. Harvard Bits

12. Harvard Bits

13. e e e e e e e e e e e e Harvard Bits

14. e t t t e t t e t e e t e e t t e e e e e t Harvard Bits

15. e h t t h t e t t e t e e t e h e t t h e e e e e t h Harvard Bits

16. e h t t h t e o t o t o e t e e t o e h e t t h o e o e o e e o e t h Harvard Bits

17. s s e i h t t i h t e o t o t o e t i e e t o e h e t t h o e i o e o e s i e o e t h i Harvard Bits

18. s s e r i h t t i h t e o r t o r t o e t i e e t o e h e t t h o e i o e o e r s i e o e t h i Harvard Bits

19. s a s e r i h t b l v t i h t e o r t o r t o e t n i e n f e t a o e h b l e q u a t f t h o e c i n o e o n m f e r s i e o e t h i d n Harvard Bits

20. Substitution cipher • Replace each character of the message by another character • In general • Original message is called the plaintext • Encrypted result is called the ciphertext • Substitution ciphers easily cracked by frequency analysis Harvard Bits

21. Bob Alice encrypt decrypt plaintext message plaintext message ciphertext key key Eve SENDER RECEIVER retreat at dawn retreat at dawn sb%6x*cmf ciphertext ATTACKER Cryptosystems Harvard Bits

22. Cracking ciphers • Frequency analysis has been known since the 9th century. • Al Kindi’s Manuscript on Deciphering Cryptographic Messages Yaqub Ibn Ishaq al-Kindi (801-873) Harvard Bits

23. Mary Stuart, 1587 Harvard Bits

24. Russian monoalphabetic substitution key, recovered by England’s Decyphering Branch, 1728 • From David Kahn, The Codebreakers Harvard Bits

25. Harvard Bits

26. “The so-called Binnu code assigns a number in order to each letter in the Italian alphabet and adds three to that number in the ciphertext so that "A" is 4, "B" is 5 and so on.” -- The Register • “If Caesar had anything confidential to say, he wrote it in cipher, that is, by so changing the order of the letters of the alphabet, that not a word could be made out. If anyone wishes to decipher these, and get at their meaning, he must substitute the fourth letter of the alphabet, namely D, for A, and so with the others.” Harvard Bits

27. The Koan of the Yogi • “In theory there is no difference between theory and practice. In practice, there is.” Harvard Bits

28. Cryptologic lessons • Breakthroughs can render previously reliable cryptographic methods insecure • News of cryptanalytic breakthroughs travels slowly • Making strong encryption systems available does not guarantee they will be used Harvard Bits

29. Vigenère Encryption • Use several Caesar substitutions and cycle through them • Sequence of substitutions determined by a secret key Blaise de Vigenere (1523-1596) Harvard Bits

30. Fight fiercely, Harvard! Fight! Fight! Fight! X W T N U N Z H JQRR ZPRU NOEJ GQXK LTVM IBWL YVG Harvard Bits

31. An Actual Vigenère Cipher Used for corresponsence between a businessman and a lawyer ca. 1900 Harvard Bits

32. Breaking Vigenère – (1) • If the key has length K, then the ciphertext letters K positions apart are specified by the same character in the key … • And thus is the result of a simple substitution • And thus can be attacked by frequency analysis • Example: Suppose the key length is three: DJBK FJWO VJSW FKDS GFJD RKEM CNEJ JKSJ FKDJ SJSS So the decryption reduces to doing frequency analysis K times – provided we know K Harvard Bits

33. Breaking Vigenère – (2) • To find the length of the key: • Try different values for K, looking at every Kth letter of the ciphertext, and pick the one for which the frequency distribution looks like the frequency distribution for English. • Clever methods to do this by hand: • Babbage, Kasiski: counting double letters (1850s, 1860s) • Friedman: Index of Coincidence (1920s) • With computers, we don’t need to be clever: Can do brute-force statistics (let’s try it) Harvard Bits

34. Theory vs. Practice1917 Harvard Bits

35. One-Time Pad: Key as long as plaintext • The Only Provably Secure Cryptosystem • No patterns, so nothing to analyze • But getting the keys from Alice to Bob securely is just as hard as getting an unencrypted message! • Unsuitable for e-commerce • “Meet” Amazon to get a key? Harvard Bits

36. Beware Security Through Obscurity • Kerckhoffs’ Principle (1883): “The system must not require secrecy, and it could fall into the hands of the enemy without causing trouble. If a system requiring secrecy were to find itself in the hands of too many individuals, it could be compromised upon each engagement in which any of them take part.” • Still regularly violated by Internet security start-ups and their credulous investors Harvard Bits

37. DES: The Data Encryption Standard • A 1976 public standard • 56 bit key • Long enough in 1976 • With today’s more powerful computers a brute force search through possible keys takes only a day • Superceded by Advanced Encryption Standard or “AES”: 128, 192, or 256 bit key • AES not cracked as far as we know Harvard Bits

38. But the Big Problem Remains: How to Get the Key securely from Alice to Bob? ?? Harvard Bits To be continued …