1. February 22, 2011Harvard Bits1 FNNC LNQMHMF ! Sghr kdbstqd hr zants dmbqxoshnm

2. February 22, 2011Harvard Bits2 The Caesar Cipher (Suetonius) R“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.”

3. February 22, 2011Harvard Bits3 Caesar cipher abcdefghijklmnopqrstuvwxyz DEFGHIJKLMNOPQRSRUVWXYZABC RReplace 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 encryption decryption

4. Cryptography and National Security February 22, 20114Harvard Bits

5. February 22, 2011Harvard Bits5 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

6. February 22, 2011Harvard Bits6 The Stakes Rise After 9/11 RSept. 13, 2001: Sen. Judd Gregg (NH) calls for encryption regulations, saying encryption makers should be required to include decryption methods for government agents. RUS market force would be used to constrain foreign makers of encryption products RSept. 13, 2001: Sen. Judd Gregg (NH) calls for encryption regulations, saying encryption makers should be required to include decryption methods for government agents. RUS market force would be used to constrain foreign makers of encryption products

7. February 22, 2011Harvard Bits7 A month later, encryption is OK! ROctober 24, 2001: USA PATRIOT Act passes RVastly enhanced authorization for government surveillance in the interest of national security RNot one word about encryption! RWhy did US Congress drop its efforts to control encryption, barely a month after the attack on the US? ROctober 24, 2001: USA PATRIOT Act passes RVastly enhanced authorization for government surveillance in the interest of national security RNot one word about encryption! RWhy did US Congress drop its efforts to control encryption, barely a month after the attack on the US?

8. February 22, 2011Harvard Bits8 Electronic Commerce!

9. Treatise on the Astrolabe, 1391 (once attributed to Chaucer) February 22, 20119Harvard Bits

10. February 22, 2011Harvard Bits10 Letter Frequencies Source: Wikipedia

11. February 22, 201111Harvard Bits

12. February 22, 201112Harvard Bits

13. e e e e e e e e e ee e February 22, 201113Harvard Bits

14. e e e e e e e e e ee t t t t t t t t t e t February 22, 201114Harvard Bits

15. e e e e e e e e e ee t t t t t t t t t t e h h h h h February 22, 201115Harvard Bits

16. e e e e e e e e e ee t t t t t t t t t t e h h h h h oo o o o o o o February 22, 201116Harvard Bits

17. e e e e e e e e e ee t t t t t t t t t t e h h h h h oo o o o o o o is i i i i s s i February 22, 201117Harvard Bits

18. e e e e e e e e e ee t t t t t t t t t t h e h h h h oo o o o o o o is i i i i s s i r r r r February 22, 201118Harvard Bits

19. e e e e e e e e e ee t t t t t t t t t t h e h h h oo o o o o o o is i i i i s s i r r r r h f a a b a b l l f v n n n n n f c uq d m February 22, 201119Harvard Bits

20. February 22, 2011Harvard Bits20 Substitution cipher RReplace each character of the message by another character RIn general ROriginal message is called the plaintext REncrypted result is called the ciphertext RSubstitution ciphers easily cracked by frequency analysis RReplace each character of the message by another character RIn general ROriginal message is called the plaintext REncrypted result is called the ciphertext RSubstitution ciphers easily cracked by frequency analysis

21. February 22, 2011Harvard Bits21 Cryptosystems ATTACKER key encrypt plaintext message retreat at dawn key decrypt ciphertext plaintext message retreat at dawn SENDERciphertext sb%6x*cmf RECEIVER Alice Bob Eve

22. February 22, 2011Harvard Bits22 Yaqub Ibn Ishaq al- Kindi (801-873) Cracking ciphers RFrequency analysis has been known since the 9 th century. RAl Kindi’s Manuscript on Deciphering Cryptographic Messages RFrequency analysis has been known since the 9 th century. RAl Kindi’s Manuscript on Deciphering Cryptographic Messages

23. February 22, 2011Harvard Bits23 Mary Stuart, 1587

24. RRussian monoalphabetic substitution key, recovered by England’s Decyphering Branch, 1728 RFrom David Kahn, The Codebreakers RRussian monoalphabetic substitution key, recovered by England’s Decyphering Branch, 1728 RFrom David Kahn, The Codebreakers February 22, 201124Harvard Bits

25. February 22, 2011Harvard Bits25

26. February 22, 2011Harvard Bits26 R“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.” “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

27. February 22, 2011Harvard Bits27 The Koan of the Yogi R“In theory there is no difference between theory and practice. In practice, there is.”

28. February 22, 2011Harvard Bits28 Cryptologic lessons RBreakthroughs can render previously reliable cryptographic methods insecure RNews of cryptanalytic breakthroughs travels slowly RMaking strong encryption systems available does not guarantee they will be used RBreakthroughs can render previously reliable cryptographic methods insecure RNews of cryptanalytic breakthroughs travels slowly RMaking strong encryption systems available does not guarantee they will be used

29. February 22, 2011Harvard Bits29 Vigenère Encryption RUse several Caesar substitutions and cycle through them RSequence of substitutions determined by a secret key RUse several Caesar substitutions and cycle through them RSequence of substitutions determined by a secret key Blaise de Vigenere (1523-1596)

30. abcdefghijklmnopqrstuvwxyz STUVWXYZABCDEFGHIJKLMNOPQR OPQRSTUVWXYZABCDEFGHIJKLMN NOPQRSTUVWXYZABCDEFGHIJKLM GHIJKLMNOPQRSTUVWXYZABCDEF BCDEFGHIJKLMNOPQRSTUVWXYZA IJKLMNOPQRSTUVWXYZABCDEFGH RSTUVWXYZABCDEFGHIJKLMNOPQ DEFGHIJKLMNOPQRSTUVWXYZABC Fight fiercely, Harvard! Fight! Fight! Fight! H JQRR ZPRU NOEJ GQXK LTVM IBWL YVG XWTNUNZ February 22, 201130Harvard Bits

31. February 22, 2011Harvard Bits31 An Actual Vigenère Cipher Used for corresponsence between a businessman and a lawyer ca. 1900

32. February 22, 2011Harvard Bits32 Breaking Vigenère – (1) RIf the key has length K, then the ciphertext letters K positions apart are specified by the same character in the key … RAnd thus is the result of a simple substitution RAnd thus can be attacked by frequency analysis RExample: Suppose the key length is three: RIf the key has length K, then the ciphertext letters K positions apart are specified by the same character in the key … RAnd thus is the result of a simple substitution RAnd thus can be attacked by frequency analysis RExample: 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

33. February 22, 2011Harvard Bits33 Breaking Vigenère – (2) RTo find the length of the key: RTry 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. RClever methods to do this by hand: RBabbage, Kasiski: counting double letters (1850s, 1860s) RFriedman: Index of Coincidence (1920s) RWith computers, we don’t need to be clever: Can do brute-force statistics (let’s try it) RTo find the length of the key: RTry 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. RClever methods to do this by hand: RBabbage, Kasiski: counting double letters (1850s, 1860s) RFriedman: Index of Coincidence (1920s) RWith computers, we don’t need to be clever: Can do brute-force statistics (let’s try it)

34. February 22, 2011Harvard Bits34 Theory vs. Practice 1917

35. February 22, 2011Harvard Bits35 One-Time Pad: Key as long as plaintext RThe Only Provably Secure Cryptosystem RNo patterns, so nothing to analyze RBut getting the keys from Alice to Bob securely is just as hard as getting an unencrypted message! RUnsuitable for e-commerce R“Meet” Amazon to get a key? RThe Only Provably Secure Cryptosystem RNo patterns, so nothing to analyze RBut getting the keys from Alice to Bob securely is just as hard as getting an unencrypted message! RUnsuitable for e-commerce R“Meet” Amazon to get a key?

36. February 22, 2011Harvard Bits36 Beware Security Through Obscurity RKerckhoffs’ 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.” RStill regularly violated by Internet security start-ups and their credulous investors RKerckhoffs’ 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.” RStill regularly violated by Internet security start-ups and their credulous investors

37. February 22, 2011Harvard Bits37 DES: The Data Encryption Standard RA 1976 public standard R56 bit key RLong enough in 1976 RWith today’s more powerful computers a brute force search through possible keys takes only a day RSuperceded by Advanced Encryption Standard or “AES”: 128, 192, or 256 bit key RAES not cracked as far as we know RA 1976 public standard R56 bit key RLong enough in 1976 RWith today’s more powerful computers a brute force search through possible keys takes only a day RSuperceded by Advanced Encryption Standard or “AES”: 128, 192, or 256 bit key RAES not cracked as far as we know

38. February 22, 2011Harvard Bits38 But the Big Problem Remains: How to Get the Key securely from Alice to Bob? ?? To be continued …