1. Lecture 3 Overview

2. Ciphers The intent of cryptography is to provide secrecy to messages and data Substitutions – ‘hide’ letters of plaintext Transposition – scramble adjacent characters CS 450/650 – Lecture 3: Entropy 2

3. Entropy Shannon demonstrated mathematical methods of treating communication channels, bandwidth, and the effects of random noise on signals – p i is the probability of a given message (or piece of information) – n is the number of possible messages (or pieces of information) CS 450/650 – Lecture 3: Entropy 3

4. Entropy Entropy gives an indication of the complexity, or randomness, of a message or a data set. Generally, signals or data sets with high entropy, – Have a greater chance of a data transmission error – Require greater bandwidth to transmit – Have smaller capacity for compression – Appear to have a greater degree of "disorder” CS 450/650 – Lecture 3: Entropy 4

5. Entropy and Cryptography Through cryptography, we increase the uncertainty in the message for those who do not know the key Plaintext has an entropy of zero as there is no uncertainty about it. – This class is CS 450 Encryption using one of x equally probable keys increases the entropy to x – KBXT LWER ACMF OSJU CS 450/650 – Lecture 3: Entropy 5

6. Entropy and Cryptography With a perfect cipher “all keys are essentially equivalent” A good cipher will make a message look like noise Encryption should "scramble" the original message to the maximum possible extent Algorithms should take a message through a sequence of substitutions and transpositions CS 450/650 – Lecture 3: Entropy 6

7. Shannon Characteristics of ‘Good’ Ciphers 1.“The amount of secrecy needed should determine the amount of labor appropriate for the encryption and decryption” – Hold off the interceptor for required time duration 2.“The set of keys and enciphering algorithm should be free from complexity” – There should not be restriction on choice of keys or types of plaintext 3.“The implementation of the process should be as simple as possible” – Hand implementation, software bugs CS 450/650 – Lecture 3: Entropy 7

8. Shannon Characteristics of ‘Good’ Ciphers 4.“Errors in ciphering should not propagate and cause corruption of further information in the message” – An error early in the process should not throw off the entire remaining cipher text 5.“The size of the enciphered text should be no larger than the text of original message” – A ciphertext that expands in size cannot possibly carry more information than the plaintext CS 450/650 – Lecture 3: Entropy 8

9. Trustworthy Encryption Systems Commercial grade encryption 1.Based on sound mathematics 2.Analyzed by competent experts 3.Test of time DES: Data Encryption Standard RSA: River-Shamir-Adelman AES: Advanced Encryption Standard CS 450/650 – Lecture 3: Entropy 9

10. Confusion and Diffusion Confusion – Has complex relation between plaintext, key, and ciphertext – The interceptor should not be able to predict what will happen to ciphertext by changing one chatracter in plaintext Diffusion – Cipher should spread information from plaintext over entire ciphertext – The interceptor should require access to much of ciphertext to infer algorithm CS 450/650 – Lecture 3: Entropy 10

11. Lecture 4 Data Encryption Standard (DES) CS 450/650 Fundamentals of Integrated Computer Security Slides are modified from Hesham El-Rewini and J. Orlin Grabbe

12. Data Encryption Standard Combination of substitution and transposition – Repeated for 16 cycles – Provides confusion and diffusion Product cipher – Two weak but complementary ciphers can be made more secure by being applied together CS 450/650 – Lecture 4: DES 12

13. A High Level Description of DES CS 450/650 – Lecture 4: DES 13 Input - P 16 Cycles Output - C Key IP Inverse IP

14. A Cycle in DES CS 450/650 – Lecture 4: DES 14 Right halfLeft half Key shifted And Permuted New R-halfNew L-half f

15. K 64 bits PC-1 K+ 56 bits C0 28 bitsD0 28 bits C1 28 bits D1 28 bits C2 28 bits D2 28 bits C16 28 bits D16 28 bits PC-2 K1 48 bitsK2 48 bitsK16 48 bits Shift Key Summary CS 450/650 – Lecture 4: DES 15

16. 32 bits Kn 48 bits E E(Rn-1) 48 bits E(Rn-1)+Kn 48 bits S Boxes P f CS 450/650 – Lecture 4: DES 16

17. M 64 bits I-P L0 32 bitsR0 32 bits IP 64 bits f L1 32 bitsR1 32 bits K1 48 bits Cycle 1 CS 450/650 – Lecture 4: DES 17

18. L1 32 bitsR1 32 bits f L2 32 bitsR2 32 bits K2 48 bits Cycle 2 CS 450/650 – Lecture 4: DES 18

19. L2 32 bitsR2 32 bits f L3 32 bitsR3 32 bits K3 48 bits Cycle 3 CS 450/650 – Lecture 4: DES 19

20. L15 32 bitsR15 32 bits f L16 32 bitsR16 32 bits K16 48 bits IP -1 C 64 bits L16 32 bitsR16 32 bits Cycle 16 CS 450/650 – Lecture 4: DES 20

21. DES CS 450/650 Fundamentals of Integrated Computer Security 21

22. Design of the Algorithm key elements of the algorithm design were "sensitive" and would not be made public – the rationale behind transformations by the S- boxes, the P-boxes, and the key changes trapdoors? – Congressional inquiry design flaw would be discovered by a cryptanalyst – to date, no serious flaws have been published CS 450/650 Fundamentals of Integrated Computer Security 22

23. Does DES Work? Differential Cryptanalysis Idea – Use two plaintext that barely differ – Study the difference in the corresponding cipher text – Collect the keys that could accomplish the change – Repeat CS 450/650 – Lecture 4: DES 23

24. Cracking DES During the period NBS was soliciting comments on the proposed algorithm, the creators of public key cryptography registered some objections to the use of DES. – Hellman wrote: "Whit Diffie and I have become concerned that the proposed data encryption standard, while probably secure against commercial assault, may be extremely vulnerable to attack by an intelligence organization" letter to NBS, October 22, 1975 CS 450/650 – Lecture 4: DES 24

25. Cracking DES (cont.) Diffie and Hellman then outlined a "brute force" attack on DES – By "brute force" is meant that you try as many of the 2 56 possible keys as you have to before decrypting the ciphertext into a sensible plaintext message – They proposed a special purpose "parallel computer using one million chips to try one million keys each" per second CS 450/650 – Lecture 4: DES 25

26. Cracking DES (cont.) In 1998, Electronic Frontier Foundation spent $220K and built a machine that could go through the entire 56-bit DES key space in an average of 4.5 days – On July 17, 1998, they announced they had cracked a 56-bit key in 56 hours The computer, called Deep Crack – used 27 boards each containing 64 chips – was capable of testing 90 billion keys a second CS 450/650 – Lecture 4: DES 26

27. Cracking DES (cont.) In early 1999, Distributed.net used the DES Cracker and a worldwide network of nearly 100K PCs to break DES in 22 hours – combined they were testing 245 billion keys per second It has been shown that a dedicated hardware device with a cost of $1M (is much less in 2011) can search all possible DES keys in about 3.5 hours This just serves to illustrate that any organization with moderate resources can break through DES with very little effort these days CS 450/650 – Lecture 4: DES 27

28. Triple DES Triple-DES is just DES with two 56-bit keys applied. Given a plaintext message, the first key is used to DES- encrypt the message. The second key is used to DES-decrypt the encrypted message. – Since the second key is not the right key, this decryption just scrambles the data further. The twice-scrambled message is then encrypted again with the first key to yield the final ciphertext. This three-step procedure is called triple-DES. CS 450/650 – Lecture 4: DES 28

29. Detailed DES Example Plain text message M M = 0123456789ABCDEF (hexadecimal format) M in binary format: M = 0000 0001 0010 0011 0100 0101 0110 0111 1000 1001 1010 1011 1100 1101 1110 1111 Left Half (L) and Right Half (R) L = 0000 0001 0010 0011 0100 0101 0110 0111 R = 1000 1001 1010 1011 1100 1101 1110 1111 CS 450/650 – Lecture 4: DES 29

30. Key Key K K = K = 133457799BBCDFF1 (hexadecimal format) K in binary format: K = 00010011 00110100 01010111 01111001 10011011 10111100 11011111 11110001 Note: DES operates on the 64-bit blocks using key sizes of 56- bits. The keys are actually stored as being 64 bits long, but every 8th bit in the key is not used (i.e. bits numbered 8, 16, 24, 32, 40, 48, 56, and 64). CS 450/650 – Lecture 4: DES 30

31. Step 1: Create 16 sub-keys (48-bits) 1.1 The 64-bit key is permuted according to table PC-1. CS 450/650 – Lecture 4: DES 31 5749413325179 1585042342618 1025951433527 1911360524436 63554739312315 7625446383022 1466153453729 211352820124

32. Example (cont.) From the original 64-bit key K = 00010011 00110100 01010111 01111001 10011011 10111100 11011111 11110001 Using PC-1, we get the 56-bit permutation K+ = 1111000 0110011 0010101 0101111 0101010 1011001 1001111 0001111 CS 450/650 – Lecture 4: DES 32

33. Split this key 1.2 Split this key into left and right halves, C 0 and D 0, where each half has 28 bits K+ = 1111000 0110011 0010101 0101111 0101010 1011001 1001111 0001111 From the permuted key K+, we get C 0 = 1111000 0110011 0010101 0101111 D 0 = 0101010 1011001 1001111 0001111 CS 450/650 – Lecture 4: DES 33

34. 12345678910111213141516 shift 1122222212222221 Create 16 blocks 1.3 Create 16 blocks C n and D n, 1<=n<=16. C n and D n are obtained from C n-1 and D n-1 using the following schedule of "left shifts". CS 450/650 – Lecture 4: DES 34

35. Example (Cont.) C 0 = 1111000 0110011 0010101 0101111 D 0 = 0101010 1011001 1001111 0001111 C 1 = 1110000110011001010101011111 D 1 = 1010101011001100111100011110 C 2 = 1100001100110010101010111111 D 2 = 0101010110011001111000111101 C 3 = 0000110011001010101011111111 D 3 = 0101011001100111100011110101 CS 450/650 – Lecture 4: DES 35

36. Example (Cont.) C 4 = 0011001100101010101111111100 D 4 = 0101100110011110001111010101 C 5 = 1100110010101010111111110000 D 5 = 0110011001111000111101010101 C 6 = 0011001010101011111111000011 D 6 = 1001100111100011110101010101 C 7 = 1100101010101111111100001100 D 7 = 0110011110001111010101010110 CS 450/650 – Lecture 4: DES 36

37. Example (Cont.) C 8 = 0010101010111111110000110011 D 8 = 1001111000111101010101011001 C 9 = 0101010101111111100001100110 D 9 = 0011110001111010101010110011 C 10 = 0101010111111110000110011001 D 10 = 1111000111101010101011001100 C 11 = 0101011111111000011001100101 D 11 = 1100011110101010101100110011 CS 450/650 – Lecture 4: DES 37

38. Example (Cont.) C 12 = 0101111111100001100110010101 D 12 = 0001111010101010110011001111 C 13 = 0111111110000110011001010101 D 13 = 0111101010101011001100111100 C 14 = 1111111000011001100101010101 D 14 = 1110101010101100110011110001 C 15 = 1111100001100110010101010111 D 15 = 1010101010110011001111000111 CS 450/650 – Lecture 4: DES 38

39. 1417112415 3281562110 2319124268 1672720132 415231374755 304051453348 444939563453 464250362932 Form the keys K n 1.4 Form the keys K n, for 1<=n<=16, by applying the following permutation table to each of the concatenated pairs C n D n. Each pair has 56 bits, but PC-2 only uses 48 of these. CS 450/650 – Lecture 4: DES 39

40. Example (Cont.) For the first key we have C 1 D 1 = 1110000 1100110 0101010 1011111 1010101 0110011 0011110 0011110 which, after we apply the permutation PC-2, becomes K 1 = 000110 110000 001011 101111 111111 000111 000001 110010 CS 450/650 – Lecture 4: DES 40

41. Example (Cont.) K 2 = 011110 011010 111011 011001 110110 111100 100111 100101 K 3 = 010101 011111 110010 001010 010000 101100 111110 011001 K 4 = 011100 101010 110111 010110 110110 110011 010100 011101 K 5 = 011111 001110 110000 000111 111010 110101 001110 101000 K 6 = 011000 111010 010100 111110 010100 000111 101100 101111 K 7 = 111011 001000 010010 110111 111101 100001 100010 111100 CS 450/650 – Lecture 4: DES 41

42. Example (Cont.) K 8 = 111101 111000 101000 111010 110000 010011 101111 111011 K 9 = 111000 001101 101111 101011 111011 011110 011110 000001 K 10 = 101100 011111 001101 000111 101110 100100 011001 001111 K 11 = 001000 010101 111111 010011 110111 101101 001110 000110 K 12 = 011101 010111 000111 110101 100101 000110 011111 101001 CS 450/650 – Lecture 4: DES 42

43. Example (Cont.) K 13 = 100101 111100 010111 010001 111110 101011 101001 000001 K 14 = 010111 110100 001110 110111 111100 101110 011100 111010 K 15 = 101111 111001 000110 001101 001111 010011 111100 001010 K 16 = 110010 110011 110110 001011 000011 100001 011111 110101 CS 450/650 – Lecture 4: DES 43

44. Step 2: Encode each 64-bit block of data 2.1 Do initial permutation IP of M to the following IP table. CS 450/650 – Lecture 4: DES 44 585042342618102 605244362820124 625446383022146 645648403224168 57494133251791 595143352719113 615345372921135 635547393123157

45. Example (Cont.) Applying the initial permutation to the block of text M, we get M = 0000 0001 0010 0011 0100 0101 0110 0111 1000 1001 1010 1011 1100 1101 1110 1111 IP = 1100 1100 0000 0000 1100 1100 1111 1111 1111 0000 1010 1010 CS 450/650 – Lecture 4: DES 45

46. Divide the permuted block IP 2.2 Divide the permuted block IP into a left half L 0 of 32 bits, and a right half R 0 of 32 bits IP = 1100 1100 0000 0000 1100 1100 1111 1111 1111 0000 1010 1010 1111 0000 1010 1010 From IP we get L 0 = 1100 1100 0000 0000 1100 1100 1111 1111 R 0 = 1111 0000 1010 1010 1111 0000 1010 1010 CS 450/650 – Lecture 4: DES 46

47. Proceed through 16 iterations of f 2.3 Proceed through 16 iterations, for 1<=n<=16, using a function f which operates on two blocks— a data block of 32 bits and a key K n of 48 bits to produce a block of 32 bits. L n = R n-1 R n = L n-1 + f(R n-1,K n ) -- + denote XOR K 1 = 000110 110000 001011 101111 111111 000111 000001 110010 L 1 = R 0 = 1111 0000 1010 1010 1111 0000 1010 1010 R 1 = L 0 + f(R 0,K 1 ) CS 450/650 – Lecture 4: DES 47

48. The Calculation of the function f 1 - Expand R n-1  E(R n-1 ) 2- XOR  K n + E(R n-1 ) = B 1 B 2 B 3 B 4 B 5 B 6 B 7 B 8 3- Substitution S-Boxes  S 1 (B 1 )S 2 (B 2 )S 3 (B 3 )S 4 (B 4 )S 5 (B 5 )S 6 (B 6 )S 7 (B 7 )S 8 (B 8 ) 4- P permutation  f = P(S 1 (B 1 )S 2 (B 2 )...S 8 (B 8 )) CS 450/650 – Lecture 4: DES 48

49. 3212345 456789 8910111213 121314151617 161718192021 202122232425 242526272829 28293031321 Expand each block R n-1 2.4 Expand each block R n-1 from 32 bits to 48 bits using a selection table that repeats some of the bits in R n-1. CS 450/650 – Lecture 4: DES 49

50. E R n-1 E(R n-1 ) Example (Cont.) We'll call the use of this selection table the function E. Thus E(R n-1 ) has a 32 bit input block, and a 48 bit output block. CS 450/650 – Lecture 4: DES 50

51. Example (Cont.) We calculate E(R 0 ) from R 0 as follows: R 0 = 1111 0000 1010 1010 1111 0000 1010 1010 E(R 0 ) = 011110 100001 010101 010101 011110 100001 010101 010101 Note that each block of 4 original bits has been expanded to a block of 6 output bits. CS 450/650 – Lecture 4: DES 51

52. XOR Operation In the f calculation, we XOR the output E(R n-1 ) with the key K n : K n + E(R n-1 ) K 1 = 000110 110000 001011 101111 111111 000111 000001 110010 E(R 0 ) = 011110 100001 010101 010101 011110 100001 010101 010101 K 1 +E(R 0 ) = 011000 010001 011110 111010 100001 100110 010100 100111 CS 450/650 – Lecture 4: DES 52

53. Substitution – S-Boxes We now have 48 bits, or eight groups of six bits. We use each group of 6 bits as addresses in tables called "S boxes". Each group of six bits will give us an address in a different S box. Located at that address will be a 4 bit number. This 4 bit number will replace the original 6 bits. The net result is that the eight groups of 6 bits are transformed into eight groups of 4 bits (the 4-bit outputs from the S boxes) for 32 bits total. CS 450/650 – Lecture 4: DES 53

54. Substitution – S-Boxes (Cont.) K n + E(R n-1 ) = B 1 B 2 B 3 B 4 B 5 B 6 B 7 B 8 where each B i is a group of six bits. We now calculate S 1 (B 1 )S 2 (B 2 )S 3 (B 3 )S 4 (B 4 )S 5 (B 5 )S 6 (B 6 )S 7 (B 7 )S 8 (B 8 ) where S i (B i ) referrers to the output of the i-th S box. CS 450/650 – Lecture 4: DES 54

55. Substitution – S-Boxes (Cont.) Box S1 CS 450/650 – Lecture 4: DES 55 0123456789101112131415 01441312151183106125907 10157414213110612119539 24114813621115129731050 31512824917511314100613

56. Finding S1(B1) The first and last bits of B represent in base 2 a number in the decimal range 0 to 3. – Let that number be i. The middle 4 bits of B represent in base 2 a number in the decimal range 0 to 15. – Let that number be j. Look up in the table the number in the i-th row and j-th column. The tables defining the functions S 1,...,S 8 are given in page 740 CS 450/650 – Lecture 4: DES 56

57. Example (Cont.) For input block B = 011011 the first bit is "0" and the last bit "1" giving 01 as the row. – This is row 1. The middle four bits are "1101". – This is the binary equivalent of decimal 13, so the column is column number 13. In row 1, column 13 appears 5. This determines the output; – 5 is binary 0101, so that the output is 0101. Hence S1(011011) = 0101. CS 450/650 – Lecture 4: DES 57

58. Example (Cont.) For the first round, we obtain as the output of the eight S boxes: K 1 + E(R 0 ) = 011000 010001 011110 111010 100001 100110 010100 100111 S 1 (B 1 )S 2 (B 2 )S 3 (B 3 )S 4 (B 4 )S 5 (B 5 )S 6 (B 6 )S 7 (B 7 )S 8 (B 8 ) = 0101 1100 1000 0010 1011 0101 1001 0111 CS 450/650 – Lecture 4: DES 58

59. Permutation P of the S-box output f = P(S 1 (B 1 )S 2 (B 2 )...S 8 (B 8 )) CS 450/650 – Lecture 4: DES 59 1672021 29122817 1152326 5183110 282414 322739 1913306 2211425

60. Example (Cont.) From the output of the eight S boxes: S 1 (B 1 )S 2 (B 2 )S 3 (B 3 )S 4 (B 4 )S 5 (B 5 )S 6 (B 6 )S 7 (B 7 )S 8 (B 8 ) = 0101 1100 1000 0010 1011 0101 1001 0111 we get f = 0010 0011 0100 1010 1010 1001 1011 1011 CS 450/650 – Lecture 4: DES 60

61. Example (Cont.) R 1 = L 0 + f(R 0, K 1 ) = 1100 1100 0000 0000 1100 1100 1111 1111 + 0010 0011 0100 1010 1010 1001 1011 1011 = 1110 1111 0100 1010 0110 0101 0100 0100 CS 450/650 – Lecture 4: DES 61

62. Process Repeated 16 rounds In the next round, we will have L 2 = R 1, which is the block we just calculated, and then we must calculate R 2 =L 1 + f(R 1, K 2 ), and so on for 16 rounds. CS 450/650 – Lecture 4: DES 62

63. Final Phase At the end of the sixteenth round we have L 16 and R 16. We then reverse the order of the two blocks into R 16 L 16 and apply a final permutation IP -1 as defined by the following table CS 450/650 – Lecture 4: DES 63 408481656246432 397471555236331 386461454226230 375451353216129 364441252206028 353431151195927 342421050185826 33141949175725

64. Example (cont.) If we process all 16 blocks using the method defined previously, we get, on the 16th round, L 16 = 0100 0011 0100 0010 0011 0010 0011 0100 R 16 = 0000 1010 0100 1100 1101 1001 1001 0101 CS 450/650 – Lecture 4: DES 64

65. Example (cont.) We reverse the order of these two blocks and apply the final permutation to R 16 L 16 = 00001010 01001100 11011001 10010101 01000011 01000010 00110010 00110100 IP -1 = 10000101 11101000 00010011 01010100 00001111 00001010 10110100 00000101 which in hexadecimal format is 85E813540F0AB405 CS 450/650 – Lecture 4: DES 65

66. The End M = 0123456789ABCDEF C = 85E813540F0AB405 Decryption is simply the inverse of encryption, following the same steps as above, but reversing the order in which the sub-keys are applied CS 450/650 – Lecture 4: DES 66