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# Computer Security Lecture 2: Classical Encryption Techniques 1

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We will discuss the following: Symmetric Encryption, Substitution Techniques, Caesar Cipher, Monoalphabetic Cipher, Playfair Cipher, Hill Cipher

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### Computer Security Lecture 2: Classical Encryption Techniques 1

1. 1. Classical Encryption Techniques 1 Classical Encryption Techniques 1
2. 2. Classical Encryption Techniques 1 Symmetric Encryption Substitution Techniques Caesar Cipher Monoalphabetic Cipher Playfair Cipher Hill Cipher
3. 3. Classical Encryption Techniques 1 Symmetric Encryption Substitution Techniques Caesar Cipher Monoalphabetic Cipher Playfair Cipher Hill Cipher
4. 4. Classical Encryption Techniques 1
5. 5. Classical Encryption Techniques 1  Encryption algorithm: The encryption algorithm performs various substitutions and transformations on the plaintext.  Secret key: The secret key is also input to the encryption algorithm. The key is a value independent of the plaintext and of the algorithm. The algorithm will produce a different output depending on the specific key being used at the time.
6. 6. Classical Encryption Techniques 1  Ciphertext: This is the scrambled message produced as output. It depends on the plaintext and the secret key.  Decryption algorithm: This is essentially the encryption algorithm run in reverse. It takes the ciphertext and the secret key and produces the original plaintext.
7. 7. Classical Encryption Techniques 1  Symmetric-key algorithms are algorithms for cryptography that use the same cryptographic keys for both encryption of plaintext and decryption of ciphertext.
8. 8. Classical Encryption Techniques 1 Symmetric Encryption Substitution Techniques Caesar Cipher Monoalphabetic Cipher Playfair Cipher Hill Cipher
9. 9. Classical Encryption Techniques 1 Cipherered text 3 IODQN HDVW DWWDFN DW GDZQ A B C D E F G H I J K L M N O P Q R S T U V W X Y Z A B C A B C D E F G H I J K L M N O P Q R S T U V W X Y Z The clear text message would be encoded using a key of 3. 1 FLANK EAST ATTACK AT DAWN Shift the top scroll over by three characters (key of 3), an A becomes D, B becomes E, and so on. 2 The clear text message would be encrypted as follows using a key of 3. Clear text
10. 10. Classical Encryption Techniques 1  Caesar Cipher  Monoalphabetic Ciphers  Playfair Cipher  Hill Cipher  Polyalphabetic Ciphers  Vigenère Cipher  Autokey Cipher  Vernam Cipher
11. 11. Classical Encryption Techniques 1 Symmetric Encryption Substitution Techniques Caesar Cipher Monoalphabetic Cipher Playfair Cipher Hill Cipher
12. 12. Classical Encryption Techniques 1 Caesar Cipher is one of the simplest and most widely known encryption techniques.
13. 13. Classical Encryption Techniques 1
14. 14. Classical Encryption Techniques 1
15. 15. Classical Encryption Techniques 1
16. 16. Classical Encryption Techniques 1  PlainText = dcodex  K=3 1) P=d 2) P=3 3) C=P+K mod 26=3+3 mod 26=6 mod 26 =6 4) C=g
17. 17. Classical Encryption Techniques 1  PlainText = dcodex  K=3 1) P=x 2) P=23 3) C=P+K mod 26=23+3 mod 26=26 mod 26= 0 4) C=a
18. 18. Classical Encryption Techniques 1  P= dcodex  C= gfrgha  K=3
19. 19. Classical Encryption Techniques 1  CipherText = gfrgha  K=3 1) C=g 2) C=6 3) P=C-K mod 26=6-3 mod 26=3 4) P=d
20. 20. Classical Encryption Techniques 1  CipherText = gfrgha  K=3 1) C=a 2) C=0 3) P=C-K mod 26=0-3 mod 26=-3 mod 26 =23 4) P=x
21. 21. Classical Encryption Techniques 1  C= gfrgha  P= dcodex  K=3
22. 22. Classical Encryption Techniques 1  Three important characteristics of this problem enabled us to use a bruteforce cryptanalysis: The encryption and decryption algorithms are known. There are only 25 keys to try. The language of the plaintext is known and easily recognizable.
23. 23. Classical Encryption Techniques 1
24. 24. Classical Encryption Techniques 1 How to implement Caesar Cipher technique on Arabic letters?
25. 25. Classical Encryption Techniques 1 Symmetric Encryption Substitution Techniques Caesar Cipher Monoalphabetic Cipher Playfair Cipher Hill Cipher
26. 26. Classical Encryption Techniques 1 A monoalphabetic cipher uses fixed substitution over the entire message Random Key
27. 27. Classical Encryption Techniques 1  Example: Plaintext alphabets: ABCDEFGHIJKLMNOPQRSTUVWXYZ Ciphertext alphabet: ZEBRASCDFGHIJKLMNOPQTUVWXY  P= ITEMS  Encoding  C= FQAIP  Decoding  P= ITEMS
28. 28. Classical Encryption Techniques 1  Relative Frequency of Letters in English Text
29. 29. Classical Encryption Techniques 1  C=
30. 30. Classical Encryption Techniques 1  cipher letters P and Z are the equivalents of plain letters e and t
31. 31. Classical Encryption Techniques 1  Finally, The complete plaintext
32. 32. Classical Encryption Techniques 1 How to implement Monoalphabetic Cipher technique on Arabic letters?
33. 33. Classical Encryption Techniques 1 Symmetric Encryption Substitution Techniques Caesar Cipher Monoalphabetic Cipher Playfair Cipher Hill Cipher
34. 34. Classical Encryption Techniques 1  The Playfair system was invented by Charles Wheatstone, who first described it in 1854.  Used by many countries during wartime  The Playfair algorithm is based on the use of a 5 x 5 matrix of letters constructed using a keyword.
35. 35. Classical Encryption Techniques 1  In this case, the keyword is monarchy.
36. 36. Classical Encryption Techniques 1  4 Rules: 1) If both letters are the same (or only one letter is left), add an "X" after the first letter. 2) If the letters appear on the same row of your table, replace them with the letters to their immediate right respectively
37. 37. Classical Encryption Techniques 1  4 Rules: 3) If the letters appear on the same column of your table, replace them with the letters immediately below respectively 4) If the letters are not on the same row or column, replace them with the letters on the same row respectively but at the other pair of corners of the rectangle defined by the original pair.
38. 38. Classical Encryption Techniques 1  P=Hide the gold in the tree stump (note the null "X" used to separate the repeated "E"s)  P= HI DE TH EG OL DI NT HE TR EX ES TU MP  K= playfair example
39. 39. Classical Encryption Techniques 1  How to build 5x5 Matrix (assuming that I and J are interchangeable), the table becomes (omitted letters in red):
40. 40. Classical Encryption Techniques 1  P= HI DE TH EG OL DI NT HE TR EX ES TU MP
41. 41. Classical Encryption Techniques 1  P= HI DE TH EG OL DI NT HE TR EX ES TU MP
42. 42. Classical Encryption Techniques 1  P= HI DE TH EG OL DI NT HE TR EX ES TU MP
43. 43. Classical Encryption Techniques 1  P= HI DE TH EG OL DI NT HE TR EX ES TU MP
44. 44. Classical Encryption Techniques 1  P= HI DE TH EG OL DI NT HE TR EX ES TU MP
45. 45. Classical Encryption Techniques 1 C= BM OD ZB XD NA BE KU DM UI XM MO UV IF the message "Hide the gold in the tree stump" becomes "BMODZ BXDNA BEKUD MUIXM MOUVI F"
46. 46. Classical Encryption Techniques 1 Using Playfair Cipher how to decrepit the following cipher text: C= “BMODZ BXDNA BEKUD MUIXM MOUVI F” K= playfair example
47. 47. Classical Encryption Techniques 1 Symmetric Encryption Substitution Techniques Caesar Cipher Monoalphabetic Cipher Playfair Cipher Hill Cipher
48. 48. Classical Encryption Techniques 1  The Hill Cipher was invented by Lester S. Hill in 1929  The Hill Cipher based on linear algebra  Encryption  2 x 2 Matrix Encryption  3 x 3 Matrix Encryption
49. 49. Classical Encryption Techniques 1  square matrix 𝑀 by the equation 𝑀𝑀−1 = 𝑀−1 𝑀= 𝐼, where 𝐼 is the identity matrix.  C = P*K mod 26
50. 50. Classical Encryption Techniques 1
51. 51. Classical Encryption Techniques 1  Example of Key 2 x 2  𝐾 = 𝐻 𝐼 𝐿 𝐿 = 7 8 11 11  plaintext message "short example“  𝑃= short example  𝑃 = 𝑆 ℎ 𝑜 𝑟 𝑡 𝑒 𝑥 𝑎 𝑚 𝑝 𝑙 𝑒 = 18 7 14 17 19 4 23 0 12 15 11 4
52. 52. Classical Encryption Techniques 1  𝑃 = 𝑆 ℎ 𝑜 𝑟 𝑡 𝑒 𝑥 𝑎 𝑚 𝑝 𝑙 𝑒 = 18 7 14 17 19 4 23 0 12 15 11 4  𝐶 = 𝐾 ∗ 𝑃 𝑚𝑜𝑑 26  𝑘0 𝑘1 𝑘2 𝑘3 ∗ 𝑝0 𝑝1 = 𝑘0 ∗ 𝑝0 + 𝑘1 ∗ 𝑝1 𝑘2 ∗ 𝑝0 + 𝑘3 ∗ 𝑝1  7 8 11 11 ∗ 18 7 = 7 ∗ 18 + 8 ∗ 7 11 ∗ 18 + 11 ∗ 7 = 182 275  𝐶 = 182 275 𝑚𝑜𝑑 26 = 0 15 = 𝑎 𝑝
53. 53. Classical Encryption Techniques 1  𝑃 = 𝑆 ℎ 𝑜 𝑟 𝑡 𝑒 𝑥 𝑎 𝑚 𝑝 𝑙 𝑒 = 18 7 14 17 19 4 23 0 12 15 11 4
54. 54. Classical Encryption Techniques 1  𝑃 = 𝑆 ℎ 𝑜 𝑟 𝑡 𝑒 𝑥 𝑎 𝑚 𝑝 𝑙 𝑒 = 18 7 14 17 19 4 23 0 12 15 11 4
55. 55. Classical Encryption Techniques 1  𝑃 = 𝑆 ℎ 𝑜 𝑟 𝑡 𝑒 𝑥 𝑎 𝑚 𝑝 𝑙 𝑒 = 18 7 14 17 19 4 23 0 12 15 11 4
56. 56. Classical Encryption Techniques 1  𝑃 = 𝑆 ℎ 𝑜 𝑟 𝑡 𝑒 𝑥 𝑎 𝑚 𝑝 𝑙 𝑒 = 18 7 14 17 19 4 23 0 12 15 11 4
57. 57. Classical Encryption Techniques 1  𝑃 = 𝑆 ℎ 𝑜 𝑟 𝑡 𝑒 𝑥 𝑎 𝑚 𝑝 𝑙 𝑒 = 18 7 14 17 19 4 23 0 12 15 11 4
58. 58. Classical Encryption Techniques 1  𝐶 = 𝑎 𝑝 𝑎 𝑑 𝑗 𝑡 𝑓 𝑡 𝑤 𝑙 𝑓 𝑗  This gives us a final ciphertext of "APADJ TFTWLFJ"
59. 59. Classical Encryption Techniques 1  𝐶 = 𝑎 𝑝 𝑎 𝑑 𝑗 𝑡 𝑓 𝑡 𝑤 𝑙 𝑓 𝑗  This gives us a final ciphertext of "APADJ TFTWLFJ“  𝐾 = 𝐻 𝐼 𝐿 𝐿 = 7 8 11 11  We want to find 𝐾−1
60. 60. Classical Encryption Techniques 1  𝑆𝑡𝑒𝑝 1 − 𝐹𝑖𝑛𝑑 𝑡ℎ𝑒 𝑀𝑢𝑙𝑡𝑖𝑝𝑙𝑖𝑐𝑎𝑡𝑖𝑣𝑒 𝐼𝑛𝑣𝑒𝑟𝑠𝑒 𝑜𝑓 𝑡ℎ𝑒 𝐷𝑒𝑡𝑒𝑟𝑚𝑖𝑛𝑎𝑛𝑡 𝐷 𝐾 = 7 ∗ 11 − 8 ∗ 11 = −11 𝑚𝑜𝑑 26 = 15 𝐷𝐷−1 = 1 𝑚𝑜𝑑 26 = 15 ∗ 𝐷−1 15 ∗ 𝐷−1 𝑚𝑜𝑑 26 = 1 Try and Test 1 𝑚𝑜𝑑 26 = 105 105 mod 26 =1 𝐷−1 = 7
61. 61. Classical Encryption Techniques 1  𝑆𝑡𝑒𝑝 2 − 𝐹𝑖𝑛𝑑 𝑡ℎ𝑒 𝐴𝑑𝑗𝑢𝑔𝑎𝑡𝑒 𝑀𝑎𝑡𝑟𝑖𝑥 𝑜𝑓 𝐾𝑒𝑦 𝑎𝑑𝑗 𝑎 𝑏 𝑐 𝑑 = 𝑑 −𝑏 −𝑐 𝑎 𝑎𝑑𝑗 7 8 11 11 = 11 −8 −11 7 mod 26 = 11 18 15 7
62. 62. Classical Encryption Techniques 1  𝑆𝑡𝑒𝑝 3 𝑀𝑢𝑙𝑡𝑖𝑝𝑙𝑦 𝑡ℎ𝑒 𝑀𝑢𝑙𝑡𝑖𝑝𝑙𝑖𝑐𝑎𝑡𝑖𝑣𝑒 𝐼𝑛𝑣𝑒𝑟𝑠𝑒 𝑜𝑓 𝑡ℎ𝑒 𝐷𝑒𝑡𝑒𝑟𝑚𝑖𝑛𝑎𝑛𝑡 𝑏𝑦 𝑡ℎ𝑒 𝐴𝑑𝑗𝑢𝑔𝑎𝑡𝑒 𝑀𝑎𝑡𝑟𝑖𝑥  7 ∗ 11 18 15 7 = 77 126 105 49 𝑚𝑜𝑑 26 = 25 22 1 23 = 𝐾−1
63. 63. Classical Encryption Techniques 1  𝐶 = 𝑎 𝑝 𝑎 𝑑 𝑗 𝑡 𝑓 𝑡 𝑤 𝑙 𝑓 𝑗 = 0 15 0 3 9 19 5 19 22 11 5 9
64. 64. Classical Encryption Techniques 1  𝐶 = 𝑎 𝑝 𝑎 𝑑 𝑗 𝑡 𝑓 𝑡 𝑤 𝑙 𝑓 𝑗 = 0 15 0 3 9 19 5 19 22 11 5 9
65. 65. Classical Encryption Techniques 1  𝐶 = 𝑎 𝑝 𝑎 𝑑 𝑗 𝑡 𝑓 𝑡 𝑤 𝑙 𝑓 𝑗 = 0 15 0 3 9 19 5 19 22 11 5 9
66. 66. Classical Encryption Techniques 1  𝐶 = 𝑎 𝑝 𝑎 𝑑 𝑗 𝑡 𝑓 𝑡 𝑤 𝑙 𝑓 𝑗 = 0 15 0 3 9 19 5 19 22 11 5 9
67. 67. Classical Encryption Techniques 1  𝐶 = 𝑎 𝑝 𝑎 𝑑 𝑗 𝑡 𝑓 𝑡 𝑤 𝑙 𝑓 𝑗 = 0 15 0 3 9 19 5 19 22 11 5 9
68. 68. Classical Encryption Techniques 1  𝐶 = 𝑎 𝑝 𝑎 𝑑 𝑗 𝑡 𝑓 𝑡 𝑤 𝑙 𝑓 𝑗 = 0 15 0 3 9 19 5 19 22 11 5 9
69. 69. Classical Encryption Techniques 1 Using Hill Cipher how to implement 3x3 matrix encryption ? The key for a hill cipher is a matrix e.g. 𝒌 = 2 4 5 9 2 1 3 17 7 and message= ATTACK AT DAWN
70. 70. Classical Encryption Techniques 1  Use MS Word  Send me mail to mloey@live.com with email subject “ Classical Encryption Techniques 1 “  Put your name on Arabic with department and section on word and email body  Finally, press Send  Deadline Next Lecture