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# Computer Security Lecture 3: Classical Encryption Techniques 2

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https://mloey.github.io/courses/security2017.html

We will discuss the following: Polyalphabetic Ciphers, Vigenère Cipher, Autokey Cipher, Vernam Cipher, Transposition Techniques, Rail Fence Cipher, Row Transposition Cipher

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### Computer Security Lecture 3: Classical Encryption Techniques 2

1. 1. Classical Encryption Techniques 2 Classical Encryption Techniques 2
2. 2. Classical Encryption Techniques 2 Polyalphabetic Ciphers Vigenère Cipher Autokey Cipher Vernam Cipher Transposition Techniques Rail Fence Cipher Row Transposition Cipher
3. 3. Classical Encryption Techniques 2 Polyalphabetic Ciphers Vigenère Cipher Autokey Cipher Vernam Cipher Transposition Techniques Rail Fence Cipher Row Transposition Cipher
4. 4. Classical Encryption Techniques 2  A polyalphabetic cipher is any cipher based on substitution, using multiple substitution alphabets.  polyalphabetic cipher techniques have the following features in common: A set of related monoalphabetic substitution rules is used. A key determines which particular rule is chosen for a given transformation.
5. 5. Classical Encryption Techniques 2  Assume  Then We set these Rules:
6. 6. Classical Encryption Techniques 2  Then We set Key as Rules: 1) Shift the first letter three position to the right 2) Shift the second letter five position to the right 3) Shift the third letter seven position to the right  Given Plaintext = security
7. 7. Classical Encryption Techniques 2  Given Plaintext = security 1) Divide Plaintext to three words 2) P= sec uri ty 3) C= VJJ XWP WD
8. 8. Classical Encryption Techniques 2  Then We set Key as Rules: (reverse) 1) Shift the first letter three position to the left 2) Shift the second letter five position to the left 3) Shift the third letter seven position to the left  Given Ciphertext = VJJXWPWD
9. 9. Classical Encryption Techniques 2  Given C= VJJXWPWD 1) Divide Plaintext to three words as your rules number 2) C= VJJ XWP WD 3) C= SEC URI TY
10. 10. Classical Encryption Techniques 2 Polyalphabetic Ciphers Vigenère Cipher Autokey Cipher Vernam Cipher Transposition Techniques Rail Fence Cipher Row Transposition Cipher
11. 11. Classical Encryption Techniques 2 The Vigenère cipher, was invented by a Frenchman, Blaise de Vigenère in the 16th century. Vigenère cipher is a simple polyalphabetic cipher
12. 12. Classical Encryption Techniques 2  𝐶𝑖 = 𝑃𝑖 + 𝐾 𝑚𝑜𝑑 26  𝑃𝑖 = 𝐶𝑖 − 𝐾 𝑚𝑜𝑑 26 Repeating key
13. 13. Classical Encryption Techniques 2  𝐾=deceptive  𝑃=we are discovered save yourself
14. 14. Classical Encryption Techniques 2
15. 15. Classical Encryption Techniques 2  Result
16. 16. Classical Encryption Techniques 2 The strength of Vigenère Cipher is that there are multiple ciphertext letters for each plaintext letter
17. 17. Classical Encryption Techniques 2  decryption simply works in reverse  𝑃𝑖 = 𝐶𝑖 − 𝐾 𝑚𝑜𝑑 26
18. 18. Classical Encryption Techniques 2 Polyalphabetic Ciphers Vigenère Cipher Autokey Cipher Vernam Cipher Transposition Techniques Rail Fence Cipher Row Transposition Cipher
19. 19. Classical Encryption Techniques 2 An autokey cipher (also known as the autoclave cipher) is a cipher which incorporates the message (the plaintext) into the key.  𝑃 = {𝑝1, 𝑝2, 𝑝3, … , 𝑝 𝑛}  𝐾 = {𝑘1, 𝑝1, 𝑝2, 𝑝3, … , 𝑝 𝑛−1}  𝐶 = {𝑐1, 𝑐2, 𝑐3, … , 𝑐 𝑛}
20. 20. Classical Encryption Techniques 2  𝐶𝑖 = 𝑃𝑖 + 𝐾𝑖 𝑚𝑜𝑑 26  𝑃𝑖 = 𝐶𝑖 − 𝐾𝑖 𝑚𝑜𝑑 26
21. 21. Classical Encryption Techniques 2  𝑲=m  𝑷=attack is today Plaintext a t t a c k i s t o d a y P Value 0 19 19 0 2 10 8 18 19 14 3 0 24 Key 12 0 19 19 0 2 10 8 18 19 14 3 0 C Value 12 19 12 19 2 12 18 0 11 7 17 3 24 Ciphertext m t m t c m s a l h r d y
22. 22. Classical Encryption Techniques 2  𝑲=m  𝑪=mtmtcmsalhrdy Ciphertext m t m t c m s a l h r d y C Value 12 19 12 19 2 12 18 0 11 7 17 3 24 Key 12 0 19 19 0 2 10 8 18 19 14 3 0 P Value 0 19 19 0 2 10 8 18 19 14 3 0 24 Plaintext a t t a c k i s t o d a y
23. 23. Classical Encryption Techniques 2 Polyalphabetic Ciphers Vigenère Cipher Autokey Cipher Vernam Cipher Transposition Techniques Rail Fence Cipher Row Transposition Cipher
24. 24. Classical Encryption Techniques 2  Vernam Cipher was introduced by an AT&T engineer named Gilbert Vernam in 1918.  The ultimate defense against such a cryptanalysis is to choose a keyword that is as long as the plaintext and has no statistical relationship to it.
25. 25. Classical Encryption Techniques 2
26. 26. Classical Encryption Techniques 2  Encryption 𝐶 = 𝑃 𝑋𝑂𝑅 𝐾  Decryption 𝑃 = 𝐶 𝑋𝑂𝑅 𝐾
27. 27. Classical Encryption Techniques 2  P=11100011101010101101  K=1001010101  P=11100011101010101101  K=10010101011001010101  C=01110110110011111000
28. 28. Classical Encryption Techniques 2  C=01110110110011111000  K=10010101011001010101  P=11100011101010101101
29. 29. Classical Encryption Techniques 2 Polyalphabetic Ciphers Vigenère Cipher Autokey Cipher Vernam Cipher Transposition Techniques Rail Fence Cipher Row Transposition Cipher
30. 30. Classical Encryption Techniques 2 F...K...T...T...A...W. .L.N.E.S.A.T.A.K.T.A.N ..A...A...T...C...D... Ciphered Text 3 FKTTAW LNESATAKTAN AATCD The clear text message would be encoded using a key of 3. 1 FLANK EAST ATTACK AT DAWN Use a rail fence cipher and a key of 3. 2 The clear text message would appear as follows. Clear Text
31. 31. Classical Encryption Techniques 2 Transposition Techniques performing some sort of permutation on the plaintext letters (reorder the position of letters in plaintext).  Types:  Rail Fence Cipher  Row Transposition Cipher
32. 32. Classical Encryption Techniques 2 Polyalphabetic Ciphers Vigenère Cipher Autokey Cipher Vernam Cipher Transposition Techniques Rail Fence Cipher Row Transposition Cipher
33. 33. Classical Encryption Techniques 2  P= meet me after the toga party  K=2 1) 𝑝 = 𝑚 𝑒 𝑚 𝑒 𝑡 𝑒 𝑎 𝑡 𝑟 𝑓 𝑒 𝑡 ℎ 𝑡 𝑔 𝑒 𝑜 𝑎 𝑝 𝑟 𝑦 𝑎 𝑡 2) C=mematrhtgpryetefeteoaat
34. 34. Classical Encryption Techniques 2  C=mematrhtgpryetefeteoaat  K=2 1) 𝐶 = 𝑚 𝑒 𝑚 𝑒 𝑡 𝑒 𝑎 𝑡 𝑟 𝑓 𝑒 𝑡 ℎ 𝑡 𝑔 𝑒 𝑜 𝑎 𝑝 𝑟 𝑦 𝑎 𝑡 2) P= meetmeafterthetogaparty
35. 35. Classical Encryption Techniques 2 Polyalphabetic Ciphers Vigenère Cipher Autokey Cipher Vernam Cipher Transposition Techniques Rail Fence Cipher Row Transposition Cipher
36. 36. Classical Encryption Techniques 2  P= attack postponed until two am  K= 4312567  𝐶 = 4 3 1 2 5 6 7 𝑎 𝑡 𝑡 𝑎 𝑐 𝑘 𝑝 𝑜 𝑑 𝑤 𝑠 𝑢 𝑜 𝑡 𝑛 𝑎 𝑝 𝑡 𝑚 𝑜 𝑖 𝑥 𝑛 𝑙 𝑥 𝑒 𝑡 𝑥 = 𝑡𝑡𝑛𝑎𝑎𝑝𝑡𝑚𝑡𝑠𝑢𝑜𝑎𝑜𝑑𝑤𝑐𝑜𝑖𝑥𝑘𝑛𝑙𝑥𝑝𝑒𝑡𝑥
37. 37. Classical Encryption Techniques 2  C= 𝑡𝑡𝑛𝑎𝑎𝑝𝑡𝑚𝑡𝑠𝑢𝑜𝑎𝑜𝑑𝑤𝑐𝑜𝑖𝑥𝑘𝑛𝑙𝑥𝑝𝑒𝑡𝑥 = Len(C)=28  K= 4312567  Each Column have 28/7= 4 letter  𝑃 = 4 3 1 2 5 6 7 𝑎 𝑡 𝑡 𝑎 𝑐 𝑘 𝑝 𝑜 𝑑 𝑤 𝑠 𝑢 𝑜 𝑡 𝑛 𝑎 𝑝 𝑡 𝑚 𝑜 𝑖 𝑥 𝑛 𝑙 𝑥 𝑒 𝑡 𝑥 = 𝑎𝑡𝑡𝑎𝑐𝑘𝑝𝑜𝑠𝑡𝑝𝑜𝑛𝑒𝑑𝑢𝑛𝑡𝑖𝑙𝑡𝑤𝑜𝑎𝑚𝑥𝑥𝑥
38. 38. Classical Encryption Techniques 2  As shown in Figure below, use Multiplicative Cipher to encrypt “enemy attack tonight” with key = 4. 𝑪 = 𝑷𝒙𝑲 𝒎𝒐𝒅 𝟐𝟔 𝑲 𝑷𝒍𝒂𝒊𝒏𝒕𝒆𝒙𝒕 Sender Encryption 𝑪
39. 39. Classical Encryption Techniques 2  As shown in Figure below, use Affine Cipher to encrypt “enemy attack tonight” with key pair (4,3). 𝑻 = 𝑷𝒙𝑲 𝟏 𝒎𝒐𝒅 𝟐𝟔 𝑲 𝟏 𝑷𝒍𝒂𝒊𝒏𝒕𝒆𝒙𝒕 𝑪 Sender Encryption 𝑪 = 𝑻 + 𝑲 𝟐 𝒎𝒐𝒅 𝟐𝟔 𝑲 𝟐
40. 40. Classical Encryption Techniques 2  Use MS Word  Send me mail to mloey@live.com with email subject “ Classical Encryption Techniques 2 “  Put your name on Arabic with department and section on word and email body  Finally, press Send  Deadline Next Lecture