Computer Security Lecture 3: Classical Encryption Techniques 2

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Classical Encryption Techniques 2 Classical Encryption Techniques 2

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Classical Encryption Techniques 2 Polyalphabetic Ciphers Vigenère Cipher Autokey Cipher Vernam Cipher Transposition Techniques Rail Fence Cipher Row Transposition Cipher

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Classical Encryption Techniques 2 Polyalphabetic Ciphers Vigenère Cipher Autokey Cipher Vernam Cipher Transposition Techniques Rail Fence Cipher Row Transposition Cipher

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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.

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Classical Encryption Techniques 2  Assume  Then We set these Rules:

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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

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Classical Encryption Techniques 2  Given Plaintext = security 1) Divide Plaintext to three words 2) P= sec uri ty 3) C= VJJ XWP WD

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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

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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

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Classical Encryption Techniques 2 Polyalphabetic Ciphers Vigenère Cipher Autokey Cipher Vernam Cipher Transposition Techniques Rail Fence Cipher Row Transposition Cipher

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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

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Classical Encryption Techniques 2  = + 26  = − 26 Repeating key

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Classical Encryption Techniques 2  =deceptive  =we are discovered save yourself

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Classical Encryption Techniques 2

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Classical Encryption Techniques 2  Result

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Classical Encryption Techniques 2 The strength of Vigenère Cipher is that there are multiple ciphertext letters for each plaintext letter

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Classical Encryption Techniques 2  decryption simply works in reverse  = − 26

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Classical Encryption Techniques 2 Polyalphabetic Ciphers Vigenère Cipher Autokey Cipher Vernam Cipher Transposition Techniques Rail Fence Cipher Row Transposition Cipher

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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.  = {1 , 2 , 3 , … , }  = {1 , 1 , 2 , 3 , … , −1 }  = {1 , 2 , 3 , … , }

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Classical Encryption Techniques 2  = + 26  = − 26

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Classical Encryption Techniques 2  =m  =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

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Classical Encryption Techniques 2  =m  =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

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Classical Encryption Techniques 2 Polyalphabetic Ciphers Vigenère Cipher Autokey Cipher Vernam Cipher Transposition Techniques Rail Fence Cipher Row Transposition Cipher

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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.

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Classical Encryption Techniques 2

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Classical Encryption Techniques 2  Encryption  =  Decryption  =

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Classical Encryption Techniques 2  P=11100011101010101101  K=1001010101  P=11100011101010101101  K=10010101011001010101  C=01110110110011111000

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Classical Encryption Techniques 2  C=01110110110011111000  K=10010101011001010101  P=11100011101010101101

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Classical Encryption Techniques 2 Polyalphabetic Ciphers Vigenère Cipher Autokey Cipher Vernam Cipher Transposition Techniques Rail Fence Cipher Row Transposition Cipher

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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

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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

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Classical Encryption Techniques 2 Polyalphabetic Ciphers Vigenère Cipher Autokey Cipher Vernam Cipher Transposition Techniques Rail Fence Cipher Row Transposition Cipher

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Classical Encryption Techniques 2  P= meet me after the toga party  K=2 1) = ℎ 2) C=mematrhtgpryetefeteoaat

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Classical Encryption Techniques 2  C=mematrhtgpryetefeteoaat  K=2 1) = ℎ 2) P= meetmeafterthetogaparty

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Classical Encryption Techniques 2 Polyalphabetic Ciphers Vigenère Cipher Autokey Cipher Vernam Cipher Transposition Techniques Rail Fence Cipher Row Transposition Cipher

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Classical Encryption Techniques 2  P= attack postponed until two am  K= 4312567  = 4 3 1 2 5 6 7 =

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Classical Encryption Techniques 2  C= = Len(C)=28  K= 4312567  Each Column have 28/7= 4 letter  = 4 3 1 2 5 6 7 =

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Classical Encryption Techniques 2  As shown in Figure below, use Multiplicative Cipher to encrypt “enemy attack tonight” with key = 4. = Sender Encryption

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Classical Encryption Techniques 2  As shown in Figure below, use Affine Cipher to encrypt “enemy attack tonight” with key pair (4,3). = Sender Encryption = +

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Classical Encryption Techniques 2  Use MS Word  Send me mail to [email protected] 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

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Classical Encryption Techniques 2 facebook.com/mloey [email protected] twitter.com/mloey linkedin.com/in/mloey [email protected] mloey.github.io