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# Encryptolog y-1216310707267721-9

## on Aug 23, 2011

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## Encryptolog y-1216310707267721-9Presentation Transcript

• Cryptology and Its Applications
• Presented By
• CSC STUDENTS
[ ]
• [ ] Introduction
• The science of cryptology is the science of secure communications, formed from the Greek
• words kryptós , &quot;hidden&quot;, and logos , &quot;word&quot;.
• Useful Terminology
• CIPHERTEXT
• CODE
• CRYPTANALYSIS
• CRYPTOGRAPHY
• CRYPTOSYSTEM
• DECRYPTION
• ENCRYPTION
• Cryptography
• PRODUCT CIPHERS
[ ] A D F G B X A S U B J E C D T A D F G H F I K L M N O G P Q R V W X V Y Z 0 1 2 3 X 4 5 6 7 8 9 M E R C H A N T T A Y L O R S FG AV GF AX DX DD FV DA DA DD VA FF FX GF AA
• Cryptography
• Block Ciphers
• It is a type of symmetric-key encryption algorithm that changes a fixed length of block text into same length of Cipher text.
• Encryption works by means of key and Decryption is the reverse of encryption process using the same key.
• Stream Ciphers
• The encryption method is based on simple XOR of binary bits. It encrypts one bit at a time.
• The key sequence is generated on highly complex mathematical algorithms. LFSR based on primitive polynomials and combination of several LFSR using linear and nonlinear logic provides a very high level of complexity.
[ ]
• Cryptography
• Public Key Cryptography
• RSA Algorithm
• In this system a user chooses a pair of prime numbers so large that factorizing the product is beyond all computing capabilities.
• Stages of sending the message by this method.
• The receiver, M, distributes his public key pair.
• The sender, F, composes a plain text message, m , and then uses Ms public key to encrypt the message and from cipher text, c . c is the remainder left when m is raised to the power of e and divided by the modulus n .
[ ]
• Cryptography
• c = me mod n (where e and n are Ms public key pair).
• F sends the ciphertext, c , to M.
• The receiver, M, decrypts the ciphertext and retrieves the plaintext message, m . m is the remainder obtained when c is raised to the power of d and divided by n .
• m = cd mod n
• As you can see, this process requires d , which only M knows. Another person, I, who intercepts the message, can not decrypt it.
• Example:
• Let p = 5, q = 11, n = pq = 55
• The least common multiple of ( p-1 )( q-1 ) is 20 = 22 ´ 5.
[ ]
• Cryptography
• Therefore, in this case, any key, e , not divisible by 2 or 5 will have a matching key, d ( e must be relatively prime to ( p-1 )( q-1 ) for it to be the key).
• Let e = 7
• (ed -1) mod (p-1)(q-1) = 0 d = 3
• Let the plaintext message, m = b = 2
• The ciphertext, c = me mod n = 27 mod 55 = 18
• To decrypt this information it is necessary to know d.
• m’ = cd mod n = 183 mod 55 = 2 The message has been successfully decrypted.
[ ]
• Cryptography in the &quot;Real World&quot;
• Applications of Cryptography
• military and diplomatic services
• protecting confidential company information, telephone calls
• lottery tickets
[ ]
• Cryptanalysis
• Unlike cryptography which is a clearly defined science, cryptanalysis is as
• much an art as it is a science.
• Types of Cryptanalysis: The type used depends on the type of cipher and how much information the cryptanalyst has .
• Types Of Cryptanalytic Attacks
• A brute force attack
• Dictionary attack
• Ciphertext only attack
• Chosen plaintext
[ ]
• Cryptanalysis
• A Triumph of Cryptanalysis – Enigma
• What Was Enigma?
• The Enigma machine consisted of a 26 letter keyboard for
• input. The output was read off 26 lamps which each
• corresponded to a letter. The encipherment was performed
• by a device called a &quot;scrambler&quot; that was made of three
• rotating wheels on a common spindle and a plug board
• security.
[ ]
• Cryptanalysis