Cryptography

Cryptography

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  CRYPTOGRAPHY Ms. DateSaroj ME CSE (Part Time )
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  CRYPTOGRAPHY comes fromthe Greek words for “ secret writing”. It is the science of devising methods that allow information to be sent in a secure form in such a way that the only person, who possess the key can read it. It is the collection of algorithm & associated procedures for hiding & unhiding information
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  Plaintext :Messages tobe encrypted. Cipher text: Output of encryption process. Key Intruder( Active & Passive) Channel
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  Encryption Model, E Channel Decryption Model, D Introduction to Cryptography Ek DK Intruder p L A I N T E XT P L A I N T E X T CIPHERTEXT Active Intruder Passive Intruder FIG. THE ENCRYPTION MODEL (For a Symmetric-Key Cipher)
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  The messages tobe encrypted ,known as Plaintext , are transformed by a key. The output of encryption process, known as Cipher text , is the transmitted through the channel . We assume that the intruder hears & accurately copies down the complete Cipher text. However, unlike the intended receiver, the intruder does not know what the decryption key is and so can’t decrypt the Cipher text easily. Cryptography
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  Cryptography Passive intruders can only listen to the communication channel Active intruders can modify the legitimate messages before they get to the receiver. Cryptanalysis: The art of breaking ciphers. Cryptography: The art of devising ciphers. & Cryptology means ----------- CRYPTOLOGY CRYPTANALYSIS CRYPTOGRAPHY = +
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  It will beoften useful to have a notation for relating plaintext, cipher text & Keys. Let’s assume that ,the plaintext has n characters , drawn from an alphabet X. We represent the plaintext msg. as a vector of n symbols— X={x1,x2,x3,……xn} Before transmitting, the sender Encrypts the msg. This is done by a rule that maps X into a sequence of new characters called as Cipher text. Y={y1,y2,y3,…..,yn} Notation:
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  The mapping fromX to Y is based on an encryption rule ,which we may write as Y=E k (X) The subscript k (Key) denotes a particular rule, employed in a ciphersystem to determine the cipher text. The cipher text msg. is transmitted to the receiver . In order to read the original msg.,the receiver must use another rule ,called a decryption rule . Notation: Contd…..
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  This decryption rulecan be looked as an inverse mapping defined by encryption rule. X=Dk (Y) Notation: Contd……. Y = Ek (X) X = Dk (Y) X = Dk(Ek(X))
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  Let’s look atthe methods by which cryptosystems can be attacked: 1. Cipher text-only attack 2. Known plaintext attack. 3. Chosen plaintext attack 4. Chosen cipher text attack. Regardless of any attack intruder’s task is to determine the key. If the key is known, the cryptosystem is broken.
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  Encryptions methods havehistorically been divided into two categories: 1. Substitution Ciphers 2.Transposition Ciphers
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  In a substitutioncipher each letter or group of letters is replaced by another letter or group of letters. One of the oldest known ciphers is the Caesar Cipher . In this method a becomes D, b becomes E,….. , z becomes C. For example attack becomes DWWDFN. A generalization is shift cipher text alphabets by k letters.
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  Transposition Ciphers Substitutionciphers preserves the order of alphabets. Transposition ciphers reorder the letters .
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  Some simple ciphersystems 1. The shift Cipher 2. The Vigenere Cipher 3. The permutation Cipher 4. The Substitution Cipher
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  Codes It isalso possible to increase the security of encryption by encrypting the entire words, phrases, sentences, before the applying to the encryption model. Secret codebook is shared between sender & receiver. Even if the cipher has been broken , Intruder will still face the problem of decoding it. Provides data compression to the plaintext.
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  Codes contd….The combination of coding & ciphering, if used properly, can produce a cryptosystem that is far more difficult to break than a cryptosystem that uses either. DISADVANTAGE To prevent from the attacks, the codebook needs to be changed frequently. But changing & securely communicating codebooks is difficult challenge. So, codes are not well adapted in modern communication systems.
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  THANK YOU !........