Cryptography-Known plain text attack

Brief description about cryptanalysis , cryptography and known plain text attack.

1. KNOWN PLAIN TEXT CRYPTANALYSIS PRESENTED BY:- AMITESH GAURAV-125111 SATYAM AGARWAL-125112 ASHOK RAJAK-125113 SPANDAN MAJI-125114 SUBHAM MUKHERJEE-125115

2. What is cryptography? Cryptography enables you to store sensitive information or transmit it across insecure networks so that it cannot be read by anyone except the intended recipient. Cryptography concerns itself with the following four objectives: 1)  Confidentiality   2 )  Integrity 3 )  Non-repudiation   4) Authentication

3. Encryption & Decryption

4. Cipher Text and Plain Text Cipher text  is also known as encrypted or encoded information because it contains a form of the original plaintext  that is unreadable by a human or computer without the proper cipher to decrypt it. Decryption, the inverse of encryption, is the process of turning cipher text  into readable  plaintext .

5. Transmission technique

6. Public Key and Private Key In  cryptography , a  key  is a piece of information (a parameter) that determines the functional output of a  cryptographic  algorithm or cipher. Without a  key , the algorithm would produce no useful result . A cryptographic system that uses two  keys  -- a  public key  known to everyone and a private or secret  key  known only to the recipient of the message. When John wants to send a secure message to Jane, he uses Jane's  public key  to  encrypt  the message. Jane then uses her private key  to decrypt it .

7. What is cryptanalysis? Cryptanalysis refers to the study of ciphers, cipher text, or cryptosystems (that is, to secret code systems) with a view to finding weaknesses in them that will permit retrieval of the plaintext from the cipher text, without necessarily knowing the key or the algorithm.

8. Role of Cryptanalysis The cryptographer's goal is to provide security for information by developing strong cryptosystems, while the cryptanalyst's goal is to discover weaknesses or flaws in cryptosystems and break the security provided by those systems . Cryptanalysts can use powerful computing equipment and a variety of procedures, processes, and techniques to launch attacks against cryptosystems. A good cryptanalyst can even determine plaintext from samples of ciphertext without even knowing the cipher that was used to produce it.

9. What is an Attack? In computer  and  computer networks  an  attack  is any attempt to destroy, expose, alter, disable, steal or gain unauthorized access to or make unauthorized use of an asset. An attack should lead to a  security incident  i.e. a  security event  that involves a  security violation .

10. Active and Passive Attack An active attack is a network attack characterized by the attacker attempting to break into the system. During an active attack, the intruder will introduce data into the system as well as potentially change data within the system. A passive attack is a network attack in which the purpose is solely to gain information about the target and no data is changed on the target.

11. Types of Attacks on Encrypted messages Known Plaintext Attack Chosen Plaintext Attack

12. Known Plain Text Attack The  known-plaintext attack (KPA)  is an attack model for cryptanalysis where the attacker has access to both the plaintext  and its encrypted version (cipher text).   In cryptography, the known plaintext attack, or KPA, is an attack based on having samples of both the plaintext and corresponding encrypted or cipher text for that information available.

13. Understanding Known Plain Text Attack Alice sends a message to Bob encrypted with his public key. Eve overhears an encrypted communication from Bob to Alice, and later observes them meeting at Baker Street . Eve can now guess that the communication contained the word "baker street" somewhere, a form of known plaintext attack .

14. Known Plain Text Attack (Contd.)

15. Known Plain Text Attack (Contd.)

16. An Example We have, Ciphertext : PQ Plaintext: if Formula: Y = aX + b ( Y is cipher, X is plain) Two equations: 16 = a.9 + b ( i P) 17 = a.6 + b (f Q)

17. An Example 16 = a.9 + b 17 = a.6 + b -1 = a.3 + 0 ( by subtracting) 25 (mod 26) = a.3 a = 25/3 a= 25.(3^-1) a = 25.9 as 3^-1 (mod 26) = 9 using Euclidean algorithm a = 17 (mod 26) Also by solving two equations we get, b = 9 (mod 26) So, we get the final equation Y = 17.X + 9 Key is (17,9)

18. Controlling Plain Text Attack Reduce frequent word in the message Reduce special word or phrase in the message Less number of pairs of plaintext- ciphertext Minimize validity of key

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