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Chapter:1 Introduction

Chapter:1 Introduction. Rachana Y. Patil. What is “Security”. Freedom from risk or danger; safety. - Freedom from doubt, anxiety, or fear; confidence. Something that gives or assures safety, etc…. Why do we need security?.

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Chapter:1 Introduction

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  1. Chapter:1Introduction Rachana Y. Patil

  2. What is “Security” • Freedom from risk or danger; safety. - Freedom from doubt, anxiety, or fear; confidence. • Something that gives or assures safety, etc…..

  3. Why do we need security? • Protect vital information while still allowing access to those who need it • Trade secrets, medical records, etc. • Provide authentication and access control for resources • Guarantee availability of resources

  4. Security Goals Confidentiality Secure Integrity Avalaibility

  5. Goal: Keep the contents of communication or data on storage secret Example: Alice and Bob want their communications to be secret from Eve Confidentiality Bob Alice

  6. Integrity Changes need to be done only by authorized entities and through authorized mechanisms

  7. Availability • The information created and stored by an organization needs to be available to authorized entities

  8. ATTACKS

  9. Attacks threatening confidentiality • In general, two types of attack threaten the confidentiality of information: • Snooping :- refers to unauthorized access to or • interception of data. • Traffic analysis :-refers to obtaining some other type of • information by monitoring online • traffic.

  10. Attacks threatening Integrity Modification : the attacker intercepts the message and changes it.Masquerading or spoofing happens when the attacker impersonates somebody else. Replaying : the attacker obtains a copy of  a message sent by a user and later tries to replay it. Repudiation : the sender of the message might later deny that she has sent the message; the receiver of the message might later deny that he has received the message.

  11. Attacks threatening Availability Denial of service (DoS) attacks may slow down or totally interrupt the service of a system. The attacker can use several strategies to achieve this. They might make the system so busy that it collapses, or They might intercept messages sent in one direction and make the sending system believe that one of the parties involved in the communication or message has lost the message and that it should be resent

  12. Passive vs Active Attacks

  13. Security services Standards have been defined for security services to achieve security goals and prevent security attacks.

  14. Techniques The actual implementation of security goals needs some help from mathematics. Two techniques are prevalent today: one is very general—cryptography—and one is specific—steganography. Cryptography Some security services can be implemented using cryptography. Cryptography, a word with Greek origins, means “secret writing”. Steganography The word steganography, with its origin in Greek, means “covered writing”, in contrast to cryptography, which means “secret writing”.

  15. Cryptography Imagine two people wanting to communicate with each other secretly . Alice want to send a message to bob that no body else can read Meet me At noon Bob Alice Alice must somehow transform her message called plaintext so that an adversary can not read it

  16. Bob Alice Meet me At noon Encrypt Alice uses an encryption algorithm to transform her plaintext Message into cipher text. Cipher text is a scramble form of her original message that hopefully prevents an adversary from eavesdropping.

  17. Key Bob Alice Meet me At noon Encrypt Phhw ph dw qrrq In order to encrypt the message, the encryption algorithm Takes as input both the plaintext of the original message, plus a key that tells the algorithm how to “scramble” the plaintext into cipher text

  18. Key Bob Alice Meet me At noon Encrypt Phhw ph dw qrrq There is just one problem .the message has been encrypted to prevent adversaries from evesdroping,but now bob can’t read the message. what we need is a way to retrieve the original plaintext message from the cipher text

  19. Key Bob Key Alice Meet me At noon Meet me At noon Phhw ph dwqrrq Encrypt Decrypt Both Alice and Bob use the same algorithm to encrypt and decrypt. They both use a key, that must be kept secrate,to transform their communication. If any one else discovers the key, their communications would be compromised.

  20. Basic Terminology • Cryptography • The art or science encompassing the principles and methods of transforming message an intelligible into one that is unintelligible, and then retransforming that message back to its original form • Plaintext • The original intelligible message • Ciphertext • The transformed message • Cipher • An algorithm for transforming an intelligible message into one that is unintelligible by transposition and/or substitution methods • Key • Some critical information used by the cipher, known only to the sender & receiver

  21. Basic Terminology • Encipher (encode) • Process of converting plaintext to ciphertext using a cipher and a key • Decipher (decode) • The process of converting ciphertext back into plaintext using a cipher and a key • Cryptanalysis (code breaking) • The study of principles and methods of transforming an unintelligible message back into an intelligible message without knowledge of the key. • Cryptology • The field encompassing both cryptography and cryptanalysis

  22. Basic Terminology • Encryption • The mathematical function mapping plaintext to ciphertext using the specified key: Y = EK(X) or E(K, X) • Decryption • The mathematical function mapping ciphertext to plaintext using the specified key: X = DK(Y) or D(K, X) = EK-1(Y)

  23. Symmetric Key Cipher General idea of symmetric-key cipher

  24. Continued If P is the plaintext, C is the ciphertext, and K is the key, We assume that Bob creates P1; we prove that P1 = P:

  25. Continued Locking and unlocking with the same key

  26. Kerckhoff’s Principle Based on Kerckhoff’s principle, one should always assume that the adversary, Eve, knows the encryption/decryption algorithm. The resistance of the cipher to attack must be based only on the secrecy of the key.

  27. Cryptanalysis As cryptography is the science and art of creating secret codes, cryptanalysis is the science and art of breaking those codes. Cryptanalysis attacks

  28. Continued Ciphertext-Only Attack Ciphertext-only attack

  29. Ciphertext-Only Attack Brute force Statistical attack Pattern attack

  30. Continued Known-Plaintext Attack Known-plaintext attack

  31. Continued Chosen-Plaintext Attack Chosen-plaintext attack

  32. Continued Chosen-Ciphertext Attack Chosen-ciphertext attack

  33. Categories of Traditional Ciphers • Substitution Cipher • Replace one symbol with another • Transposition Cipher • Reorders Symbols

  34. Substitution Cipher • Mono-alphabetic Substitution • The relationship between symbols in plain text to a symbols In cipher text is always one to one • Poly-alphabetic substitution cipher • Each occurrence of a character may have different substitution • The relationship between symbols in plain text to a symbols In cipher text is always one to many

  35. Mono-alphabetic Substitution • Additive Cipher • Multiplicative Cipher • Affine Cipher • Monoaplhabetic Substitution Cipher

  36. Additive Cipher / Shift Cipher / Caesar Cipher Key k Key K Bob Alice Plaintext P Plaintext P Cipher text C C=(P+K) mod 26 P=(C-K) mod 26 Encryption Decryption

  37. Additive Cipher / Shift Cipher / Caesar Cipher • EXAMPLE Plaintext = apple Key = 5 Ciphertext=? Plaintext a00 Encryption(00+05) mod 26 Ciphertext: 05 F Plaintext p15 Encryption(15+05) mod 26 Ciphertext: 20 U Plaintext p15 Encryption(15+05) mod 26 Ciphertext: 20 U Plaintext l 11 Encryption(11+05) mod 26 Ciphertext: 15 P Plaintext e 04 Encryption(04+05) mod 26 Ciphertext: 09 J Ciphertext = FUUPJ

  38. Additive Cipher / Shift Cipher / Caesar Cipher • EXAMPLE Ciphertext = FUUPJ Key = 5 Plaintext=? • EXAMPLE • Plaintext = hello • Key = 15 • Ciphertext=?

  39. Multiplicative Cipher Key k Key K Bob Alice Plaintext P Plaintext P Cipher text C C=(P*K) mod 26 P=(C*K-1) mod 26 Encryption Decryption

  40. Affine Cipher Key k1 Key K2 Alice Plaintext P T C=(P*K1) mod 26 P=(C+K2) mod 26 Ciphertext C Encryption Bob Plaintext P C=(P*K-1) mod 26 P=(C-K2) mod 26 Decryption Key k1 Key K2

  41. Affine Cipher • EXAMPLE plaintext = hello Key pair = (7,2) Ciphertext=?

  42. Monoaplhabetic Substitution Cipher • The shift pattern could be random assignment of characters for each alphabet EX. • This would also give 26! possibilities A B C D E F G H I J K L M N O P Q R S T U V W X Y Z P M J S Q O L E Y T V U A X I K C G B W D R N H Z F

  43. Poly-alphabetic substitution cipher • AutoKey Cipher • Playfair Cipher • Vigenere Cipher

  44. Autokey cipher • In this cipher the key is a stream of subkeys,in which each sub key is use to encrypt the corresponding character in the plaintext • The first sub key is predetermined value secretly agreed upon by Alice and Bob. • The second sub key is the value of first plaintext character P=P1P2p3……. C=C1C2C3…….. K=(K1,P1,P2,…..) Encryption : Ci = (Pi+Ki) mod 26 Decryption : Pi = (Ci-Ki)mod 26

  45. playfair cipher • Best-known multiple-letter substitution cipher • Digram cipher (digram to digram, i.e., E(pipi+1) = cici+1 through keyword-based 5x5 transformation table) • Great advance over simple monoalphabetic cipher (26 letters  26x26=676 digrams)

  46. playfair cipher • Before encryption ,if two letters in the pair are same, a bogus letter is inserted to separate them • After inserting a bogus letter if the number of character in the plaintext is odd, one extra bogus character is added at the end of the string • The cipher uses three rules for encryption 1. If the two letter in the pair are located in same row of the key, then The corresponding encrypted character for each letter is the next letter to the right in the same row

  47. playfair cipher 2. If two letter in the pair are in the same column of the key ,the corresponding encrypted character for each letter is the letter beneath 3. If two letter in the pair are are not in the same row or column of the key ,the corresponding encrypted character for each letter is the letter that is in it’s own row but in the same column as the other letter.

  48. playfair cipher • EXAMPLE plaintext = hello Cipher text=? Key= Plaintext : hello Add bogus character helxlo he lx lo he  EC lx  QZ lo  BX Ciphertext : ECQZBX

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