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CSCI-100 Introduction to Computing

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CSCI-100 Introduction to Computing Privacy & Security Part I Simplified Communications Model - Diagram Encryption Encryption is the coding and scrambling process by ... – PowerPoint PPT presentation

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Title: CSCI-100 Introduction to Computing


1
CSCI-100 Introduction to Computing
  • Privacy Security
  • Part I

2
  • Simplified Communications Model - Diagram

3
  • Encryption
  • Encryption is the coding and scrambling process
    by which a message is made unreadable except by
    the intended recipient
  • Encryption is needed for electronic commerce
  • Encryption Basics
  • A readable message is called plaintext
  • An encryption algorithm is a formula used to make
    plaintext unreadable
  • The coded message is called ciphertext

I LOVE YOU
V YBIR LBH
4
  • Symmetric key encryption are encryption
    techniques that use the same key to encrypt and
    decrypt a message
  • Strong encryption refers to encryption methods
    that are used by banks and military agencies and
    are nearly impossible to break

5
  • Basic Terminology
  • plaintext - the original message
  • ciphertext - the coded message
  • cipher - algorithm for transforming plaintext to
    ciphertext
  • key - info used in cipher known only to
    sender/receiver
  • encipher (encrypt) - converting plaintext to
    ciphertext
  • decipher (decrypt) - recovering ciphertext from
    plaintext
  • cryptography - study of encryption
    principles/methods
  • cryptanalysis (codebreaking) - the study of
    principles/ methods of deciphering ciphertext
    without knowing key
  • cryptology - the field of both cryptography and
    cryptanalysis

6
  • Symmetric Cipher Model

7
  • Two requirements for secure use of symmetric
    encryption
  • a strong encryption algorithm
  • a secret key known only to sender / receiver
  • Y EK(X)
  • X DK(Y)
  • Assume encryption algorithm is known
  • Implies a secure channel to distribute key

8
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9
  • Classical Substitution Ciphers
  • Where letters of plaintext are replaced by other
    letters or by numbers or symbols
  • Caesar Cipher
  • Earliest known substitution cipher
  • By Julius Caesar
  • First attested use in military affairs
  • Replaces each letter by k-th letter on
  • Example ( what is k ? )
  • meet me after the toga party
  • PHHW PH DIWHU WKH WRJD SDUWB

10
  • Can define transformation (with k 3) as
  • 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
  • D E F G H I J K L M N O P Q R S T U V W X Y Z A B
    C
  • Mathematically give each letter a number
  • a b c d e f g h i j k l m
  • 0 1 2 3 4 5 6 7 8 9 10 11 12
  • n o p q r s t u v w x y Z
  • 13 14 15 16 17 18 19 20 21 22 23 24 25
  • Then have Caesar cipher as
  • Y EK(X) (X k) mod 26
  • X DK(Y) (Y k) mod 26
  • EXAMPLE Encrypt howdy using key k 5

11
  • Cryptanalysis of Caesar Cipher
  • Only have 26 possible ciphers
  • A maps to A,B,..Z
  • Could simply try each in turn
  • A brute force search
  • Given ciphertext, just try all shifts of letters

12
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13
  • Brute Force Search
  • Always possible to simply try every key
  • Most basic attack, proportional to key size
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