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Cryptography CS 555 Topic 1: Course Overview & What is Cryptography Cryptography CS 555 Topic 1: Course Overview & What is Cryptography

Cryptography CS 555 Topic 1: Course Overview & What is Cryptography - PowerPoint Presentation

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Uploaded On 2018-10-10

Cryptography CS 555 Topic 1: Course Overview & What is Cryptography - PPT Presentation

1 Administrative Note Professor Blocki is traveling and will be back on Wednesday Email jblockipurdueedu Thanks to Professor Spafford for covering the first lecture 2 httpswwwcspurdueeduhomesjblockicourses555Spring17indexhtml ID: 687515

cryptography key secret message key cryptography message secret space plaintext bob modern output encryption cipher confidentiality security communication ciphertext

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Slide1

CryptographyCS 555

Topic 1: Course Overview & What is Cryptography

1Slide2

Administrative Note

Professor Blocki is traveling and will be back on Wednesday. E-mail: jblocki@purdue.edu

Thanks to Professor Spafford for covering the first lecture!

2

https://www.cs.purdue.edu/homes/jblocki/courses/555_Spring17/index.html

(also on syllabus)

Slide3

What is Cryptography?

“the art of writing or solving codes” – Concise Oxford English Dictionary

3Slide4

What is Cryptography?

“the art of writing or solving codes” – Concise Oxford English Dictionary

“The study of mathematical techniques for securing digital information, systems and distributed computation against adversarial attacks.”

-- Intro to Modern Cryptography

Late 20

th

century

Art

Science

4Slide5

What Does It Mean to “Secure Information”

Confidentiality (Security/Privacy)Only intended recipient can see the communication

5Slide6

What Does It Mean to “Secure Information”

Confidentiality (Security/Privacy)Only intended recipient can see the communication

Integrity (Authenticity)

The message was actually sent by the alleged sender

Bob

Alice

I love you Alice… - Bob

We need to break up -Bob

6Slide7

Two Attacker Models

Passive AttackerAttacker can eavesdrop Protection Requires?

Confidentiality

Active Attacker

Has full control over communication channel

Protection Requires? Confidentiality & Integrity

7Slide8

Steganography vs Cryptography

SteganographyGoal: Hide existence of a messageInvisible Ink, Tattoo Underneath Hair, …

Assumption: Method is secret

8Slide9

Steganography vs Cryptography

SteganographyGoal: Hide existence of a message

Invisible Ink, Tattoo Underneath Hair, …

Assumption:

Method is secretCryptography

Goal: Hide the meaning of a messageDepends only on secrecy of a (short) keyKerckhoff’s

Principle:

Cipher method should

not be required to be secret.

9Slide10

Symmetric Key Encryption

What cryptography has historically been all about (Pre 1970)Two parties (sender and receiver) share secret key

Sender uses key to encrypt (“scramble”) the message before transmission

Receiver uses the key to decrypt (“unscramble”) and recover the original message

10Slide11

Encryption: Basic Terminology

Plaintext

The original message m

Plaintext Space (Message Space)

The set

of all possible plaintext messagesExample 1:

Example 2:

-

Ciphertext

An encrypted (“scrambled”) message

(

ciphertext

space)

Key/

Keyspace

 

11Slide12

Private Key Encryption Syntax

Message Space:

Key Space:

Three Algorithms

(Key-generation algorithm)

Input: Random Bits R

Output: Secret key

(Encryption

algorithm)

Input: Secret key

and message

Output:

ciphertext

c

(Decryption

algorithm)

Input: Secret key

and a ciphertex

Output: a plaintext message

Invariant: Dec

k

(

Enc

k

(m))=m

 

Typically picks

uniformly at random

 

Trusted Parties (e.g., Alice and Bob) must run Gen in advance to obtain secret k.

Assumption: Adversary does not get to see output of Gen

12Slide13

Cryptography History

2500+ yearsOngoing battleCodemakers

and codebreakers

13

Shannon Entropy/Perfect Secrecy

(~1950)

Caesar Shift Cipher (50 BC)

Frequency Analysis

Cipher Machines (1900s)

1970s

Public Key Crypto/RSA

Formalization of Modern Crypto (1976+)Slide14

Who Uses Cryptography

Traditionally: MilitiasModern Times: Everyone!

14

Revolutionary War

Caesar Shift Cipher (50 BC)

Modern CryptoSlide15

Course Goals

Understand the mathematics underlying cryptographic algorithms and protocolsUnderstand the power (and limitations) of common cryptographic tools

Understand the formal approach to security in modern cryptography

15Slide16

Course Background

Some probabilityAlgorithms and complexityGeneral Mathematical Maturity

Understand what is (is not) a proper definition

Know how to write a proof

16Slide17

Coming Up…

Classic Ciphers + Frequency AnalysisBefore Next Class Read: Katz and Lindell 1.3

Plus Katz and Lindell 1.1-1.2 if you haven’t already

17Slide18

18