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
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CryptographyCS 555
Topic 1: Course Overview & What is Cryptography
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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)
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What is Cryptography?
“the art of writing or solving codes” – Concise Oxford English Dictionary
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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
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What Does It Mean to “Secure Information”
Confidentiality (Security/Privacy)Only intended recipient can see the communication
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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
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Two Attacker Models
Passive AttackerAttacker can eavesdrop Protection Requires?
Confidentiality
Active Attacker
Has full control over communication channel
Protection Requires? Confidentiality & Integrity
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Steganography vs Cryptography
SteganographyGoal: Hide existence of a messageInvisible Ink, Tattoo Underneath Hair, …
Assumption: Method is secret
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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.
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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
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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
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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
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Cryptography History
2500+ yearsOngoing battleCodemakers
and codebreakers
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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!
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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
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Course Background
Some probabilityAlgorithms and complexityGeneral Mathematical Maturity
Understand what is (is not) a proper definition
Know how to write a proof
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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
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