cryptography_priceton_university_fall_2007.ppt

Princeton University • COS 433 • Cryptography • Fall 2007 • Boaz Barak
COS 433: Cryptography
Princeton University
Fall 2007
Boaz Barak
Two important quick notes:
 Slides will be on course web site
 Please stop me if you have questions!

2
Cryptography
History of 2500- 4000 years.
Recurring theme: (until 1970’s)
 Secret code invented
 Typically claimed “unbreakable” by inventor
 Used by spies, ambassadors, kings, generals for crucial tasks.
 Broken by enemy using cryptanalysis.
Throughout most of this history:
cryptography = “secret writing”:
“Scramble” (encrypt) text such that it is hopefully unreadable by
anyone except the intended receiver that can decrypt it.

3
Examples
1587: Ciphers from Mary of Scots plotting assassination of queen
Elizabeth broken; used as evidence to convict her of treason.
1860’s (civil war): Confederacy used good cipher (Vigenere) in a bad
way. Messages routinely broken by team of young union
cryptanalysts; in particular leading to a Manhattan manufacturer of
plates for printing rebel currency.
1878: New York Tribune decodes telegram proving Democrats’ attempt
to buy an electoral vote in presidential election for $10K.
1914: With aid of partial info from sunken German ships, British
intelligence broke all German codes.
Cracked telegram of German plan to form alliance with Mexico and
conquer back territory from U.S. As a result, U.S. joined WWI.
WWII: Cryptanalysis used by both sides. Polish & British cryptanalysts
break supposedly unbreakable Enigma cipher using mix of
ingenuity, German negligence, and mechanical computation.
Churchill credits cryptanalysts with winning the war.

4
This Course
What you’ll learn:
 Foundations and principles of the science
 Definitions and proofs of security
 High-level applications
 Critical view of security suggestions and products
What you will not learn:
 The most efficient and practical versions of components.
 Designing secure systems*
 “Hacking” – breaking into systems.
 Everything important about crypto
 Basic primitives and components.
 Viruses, worms, Windows/Unix bugs, buffer overflow etc..
 Buzzwords
Will help you avoid designing
insecure systems.

5
This Course
Modern (post 1970’s) cryptography:
Provable security – breaking the “invent-break-tweak” cycle
 Perfect security (Shannon) and its limitations
 Computational security
 Pseudorandom generators, one way functions
Beyond encryption – public-key crypto and other wonderful creatures
 Public-key encryption based on factoring and RSA problem
 Digital signatures, hash functions
 Zero-knowledge proofs
 Active security – Chosen-Ciphertext Attack
Advanced topics (won’t have time for all  )
 The SSL Protocol and attacks on it
 Secret Sharing
 Multi-party secure computation
 Quantum cryptography
 Password-based key-exchange, broadcast encryption, obfuscation

6
Administrative Info
Lectures: Tue,Thu 1:30-2:50pm (start on time!)
Instructor: Boaz Barak: boaz@cs
Web page: http://www.cs.princeton.edu/courses/archive/fall07/cos433/
Or: Google “Boaz Barak” and click “courses”
TA: Rajsekar Manokaran ( rajsekar@cs )
Important: join mailing list, email me to set appointment
before next class
Office hrs: Thu after class (3pm) or by appointment.
Precepts: ---
Office hrs: ---

7
Prerequisites
1. Ability to read and write mathematical proofs and definitions.
2. Familiarity with algorithms – proving correctness and
analyzing running time (O notation).
Required:
Helpful but not necessary:
Complexity. NP-Completeness, reductions, P, BPP, P/poly
Probabilistic Algorithms. Primality testing, hashing,
Number theory. Modular arithmetic, prime numbers
See web-site for links and resources.
3. Familiarity with basic probability theory (random variables,
expectations – see handout).

8
Reading
Foundations of Cryptography / Goldreich.
Graduate-level text, will be sometimes used.
Lecture notes on web: (links on web site)
Computational Intro to Algebra and Number Theory / Shoup.
(Available also on the web)
Introduction to the Theory of Computation / Sipser.
For complexity background
Introduction to Modern Cryptography / Katz & Lindell
Main text used, though not 100% followed

9
Requirements
Exercises: Weekly from Thursday till Thursday before class.
Submit by email / mailbox / in class to Rajsekar.
Flexibility: 4 late days, bonus questions
Take home final.
Final grade: 50% homework, 50% final
Honor code. Collaboration on homework with other students encouraged.
However, write alone and give credit.
Work on final alone and as directed.

10
This course is hard
 Challenging weekly exercises
 Emphasis on mathematical proofs
 Counterintuitive concepts.
 Extensive use of quantifiers/probability
But it’s not my fault :)
 Good coverage of crypto (meat, vegetables and desert) takes a year.
 Simulation / experimentation can’t be used to show security.
 Need to acquire “crypto-intuition”
 Quantifiers, proofs by contradiction, reductions, probability are inherent.
Mitigating hardness
 Avoid excessive exercises – only questions that teach you something.
 Try best to explain intuition behind proofs
 Me and Rajsekar available for any questions and clarifications.

11
Encryption Schemes
Alice wants to send Bob a secret message.
They agree in advance on 3 components:
 Encryption algorithm: E
 Decryption algorithm: D
 Secret key: k
To encrypt plaintext m, Alice sends c = E(m,k) to Bob.
To decrypt a cyphertext c, Bob computes m’ = D(c,k).
c = E(m,k)
c
m’ = D(c,k)
 A scheme is valid if m’=m
 Intuitively, a scheme is secure if eavesdropper can not learn m from c.

12
Example 1: Caesar’s Cipher
Key: k = no. between 0 and 25.
Encryption: encode the ith letter as the (i+k) th letter.
(working mod 26: z+1=a )
Decryption: decode the jth letter to the (j-k) th letter.
S E N D R E I N F O R C E M E N T
Plain-text:
Key: 2
Cipher-text: U G P F T F K P H Q T E G O G P V
Problem: only 26 possibilities for key – can be broken in short time.
In other words: “security through obscurity” does not work.
Kerchoff’s Principle (1883): System should be secure even if
algorithms are known, as long as key is secret.

13
Example 2: Substitution Cipher
Key: k = table mapping each letter to another letter
A B C Z
U R B E
Encryption and decryption: letter by letter according to table.
# of possible keys: 26! ( = 403,291,461,126,605,635,584,000,000 )
However – substitution cipher is still insecure!
Key observation: can recover plaintext using statistics on letter
frequencies.
LIVITCSWPIYVEWHEVSRIQMXLEYVEOIEWHRXEXIPFEMVEWHKVSTYLX
ZIXLIKIIXPIJVSZEYPERRGERIMWQLMGLMXQERIWGPSRIHMXQEREKI
He e e e h e t t ht
ethe eet e e h h t e e t e
I – most common letter
LI – most common pair
XLI – most common triple
Here e r e h e t t r r ht
ethe eet e r e h h t e e t e
I=e L=h X=t
Here e ra a e ha a ea tat a ra r ht
ethe eet e r a a e h h t a e e t a a e
V=r E=a Y=g
HereUpOnLeGrandAroseWithAGraveAndStatelyAirAndBrought
MeTheBeetleFromAGlassCaseInWhichItWasEnclosedItWasABe

14
Example 3- Vigenere
“Multi-Caesar Cipher” – A statefull cipher
Key: k = (k1,k2,…,km) list of m numbers between 0 and 25
Encryption: 1st letter encoded as Caesar w/ key=k1 : i  I + k1 (mod 26)
2nd letter encoded as Caesar w/ key=k2 : i  I + k2 (mod 26)
mth letter encoded as Caesar w/ key=km : i  I + km (mod 26)
m+1th letter encoded as Caesar w/ key=k1 : i  I + k1 (mod 26)
Decryption: In the natural way
…
Important Property: Can no longer break using letter frequencies alone.
‘e’ will be mapped to ‘e’+k1,‘e’+k2,…,‘e’+km according to location.
nth letter encoded w/ key=k(n mod m) : i  I + k(n mod m) (mod 26)
Considered “unbreakable” for 300 years (broken by Babbage, Kasiski 1850’s)
(Belaso, 1553)

15
Example 3- Vigenere
Encryption:
Breaking Vigenere:
nth letter encoded w/ key=k(n mod m) : i  I + k(n mod m) (mod 26)
(Belaso, 1553)
LIVITCSWPIYVEWHEVSRIQMXLEYVEOIEWHRXEXIPFEM VEWHKV
Step 1: Guess the length of the key m
Step 2: Group together positions {1, m+1, 2m+1, 3m+1,…}
{m-1, 2m+m-1, 3m+m-1,…}
…
{2, m+2, 2m+2, 3m+2,…}

16
Example 3- Vigenere
Encryption:
Breaking Vigenere:
nth letter encoded w/ key=k(n mod m) : i  i + k(n mod m) (mod 26)
(Belaso, 1553)
LIVITC
SWPIYV
EWHEVS
RIQMXL
EYVEOI
EWHRXE
XIPFEM
VEWHKV
Step 1: Guess the length of the key m
Step 2: Group together positions 1, m+1, 2m+1, 3m+1,…
Step 3: Frequency-analyze each group independently.
{m-1, 2m+m-1, 3m+m-1,…}
…
{2, m+2, 2m+2, 3m+2,…}

17
Example 4 - The Enigma
A mechanical statefull cipher.
Roughly: composition of 3-5 substitution ciphers
implemented by wiring.
Wiring on rotors moving in different schedules,
making cipher statefull
Key: 1) Wiring of machine (changed infrequently)
2) Daily key from code books
3) New operator-chosen key for each message
Tools used by Poles & British to break Enigma:
1) Mathematical analysis combined w/ mechanical computers
2) Captured machines and code-books
3) German operators negligence
4) Known plaintext attacks (greetings, weather reports)
5) Chosen plaintext attacks
Used by Germany in WWII for top-secret communication.

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Post 1970’s Crypto
Two major developments:
1) Provably secure cryptography
Encryptions w/ mathematical proof that are unbreakable*
* Currently use conjectures/axioms,
however defeated all cryptanalysis effort so far.
2) Cryptography beyond “secret writing”
Public-key encryptions
Digital signatures
Zero-knowledge proofs
Anonymous electronic elections
Privacy-preserving data mining
e-cash
…

19
Review of Encryption Schemes
 Secret key: k
To encrypt m, Alice sends c = E(m,k) to Bob.
To decrypt c, Bob computes m’ = D(c,k).
c = E(m,k)
c
m’ = D(c,k)
Q: Can Bob send Alice the secret key over the net?
A: Of course not!! Eve could decrypt c!
Q: What if Bob could send Alice a “crippled key”
useful only for encryption but no help for decryption

20
Public Key Cryptography [DH76,RSA77]
To encrypt m, Alice sends c = E(m,e) to Bob.
To decrypt c, Bob computes m’ = D(c,d).
c = E(m,e)
c
m’ = D(c,d)
 Key: Bob chooses two keys:  Secret key d for decrypting messages.
 Public key e for encrypting messages.
choose d,e
e
Should be safe to send e “in the clear”!
 A scheme is valid if m’=m
 Intuitively, a scheme is secure if eavesdropper can not learn m from c.
Even if Eve knows the key e!

21
Other Crypto Wonders
Digital Signatures. Electronically sign documents in unforgeable way.
Zero-knowledge proofs. Alice proves to Bob that she earns <$50K
without Bob learning her income.
Privacy-preserving data mining. Bob holds DB. Alice gets answer to one
query, without Bob knowing what she asked.
Playing poker over the net. Alice, Bob, Carol and David can play poker
over the net without trusting each other or any central server.
Distributed systems. Distribute sensitive data to 7 servers
s.t. as long as <3 are broken, no harm to security occurs.
Electronic auctions. Can run auctions s.t. no one (even not seller)
learns anything other than winning party and bid.

22
Cryptography & Security
Prev slides: Have provably secure algorithm for every crypto task imaginable.
Q: How come nothing is secure?
A1: Not all of these are used or used correctly:
 Strange tendency to use “home-brewed” cryptosystems.
 Combining secure primitives in insecure way
 Strict efficiency requirements for crypto/security:
 Many provably secure algs not efficient enough
 The cost is visible but benefit invisible.
 Easy to get implementation wrong – many subtleties
 Compatibility issues, legacy systems,
 Misunderstanding properties of crypto components.

23
Cryptography & Security
Prev slides: Have provably secure algorithm for every crypto task imaginable.
Q: How come nothing is secure?
A2: Cryptography is only part of designing secure systems
 Chain is only as strong as weakest link.
 A “dormant bug” is often a security hole.
 Security is hard to “modularize”
 Human element
(hard to add to existing system, changes in system
features can have unexpected consequences)
 Many subtle issues (e.g., caching & virtual memory, side
channel attacks)
 Key storage and protection issues.

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