Crypto

Online Cryptography Course

Dan Boneh

Introduction

Course Overview

Dan Boneh

Welcome
Course objectives:
• Learn how crypto primitives work
• Learn how to use them correctly and reason about security
My recommendations:
• Take notes
• Pause video frequently to think about the material
• Answer the in-video questions
Dan Boneh

Cryptography is everywhere
Securecommunication:
– web traffic: HTTPS
– wireless traffic: 802.11i WPA2 (and WEP), GSM, Bluetooth

Encrypting files on disk: EFS, TrueCrypt
Content protection (e.g. DVD, Blu-ray): CSS, AACS
User authentication

… and much much more
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Secure communication

no eavesdropping
no tampering

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Secure Sockets Layer / TLS
Two main parts
1. Handshake Protocol: Establish sharedsecret key
using public-key cryptography (2nd part of course)
2. Record Layer: Transmit data using shared secret key
Ensure confidentiality and integrity (1st part of course)
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Protected files on disk
Disk
Alice

File 1

File 2

Alice
No eavesdropping
No tampering

Analogous to secure communication:
Alice today sends a message to Alice tomorrow
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Buildingblock: sym. encryption
Alice
m

E

Bob
E(k,m)=c

k

c

D

D(k,c)=m

k

E, D: cipher
k: secret key (e.g. 128 bits)
m, c: plaintext, ciphertext
Encryption algorithm is publicly known
• Never use a proprietary cipher
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Use Cases
Single use key: (one time key)
• Key is only used to encrypt one message
• encrypted email: new key generated for every email
Multi usekey: (many time key)
• Key used to encrypt multiple messages
• encrypted files: same key used to encrypt many files
• Need more machinery than for one-time key
Dan Boneh

Things to remember
Cryptography is:
– A tremendous tool
– The basis for many security mechanisms
Cryptography is not:
– The solution to all security problems
– Reliable unless implemented and used properly
–Something you should try to invent yourself
• many many examples of broken ad-hoc designs
Dan Boneh

End of Segment

Dan Boneh

Online Cryptography Course

Dan Boneh

Introduction
What is cryptography?

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Crypto core
Secret key establishment:

Talking
to Alice

Talking
to Bob
Alice

Bob

attacker???

Secure communication:

k

m1

k

m2
confidentialityand integrity
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But crypto can do much more
• Digital signatures

• Anonymous communication
Alice
signature

Who did I
just talk to?

Alice
Bob

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But crypto can do much more
• Digital signatures

• Anonymous communication
• Anonymous digital cash
– Can I spend a “digital coin” without anyone knowing who I am?
– How to prevent double spending?

1$Alice

Internet

Who was
that?

(anon. comm.)
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Protocols
• Elections
• Private auctions

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Protocols
• Elections
• Private auctions

Goal: compute f(x1, x2, x3, x4)

trusted
authority

“ Thm:” anything the can done with trusted auth. can also
be done without
• Secure multi-party computation
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Crypto magic
• Privately outsourcingcomputation
search
query

What did she
search for?

E[ query ]

Alice

E[ results ]
results

• Zero knowledge (proof of knowledge)
N=p∙q

Alice

???

I know the factors of N !!
proof π

Bob

N
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A rigorous science
The three steps in cryptography:

• Precisely specify threat model
• Propose a construction
• Prove that breaking construction under
threat mode willsolve an underlying hard problem
Dan Boneh

End of Segment

Dan Boneh

Online Cryptography Course

Dan Boneh

Introduction

History

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History
David Kahn, “The code breakers” (1996)

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Symmetric Ciphers

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Few Historic Examples

(all badly broken)

1. Substitution cipher

k :=

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Caesar Cipher

(no key)

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