Attacking Cryptography Flip coins 64 coin flips Some will be - - PowerPoint PPT Presentation

Attacking Cryptography

Flip coins

• 64 coin flips
• Some will be assigned to make it up.
• Others will write a simple program to do it.
• 010101 string
• Turn into coin flip channel on Slack.

Measuring Randomness

• One of the following I made up; the other one I

randomly generated.

• 001010101011001101
• 010101101010001001

Cryptography Model

Plaintext Cyphertext

CRYPTOGRAPHY

Random Oracle Model

Plaintext RANDOM

Definitions

• Hash function
• Stream cipher
• Block cipher
• Symmetric key encryption
• Public key encryption
• Digital signature scheme
• Pseudorandom

Hash Functions

• Random function
• Password storage
• Checksums on files
• sys files
• user files
• integrity of sys files
• Message digests h(M)
• Sign the digest
• Timestamp the digest

Properties of Random Functions

• One way function:
• Given x, compute h(X). Given h(x) hard to generate the

preimage, x.

• Output doesn’t leak information about input
• Given a message x and a key k, compute one-way

encryption h(x,k).

• Hard to find collisions
• Hard to find messages x,y such that x≠y but h(x)=h(y).

• If |h(m)|=n, then 2

n

hash values. Attacker needs to guess 2

n/2

values before guessing m.

• Birthday paradox
• given 30 or more people, some pair will have the same birthday
• given 23 or more, more 50% odds
• Capture-recapture
• k/K = n/N
• N= Kn/k
• where k=overlap individuals, K=recapture population,

n=capture population, N=total population

Public key encryption

• Trapdoor one way function
• KR - public key
• KR-1 - private key
• Given KR, can’t compute KR-1
• C={M}KR
• M = {C}KR-1

Attacks

• Known plaintext
• Chosen plaintext
• Chosen ciphertext
• Related key attack
• Forgery attack
• Key recovery attack