Crypto Talk

TSC

Goal of Talk
- What different primitives are and what they are for
- Not for building crypto - don't do that
Purpose of Crypto
- Privacy
- Validation
- Authenticity
History
- Substitution cipher → freq analysis
- Vigener Cipher → freq analysis again
Symmetric Cipher
- Claude Shannon and perfect security
- XOR OTP example
Problems with OTP/sym.
- key must be exchanged
  - PRG
  - Still must be OTP
- Repeat attack
- Substitution attack
Hashing
- What is a hash
- MAC
- Advantage on non-crypto hash
- Pigeon hole and collision resistance
- Block Chain and Merkle Tree
- Password hashing
Block Ciphers
- AES
- CBC vs CTR
- Nonce
Stream Ciphers
- Simpler to understand, harder to implement
Comparison
- Mostly, don't worry
- Block ciphers are easier to design
- Stream ciphers are faster
Asymmetric
- Diffe-Hellman - won't cover, but they got there first
- Key exchange
- RSA
- Elliptic
- Encryption and signature
Future
- Quantum computing
- Homomorphic cryptography

Alice or (Alice and Bob) have a 256bit key - 100 digit number
Not covering key exchange (maybe later)
Encryption is many things
- Already discussed encryption and key exchange
- Also authentication - message is unchanged and from source
Hash - like fingerprint
Block hash using exponents
- r ^ n % p = h
- p : prime
- r : primitive root
- n : input
- h : hash
- algorithm to do this quickly
- given r, p and h, cannot be reversed (discrete log problem)
Chaining block hash with key for HMAC
- nonce
- XOR
- h[n] = hash(m[n] XOR h[n-1])
XOR using hash as PRNG
Problem: Anything that can authenticate can encrypt