The Internet, Cryptography, Git

(all really fast)

CS 300 Lecture 4-1
Bart Massey

How a network works

• Serial: Wire changes electrical level over time

  • Wireless is the same, but without a wire
  • Noise, range
  • big-endian vs little-endian

• Hardware on end to generate / reconstruct bytes

• Usually point-to-point interfaces

• One host may have many interfaces

  

Packets and Addressing

• Useful to send sequences of bytes as a logical "packet"

  • Includes error-checking
  • Probably variable-length

• Give each endpoint a unique identifier: "address"

  • Mark packets with source and destination address

• Route packets according to address

  • This is magic and beyond scope

    

Protocol Layering

• Hardware network is

  • Unreliable
  • Out-of-order
  • Limited max packet size

• Add logic to retransmit packets, make larger packets, etc

  • This logic is a protocol
  • Typically not very dependent on underlying hardware
  • In our case, TCP/IP

• Addressing

  • MAC-level for hardware ala ethernet 70:56:81:ba:39:1b
  • IPv4 addressing: 192.168.1.1

  • IPv6 addressing: out of scope

    

TCP

• Adds virtual reliable stream abstraction: packets are resent, reordered and buffered as needed

• Adds concept of software endpoint "port": OS kernel can tell which software is to receive stream

• Generally treated like files by programming languages: get a file descriptor, read / write it

  

Layering On TCP

• Figure out what information you want to send/receive

• Design a protocol by which that info is sent/received

  • The protocol needs to be serialized

    

Basic Cryptography

• Idea: Given a public algorithm and a secret key, allow two parties with the key to communicate such that anyone without the key can't discover the communication

• Very old idea

• Plaintext, ciphertext

• Modern cipher: takes a key, does key setup, then

  • Runs transmitted bytes through an encryption algorithm first

  • Runs received bytes through a decryption algorithm first

    

Some Crypto Technicalities

• Symmetric Key vs Public Key

• Block cipher

  • Block chaining
  • IVs and setup

  • Feistel ciphers

    

RC4

• Stream cipher

  • Key setup initializes crypto-secure PRNG
  • Xor plaintext with keystream to encrypt
  • Xor ciphertext with keystream to decrypt

• Tricks for setup

• Some attacks

  

Public-key crypto

• Separate encryption key and decryption key

• Means everyone can publish encryption key

  • Anyone can send a message readable only by recipient

• SSH: RSA and DSA