quartz/content/notes/412-lectures.md

title	aliases	tags
412-lectures

Lecture 7

copmuter security

• physical
• software (authenticity correctness)
• information (CIA/ IAS octave)
• network

cryptography can be bad because

• you are still giving access to data in some form
• how long will the cipher remain secure
• managing keys can be challenging

crypto in CIA

• c - hide information
• i - check for changes, checksums, MACs, Digital signatures
• a - not directly, but indirectly, validate authentity of network link usage

fallible machines

• ram corruption, fade or malfunction of storage media, software issues (fs bugs, compression bugs, etc)
• solution
  • estimate probability of failures, determine how many trials to achieve a certain level of confidence
• aside: mahcines are designed to fail frequently
  • reliability tradeoff with increases in speed/power consuption (e.g., overclocking)

distributed consensus

• e.g., Master/master relational database
• NoSQL gossip protocols, and eventual consistency
• network instrastructure e.g., routers as hot spares
• consensus gathering systems
  • apache zookeeper distributed synchronisation
  • etcd (used in kubernetes)

dist cons algorithms

• fischer lynch paterson impossibility result
• paxos - fault tolerant consensus over distributed nodes (use din apache zoo)
• raft - alternative to paxos used in etcd
  • easier than paxos to understand and implement
    • sub problems: leader election, log (data) replication by leader to followers
• EPaxos - more compex and efficient that paxos

all used when all servers are trusted. when there are malicious parties the consensus set size must grow. you need a majority of votes from the asumed-benign server set.

permissionless blockchain - do not control the server set, safety presumed if 50% of nodes are benign

fault tolerance

• CFT - when a node dissapears
• BFT - nodes acting maliciously
• raft and paxos only CFT, variants of Paxos are BFT

building a cryptocurrency

• how to make a 'coin'
• how to id coin owners
• how to protect from forgery
• how to record ownership and transfer of ownership
• how are coins single-use

distributed consensus in bitcoin

• track who has what (in normal currency done by banks)
• bitcion has all validating nodes store the whole ledger - indicates order of transactions
  • collectively agreeing its content avoid double spending
• a wallet is a hash of a public key a client generates

proof of work

• to protect from sybil attacks it is computationaly costly to add transactions
• reward for incorporative a new transaction
• to validate a transaction
  • check for double spending internally
  • make merkle tree over transaction hashes
  • to close block, apply proof of work algorithm
• must be easy to check and hard to compute
  • bitcoin: find a nonce that when appended to the block of transactions gives a hash value less than the target (SHA-256, target is dynamic ~10min to compute)
  • block has is included in next block

miners

• validate blocks
• reward of 6.25 bitcoin
• ability to levy fees
• broadcast communication between miners uses a p2p protocol
• variance in mining time is larger than message broadcast time - helps serialisation
• possible for multiple answers to be broadcast
  • nodes who 'hear' multiple solutions keep them all
  • subsequent mining is done on the longest fork (parallel forks tend to die quickly)

approving transactions

• transaction is recorded in a block
• relevant block must be in longest blockchain and five for more blocks must follow it
• transaction clearing delay (in effect)

content of transaction

• no persistens coins: serial numbers are transaction hashes
• transaction specifies number of inputs and outputs,
  • inputs usually previous transactions
  • can ouput back to yourself, thus pocketing 'change'
  • remainder of input, after subtracting output, is transaction fee
• can search back in time to find a transaction
  • either genesis block (50 bitcoin) ir a coinbase mining reward

main node roles

• network - all nodes help routing with p2p protocol
• wallet - manage keys that show ownership of transactions
• miner - participate in proof of work block verifications
• blockchain - can carry full blockchain

bitcoin core refereence client contains all four functions

• miners (may) leave out wallet
• lightweight wallet only has wallet and network components
• some nodes store blockchain but do not do mining

Lecture 9

Merkle tree

• hash value compted for each data block
• tree built with parent