Blockchain Technology Flashcards
1
Q
bitcoin vs bitcoin cash
A
- split based on how to deal with slow transactions
Bitcoin
- started using segregated witness (SegWit2x)
- reduces amount of verification data for each block
- Bitcoin also plans to double block size (to 2mb)
Bitcoin Cash
- felt SegWit2 didn’t go far enough
- increases block size to 8mb (up from 1mb)
2
Q
hash functions
A
Consists of:
- input: message
- can be any length
- output: digest, tag, hash, fingerprint
- fixed in length
Desired traits
- should be fast
- collision resistant
- hard to infer anything about input
- was it even or odd
- well distributed (should look random)
3
Q
51% Attack
A
- hypothetical in which one actor aquires control of over 50% of mining rigs
- can double spend
- can prevent others’ transactions
4
Q
digital signature
A
- scheme for verifying the completeness of a message and the identity of its sender
- Examples: RSA, DSS
Steps
- Alice wants to sign a document
- Alice generates 2 keys
- Private key
- Public key
- They’re generated at the same time
- Cannot get private key from the public key
- Alice hashes the document with private key to produce signature
- Bob wishes to verify the message (does this message really generate this signature)
- Bob uses a verification function on the message, signature, and public key determine if message is valid
5
Q
bitcoin block
A
- a grouping of individual transations
Includes
- mutliple unrecorded transactions
- a special transaction representing miners’ reward
- an encoding of previous transaction block (to ensure continuity)
- a proof-of-work puzzle
6
Q
bitcoin transaction - steps
A
Steps
- Alice wishes to send money to Bob
- Alice uses a bitcoin client to record transations in ledger
- Alice specifies
- Hashes from previous transactions to her account (verifying that she has the money to begin with)
- List of recipients for bitcoin (their public keys)
- Amount to transfer (usually less than her present balance)
- Specify change going back to her
- Leftover money is used for transaction fee
- Takes transaction details and generates a digital signature
- Alice appends transaction with digital signature
- Alice broadcasts complete transaction
7
Q
bitcoin mining - steps
A
- miners confirm the order in which transactions take place in order to prevent double-spending
Steps
- Miners gather transactions that haven’t been mined into a block
- Also adds an additional transaction representing his reward
- coinbase/generation transaction
- how new bitcoins are created
- Starts hashing them in pairs to get a single digest value
- Hashes this transaction with most recent block in network to produce a number
- Convert that number into a challenge (for proof-of-work)
- Miner performs proof-of-work
- He generates many potential proofs to the challenge until one works
- Once found, miner announces proof
- This miner’s blockchain becomes the new official chain; other miners build on it
- The more leading zeros required, the more difficult it is to generate a proof
- Number of zeros is calibrated so that it takes a node, on average, 10 minutes to find a proof
8
Q
longest chain
A
- chain with PoW puzzles that were the most difficult to solve
9
Q
bitcoin money supply
A
- money supply limit is 21M bitcoin
- every time 210K blocks is generated, reward gets cut in half
- it takes about 4 years to generate 210K block
- entire supply will be generated by 2140
10
Q
proof of work
A
- a security measure/intentional hurdle incorporated into most crytocurrency protocols
- PoW puzzles have a difficulty score that describes how hard puzzle is
- puzzles are difficult to solve but easy to verify
Steps
- Imagine there’s a challenge string
- Miner generates a random number, called a “proof” or a “nonce”
- He then concatenates the challenge string and the proof
- And hashes the result
- If the hash has the requisite number ofleading zeros
- Then the challenge has been solved
- There’s no effective way to guess or infer the solution, so the miner has to do this with brute force
11
Q
proof of stake
A
- bitcoins are assigned a stake value
- value grows as long as bitcoin isn’t used
- when bitcoin is used for transaction, “stake” value is reset
- people who make fraudulent confirmations will lose money
- in PoS, blocks are forged, rather than mined
- the probability that an individual can forge a block is proportional to the stake they have
12
Q
ethereum tokens
A
- boost ether’s value by creating a need for the currency
- issued via crowdsale called Initial Coin Offering (ICO)
- ERC20 is a standard interface for tokens
- Must implement the following:
- get the total token supply
- get the account balance
- transfer the token
- approve spending the token
13
Q
hashrate
A
- measure of mining power
- number of nonces tried per second normalized by rashrate of the network
14
Q
asic
A
- application-specific integrated circuit
- circuit that is designed for a specifc purpose
- ex: video graphic cards
- asic resistence
- memory-hard puzzles
- memory doesn’t improve as quickly as processing speed
- puzzles that require memory won’t advantage asic as much
15
Q
externally owned accounts
A
- EOA
- accounts that are owned by a private key
- cannot contain EVM code
16
Q
byzantine fault tolerance
A
- ability of a distributed network to handle the byzantine generals problem
17
Q
two generals probelms
A
- example of a consensus problem
- there is no perfect solution
Setup
- Two aligned armies (A and B) have to decide whether to attack their opponent (C) tomorrow morning
- They will win if they attack together, but lose separately
- The armies are separated by the territory of army C
- They communicate through courriers, which may or may not make it
- They attempt to send messages back and forth in order to reach consensus
18
Q
byzantine generals problem
A
- example of a consensus problem
- generalization of the two generals problem
Setup
- a number of byzantine generals getting ready to attack a fortress
- must decide if they will attack or retreat
- but some generals might be traitors
- the goal is to reach consensus even though one or more generals is a traitor
- traitors can tell send different messages to different generals
- how many traitors can the system tolerate?
- (must be < 1/3)
19
Q
double spend
A
- a deliberate fork
- first, agent spends bitcoin
- then agent creates another block at same level to send bitcoin to another person, and starts mining on that chain
20
Q
Gas
A
- costs around 20K gas to set a value
- storing data is generally more expensive that performing an action
*
21
Q
EVM
A
- Runs opcodes
22
Q
Ether
A
- Currency of ethereum
- Used to pay for computation
- Smallest unit (base unit) called wei
23
Q
ABI
A
- Application Binary Interface
- Tells web3 how to interact with binary code on blockchain
- JSON array
- First element: send
- Function name
- Balance
- data field
- first 20 bytes are function call
- rest is a parameter
24
Q
Three Types of Blockchains
A
- Public blockchains
- Normal blockchains
- Consortium blockchains
- Controlled by a preselected set of nodes (computers)
- Only those preapproved nodes can sign transaction
- Considered partially decentralized
- Private blockchains (aka private networks)
- Not necessarily considered legitimate blockchains
- All permissions are kept centralized, by 1 node
- Banks are experimenting with this
- Applications: database management, auditing, and things specific to that organization
25
Q
Smart Contracts
A
- Aid in transferring something of value in transparent and secure way
- Eliminates middle party
Steps
- Buyer pays for product
- Receipt is held in virtual smart contract
- Seller gives buyer entry key on date specified beforehand
- If seller reneges, blockchain will automatically give refund on date key should have arrived
- If seller gives key early, blockchain holds it until agreed date
- Contract is saved on blockchain, so it cannot be altered once created
- If attempt is made to alter it, all parties are notified
Pros
- Autonomy: cannot be manipualted by third parties
- Trust: impossible to lose
- Backups
- Safety
- Speed
- Saves money: no intermediary
- Accuracy
26
Q
dApp
A
- Distributed application
- Runs on ethereum virtual machine
27
Q
DAO
(Democractic Autonomous Organization)
A
- virtual organization where members can vote on issues
*
28
Q
Wallets
A
2 Types of Wallets
- Externally Owned Accounts (EOA)
- Create in your wallets
- You own private key, public key and password
- Have an ether balance
- Can send trasactions
- Have not code associated with them
- Contracts Accounts
- Deployed in blockchain
- Contain code
- Have an ether balance
- Code is triggered by transactions or messages
- Can manipulate its own persistent storage
29
Q
Keypair
A
30
Q
private network
A
31
Q
Solidity
functions
A
- functions can be make to return variables or not
- semicolons are required
- constructor
- just like Java constructor
- runs on deployment, and never again
- costs gas
- constant functions
- can return values, but cannot change anything on blockchain
- can return more than one thing
- don’t cost gas to run
- syntax:
- function getSomeVar() constant returns (uint) {
return someVar
}
*
32
Q
Solidity
Types
A
- bool
- auto-initialized to false
- int (integer)
- ranges from int8 - int256
- uint (unsigned integer)
- ranges from uint8 - uint256
- auto initialized to zero
- can only be positive
- bytes
- address
- holds a 20 byte value (size of ethereum address)
- has properties balance (gets balance at that addres) and transfer (function that sends wei from contract address to involking address)
- string
- arrays
- can be either fixed or dynamic
- enums
- used to create user-defined types
- structs
- mappings
- hash tables
33
Q
Solidity
access contact from another contract
A
2 Options
- Instantiate a new instance of contract
- myContract = new ContractA()
- Store reference to existing contract
- myContract = ContractA(addressA)
34
Q
Solidity
msg
A
- special object
- properties
- sender
- address of individual that sends funds
- sender
35
Q
Solidity
Inheritance
A
- contracts can inherit from each other like classes in other languages
- use keyword is to inherit
- ex: contract con1 is con2 { }
36
Q
Solidity
Data Location
A
- Three places to store data in solidty:
- storage (persisting)
- memory (non-persisting)
- call-data (where external function params are stored)
- When assigning from a storage to memory variable, always creates an independent copy
- Assignments to local storage variables are references
- Forced data location
- parameters (not return) of external functions: calldata
- state variables: storage
- Default data location:
- parameters (also return) of functions: memory
- all other local variables: storage
37
Q
Web3
A
- Interface between ethereum and javascript (and html)
- Get access contracts as objects
- Create contractor from contract, and then instantiate
- Syntax:
- var MyContract = (web3.eth.contract(abiArray))
- var contractInstance = MyContract.ad([address])
38
Q
public variable
A
- can access public variables without using gas
- an accessor is created for each public variable
- functions are public by default
39
Q
testrpc
A
- simulated blockchain that runs on memory on computer
40
Q
interface
A
- essentially an api
- a json object that describes all of the functions in contract
- their params, arguments, what they return, etc.
41
Q
migration
A
- scripts that allow you to automate tasks related to setting up contracts
- mass-deployments
*
- mass-deployments
42
Q
library
A
- makes functionality available to multiple contracts at once
43
Q
deployment steps
A
Steps
- Write contract
- Place contract in contracts folder
- Run truffle compile
- Add your contract to 2_deploy_contracts.js
- Turn on testrpc
- Run truffle migrate
- truffle migrate
44
Q
transaction/call
A
- transaction
- writing to network/changing data
- ex: sending ether; creating contracts
- transactions cost ether and take time
- typically aren’t processed immediately
- typically return transaction id, rather than return value
- in function definition: just returns
- call
- reading from network
- free to run
- processed immediately
- in function definition: constant returns
- in function call: myContract.getBalance.call()
45
Q
abstractions
A
- JavaScript wrappers for solidity contracts
- Allow you to interact with contract in js
46
Q
module loader/module bundler
A
- allows you to code modules separately, and combine them in the end
- generates static assets
- examples:
- webpack
- require.js
- browserify
47
Q
package manger
A
- npm
- mainly for node packages
- bower
- mainly for frontend packages
- yarn
48
Q
task runners
A
- used to automate tasks, such as converting from SASS to CSS, or minifying
- examples:
- gulp
- grunt
49
Q
bitcoin script
A
- uses stacks
- composed of op codes
- turing incomplete
- primarily used to validate bitcoin transations
