Project 2 - Mining Points

Purpose

• Understand how cryptocurrency mining works by building and running a miner for a new cryptocurrency.

• Explore some potential threats to bitcoin by attempting attacks on our class cryptocurrency.

• Get some experience using cloud computing resources and understanding computing costs in practice.

Collaboration Policy

For Part 1 of this assignment, everyone should create and run their own PointCoin miner, and writeup their own answers to the questions.

You may, and are encouraged to, discuss all of the problems with anyone else you want (both on-line using the course web site or any other means you choose, and in person), and it is okay to share code with others so long as you understand everything in all of the code you use.

For Part 2, you may work with others in the class (indeed, part of your goal for part 2 is to form a mining coallition with enough power to disrupt the normal behavior of the network). You should not start Part 2 or make any attempts to (intentionally) disrupt the PointCoin network until the announced starting time for Part 2 (which will be announced later in class).

Part 1: Mining PointCoin

To set up your pointcoind node:

1. Setup an EC2 instance, install go and pointcoin following these directions: Installing PointCoin on AWS.

2. Setup your node and wallet: Using PointCoin

The main goal for this part is for everyone to write your own PointCoin miner. We have provided some code in the Project 2 repo for you to get started:

• miner.go - an initial template for a PointCoin miner. You will implement your miner by modifying this file.
• support.go - some functions that you will find useful in implementing your miner. (You are not expected to change these functions, but can change them if you want to.)

Next, we provide some hints that may be helpful in building your miner.

Coinbase Transaction

Although some people might mine cryptocurrency for purely altruistic reasons, the main economic incentive for mining is that the miner who finds a new block can add a transaction to that block transfering the mining fee to their address.

We have provided the createCoinbaseTx function in support.go to create this transaction. In includes this code for setting up the inputs and outputs of the transaction:

        tx.AddTxIn(&btcwire.TxIn{
                // Coinbase transactions have no inputs, so previous outpoint is
                // zero hash and max index.
                PreviousOutPoint: *btcwire.NewOutPoint(&btcwire.ShaHash{},
                                                       btcwire.MaxPrevOutIndex),
                SignatureScript: coinbaseScript,
                Sequence:        btcwire.MaxTxInSequenceNum,
        })
        tx.AddTxOut(&btcwire.TxOut{
                Value: blockchain.CalcBlockSubsidy(nextBlockHeight,
                                                   &btcnet.MainNetParams),
                PkScript: pkScript,
        })

In PointCoin, the mining reward is 1 pointcoin (see this commit for how it was modified from how it is set in bitcoin).

If you want to receive the rewards for your mining efforts, you need to set the address used in the coinbase transaction to be the address for your PointCoin wallet.

Merkle Root

The block header contains the Merkle Root of all the transactions in the block. This should be all the valid transactions submitted to the network, with your coinbase transaction added.

We have provided the createMerkleRoot function in support.go for computing the Merkle root of a list of transactions:

func createMerkleRoot(txs []*btcwire.MsgTx) *btcwire.ShaHash {
    txutil := []*btcutil.Tx{}
    for _, tx := range txs {
        convtx := btcutil.NewTx(tx)
        txutil = append(txutil, convtx)
    }

    store := blockchain.BuildMerkleTreeStore(txutil)
    merkleRoot := store[len(store)-1]
    return merkleRoot
}

Creating a Block

A block in PointCoin is similar to a block in bitcoin. It contains a header consisting of:

• The hash of the previous block
• The hash of the Merkle tree of all the transactions
• The difficulty
• A nonce (32 bits)

In addition to the header, it includes the list of transactions.

We have provided the CreateBlock function in support.go to build a block given these inputs:

func CreateBlock(prevHash string, merkleRoot *btcwire.ShaHash, difficulty big.Int, 
                 nonce uint32, txs []*btcwire.MsgTx) *btcwire.MsgBlock {
    prevH, _ := btcwire.NewShaHashFromStr(prevHash)
    d := blockchain.BigToCompact(&difficulty)
    header := btcwire.NewBlockHeader(prevH, merkleRoot, d, nonce)

    msgBlock := btcwire.NewMsgBlock(header)
    for _, tx := range txs {
        msgBlock.AddTransaction(tx)
    }

    return msgBlock
}

Mining a Block

Of course, the block you create is unlikely to be valid with a randomly selected nonce. To find a valid block, it is necessary to find a nonce such that the hash of the block header (with that nonce included) is below the target difficulty.

The block nonce is a uint32 in block.Header.Nonce. You can update this value with a simple assignment to try a different nonce value.

The function BlockSha returns the Double-SHA256 hash of the block header, so you can compute the hash of your block using block.Header.BlockSha().

Because of the different ways numbers and hashes are represented, the difficulty comparison is more awkward than one would like. You can use the provided lessThanDiff function to check if the returned hash is less than the target difficulty:

func lessThanDiff(hash btcwire.ShaHash, difficulty big.Int) bool {
    bigI := blockchain.ShaHashToBig(&hash)
    return bigI.Cmp(&difficulty) <= 0
}

If you've succeeded in finding a good nonce, submit the block!

This code will submit the block (since client is setup as an RPC to the node running on your own instance, it is submitting it to your own node first, which, if the block is valid, will submit it to the rest of the network):

   err := client.SubmitBlock(btcutil.NewBlock(block), nil)
   if err != nil { // something failed

Note that one reason your submission may fail is if the blockchain has already advanced. This would happen if another miner found and submitted a block (that was received by your node) since the call you made to client.GetBlockTemplate(&btcjson.TemplateRequest{}) to obtain the previous block (used in the template header).

Becoming a PointCoin Tycoon

You should start by making your miner as simple as possible, and getting a simple miner working before attempting to do anything more complicated.

If you are ambitious, though, there are lots of ways to improve the performance of your miner (still running on the low-powered micro EC2 node).

Here are a few possibilities:

• Explore the tradeoff between frequently checking the network to update your block template, and computing lots of hashes (that might be wasted if the blockchain has advanced). Your miner will not perform well if you update your template after every hash attempt, or if you keep trying until you find a good block without ever updating your template.

• Even the micro instances have more than one core. You are missing out on a lot of potential mining power if you are only using one thread. Actually, micro nodes do not even give you a full core! You will not get better performace on a micro node by using multi-threading, since you only have access to a single CPU core. Your EC2 node is actually being throttled by Amazon since it is sharing a physical node with other VMs, and your compute use will be throttled if it is excessive (which a running miner should be!). You may be able to use this to your advantage (since everyone elses miner is also running on a micro node, except possible course staff miners which get an unfair computing power advantage!)

(Note the revision to Problem 1. The original target of acquiring 100 PointCoin has been reduced because of the problems we had with the intial PointCoin network and need to relaunch. It is sufficient now to mine one block successfully. Earning more PointCoin will still be beneficial though! You'll have other opportunities to use it.)

Submit the Project 2/Part 1 Submission Form (by 11:59pm on Sunday, 22 February). (You can submit your answers to Problems 1-4 above either by submitting text in the form, or submitting a link to a PDF containing all of your answers).

Part 2: Investigating Mining

Collaboration Policy

For Part 2 of this assignment, everyone should their own submission. You may work with others in the class (indeed, part of your goal for part 2 is to form a mining coallition with enough power to disrupt the normal behavior of the network). For the written and coding problems, it is okay to collaborate with other students to write up one answer and submit the same answer so long as you clearly credit the students who you worked with, you fully contributed to the answer, and you fully understand everything in the answers you submit.

You should write up answers to the questions 5-9 as a PDF file, and submit it by sending an email to evans@cs.virginia.edu with subject line Project 2 Submission.

Analyzing the Double Spending Attack

For this problem, your goal is to analyze how much of a threat it is for one miner (or pool controlled by a single authority) to control a large faction of the hashing power.

Assume the attacker's goal is to get a victim to accept a payment (e.g., transfer pointcoin for cash-equivalent) that is valid in the (temporary) blockchain, and then release mined blocks to replace that blockchain with one that does not include the payment.

You may assume (inaccurately!) that there is no cost to the attacker to do the transaction. That is, the attacker can create a transaction selling P pointcoin for US dollars, and inverse that transaction (sell the US dollars to buy pointcoin) at a future time, and end up with P pointcoin after the two transactions. (Of course, in any real financial system, there are transaction costs. A more realistic answer would include this in your analysis.)

Your answer to problem 5 can either be a closed form mathematical equation, e.g.,

Forming Mining Pools

Your answers to problems 5 and 6 should convince you that it is unlikely that a single miner with less than 1/30 of the hashing power of the PointCoin network will have a good likelihood of successfully double-spending.

Develop a strategy for forming a mining pool. You may collaborate with as many students as you wish to develop your mining pool, but you should be mutually distrustful of each other and you are encouraged to find ways to take advantage of other students. That is to say, the effectiveness of your mining pool cannot depend on everyone behaving honestly, but must take measures to prevent dishonest behavior in your mining pool. You may, however, assume that everyone can only use the computing power of their own EC2 micro node for mining (and it would be considered cheating to use additional power for mining).

To actually execute your strategy, you should make it as simple as possible (at least to get started), and then add more complexity only after successfully accompilshing the simplest possible start.

Free-for-All!

For the last problem, your goal is to attack the PointCoin network in a way that benefits yourself. You can be creative on this and do not need to limit yourself to attacks suggested in class or this page, but should follow the rules below carefully and check with Dave before doing anything questionable.

Rules

The only rules are:

1. You must disclose honestly everything that you did when you submit the assignment (and in response to any questions before this).

2. You may not do anything that violates any law or the University honor code (although the normal "lying" and "stealing" provisions do not apply to your nodes' behavior in pointcoin protocols or things you tell other students in the class about your network activities). It is considered fair (indeed, encouraged) to "lie" or "mislead" your fellow students regarding your mining behavior. For example, it would be totally okay and encouraged for you to attempt to trick others into joining your mining pool by offering them a share of the points mined, but the not to deliver those points. For the purposes of this assignment, you should consider your classmates to be mutually distrusting individuals.

3. You should not use any computing resources for PointCoin mining other than your one micro EC2 node. (You may use other computing resources for development, offline analysis, other computation, but not to directly mine PointCoin.)

4. You should not do anything with your EC2 node that violates Amazon's Acceptable Use Policy. In particular, this excludes denial of service attacks or both other students' nodes and the staff nodes.

5. You should not do anything a reasonable person would consider harmful, unethical, or in violation of the spirit of this assignment. This is a very vague statement. What it really means is that if you are uncertain about whether something you plan to do is consistent with the spirit of this project, you should consult with Dave before doing it. Examples of things that would be clearly unacceptable include installing malware on another student's computer or using physical access to their machine to copy or modify key files (or any other data) on their machine.

Submission

Submit your answers to questions 5-9 as a PDF file attachment in an email to evans@cs.virginia.edu with subject line Project 2 Submission by 11:59pm on Thursday, 5 March.