Each lab will consist of a small problem and details of how to proceed. Each lab is intended to give every student hands-on experience with the core concepts and technologies covered during the course.  In contrast to project-related and team-related work where a student may concentrate on one technology as a team member, labs are designed to give each and every student exposure to all the technologies that come into play in the course.  You need to submit labs to the TAs for grading--see submission instructions below.  Generally, unless otherwise specified, you will have one week to complete each assigned lab.

See the syllabus for information on grading.  Turning in lab assignments on time is required, without exception, and all late deliveries will be penalized, regardless of cause.  Submit your assignments to your Subversion repository according to the directions on the syllabus page.

You may write these solutions in any programming language of your choice.  Our suggestion is that now is not the time to learn a new programming language along with the concepts themselves.  So use whatever programming language you  know best.

Lab 6   Due: 5:00 pm, Monday, May 13, 2019

Problem 1:  Cryptocurrency and Mining: 

BACKGROUND:

WHAT YOU NEED TO DO:

STEP 1:

You may have noticed that in the last lab, the blockchain magically got created by just adding new transactions.  We are going to modify that in this lab, to make it a tad bit more realistic.

First, you will need to think up a name for your new cryptocurrency you will be mining on your blockchain.  You can't call it Bitcoin or Ether or any other existent cryptocurrency.  Barring that, there are no rules.  You could call it "Barbaracoin" or "Yapstone" or "Foolsgold" or "Capnjack" or whatever you'd like.  You will create a Miner class (see below).  Your Miner will take newly-created transactions and collect transactions into blocks.  Each new block will include a "coinbase" transaction that creates "Barbaracoins" (or whatever).  The naming of your cryptocurrency is entirely up to you.  Have fun.

Your cryptocurrency should be denominated in fractional form (similar to satoshis in bitcoin), and we will keep it simple:  you will need to denominate your cryptocurrency in 1/1000s (milli-coins).  So, as an example, 1 "Barbarinsky" is 10^-3, or 0.001, "Barbaracoins".  You get the idea.  You will use the fractional form when designating the Value of an Output (see below). 

You are to create a new class/structure called TxnMemoryPool, which can be of any type you wish, but a simple list of Transaction objects/structures will do (order is unimportant), and this will store your transactions in memory during processing.  Note you will no longer be automatically adding transactions to blocks on creation of the transactions.  Rather, you will add them to the TxnMemoryPool on creation.  This will simulate receiving them from the network. 

If you plan to use multiple threads, you should set up a method or function that creates a new unique transaction every 1-10 seconds (if using threads, you can make this decision as you progress through your testing) and adds each new transaction to the TxnMemoryPool.  If you plan not to use multiple threads, you may simply pre-create 91 new transactions and add them to your TxnMemoryPool before beginning mining, which may prove easier.  You should modify your blockchain code to include a new MAX_TXNS field, which is the maximum number of transactions per block.  You should set this to 10 (including the coinbase transaction).  So 9 transactions + 1 coinbase transaction = 10 Max.

If you're wondering what in the world will go in your inputs and outputs, you might remember that even a SHA-256 hash of a single character, such as 'a', will always result in a 256-bit, 32-byte hash, every time, ad infinitum.  This means that you could simply stuff a hash of the current timestamp+some_randomly_generated_number into the inputs, and no one will be the wiser.  Again, we are not concerning ourselves at this point with "connecting-up" the outputs to the inputs.  Now, speaking of outputs, it's time now to create a more formal definition of an output:  an Output class/structure.  Let's make our new Output look like this:

Note that it is up to you to determine the best "decimal" format for your particular language.  That may be a Decimal object (serialized), or an int (knowing that any Value is denominated in 1/1000s of your currency), etc.  That's your call.  So if you placed a "1500" in your Value, that would mean 1.5 "Barbaracoins" (or "CapnJacks", etc.).  At this point, you can put anything you want in the string Script.  See above conversation on inputs.

STEP 2:

You will create a new Miner class/function, that will go through the TxnMemoryPool (which will eventually be growing at a steady rate), and your Miner will attempt to create a block whose Blockhash is less than the target you will set.  In this lab, our target will be calculated as:

_{Target = coefficient * 2}^{(0x8 * (exponent – 0x3))}

Your Header.Bits (for all blocks) will default to 0x207fffff, which is the difficulty bits for Regtest.  This means that your coefficient will be '0x7fffff' and your exponent will be '0x20'.  Note that this is just a starting default.  It's designed to make mining a slight challenge but nothing you're going to spend eternity waiting on (on average...your mileage may vary...).  Feel free to adjust your bits in order to play around with different levels of difficulty if you get bored.  In our case for this lab, you will not need to worry about automatically adjusting the mining difficulty.  For example, for a little more challenge, you could try the difficulty bits value used on Testnet:  0x1d00ffff.  When you are done experimenting, you will want to reset your Header.Bits to the default of 0x207fffff for final submission.

As you build your candidate blocks, keep in mind you will need a coinbase transaction as the first transaction in each of the blocks.  This coinbase transaction will create an amount of your cryptocurrency that is also up to you.  You could follow the bitcoin model and start out with 50 bitcoins, or you could be more or less generous.  That's entirely up to you.

Hopefully you adequately encapsulated the creation of the genesis block in your blockchain.

Beyond this, nothing much changes from Lab 5.  And as usual, unless specified, everything else is up to you.

STEP 3:

You are to write a test case (can be a simple main function or an actual test harness from a framework) in which you run your new Blockchain mining.  You will create a new Blockchain and by doing so you will create your genesis block.  After mining the transactions in the TxnMemoryPool, print out the block height of the tip of the chain.

Submit all code and related files (if any) as specified below.

Finally, and importantly, you are of course free, and encouraged, to look at the actual bitcoin source code for mining.  But remember, your submission had best not closely resemble any of that source in such a way that it would indicate direct copying.

You may find the following references helpful (in addition to the links from previous labs):

The Bitcoinwiki Article on Target
The Bitcoinwiki on Difficulty
Bit-Ish Data Article on Building a Blockchain:  Difficulty Explained
freeCodeCamp Article on Mining
The Bitcoinwiki Page on Block Hashing Algorithm
The Bitcoinwiki Page on Transactions
The Bitcoinwiki Page on Raw Transactions
The Bitcoinwiki Page on Blocks
Sample program that mines
Bitcoin Developer Reference on Target
Stack Exchange conversation on mining

Use the folder named "lab6" in your Subversion repository. See the syllabus for more info about submission using Subversion. Upload your Lab 6 code and any supporting materials to the repo.  Please include a README text file to explain what parts of the work you are submitting, where the different parts of the work are located and please provide a little info on how to compile and run your code.