Assignment 1: ScroogeCoin

Bitcoin and Cryptocurrency Technologies
Assignment 1: ScroogeCoin
In ScroogeCoin (Lecture 1), the central authority Scrooge receives transactions from users. You will
implement the logic used by Scrooge to process transactions and produce the ledger. Scrooge
organizes transactions into time periods or blocks. In each block, Scrooge will receive a list of
transactions, validate the transactions he receives, and publish a list of validated transactions.
Note that a transaction can reference another in the same block. Also, among the transactions
received by Scrooge in a single block, more than one transaction may spend the same output. This
would of course be a double-spend, and hence invalid. This means that transactions can’t be validated
in isolation; it is a tricky problem to choose a subset of transactions that are together valid.
You will be provided with a Transaction class that represents a ScroogeCoin transaction and has
inner classes Transaction.Output and Transaction.Input.
A transaction output consists of a value and a public key to which it is being paid. For the public keys,
we use the built-in Java PublicKey class.
A transaction input consists of the hash of the transaction that contains the corresponding output, the
index of this output in that transaction (indices are simply integers starting from 0), and a digital
signature. For the input to be valid, the signature it contains must be a valid signature over the
current transaction with the public key in the spent output.
More specifically, the raw data that is signed is obtained from the getRawDataToSign(int
index) method. To verify a signature, you will use the verifySignature() method included in the
provided file Crypto.java:
public static boolean verifySignature(PublicKey pubKey, byte[] message,
byte[] signature)
This method takes a public key, a message and a signature, and returns true if and only signature
correctly verifies over message with the public key pubKey.
Note that you are only given code to verify signatures, and this is all that you will need for this
assignment. The computation of signatures is done outside the Transaction class by an entity that
knows the appropriate private keys.
A transaction consists of a list of inputs, a list of outputs and a unique ID (see the getRawTx()
method). The class also contains methods to add and remove an input, add an output, compute
digests to sign/hash, add a signature to an input, and compute and store the hash of the transaction
once all inputs/outputs/signatures have been added.
You will also be provided with a UTXO class that represents an unspent transaction output. A UTXO
contains the hash of the transaction from which it originates as well as its index within that
transaction. We have included equals, hashCode, and compareTo functions in UTXO that allow the
testing of equality and comparison between two UTXOs based on their indices and the contents of
their txHash arrays.
Further, you will be provided with a UTXOPool class that represents the current set of outstanding
UTXOs and contains a map from each UTXO to its corresponding transaction output. This class
contains constructors to create a new empty UTXOPool or a copy of a given UTXOPool, and methods
to add and remove UTXOs from the pool, get the output corresponding to a given UTXO, check if a
UTXO is in the pool, and get a list of all UTXOs in the pool.
You will be responsible for creating a file called TxHandler.java that implements the following API:
public class TxHandler {
 /** Creates a public ledger whose current UTXOPool (collection of unspent
 * transaction outputs) is utxoPool. This should make a defensive copy of
 * utxoPool by using the UTXOPool(UTXOPool uPool) constructor.
 */
 public TxHandler(UTXOPool utxoPool);
 /** Returns true if
 * (1) all outputs claimed by tx are in the current UTXO pool,
 * (2) the signatures on each input of tx are valid,
 * (3) no UTXO is claimed multiple times by tx,
 * (4) all of tx’s output values are non-negative, and
 * (5) the sum of tx’s input values is greater than or equal to the sum of
 its output values; and false otherwise.
 */
 public boolean isValidTx(Transaction tx);
 /** Handles each epoch by receiving an unordered array of proposed
 * transactions, checking each transaction for correctness,
 * returning a mutually valid array of accepted transactions,
 * and updating the current UTXO pool as appropriate.
 */
 public Transaction[] handleTxs(Transaction[] possibleTxs);
}
Your implementation of handleTxs() should return a mutually valid transaction set of maximal size
(one that can’t be enlarged simply by adding more transactions). It need not compute a set of
maximum size (one for which there is no larger mutually valid transaction set).
Based on the transactions it has chosen to accept, handleTxs should also update its internal
UTXOPool to reflect the current set of unspent transaction outputs, so that future calls to
handleTxs() and isValidTx() are able to correctly process/validate transactions that claim
outputs from transactions that were accepted in a previous call to handleTxs().
Extra Credit: Create a second file called MaxFeeTxHandler.java whose handleTxs() method
finds a set of transactions with maximum total transaction fees -- i.e. maximize the sum over all
transactions in the set of (sum of input values - sum of output values)).