Blockchain Noobchain Example

The noobchain examples are based on the following blogs:

1. Creating Your First Blockchain with Java. Part 1

2. Creating Your First Blockchain with Java. Part 2 — Transactions.

Build

To build the JAR, execute the following command from the parent directory:

mvn clean install

Usage

Run the NoobChain class for each of the examples from an IDE like IntelliJ.

Blockchain Glossary

A blockchain is a chain/list of blocks where:

• Every block contains a hash (digital signature) of the previous block.
• A block hash is computed from the hash of its previous block.
• Each block is guaranteed to come after the previous block chronologically since a block's hash is based on the previous block's hash.

A block in a blockchain is computationally impractical to modify since every block after it has to be regenerated.

A genesis block is the first block of a blockchain.

Proof of Work (POW) is a way of ensuring that a new block is difficult to build by making the block creation process costly and time consuming. However it must be relatively trivial to check if a blockchain satifies these requriiments. This helps to avoid blockchain tampering.

Hashcash is the Proof of Work used by Bitcoin.

A blockchain wallet is a digital wallet that allows users to manage crypto-currencies such as bitcoin. Coin ownership in a blockchain is transferred as transactions. Every participant in a blockchain has a unique address for sending and receiving crypto-currencies.

A Merkle Tree is a hash tree where every leaf node is labelled with the hash of a data block and every non-leaf node is labelled with the cryptographic hash of the labels of its child nodes. The root node is called the Merkle Root which is labelled with the hash of all its child nodes.

Merkle Tree is an efficient way of verifying large data structures or lot of transactions in the case of a blockchain.

A Merkle Tree in blockchain is constructed by hashing paired data (the leaves), then pairing and hashing the results until a single hash remains. The remaining single hash is called the Merkle Root.

Noobchain Example 1

Noobchain example one includes the following:

• A simple block (Block.java) containing previous block's hash, its own hash, and simple data.

public String calculateHash() {
    String calculatedhash = StringUtil.applySha256(
        previousHash +
        Long.toString(timeStamp) +
        Integer.toString(nonce) +
        data);

    return calculatedhash;
}

• The hash is generated using SHA-256 cryptograhic hash algorithm. The code can be found in StringUtil.java.

• A blockchain, NoobChain.java, built with the aforementioned blocks.

• The first block is called the genesis block. The previous hash of the genesis block is defaulted to 0.

• Checking the validity of the noobchain by iterating over all the blocks and comparing the current block hash with the calculated hash. It also checks the current block's reference to the previous block hash with the previous block hash. Any tampering of a block's hash is easily detected.

public Boolean isChainValid() {
    // loop through blockchain to check hashes
    for (int i = 1; i < blockchain.size(); i++) {
        Block currentBlock = blockchain.get(i);
        Block previousBlock = blockchain.get(i - 1);

        // compare current block's hash with calculated hash
        if (!currentBlock.getHash().equals(currentBlock.calculateHash())) {
            log.info("Calculated hash doesn't match block's hash.");
            return false;
        }

        // compare previous hash with current block's previous hash
        if (!previousBlock.getHash().equals(currentBlock.getPreviousHash())) {
            log.info(
                "Previous block hash doesn't match curent block's previous hash.");
            return false;
        }
    }
    return true;
}

• The Proof of Work in this example is measured by having a certain number of 0s at the beginning of the hash. This is done by introducing a nonce (number used once) in the hash claculation. The nonce is modified until the required result is achieved. You can find the code in the mineBlock method of Block class. The mineBlock takes a parameter named difficulty which specifies the number of starting 0s required for a hash.

public void mineBlock(int difficulty) {
   //Create a string with difficulty * "0"
   String target = new String(new char[difficulty]).replace('\0', '0');

   while (!hash.substring(0, difficulty).equals(target)) {
       nonce++;
       hash = calculateHash();
   }
}

Noobchain Example 1 Output

Block Mined!!! : 00000dd85c854b8dfdba7b61358d2444d2afd613dbce55317f81a416551a9497
Block Mined!!! : 00000a0ae7bbf378b5643762178656a6a2be3797d08b5b39a550ee7809ea3517
Block Mined!!! : 000004720fe2ccc9b8112aa1b0e6e48412c86c1db41df566bd3b4d72a4801cb7
[
  {
    "hash": "00000dd85c854b8dfdba7b61358d2444d2afd613dbce55317f81a416551a9497",
    "previousHash": "0",
    "data": "Hi I'm the first block",
    "timeStamp": 1523425003386,
    "nonce": 106963
  },
  {
    "hash": "00000a0ae7bbf378b5643762178656a6a2be3797d08b5b39a550ee7809ea3517",
    "previousHash": "00000dd85c854b8dfdba7b61358d2444d2afd613dbce55317f81a416551a9497",
    "data": "Yo I'm the second block",
    "timeStamp": 1523425003789,
    "nonce": 108288
  },
  {
    "hash": "000004720fe2ccc9b8112aa1b0e6e48412c86c1db41df566bd3b4d72a4801cb7",
    "previousHash": "00000a0ae7bbf378b5643762178656a6a2be3797d08b5b39a550ee7809ea3517",
    "data": "Hey I'm the third block",
    "timeStamp": 1523425004150,
    "nonce": 1812067
  }
]
valid block chain: true

Noobchain Example 2

Noobchain example two includes the following:

• A simple block (Block.java) containing previous block hash, its own hash, and list of transactions.

• The hash is generated using SHA-256 cryptograhic hash algorithm from previous block hash, timestamp, nonce, and Merkle Root from the transaction ids.

• A simple wallet which can send and receive transactions of noobcoins.

• Every wallet has a public key and a private key. The public key acts as the wallet's address. The private key is used to sign transactions. Signing prevents tampering with a owner's noobcoins. A sender's public key is sent along every transaction for verification. The public key is used to verify the integrity.

• A private and public keys are generated as a key pair. Elliptic Curve Digital Signature Algorithm (ECDSA)signature is used to generate the key pair in methodgenerateKeyPairofWallet` class.

• Transactions are sent using NoobChain. Each transaction contains the following information:

  • a unique transaction id
  • public keys of the sender and the receiver of funds
  • amount to be transferred
  • previous transactions (inputs) which helps in proving that there is enough fund to send
  • outgoing transactions (outputs) with the amount to be transferred
  • a crytographic signature to prevent transaction from tampering and also prevents unauthorized user to spend noobcoins.

• The following action take place when a sender wallet sends noobcoins to a receiver wallet:

  1. Checks the balance of the sender wallet by summing up all the unspent transaction outputs which having the same public key as the sender wallet.
  2. If the amount being sent is greater than the balance, a transaction is created and signed.
  3. The newly created transaction is added to a newly created block and processed.

Noobchain Example 2 Output

2018-04-10T22:35:23.727-07:00: [INFO] main com.basaki.noobchain.NoobChain - Creating and Mining Genesis block... 
2018-04-10T22:35:23.732-07:00: [INFO] main com.basaki.noobchain.Block - Transaction Successfully added to Block
2018-04-10T22:35:23.748-07:00: [INFO] main com.basaki.noobchain.Block - Block Mined!!! : 000dea2b320f1649463bc2e932fe44e3fa0773b54889bd73a38b7382f54e27d9
2018-04-10T22:35:23.798-07:00: [INFO] main com.basaki.noobchain.NoobChain - WalletA's balance is: 100.0
2018-04-10T22:35:23.798-07:00: [INFO] main com.basaki.noobchain.NoobChain - WalletA is Attempting to send funds (40) to WalletB...
2018-04-10T22:35:23.802-07:00: [INFO] main com.basaki.noobchain.Block - Transaction Successfully added to Block
2018-04-10T22:35:23.817-07:00: [INFO] main com.basaki.noobchain.Block - Block Mined!!! : 000a8a6d690c97f2dcb75e1e044706aae90d3b7619be1ef53a17e8522a757371
2018-04-10T22:35:23.817-07:00: [INFO] main com.basaki.noobchain.NoobChain - WalletA's balance is: 60.0
2018-04-10T22:35:23.817-07:00: [INFO] main com.basaki.noobchain.NoobChain - WalletB's balance is: 40.0
2018-04-10T22:35:23.817-07:00: [INFO] main com.basaki.noobchain.NoobChain - WalletA Attempting to send more funds (1000) than it has...
2018-04-10T22:35:23.817-07:00: [INFO] main com.basaki.noobchain.Wallet - Transaction discarded as there isn't enough fund...
2018-04-10T22:35:23.869-07:00: [INFO] main com.basaki.noobchain.Block - Block Mined!!! : 000fbb6ec4e745447405790e4813fc61bc0b41d2bcca5a5d9532ac87fde75835
2018-04-10T22:35:23.869-07:00: [INFO] main com.basaki.noobchain.NoobChain - WalletA's balance is: 60.0
2018-04-10T22:35:23.869-07:00: [INFO] main com.basaki.noobchain.NoobChain - WalletB's balance is: 40.0
2018-04-10T22:35:23.869-07:00: [INFO] main com.basaki.noobchain.NoobChain - WalletB is Attempting to send funds (20) to WalletA...
2018-04-10T22:35:23.872-07:00: [INFO] main com.basaki.noobchain.Block - Transaction Successfully added to Block
2018-04-10T22:35:23.907-07:00: [INFO] main com.basaki.noobchain.Block - Block Mined!!! : 000c9ff1c10a0fc2dfd1bf7ae56d8b3dd744bda52e68657483704738c3a181d2
2018-04-10T22:35:23.907-07:00: [INFO] main com.basaki.noobchain.NoobChain - WalletA's balance is 80.0
2018-04-10T22:35:23.907-07:00: [INFO] main com.basaki.noobchain.NoobChain - WalletB's balance is 20.0
2018-04-10T22:35:23.910-07:00: [INFO] main com.basaki.noobchain.NoobChain - Block Chain Valid: true