I’m currently developing a java implementation of bitcoin, and I’m facing challenges related to handling coinbases transactions and ensuring the verification of these transactions within the blockchain. The coinbase transactions are there to reward the miners for successfully mining the blocks. These coinbase transactions generate new coins, which are then added to the miner’s wallet. However, I’m countering difficulties in correctly identifying and processing these transactions.

public static void main(String[] args) {
        Security.addProvider(new org.bouncycastle.jce.provider.BouncyCastleProvider());
        Wallet walletA = new Wallet();
        Wallet walletB = new Wallet();
        coinbase = new Wallet();

        Block genesis = new Block("0");
        genesisTransaction = mintBlock(genesis, walletA);

        Block block1 = new Block(blockchain.getLast().hash);
        block1.addTransaction(walletA.sendFunds(walletB.publicKey, 40f));
        mintBlock(block1, walletA);

        Block block2 = new Block(blockchain.getLast().hash);
        block2.addTransaction(walletA.sendFunds(walletB.publicKey, 1000f));
        mintBlock(block2, walletA);

        Block block3 = new Block(blockchain.getLast().hash);
        block3.addTransaction(walletB.sendFunds(walletA.publicKey, 20f));
        block3.addTransaction(walletB.sendFunds(walletA.publicKey, 10f));
        mintBlock(block3, walletB);

        for (int i = 0; i < 2; i++) {
            mintBlock(new Block(blockchain.getLast().hash), walletB);
        }
        System.out.println("n[*] Blockchain is valid: " + chainValidity());
}

public static Transaction mintBlock(Block newBlock, Wallet minerAddress) {
        Transaction coinbaseTx = new Transaction(coinbase.publicKey, minerAddress.publicKey, 100f, null);
        coinbaseTx.generateSignature(coinbase.privateKey);
        coinbaseTx.transactionID = "COINBASE_" + newBlock.hashCode();
        coinbaseTx.outputs.add(new TransOutput(coinbaseTx.recipient, coinbaseTx.value, coinbaseTx.transactionID));
        UTXOs.put(coinbaseTx.outputs.getFirst().ID, coinbaseTx.outputs.getFirst());
        
        newBlock.addTransaction(coinbaseTx);
        newBlock.mineBlock(difficulty);
        blockchain.add(newBlock);
        return coinbaseTx; // genesisTransaction
}

These are some of the tests I wrote to check whether the code is working, and numerically all the data works. As in each wallet has the expected transactions with the correct rewards, but there always runs into an error saying Transaction inputs too small; there might be an issue with the transaction inputs or how I’m handling the coinbase results. Below is the transaction class.

public class Transaction {
    public String transactionID;
    public PublicKey sender;
    public PublicKey recipient;
    public float value;
    public byte[] signature;

    public List<TransInput> inputs = new ArrayList<>();
    public List<TransOutput> outputs = new ArrayList<>();
    public static int sequence = 0;

    public Transaction(PublicKey from, PublicKey to, float value, List<TransInput> inputs) {
        this.sender = from;
        this.recipient = to;
        this.value = value;
        this.inputs = inputs == null ? new ArrayList<>() : inputs;
    }
    .. code excluded...
    public boolean processTransaction() {
        if (!this.verifySignature()) {
            System.out.println("[!] Transaction Signature failed to verify.");
            return false;
        }

        for (TransInput i : inputs) {
            i.UTXO = AevumChain.UTXOs.get(i.transOutputID);
        }

        if (this.getInputsValue() < AevumChain.minTransaction) {
            System.out.println("[!] Transaction Inputs too small: " + this.getInputsValue());
            return false;
        }

        float leftOver = this.getInputsValue() - this.value;
        this.transactionID = this.calculateHash();
        this.outputs.add(new TransOutput(this.recipient, value, transactionID));
        if (leftOver > 0) {
            this.outputs.add(new TransOutput(this.sender, leftOver, transactionID)); // send the left over 'change' back to sender
        }

        for (TransOutput o : outputs) {
            AevumChain.UTXOs.put(o.ID, o);
        }

        for (TransInput i : inputs) {
            if (i.UTXO == null) continue;
            AevumChain.UTXOs.remove(i.UTXO.ID);
        }
        return true;
    }

This is the output that I keep getting. Although the transaction go through correctly, there is a transaction inputs too small error everytime the transaction for the coinbase goes through. This is only an issue with the coinbase because the error only occurs during that instance.

[*] Creating and Mining Genesis block...
[+] Block mined: 000002ff3d5c92f121cdf0ebb9f79afa3c1f1c4a9c430b7107e1213d97ce4fee
[*] WalletA's balance is: 100.0
[*] WalletB's balance is: 0.0

[*] WalletA is attempting to send funds (40) to WalletB...
[!] Transaction Inputs too small: 0.0
[!] Transaction failed to process. Discarded.
[+] Block mined: 000008c35d6dd48ff3827eec2031083889ab4373b94998b385fa70e523c7b208
[*] WalletA's balance is: 160.0
[*] WalletB's balance is: 40.0

[*] WalletA is attempting to send more funds (1000) than it has...
[!] Not enough funds to send transaction. Transaction Discarded.
[!] Transaction Inputs too small: 0.0
[!] Transaction failed to process. Discarded.
[+] Block mined: 00000f56d8fe0428aa21bc012045f99f9e494b6bc3935e25c5287a4abde77a3c
[*] WalletA's balance is: 260.0
[*] WalletB's balance is: 40.0

[*] WalletB is attempting to send funds (20) to WalletA...
[*] WalletB is attempting to send funds (10) to WalletA...
[!] Transaction Inputs too small: 0.0
[!] Transaction failed to process. Discarded.
[+] Block mined: 000001f560bfd1b70363b40193ab8a977a94693d83e3533ecaacf43ea2e14521
[*] WalletA's balance is: 290.0
[*] WalletB's balance is: 110.0

[*] WalletB runs a mining algorithm for 10 coins...
[!] Transaction Inputs too small: 0.0
[!] Transaction failed to process. Discarded.
[+] Block mined: 00000a2f483d210b37f656c72c902f29f6b08af5832513f210b029b075a69c3e
[!] Transaction Inputs too small: 0.0
[!] Transaction failed to process. Discarded.
[+] Block mined: 00000b52437c5279e0dd6bde9a1031277e8604278b43ac0fb21cab3b111f02f4
[*] WalletA's balance is: 290.0
[*] WalletB's balance is: 310.0

[*] Blockchain is valid: true

Apologies that I am not able to paste all my code in here, but most of the code is very similar if not the same as mentioned in https://medium.com/programmers-blockchain/create-simple-blockchain-java-tutorial-from-scratch-6eeed3cb03fa. The implementation is the same except I’m trying to reward the miners with 100 coins for actually mining.