Simple Blockchain overview

In a nutshell

Let’s not go into too many details here, just a simplified version of how a blockchain generally work. There are a lot of information out there.

A blockchain is not only for cryptocurrencies, it is a like a digital ledger that keeps track of all the information taking place on a peer to peer network.

That information or data that is shared can be currency, contracts, token of ownership ( like NFTs).

The transactions of these data are sent from peer to peer directly and are confirmed by the other peers on the network, it’s decentralized, and the certifying authority’s responsibility (which makes a transaction legit) is shared among all peers.

Cryptocurrency case

Let’s look at how the cryptocurrency generally works.

Let’s say we have Alistair 🙋‍♂️ who wishes to send bitcoins to Belinda 🙆‍♀️ in exchange for some Gucci goodies:

graph LR A["👨"] ---|Send bitcoins|B["👩"]

Nice, now that we have that done, Belinda 🙆‍♀️‍ is just going to wait for confirmation (around 6 of them) that Alistair 🙋‍♂️ did really send the money and is not trying to scam her. Let’s look at how this transaction gets processed:

sequenceDiagram participant T as Transaction participant M as Memory pool participant BC as blockchain participant P as Peers 👨‍👩‍👧‍👦 T -->> M : Transaction gets added to a memory pool for next block M ->> P : Peers mine the memory pool to find the next block of transaction P -->> M : One miner solves the puzzle Note over P: That creates a block P ->> M: Other peers on the network accept that block M ->> BC: Block gets added to the blockchain BC ->> P: Gives the reward to the miner + transactions fees Note over M,P: The transaction is confirmed once and the mining process keeps on going to add more blocks
each blocks adding confirmation to the previous one

So each new block is a confirmation of payment. In the case of a fraudulent transaction is made, where there is double spending (meaning two transactions are made) then the memory pool would have those two transactions! If mined into two blocks you could have:

  • a block with Alistair 🙋‍♂️‍ Transaction to Belinda 🙆‍♀️
  • another block where the transaction goes to buy pizza 🍕!!

In order to avoid that, and preserve the chain integrity two things can happen:

  • the latest transaction will be refused automatically, and not added to a block.
    • In this case you’ll see it right away if there was a fraud attempt
  • If both transaction are in a block, the transaction in the most accepted block by the community will remain in the chain
    • In this case a transaction is validated by the peers. It takes multiple other blocks on top to be sure that it’s accepted.

The older your transaction is in terms of block the more certain you are that it’s legit. Usually it takes 6 confirmations (6 blocks added to the blockchain) for a merchant to accept payment, it is deemed highly unlikely to be able to push a fraudulent transaction in the blockchain from 6 blocks ago.

Bitcoin specificity

In the bitcoins world, you can have around 2.7k transactions per blocks, and new blocks are added to the bitcoin blockchain every 10min by miners. Mining requires high performance GPUs to solve mathematical problems and find blocks.

Miners make the bitcoin works, and the reward for that is an amount of bitcoin (that get halved every 210’000 blocks). However, there’s a finite amount of 21 millions bitcoins (and we’ve already mined more than 18 millions). Once all the bitcoin are available, mining may become less interesting, the miners still get the transaction fees of the block. In the case the amount of mining decreases (hence the number of possible transactions) the protocol may change through BIPs (Bitcoin Improvement Proposals). 🤷‍♀️

Which leads to one downside of bitcoin (besides its awful ecological footprint 🌳), it’s that if there’s a majority pool of miners, they can decide which blocks go in the blockchain.

This is well explained in O’Reilly mastering bitcoin and it’s all about forks of blocks in the blockchain and convergence between the multiple pools of miners. If you have enough computational power to force the blocks you want in the blockchain, it might lose its integrity.


You would need a library to interface with the bitcoin network and manage your coins. For that a popular choice is bitcoinj which comes with its caveat but is pretty safe for experimentation.

Add it to your java project using gradle:

implementation 'org.bitcoinj:bitcoinj-core:0.15.10'

Most of the examples are also in sylhare/Blockboot for reference. Though the best are the ones directly from the library itself bitcoinj’s examples!

Key components

First, you’ll need two things, the networkParameters which is basically which network you’re using either test for trials or prod for real money. The script type, here P2WPKH ( Pay to Witness Public Key Hash), because transaction fees are lower, it’s fraud proof and is compatible with newer featured introduced in 2015 with SegwitAddress (Segregated Witness)

final static NetworkParameters networkParameters = NetworkParameters.fromID(NetworkParameters.ID_TESTNET);
final static Script.ScriptType scriptType = Script.ScriptType.P2WPKH;

Now that’s set up, here are the other main components. Check bitcoinj work with wallet, but some example might be out of date.


A Wallet is used to store your ECKeys and other data.

When you create a wallet you get multiple account, usually those are managed by the online coin broker you may be using, so you don’t get to know them very well.

Wallet wallet = Wallet.createDeterministic(networkParameters, scriptType);

Deterministic wallets come with address key hierarchy already set up. Public key are expendable, they should only be used once, so your transactions can’t be tracked on the blockchain.

Blockchain and BlockStore

The BlockChain instance manages the shared, global data structure. However, the actual data of the blockchain is accessible via the BlockStore (which can be on your disk).

MemoryBlockStore blockStore = new MemoryBlockStore(networkParameters);
BlockChain blockchain = new BlockChain(networkParameters, wallet, blockStore);

The created blockchain needs to be connected to a given wallet.


A PeerGroup instance manages the network connections:

PeerGroup peerGroup = new PeerGroup(networkParameters, blockChain);

You pass your blockchain to your peerGroup, so you can download some blocks from the peers and sync. The peerGroup is necessary when you will want to make a Transaction:

Wallet.SendResult result = wallet.sendCoins(peerGroup, targetAddress, Coin.parseCoin("0.0005"));"Transaction {} was made - status: {}", result.tx.getTxId(), result.broadcastComplete.get());

With that you’re all set up to start sending coins! Sending coin is not automatic (has said before, the transaction needs to be mined into a block) which means the whole process needs to be asynchronous.

The satoshi is the smallest unit of the bitcoin cryptocurrency named after the bitcoin’s creator

Now, to simplify setting them up, there is also a WalletAppKit object that creates all you need above and connects them together.

Find how to use it in bitcoinj getting started.

If you’re playing with the test network blockchain, and you got some BTC from a faucet (that gives some for free), you may want to find your wallet information using your address with:

With that, it’s time to let your crypto money fly away 💸