Introduction

Blockchain is one of the most revolutionary technologies of the 21st century, which is still maturing and which potential is not fully realized yet. In its essence, blockchain is just a distributed database of records. But what makes it unique is that it’s not a private database, but a public one, i.e. everyone who uses it has a full or partial copy of it. And a new record can be added only with a consent of other keepers of the database. Also, it’s blockchain that made cryptocurrencies and smart contracts possible.In this series of articles we’ll build a simplified cryptocurrency that’s based on a simple blockchain implementation.

Block

Let’s start with the “block” part of “blockchain”. In blockchain it’s blocks that store valuable information. For example, bitcoin blocks store transactions, the essence of any cryptocurrency. Besides this, a block contains some technical information, like its version, current timestamp and the hash of the previous block.
In this article we’re not going to implement the block as it’s described in blockchain or Bitcoin specifications, instead we’ll use a simplified version of it, which contains only significant information. Here’s what it looks like:

type Block struct {

	Timestamp     int64

	Data          []byte

	PrevBlockHash []byte

	Hash          []byte

}

Timestamp is the current timestamp (when the block is created), Data is the actual valuable information containing in the block, PrevBlockHash stores the hash of the previous block, and Hash is the hash of the block. In Bitcoint specification Timestamp,PrevBlockHash, and Hash are block headers, which form a separate data structure, and transactions (Data in our case) is a separate data structure. So we’re mixing them here for simplicity.So how do we calculate the hashes? The way hashes are calculates is very important feature of blockchain, and it’s this feature that makes blockchain secure. The thing is that calculating a hash is a computationally difficult operation, it takes some time even on fast computers (that’s why people buy powerful GPUs to mine Bitcoin). This is an intentional architectural design, which makes adding new blocks difficult, thus preventing their modification after they’re added. We’ll discuss and implement this mechanism in a future article.For now, we’ll just take block fields, concatenate them, and calculate a SHA-256 hash on the concatenated combination. Let’s do this in SetHash method:

func (b *Block) SetHash() {

	timestamp := []byte(strconv.FormatInt(b.Timestamp, 10))

	headers := bytes.Join([][]byte{b.PrevBlockHash, b.Data, timestamp}, []byte{})

	hash := sha256.Sum256(headers)



	b.Hash = hash[:]

}

Next, following a Golang convention, we’ll implement a function that’ll simplify the creation of a block:

func NewBlock(data string, prevBlockHash []byte) *Block {

	block := &Block{time.Now().Unix(), []byte(data), prevBlockHash, []byte{}}

	block.SetHash()

	return block

}

And that’s it for the block!

Blockchain

Now let’s implement a blockchain. In its essence blockchain is just a database with certain structure: it’s an ordered, back-linked list. Which means that blocks are stored in the insertion order and that each block is linked to the previous one. This structure allows to quickly get the latest block in a chain and to (efficiently) get a block by its hash.In Golang this structure can be implemented by using an array and a map: the array would keep ordered hashes (arrays are ordered in Go), and the map would keep hash → blockpairs (maps are unordered). But for our blockchain prototype we’ll just use an array, because we don’t need to get blocks by their hash for now.

type Blockchain struct {

	blocks []*Block

}

This is our first blockchain! I’ve never thought it would be so easy 😉Now let’s make it possible to add blocks to it:

func (bc *Blockchain) AddBlock(data string) {

	prevBlock := bc.blocks[len(bc.blocks)-1]

	newBlock := NewBlock(data, prevBlock.Hash)

	bc.blocks = append(bc.blocks, newBlock)

}

That’s it! Or not?..To add a new block we need an existing block, but there’re not blocks in our blockchain! So, in any blockchain, there must be at least one block, and such block, the first in the chain, is called genesis block. Let’s implement a method that creates such a block:

func NewGenesisBlock() *Block {

	return NewBlock("Genesis Block", []byte{})

}

Now, we can implement a function that creates a blockchain with the genesis block:

func NewBlockchain() *Blockchain {

	return &Blockchain{[]*Block{NewGenesisBlock()}}

}

Let’s check that the blockchain works correctly:

func main() {

	bc := NewBlockchain()



	bc.AddBlock("Send 1 BTC to Ivan")

	bc.AddBlock("Send 2 more BTC to Ivan")



	for _, block := range bc.blocks {

		fmt.Printf("Prev. hash: %x\n", block.PrevBlockHash)

		fmt.Printf("Data: %s\n", block.Data)

		fmt.Printf("Hash: %x\n", block.Hash)

		fmt.Println()

	}

}

Output:

Prev. hash:

Data: Genesis Block

Hash: aff955a50dc6cd2abfe81b8849eab15f99ed1dc333d38487024223b5fe0f1168



Prev. hash: aff955a50dc6cd2abfe81b8849eab15f99ed1dc333d38487024223b5fe0f1168

Data: Send 1 BTC to Ivan

Hash: d75ce22a840abb9b4e8fc3b60767c4ba3f46a0432d3ea15b71aef9fde6a314e1



Prev. hash: d75ce22a840abb9b4e8fc3b60767c4ba3f46a0432d3ea15b71aef9fde6a314e1

Data: Send 2 more BTC to Ivan

Hash: 561237522bb7fcfbccbc6fe0e98bbbde7427ffe01c6fb223f7562288ca2295d1

That’s it!

Conclusion

We built a very simple blockchain prototype: it’s just an array of blocks, with each block having a connection to the previous one. The actual blockchain is much more complex though. In our blockchain adding new blocks is easy and fast, but in real blockchain adding new blocks requires some work: one has to perform some heavy computations before getting a permission to add block (this mechanism is called Proof-of-Work). Also, blockchain is a distributed database that has no single decision maker. Thus, a new block must be confirmed and approved by other participants of the network (this mechanism is called consensus). And there’re no transactions in our blockchain yet!In future articles we’ll cover each of these features.Links:

Full source codes: https://github.com/Jeiwan/blockchain_go/tree/part_1
Block hashing algorithm: https://en.bitcoin.it/wiki/Block_hashing_algorithm