Create a Blockchain Explorer in Csharp

Create a Blockchain Explorer in Csharp

Blockchain technology is already central to major cryptocurrencies like Bitcoin and its applications are ever growing. A blockchain is simply a list of records, called blocks

Each block contains information about transactions - and also confirms the validity of the previous block with something called a hash function. A hash function calculates a number from all of the data in a block. It works in such a way that if any of the bytes in the block data changes, the result of the hash function will be a different number.In a blockchain, one cannot change a block without recalculating all hashes from the changed block and to the end of the chain. Together with the fact that a blockchain is distributed to a many computers in a peer-to-peer network, this makes blockchains resistant to modifications of the data and all kinds of fraud.

A great way to learn about the blockchains for a programmer is to dive in and make a Bitcoin Blockchain Explorer - and that’s exactly what you will do right now if you keep reading on!

Start by downloading Visual Studio 2019 Community Edition from Make sure to check the workload “ASP.NET and web development”:

If you forget, no problem. Just open “Visual Studio Installer” from your start menu, select Modify, and then you are back at the screen where you can tick off the workloads you want. You may also use Visual Studio 2017, but it would be wise to update it - to make sure you have the latest version of ASP.NET Core and .NET Core. Start the Visual Studio installer and select Update if that is an option.

Scaffold Your C# Blockchain Application

Start Visual Studio, then select Create a new project, and select the template ASP.NET Core Web Application and click Next:

Enter “BlockchainExplorer” for Project name and click Create.

Next, select the template Web Application. Note that another template is named “Web Application (Model-View-Controller)”. That is not the one you want. Select the one that is only Web Application. Make sure the Configure for HTTPS is checked and click Create.

Visual Studio will scaffold some nice code to start out with. Try to run it by pressing ```Ctrl+F5````. The first time it runs, you should get this warning:

Check Don’t ask me again and click Yes. This enables running the application through HTTPS without a separate SSL certificate. A new dialog will pop up. Answer Yes:

You may get a warning in the browser telling you that “Your connection is not private”. If so, click Advanced in the lower left corner of the page. Then some more text appears, and at the bottom, there will be a link with the text Proceed to localhost (unsafe). Click it.

You are going to call the Blockchain API at, but there is a convenient NuGet package which makes it really easy. Go to the Package Manager Console by selecting the Tools menu, and then NuGet Package Manager, and then Package Manager Console.

Run the following command in the console:

Install-Package BlockchainAPI -Version 4.0.0 

The user will select from a menu of different methods in the Blockchain API. You will implement a recent searches function, and it will keep each user’s recent searches in an in-memory database. For this to work, you will need a way for the users to authenticate.

Add Login to Your ASP.NET Core C# Web application

No reason to write the authentication yourself. You can easily integrate Okta to handle the authentication for you:

Sign up for a forever-free developer account (or login if you already have one). Once you have signed up and logged in, you’ll be taken to your dashboard. Make note of your Org URL in the top right corner. It looks something like this: **Org URL: **

You must also register your application in the Okta dashboard. Select Applications at the top, and then click the green button, Add Application. Select Web and click Next.

Enter “Blockchain Explorer” for Name, and click Done. Then click Edit to go back to the form, but before we enter some values, go back to Visual Studio. Right-click the solution, BlockchainExplorer and select Properties. Then go to the tab, Debug and observe which port number is used in the URL mentioned after the checkbox Enable SSL. It is usually around 44300.

Back in the Okta Dashboard, change Login redirect URIs to use https and the same port number as above. Do the same for Initiate login URI, and add a Logout redirect URIs with a value like [https://localhost:44354/signout/callback](https://localhost:44354/signout/callback "https://localhost:44354/signout/callback"), but with the port number you have already used. Click Save.

Also, go to API in the top menu of the dashboard and click Trusted Origins. There should be a table row with a Name containing Blockchain Explorer. Click the edit icon, and change the port number to the same you used above.

Back in the Package Manager Console, also install the package Okta.AspNetCore which will make it easy to add authentication to your application. Use this command:

Install-Package Okta.AspNetCore -Version 1.2.0 

Now, open appsettings.json, and add the lines you are missing from the code below:

  "Logging": {
    "LogLevel": {
      "Default": "Warning"
  "Okta": {
    "ClientId": "{yourClientId}",
    "ClientSecret": "{yourClientSecret}",
    "OktaDomain": "{yourOktaDomain}",
    "PostLogoutRedirectUri": "https://localhost:5001/"
  "AllowedHosts": "*"

Get the values for ClientId and ClientSecret from the Okta Dashboard page that you just visited. Get the value for OktaDomain from the main page on the Okta Dashboard. (Click Dashboard on the top, and you will see your OktaDomain on the top right.)

Open “Startup.cs” and add these using statements:

using Microsoft.AspNetCore.Authentication.Cookies;
using Okta.AspNetCore;

Edit the method ConfigureServices() so that you end up with this:

public void ConfigureServices(IServiceCollection services)
    var oktaMvcOptions = new OktaMvcOptions();
    oktaMvcOptions.Scope = new List<string> { "openid", "profile", "email" };
    oktaMvcOptions.GetClaimsFromUserInfoEndpoint = true;

    services.AddAuthentication(options =>
            options.DefaultAuthenticateScheme = CookieAuthenticationDefaults.AuthenticationScheme;
            options.DefaultSignInScheme = CookieAuthenticationDefaults.AuthenticationScheme;
            options.DefaultChallengeScheme = OktaDefaults.MvcAuthenticationScheme;
    services.Configure<CookiePolicyOptions>(options =>
        options.CheckConsentNeeded = context => true;
        options.MinimumSameSitePolicy = SameSiteMode.None;

        .AddRazorPagesOptions(options =>

One important line is options.Conventions.AuthorizePage("/Index");, which declares that the Index page requires the user to login. Okta will automatically redirect any user who is not logged in to the login page, and back to Index after they successfully log in.

In the method Configure(), add app.UseAuthentication(); just before app.UseMvc();.

Now, start the application with Ctrl+F5, and you will be redirected to the Okta Login Page. After logging in, you will be redirected back to the application. Note that you may already be logged in and not see the login page. Try running again in an Incognito browser window and you will definitely see it.

In the Okta Dashboard, you may add more users to the application, but your own Okta user should already be given access to this application.

Explore the Blockchain from C#

Let’s get started exploring the blockchain. You will use the class BlockExplorer to call the Blockchain API - it has a method for each API call. Now you will enable them all for your users with some C# reflection.

Open Index.cshtml.cs, and replace the using statements with:

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Reflection;
using System.Threading.Tasks;
using BlockchainExplorer.Model;
using Info.Blockchain.API.BlockExplorer;
using Info.Blockchain.API.Models;
using Microsoft.AspNetCore.Mvc.RazorPages;

Add this property to save all the possible actions:

public IEnumerable<MethodInfo> Actions { get; private set; } 

Add the methods below to the class:

private void CreateActions()
    Actions = typeof(BlockExplorer)
        .GetMethods(BindingFlags.DeclaredOnly | BindingFlags.Public | BindingFlags.Instance)
        .Where(m => m.GetParameters().Length <= 1);

public string ShortenName(string s)
    if (s == null) return s;
    return s.Replace("Get", "").Replace("Async", "");

public string InputTypeFromCsType(Type t)
    if (t == typeof(DateTime)) return "date";
    if (t == typeof(long)) return "number";
    return "text";

The method CreateActions() gets all the public methods from the BlockExplorer class that has none or one parameter. Those are the interesting ones. *Add a call to *CreateActions* from the method **OnGet()*.

The method ShortenName takes away the words “Get” and “Async” from the names of the actions. It makes the UI prettier.

InputTypeFromCsType() helps to find the correct value for type in <input type="..."> in the UI, so that dates get date inputs, numbers get number inputs and all others get text inputs.

Open Index.cshtml, replace the contents with the following:

@using System.Reflection
@using Microsoft.AspNetCore.Mvc.Rendering
@model IndexModel

<div style="display: flex;flex-direction: row">
        <div style="">
            <select onchange="show(this.value)">
                <option value="blank"></option>
                @foreach (var action in Model.Actions)
                    var actionName = Model.ShortenName(action.Name);
                    <option value="@action.Name">@actionName</option>
            @foreach (var action in Model.Actions)
                var actionName = "Get" + Model.ShortenName(action.Name);
                actionName = string.Concat(actionName.Select(x => Char.IsUpper(x) ? " " + x : x.ToString())).TrimStart(' ');
                <form id="@action.Name" style="display: none">
                    @Html.Hidden("actionName", action.Name)
                    @foreach (var param in action.GetParameters())
                        <input name="paramValue" type="@Model.InputTypeFromCsType(@param.ParameterType)" />
                    <input type="submit" value="@actionName" />
        <table class="values"></table>

    function show(actionName) {
        for (var form of document.getElementsByTagName('form')) = 'none';
        document.getElementById(actionName).style.display = 'inline';

This HTML makes a drop down where the user can select which action to do. There is one HTML <form> for each action, but they are all hidden when the page is loaded. When selecting an action, only the corresponding form is made visible. That is done by the JavaScript at the bottom. Run the application, and see for yourself!

Call the Blockchain API from C#

Next, you want to actually call the API and see the results. Add a new folder Model and add a class Search inside it. The contents will be:

using System.Collections.Generic;
using System.ComponentModel.DataAnnotations.Schema;

namespace BlockchainExplorer.Model
    public class Search
        public long Id { get; set; }
        public string ActionName { get; set; }
        public string ParamValue { get; set; }
        public string User { get; set; }
        public List<int> Indexes { get; set; }

        public int GetCollectionNo(int index)
            return index >= Indexes.Count ? 1 : Indexes[index] + 1;

        public object GetParamsObj(int delta, int index = 0)
            while (Indexes.Count < index + 1) Indexes.Add(0);
            var indexes = Indexes.ToArray();
            indexes[index] += delta;

            return new
                actionName = ActionName,
                paramValue = ParamValue,
                indexes = indexes

The field Id is only used later when these objects are saved in an in-memory database with the help of EntityFramework Core. ActionName is the name of the action, and ParamValue is the parameter value if it needs one. User is used later when recording each user’s recent searches. Indexes is used for paging, when the results contain collections. More about that later.

Go back to Index.cshtml.cs, and add these fields to the class:

private object _object;
public string ActionName { get; private set; }
public PropertyInfo[] SimpleProps { get; private set; }
public PropertyInfo[] CollectionProps { get; private set; }

public ICollection Collection { get; private set; }
public IEnumerable<Search> RecentSearches { get; set; }
public Search CurrentSearch { get; set; }

All of these are set in Index.cshtml.cs but used in the Index.cshtml file, so that it can combine the data with the HTML required for the page.

Add these helper methods to the class:

private async Task<object> DoAction(string paramValue, MethodInfo action)
    var explorer = new BlockExplorer();
    var param = action.GetParameters().FirstOrDefault();
    var paramsObj = param == null ? new object[] { } : new[] { ConvertValue(paramValue, param.ParameterType) };
    var task = action.Invoke(explorer, paramsObj) as Task;
    await task.ConfigureAwait(false);
    var resultProperty = task.GetType().GetProperty("Result");
    var obj = resultProperty.GetValue(task);
    obj = IfCollection(obj);
    obj = await IfLatestBlock(obj, explorer);
    return obj;

private void Save(object obj)
    var type = obj.GetType();
    var allProps = type.GetProperties();
    SimpleProps = allProps.Where(p => !p.PropertyType.Name.Contains("Collection")).ToArray();
    CollectionProps = allProps.Where(p => p.PropertyType.Name.Contains("Collection")).ToArray();
    _object = obj;

private object ConvertValue(string value, Type type)
    if (type == typeof(long)) return Convert.ToInt64(value);
    if (type != typeof(DateTime)) return value;
    DateTime.TryParse(value, out var result);
    return result;

private object IfCollection(object o)
    var collection = o as ICollection;
    if (collection == null) return o;
    Collection = collection;
    var enumerator = collection.GetEnumerator();
    if (CurrentSearch.Indexes.Count > 0)
        var skipCount = CurrentSearch.Indexes[0];
        while (skipCount-- > 0) enumerator.MoveNext();
    return enumerator.Current;

private async Task<object> IfLatestBlock(object obj, BlockExplorer explorer)
    var latestBlock = obj as LatestBlock;
    if (latestBlock == null) return obj;
    return await explorer.GetBlockByHashAsync(latestBlock.Hash);

public object GetCollectionElement(PropertyInfo prop, int collectionIndex)
    var collection = prop.GetValue(_object) as IEnumerable<object>;
    if (collection == null) return null;
    var skipCount = CurrentSearch.GetCollectionNo(collectionIndex) - 1;
    return collection.Skip(skipCount).FirstOrDefault();

public int GetCollectionCount(PropertyInfo prop, int collectionIndex)
    var collection = prop.GetValue(_object) as IEnumerable<object>;
    return collection?.Count() ?? 0;

public string GetValue(PropertyInfo prop)
    return prop.GetValue(_object).ToString();

Then change OnGet() to this:

public async Task OnGet(string actionName, string paramValue, int[] indexes)
    if (actionName == null) return;
    ActionName = ShortenName(actionName);
    var action = Actions.SingleOrDefault(a => a.Name.Contains(actionName));
    if (action == null) return;
    CurrentSearch = new Search { ActionName = actionName, ParamValue = paramValue, Indexes = new List<int>(indexes) };
    var obj = await DoAction(paramValue, action);

After creating the actions, the code will check to see if the user wants to do an action. If not, the method is finished - and returns. The property ActionName contains a shortened version of the action name.

Next, the code will look up the correct action -quit if it finds nothing - or continue by setting the CurrentSearch property on an object of Search with the parameters to OnGet() saved. Then it will perform the action and get an object with all the values back. The object is saved and all of its values are shown in the view using reflection.

Go to index.cshtml and switch <table class="values"></table> to this:

<table class="values">
    @foreach (var prop in Model.SimpleProps ?? Enumerable.Empty<PropertyInfo>())
        var value = Model.GetValue(prop);
            <td class="content">@value</td>

@if (Model.CollectionProps != null && Model.CollectionProps.Length > 0)
    for (var index = 0; index < Model.CollectionProps.Length; index++)
        var prop = Model.CollectionProps[index];
        var collectionCount = Model.GetCollectionCount(prop, index);
        var collection = Model.GetCollectionElement(prop, index);

        var props = collection.GetType().GetProperties().Where(p => !p.PropertyType.Name.Contains("Collection"));
        var collectionNo = Model.CurrentSearch?.GetCollectionNo(index + 1);

        <span>Showing #@collectionNo of @collectionCount</span>
        @if (collectionNo > 1)
            @Html.ActionLink("Previous", "", Model.CurrentSearch.GetParamsObj(-1, index + 1));
        @if (collectionNo < collectionCount)
            @Html.ActionLink("Next", "", Model.CurrentSearch.GetParamsObj(1, index + 1));

        <table class="values">
            @foreach (var collProp in props)
                var value = collProp.GetValue(collection)?.ToString();

                    <td class="content">@value</td>

The code will make a simple HTML table with one column for labels and one for values. Collection properties are treated separately. Only one item from each collection is shown, and the int[] indexes property in CurrentSearch specifies which index to show for each collection. The collections all get buttons for next and previous.

You can now run the application again and start exploring the blockchain API. LatestBlock is nice to start with, since it doesn’t require any parameters. Start the application, open the drop-down below the menu-header, and select LatestBlock. Then click the button that appears, Get Latest Block. After running it, it should look something like this:

Further reading:

Build a simple Cryptocurrency App using Angular 8

Blockchain and AI: Stronger Together

Blockchain Applications and Its Future

A Simple Blockchain Application for Data Verification.

Top 10 Best Blockchain Programming Language for Blockchain Programmer

Top 10 Best Blockchain Programming Language for Blockchain Programmer

We aim at equipping you with every necessary knowledge of the best programming languages for blockchain and you'll learn more 10 Best Blockchain Programming Language for Programmer: C++, Java, Python, Ruby, Solidity, Go, JavaScript...

We aim at equipping you with every necessary knowledge of the best programming languages for blockchain and you'll learn more 10 Best Blockchain Programming Language for Programmer: C++, Java, Python, Ruby, Solidity, Go, JavaScript...

If you’re a tech-oriented person like me, chances are you’ve wondered at least once about all those latest fusses regarding blockchain and cryptocurrencies. So, what is this blockchain and why’d you be tempted to learn more about it? Blockchain, as the name suggests, is a chain of blocks; connected sequentially using complex cryptographic logic.

This technology was implemented first by Satoshi Nakamoto and was first used in the implementation of the popular BitCoin cryptocurrency. The blockchain technology is being used heavily in the industry, thanks to the high-level of security it provides in business transactions. From corporate firms to industrial banks, blockchain developers are sought everywhere equally. So, wielding this modern-day skill by learning the best blockchain programming language guarantee you an edge over your fellow developers.

Which are the best programming languages for blockchain? Developers are presently utilizing prevalent programming languages like C++ and Java to manufacture custom blockchain programs. What’s more, digital money specialists have made languages like Simplicity and Solidity that are explicitly intended for blockchain improvement.

The worldwide Blockchain market is right now worth an expected $1.2 billion and specialists foresee that it will arrive at a $57 billion valuation by 2025, developing at over 69% every year.

Significant enterprises and investors are teaming up with Blockchain counseling organizations to grow new digital currency innovation, savvy contracts, conveyed records for customary banks, gaming tokens, and inventory network the executives frameworks.

What Is Blockchain?

Customary financial uses a bank as the record and mediator. To move cash to a companion, an individual should initially contact their own bank and request that they move cash to a particular record number. The bank checks the sender’s record for assets, moves those assets to the goal, and records the exchange on the sender’s record. The accepting bank must accomplish something very similar.

In any case, the issue with this customary financial framework is that records are put away inside and are defenseless against hacking and control.

Blockchain disposes of this hazard by putting away all records online in a decentralized, unknown record that can be gotten to by anybody. Blockchain uses squares, or accumulations of information, like spreadsheet lines and segments, to store information. Squares are added to the “chain” in successive request.

In contrast to conventional bank records, which are put away inside, each blockchain client has a total record of the whole blockchain on their PC. This implies they can rapidly discover any exchange that has ever happened in the event that they have the comparing hash code. Since that information is put away freely, it can never be changed or erased — giving clients genuine feelings of serenity and trust in the framework.

Organizations keen on exploiting the blockchain upset should scan for up-and-comers with skill in the accompanying programming languages.

Here are the best programming languages for Blockchain

1. C++

C++ keeps on being one of the most famous programming languages in the tech world and is a prevailing power in the blockchain business also. The article arranged language is ideal for blockchain improvement, since it utilizes similar standards, for example, epitome, deliberation, polymorphism, and information covering up, as blockchain to avoid incidental alters to information.

Engineers additionally prize C++ in view of its memory control abilities. The language helps keep squares secure and deal with an enormous number of asset demands by enabling each system hub to acknowledge or dismiss individual squares.

C++ is additionally utilized broadly by blockchain advancement administrations due to the manner in which it handles parallel undertakings and stringing. The language is equipped for taking care of both parallel and non-parallel assignments, notwithstanding improving single-string execution.

EOS is an awesome case of a blockchain program worked with C++. The open source programming was discharged by Square in 2018 and is intended to process exchanges more rapidly than choices by restricting the product to only 21 square creating hubs. This enables the product to affirm an exchange in under a second and settle it in only two minutes.

2. JavaScript

GitHub as of late positioned JavaScript as the most mainstream language for developers — with a fantastic 95% of sites utilizing it here and there. Be that as it may, JavaScript isn’t just the lord of web advancement; the adaptable programming language is additionally utilized broadly for blockchain improvement.

One reason why blockchain designers prize JavaScript is a direct result of the manner in which it handles offbeat code. This is significant in blockchain, as thousands or even a great many exchanges might be started simultaneously. Offbeat, parallel programming empowers a program to finish numerous activities all the while. Standard, synchronous programming just can’t deal with that volume.

By running numerous activities on the double, offbeat code can improve programming responsiveness and application execution. This empowers blockchain projects to deal with the enormous volume of activities without hindering execution and disappointing clients.

You may also like: How to Build a Blockchain in JavaScript.

3. Java

The only language that can challenge the reign of C++ in the industry is Java, and for good reasons so. Java is in many ways similar to C++ regarding its object-oriented approach and a vast community of third-party applications and platforms. The main reason to use Java as the de-facto blockchain programming language in the industry is, however, its highly-capable portability.

Programs written in Java are portable across any computational device, as they don’t rely on system-specific architecture, instead uses the universal JVM(Java Virtual Machine) for execution. This makes Java one of the best programming languages for blockchain.

4. Python

Python is probably THE most trending programming language you can learn these days!

It's very popular because it's easy to learn and use, runs on all operating systems and allows you to build a broad variety of programs: Be that web applications, desktop applications, utility scripts or using it for data science and machine learning.

You'll do so whilst building your own Blockchain and Cryptocurrency. These are of course also highly trending topics and not a lot of people understand what a Blockchain really is, you'll learn a lot about the core concepts of the Blockchain and you'll see how Python can be used for the many aspects that make up a Blockchain and Cryptocurrency.

You may also like: Building a Blockchain with Python.

5. Solidity

Solidity is a savvy contract and blockchain improvement language that is utilized broadly by Ethereum designers. The area explicit language utilizes a significant number of indistinguishable standards and punctuation from JavaScript to make high-caliber, decentralized applications.

Engineers lean toward the language since it enables them to compose elevated level code for the Ethereum blockchain arrange, the second-most famous blockchain digital currency, which can be assembled into low-level machine code. It additionally enables people to use the Ethereum advanced exchange record to make brilliant agreements between organizations.

The agreement situated language utilizes invariants, preconditions, and post-conditions to streamline the advancement procedure and to make the agreement age process easier for clients.

Solidity is at present accessible on a scope of blockchain stages, including Ethereum, Ethereum Great, Tendermint, and Counterparty. It’s utilized for a scope of utilizations, including business contracts, barters, crowdfunding, and that’s just the beginning.

6. Ruby

Although quite old and tested by the industry, Ruby gained momentum as a blockchain programming language in the last couple of years or so. Ruby, an interpreted high-level language with object-oriented features, much like Python, can be a viable blockchain coding language for uncountable reasons. It offers developers the ability to prototype their vision rapidly using open source third-party APIs and plugins.

The Ruby ecosystem is thriving with loyal contributors since its inception as the de-facto web language starting from the first half of this millennium. It’s especially prevalent within the Asian developers, the most substantial fraction of open source blockchain developers.

7. Simplicity

Simplicity is a fresh out of the plastic new programming language that was discharged in November 2017 and planned explicitly for shrewd agreements and blockchain improvement. The language conceals low-level consistent parts from architects so as to expand efficiency and stay away from engineer interruptions, which is one motivation behind why it is quickly winding up well known in the network.

Like C++, Effortlessness is an item arranged language that uses indistinguishable standards from blockchain to forestall blunders and changes to information. It additionally utilizes Merklized Theoretical Sentence structure Trees to sort out the projects into trees — along these lines taking into account littler exchange sizes and lessening square space prerequisites.

The language’s makers, Blockstream, are as yet extending the language and its abilities. Designers can hope to see Simplicity being utilized in more applications towards mid-2020 once the language is incorporated into Bitcoin and its highlights are concluded.

8. Go

The brainchild of Robert Griesemer, Rob Pike, and Ken Thompson, pioneer of modern programming languages, Go is the best blockchain programming language for building hyper ledger fabric. The statically-typed yet compiled language is on par the performance level needed by a blockchain coding language. Go comes with every advanced feature you’d need when building your first blockchain, namely classes and inheritance, generics, annotations, constructors, and exceptions.

Go offers top-notch concurrency support in blockchain applications, thanks to its smart implementation of channels and interfaces. So, it’s one of the best programming languages for blockchain when it comes to developing a system that is not only efficient but also lightning fast.

9. Rust

The newest blockchain programming language on the block currently, Rust aims at providing open source devs the capability to build fast and efficient blockchain systems. We found Rust to be exceptionally good when it comes to CPU-bound tasks. You can take either a functional approach or an imperative one with Rust for developing your blockchain.

It’s one of the best programming languages for blockchain due to its highly-capable mechanism of handling mutable states. The Rust compiler provides awe-inspiring optimization of your blockchain. The fast, memory safe, and exclusively concurrent nature of this blockchain coding language makes it most suitable for developing real-world blockchains.

10. PHP

Although dimmed not suitable for modern web anymore, PHP still covers the majority of web systems. It can be utilized to build simple to complex blockchain systems as well, thanks to its object-oriented features and a vast active open source community.

If you’re a new programmer looking for getting your hands dirty at blockchain coding, PHP might turn out to be the best option for you. A considerable number of PHP developers will guarantee a ready workforce in case you develop something highly capable and intend on going corporate.


Blockchain is here to stay. The popular record-keeping technology is what makes cryptocurrency exchanges possible and is widely used by corporations, individuals, and blockchain consulting services for software development.

Developers can easily use popular programming languages like C++ and Java for blockchain development. Alternatively, the community has recently created blockchain-specific languages like Solidity and Simplicity which make cryptocurrency development a smooth process.

Expect to see more original languages spring up over the next several years, as the blockchain market continues to grow rapidly and cryptocurrency begins to be used by ever-larger numbers of people.

Thank for reading!

What is in a block? - Blockchain

In my previous article, I tried to described the concept of a blockchain with code. This time, I'll try to describe the structure of a single block. I will use the Bitcoin blockchain to explain blocks, but keep in mind that the concepts will remain more or less the same. It could be useful to read my&nbsp;<a href="" target="_blank">last article</a>to understand a few things first.


In my previous article, I tried to described the concept of a blockchain with code. This time, I'll try to describe the structure of a single block. I will use the Bitcoin blockchain to explain blocks, but keep in mind that the concepts will remain more or less the same. It could be useful to read my last articleto understand a few things first.

Structure of a block

A block is a container data structure. In the Bitcoin world, a block contains more than 500 transactions on average. The average size of a block seems to be 1MB (source). In Bitcoin Cash ( a hard fork from the Bitcoin blockchain ), the size of a block can go up to 8MB. This enables more transactions to be processed per second.

Anyway, a block is composed of a header and a long list of transactions. Let's start with the header.

Block Header

The header contains metadata about a block. There are three different sets of metadata:

  • The previous block hash. Remember that in a blockchain, every block is inherits from the previous block because we use the previous block's hash to create the new block's hash. For every block N, we feed it the hash of the block N-1.
  • Mining competition. For a block to be part of the blockchain, it needs to be given a valid hash. This contains the timestamp, the nonce and the difficulty. Mining is another crucial part of the blockchain technology, but it is outside the scope of this article.
  • The third part is a merkle tree root. This is a data structure to summarize the transactions in the block. And we will leave it at that for now. More on this later.
Block identifiers

To identify a block, you have a cryptographic hash, a digital signature if you will. This is created by hashing the block header twice with the SHA256 algorithm. For example, this is a block. I will refer to this block as an example for this article.

The block header hash for this particular block is (right column): 000000000000000000301fcfeb141088a93b77dc0d52571a1185b425256ae2fb

We also can see the previous block's hash (right column): 0000000000000000004b1ef0105dc1275b3adfd067aed63a43324929bed64fd7

Remember that we used the second hash to create the first. Every block uses the previous block's hash to construct its own hash. The block hash is a unique identifier. You won't find two blocks with the same hash.

The other way to identify a specific block is the block height. The is the position of the block in the blockchain. Our example's block is in the 500312 position. This means that there are 500311 blocks before this one. Since the creation of the Bitcoin blockchain in 2009, 500312 blocks have been created ( at the time of writing obviously ).

A block height is not unique. Several blocks can compete for the same position in the case of a fork, like Bitcoin Cash for example.

Merkle Trees

The transactions in a block are contained in a structure called a merkle tree or binary hash tree.

I feel that topics like that are easier to understand with actual examples. So we'll go coding for this. A merkle tree is constructed by recursively hashing pairs of nodes ( in this case, transactions ), until there is only one hash, called the root or merkle root. If we stay in the Bitcoin world, the cryptographic hash algorithm used is SHA256. This is applied twice each time.

An example: We have a block with 4 transactions. For the sake of simplicity, each transaction is a string:

const tA = 'Hello'
const tB = 'How are you?'
const tC = 'This is Thursday'
const tD = 'Happy new Year'

To construct our merkle tree, we start from the bottom. We take each transaction and double-hash them. I'll use the js-sha256 package here.

const sha256 = require('js-sha256').sha256

// Double-hashing here
const hA = sha256(sha256(tA))
const hB = sha256(sha256(tB))
const hC = sha256(sha256(tC))
const hD = sha256(sha256(tD))

52c87cd40ccfbd7873af4180fced6d38803d4c3684ed60f6513e8d16077e5b8e //hA
426436adcaca92d2f41d221e0dd48d1518b524c56e4e93fd324d10cb4ff8bfb9 //hB
6eeb307fb7fbc0b0fdb8bcfdcd2d455e4f6f347ff8007ed47475481a462e1aeb //hC
fd0df328a806a6517e2eafeaacea72964f689d29560185294b4e99ca16c63f8f //hD

Ok, great. Now remember that I wrote a merkle tree is constructed hashing pairs of nodes. So, we will pair our transactions and concatenate their hashes. Then, we will double hash them too. We will create a hash using the hashes hA and hB, and another for hC and hD. Then, we repeat that process until we have only one hash left and no more pairs to work with. The last hash will be our merkle root.

With only four transactions, this will be rather quick:

//Pairing hA and hB

const hAB = sha256(sha256(hA + hB))

//Pairing hC and hD

const hCD = sha256(sha256(hC + hD))

// We do it again. We pair hAB and hCD
// This is our root!
const hABCD = sha256(sha256(hAB + hCD))

The node at the top of the merkle tree is called the root. This is the information that is stored in the block header in each block on the blockchain. This is how transactions are summarized in each block. In our example block given earlier, the merkle root can be found in the right column: 


It doesn't matter how many transactions are contained in a block, they always will be summarized by a 32 bytes hash.

Note: The merkle tree is a binary tree. If there is an odd number of transactions, the last one will be duplicated so we can construct our tree.

Because all the leaves in our tree depends on other leaves, it is impossible to alter one leaf without altering others. If you change only one leaf ( one transaction ), the hash changes, therefore the hash you constructed by pairing it with another leaf changes, therefore the merkle root will be different.

You can prove that any transaction is included in a block by creating a authentification path or merkle path. You only need to know log base 2(N) 32-byte hashes. For example:

-For my 4 transactions merkle tree:

log base 2( 4 ) = 2 => If I have a path of 2 hashes for a tree of 4 transactions, I can manage to prove if a transaction belongs to this merkle tree.

For a 16 transactions merkle tree:

log base 2( 16 ) = 4 => If I have a path of 4 hashes for a tree of 16 transactions, I can manage to prove if a transaction belongs to this merkle tree.

log base 2( 1500 ) = 10.55 => If I have a path of 11 hashes for a tree of 1500 transactions, I can manage to prove if a transaction belongs to this merkle tree.

Perhaps a little diagram will help.

There are 16 leaves in this tree. We construct our tree from the bottom up by pairing each leaf. Now, anybody can prove that the leaf I ( in orange ) is part of this block by having the path given in green. We have only 4 hashes, but that is enough to know if the leaf I belongs here. That is because with those informations, we are able to construct every single leaf we need( in yellow ). We can create IJIJKLIJKLMNOP and the root and check if those hashes correspond. This is why it is very complicated to cheat a blockchain. To change one thing means you must change everything.

Well, that's pretty much what a block contains in the Bitcoin blockchain. Hope it helped!