Top 42 Blockchain Interview Questions And Answers

Question 1

What is blockchain?

Accepted Answer

**Blockchain** is a secure distributed ledger (data structure or database) that maintains a continuously growing list of ordered records, called “blocks”,  that are linked using cryptography. Each block contains a cryptographic hash of the previous block, a timestamp, and transaction data.

By design, a blockchain is resistant to modification of the data. It is "an open, distributed ledger that can record transactions between two parties efficiently and in a verifiable and permanent way".

Once recorded, the data in any given block cannot be altered retroactively without alteration of all subsequent blocks, which requires consensus of the network majority.

Question 2

Explain the common structure of blockchains

Accepted Answer

Blockchains are composed of three core parts:
* **Block**: A list of transactions recorded into a ledger over a given period. The size, period, and triggering event for blocks is different for every blockchain.
* **Chain**: A hash that links one block to another, mathematically “chaining” them together. 
* **Network**: The network is composed of “full nodes.” Think of them as the computer running an algorithm that is securing the network. Each node contains a complete record of all the transactions that were ever recorded in that blockchain.

Question 3

What is the blockchain data structure?

Accepted Answer

Basically **the blockchain data structure** is explained as a back-linked record of blocks of transactions, which is ordered. It can be saved as a file or in a plain database. Each block can be recognized by a hash, created utilizing the SHA256 cryptographic hash algorithm on the header of the block. Each block mentions a former block, also identified as the parent block, in the “previous block hash” field, in the block header.

![](https://vitalflux.com/wp-content/uploads/2018/06/Blockchain-represented-as-Linked-List-Data-Structure.png)

Question 4

Why does Blockchain need coins or tokens?

Accepted Answer

_Tokens/Coins are used as a medium of exchange between the states_. They are digital assets built in to perform a specific function within a blockchain.

When someone does a transaction, there is a _change of state_, and coins are moved from one address to another address. Apart from that, transactions contain some additional data; this data can be mutated through the change of state. For this reason, blockchains need coins or tokens to incentivize the participants to join their networks.

Question 5

What is the purpose of a blockchain node?

Accepted Answer

A blockchain exists out of blocks of data. These blocks of data are stored on nodes (compare it to small servers). **Nodes** can be any kind of device (mostly computers, laptops or even bigger servers). Nodes form the infrastructure of a blockchain.

All nodes on a blockchain are connected to each other and they constantly exchange the latest blockchain data with each other so all nodes stay up to date. They store, spread and preserve the blockchain data, so theoretically a blockchain exists on nodes.

A **full node** is basically a device (like a computer) that contains a full copy of the transaction history of the blockchain.

Question 6

What is the Genesis Block?

Accepted Answer

The **first block in any blockchain **is termed the **genesis block**. If you start at any block and follow the chain backwards chronologically, you will arrive at the genesis block. The genesis block is statically encoded within the client software, that it cannot be changed. Every node can identify the genesis block’s hash and structure, the fixed time of creation, and the single transactions within. Thus every node has a secure “root” from which is possible to build a trusted blockchain on.

Question 7

What is deterministic behavior?

Accepted Answer

If A + B = C, then no matter what the circumstances, A+B will always be equal to C. That is called deterministic behavior.

Hash functions are deterministic, meaning A’s hash will always be H(A).

Question 8

What is proof-of-work?

Accepted Answer

A **proof of work** is a piece of data which is difficult (costly, time-consuming) to produce but easy for others to verify and which satisfies certain requirements. Producing a proof of work can be a random process with low probability so that a lot of trial and error is required on average before a valid proof of work is generated. Difficulty is a measure of how difficult it is to find a hash below a given target.

Question 9

What is blockchain transaction?

Accepted Answer

**Transactions** are the things that give a blockchain purpose. They are the smallest building blocks of a blockchain system. Transactions generally consist of:
* a recipient address, 
* a sender address, 
* and a value.

This is not too different from a standard transaction that you would find on a credit card statement.

A transaction _changes the state_ of the agreed-correct blockchain. A blockchain is a shared, decentralized, distributed state machine. This means that all nodes (users of the blockchain system) independently hold their own copy of the blockchain, and the current known "state" is calculated by processing each transaction in order as it appears in the blockchain.

Question 10

What is Merkle Trees?

Accepted Answer

Merkle trees are a fundamental part of blockchain technology. A merkle tree is a structure that allows for efficient and secure verification of content in a large body of data.

A Merkle tree summarizes all the transactions in a block by producing a digital fingerprint of the entire set of transactions, thereby enabling a user to verify whether or not a transaction is included in a block.

Merkle trees are created by repeatedly hashing pairs of nodes until there is only one hash left (this hash is called the Root Hash, or the Merkle Root). They are constructed from the bottom up, from hashes of individual transactions (known as Transaction IDs). Hashing is usually conducted using the SHA-2 cryptographic hash function, though other functions can also be used.

![](https://hackernoon.com/hn-images/1*UrjiK3IjdbgoV2dyKRvAGQ.png)

Top 42 Blockchain Interview Questions

What is blockchain?

What is blockchain transaction?

What is deterministic behavior?

Why does Blockchain need coins or tokens?

Explain the common structure of blockchains

What is the Genesis Block?

What is proof-of-work?

What is the purpose of a blockchain node?

What is the blockchain data structure?

What is a hashing function?

Explain why a blockchain needs tokens to operate

What are the major elements of the blockchain ecosystem?

What is block data structure in blockchain?

What are some advantages of using Merke Trees?

What is DApp or Decentralised Application?

What are the core components of blockchain architecture?

What is a trapdoor function, and why is it needed in blockchain development?

Explain what do nodes do?

Why is the blockchain immutable?

How do verifiers check if a block is valid?

What is mining difficulty?

What is a smart contract?

Explain why there is a fixed supply of bitcoins?

What is Merkle Trees?

What is RSA algorithm?

How Are Blockchain And Distributed Ledger Different?

Explain what is target hash?

What is off-chain transaction?

Name some widespread platforms for developing blockchain applications

What is a stealth address?

What Is a Blockchain Consensus Algorithm?

What's the difference between distributed hashtable technology and the bitcoin blockchain?

What Is a Proof of Stake?

How is hard fork different from the soft fork in blockchain?

What is the difference between PoW and PoS?

What determines the mining difficulty?

What is nonce?

What is a 51% attack?

Why is Git not considered a “block chain”?

What can an attacker with 51% of hash power do?

Is it possible to brute force bitcoin address creation in order to steal money?

What are miners really solving?