Blockchain Interview Questions and Answers for experienced

1. What is a blockchain?

   • Answer: A blockchain is a distributed, immutable ledger that records and verifies transactions across multiple computers. It uses cryptographic hashing and consensus mechanisms to ensure data integrity and security, making it resistant to tampering and single points of failure.

2. Explain the concept of immutability in blockchain.

   • Answer: Immutability means that once data is recorded on the blockchain, it cannot be altered or deleted. Each block is linked to the previous one through cryptographic hashes, creating a chain where modifying one block would require altering all subsequent blocks, which is computationally infeasible.

3. What are the different types of consensus mechanisms? Explain two in detail.

   • Answer: Several consensus mechanisms exist, including Proof-of-Work (PoW), Proof-of-Stake (PoS), Proof-of-Authority (PoA), and Delegated Proof-of-Stake (DPoS).
     • PoW: Miners compete to solve complex cryptographic puzzles. The first to solve the puzzle gets to add the next block to the chain and receives a reward. It's secure but energy-intensive.
     • PoS: Validators are selected based on the number of coins they stake. The more coins staked, the higher the probability of being selected to validate the next block. It's more energy-efficient than PoW.

4. What is a smart contract?

   • Answer: A smart contract is a self-executing contract with the terms of the agreement between buyer and seller being directly written into lines of code. It automatically executes when predetermined conditions are met, eliminating the need for intermediaries.

5. Explain the role of hashing in blockchain.

   • Answer: Hashing is a cryptographic function that transforms data of any size into a fixed-size string of characters (hash). In blockchain, hashing is crucial for linking blocks together, ensuring data integrity, and preventing tampering. Any change in the data results in a completely different hash.

6. What is a private key and a public key?

   • Answer: A private key is a secret code used to sign transactions and prove ownership of cryptocurrency. A public key is a derived code that can be shared publicly to receive cryptocurrency; transactions are verified using the corresponding private key.

7. What is a 51% attack?

   • Answer: A 51% attack occurs when a single entity or group controls more than 50% of the network's computing power (hash rate in PoW). This allows them to manipulate the blockchain, such as reversing transactions or preventing new transactions from being added.

8. What is the difference between public and private blockchains?

   • Answer: Public blockchains are permissionless, meaning anyone can participate in the network. Private blockchains are permissioned, with access controlled by a central authority.

9. What are some real-world applications of blockchain technology?

   • Answer: Supply chain management, digital identity verification, voting systems, healthcare data management, financial transactions (cryptocurrencies), intellectual property rights management, and many more.

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