Blockchains: Introduction and Basics

Which properties are provided by a cryptographic hash function? (2 correct answers). Small changes in input cause large changes in output. The original input can be reconstructed from the hash. Fixed-length output for any input. Hashes depend on the geographical location of the node. Hashes require a trusted third party.

What are characteristics of a traditional centralized database? (2 correct answers). Controlled by a trusted SysAdmin. Immutable records. Central point of failure. All nodes are equal. No ACID properties.

Which elements are part of a blockchain block? (3 correct answers). Hash of the previous block. Timestamp. User passwords. Transactions. Target value.

What is the role of miners in a Proof-of-Work blockchain? (2 correct answers). Solve cryptographic puzzles. Delete old blocks. Validate and add new blocks. Control the blockchain. Issue private keys.

Los mineros compiten resolviendo puzzles criptográficos y, cuando ganan, añaden un nuevo bloque a la cadena. No central authority exists. Only one trusted copy is stored. All nodes may store a full copy. Data can be silently changed. They require a SysAdmin.

What problems of centralized systems motivate blockchain? (2 correct answers). Single point of failure. Unlimited scalability. Cybercrime vulnerability. Low transparency. Free maintenance.

What is a Merkle root? (1 correct answer). The first transaction of a block. The final hash of all transactions in a block. The public key of the miner. The difficulty value. The genesis block.

Which are advantages of blockchain? (2 correct answers). Unforgeable records. Centralized trust. Prevention of double spending. High energy efficiency. No cryptography.

What is consensus in a blockchain? (2 correct answers). Agreement among most nodes. A centralized decision. Tamper-resistant state. Temporary storage. Manual validation.

What happens when two miners find a valid block simultaneously? (2 correct answers). The network forks temporarily. The shorter chain is always chosen. The longest chain becomes valid. The system stops. One branch is later discarded.

What is a nonce used for? (1 correct answer). Encrypt transactions. Change block hash output. Store user keys. Adjust difficulty. Sign blocks.

Which are features of Proof-of-Work? (2 correct answers). Computationally expensive. Energy efficient. Easy to verify. Requires trusted validators. No mining.

What does “difficulty” represent? (1 correct answer). Block size. Mining reward. How hard it is to find a valid block. Network delay. Number of nodes.

Which data is typically included in a transaction? (1 correct answer). Sender, receiver, amount. Node IP address. Server location. Miner reward. Difficulty.

Which are characteristics of public blockchains? (2 correct answers). Anyone can participate. Require permission. Open read access. Controlled by companies. No cryptography.

What is double spending? (1 correct answer). Paying twice with the same digital asset. Sending two transactions. Mining twice. Forking. Smart contract error.

Which statements about smart contracts are correct? (2 correct answers). They are programs on the blockchain. They require human execution. They run on all nodes. They cannot handle money. They are paper contracts.

Which are alternatives to PoW? (2 correct answers). Proof-of-Stake. Proof-of-Authority. Proof-of-Encryption. Proof-of-Storage. Proof-of-Speed.

What ensures immutability in blockchain? (2 correct answers). Hash chaining. Central administrator. Cryptographic signatures. Editable records. SQL commands.

What is the genesis block? (1 correct answer). First block in the blockchain. Largest block. Last block. Private block. Backup block.