Blockchain Technology Cryptographic Principles 4 — Questions and Answers
Question 1: What is a 'replay attack' in the context of blockchain transactions, and which cryptographic element prevents it?
- Reusing old blocks; prevented by Merkle roots
- Broadcasting a valid signed transaction on another chain; prevented by chain ID in signatures (Correct answer)
- Duplicating wallet keys; prevented by ECC
- Reusing hash values; prevented by SHA-256
Correct answer: Broadcasting a valid signed transaction on another chain; prevented by chain ID in signatures
A replay attack reuses a signed transaction on a different chain; including the chain ID in the signed data (EIP-155) binds the signature to one specific chain.
Question 2: Which concept describes the property that a signed blockchain transaction cannot be denied by the sender after the fact?
- Confidentiality
- Integrity
- Non-repudiation (Correct answer)
- Availability
Correct answer: Non-repudiation
Non-repudiation means the signer cannot later deny having signed the transaction, because the signature is mathematically tied to their private key.
Question 3: In Hierarchical Deterministic (HD) wallets, what cryptographic function derives child keys from a parent key?
- RSA key derivation
- HMAC-SHA512 (Correct answer)
- AES-CBC encryption
- ECDSA signing
Correct answer: HMAC-SHA512
HD wallets use HMAC-SHA512 with the parent key and chain code to derive child private and public keys deterministically (BIP-32).
Question 4: What is the role of a 'commitment scheme' in a blockchain smart contract auction?
- Encrypting bids with the auctioneer's public key
- Allowing bidders to commit to a hidden bid, revealed only after the bidding phase ends (Correct answer)
- Signing bids with ECDSA to prove identity
- Hashing the auction contract address to prevent front-running
Correct answer: Allowing bidders to commit to a hidden bid, revealed only after the bidding phase ends
A commit-reveal scheme lets bidders submit a hash of their bid first, then reveal the actual value later, preventing other bidders from seeing bids before committing.
Question 5: Which term describes a cryptographic proof system where the verifier learns nothing about the witness beyond the truth of the statement?
- Interactive proof
- Zero-knowledge proof (Correct answer)
- Succinct argument
- Homomorphic proof
Correct answer: Zero-knowledge proof
A zero-knowledge proof allows a prover to convince a verifier that a statement is true without revealing any information about the underlying secret (witness).
Question 6: What is the significance of the 'difficulty target' in Bitcoin's proof-of-work, expressed as a hash output constraint?
- It sets the maximum allowed transaction fee
- It requires the block hash to be numerically less than a target value (i.e., begin with enough leading zeros) (Correct answer)
- It determines the minimum number of signatures per block
- It limits the Merkle tree depth for transaction inclusion
Correct answer: It requires the block hash to be numerically less than a target value (i.e., begin with enough leading zeros)
Miners must find a nonce such that the block header's SHA-256d hash is below the network's target value, effectively requiring a certain number of leading zero bits.
Question 7: In multi-signature (multisig) wallets, which cryptographic scheme is commonly used to allow M-of-N signers to authorize a transaction without revealing individual private keys using threshold signatures?
- RSA secret sharing
- Shamir's Secret Sharing combined with threshold ECDSA (Correct answer)
- AES key splitting
- HMAC key aggregation
Correct answer: Shamir's Secret Sharing combined with threshold ECDSA
Threshold ECDSA, often built on Shamir's Secret Sharing, lets M of N parties collaboratively sign without any single party exposing their private key share.
Blockchain Technology Cryptographic Principles 4
What is a 'replay attack' in the context of blockchain transactions, and which cryptographic element prevents it?