Cryptocurrency Cryptography and Hashing 4 — Questions and Answers
Question 1: What is the 'birthday attack' vulnerability in hashing, and why does it matter for blockchain?
- An attack that exploits birthday-related metadata in transactions
- A statistical attack finding hash collisions faster than brute force due to probability theory (Correct answer)
- An attack on block timestamps near miner birthdays
- A replay attack using old transaction data
Correct answer: A statistical attack finding hash collisions faster than brute force due to probability theory
The birthday paradox shows collisions can be found in roughly 2^(n/2) attempts rather than 2^n, so 256-bit hashes provide ~128-bit collision security.
Question 2: What is a 'commitment scheme' in cryptography as used in some blockchain protocols?
- A smart contract that commits funds to a fixed interest rate
- A method to commit to a value privately and reveal it verifiably later (Correct answer)
- A protocol for miners to commit to a block before broadcasting
- A cryptographic signature scheme for multi-party transactions
Correct answer: A method to commit to a value privately and reveal it verifiably later
A commitment scheme lets a party bind to a chosen value without revealing it, then later open the commitment to prove what they chose.
Question 3: How does a Merkle proof allow lightweight clients (SPV nodes) to verify a transaction?
- By downloading the full blockchain and searching for the transaction
- By providing a branch of hashes from the transaction to the Merkle root without sending all transactions (Correct answer)
- By asking miners to re-hash the block for verification
- By comparing timestamps with the network time protocol
Correct answer: By providing a branch of hashes from the transaction to the Merkle root without sending all transactions
A Merkle proof supplies only log2(n) hashes needed to reconstruct the path to the root, enabling transaction verification without full block data.
Question 4: What makes the Discrete Logarithm Problem (DLP) important for elliptic curve cryptography in blockchains?
- It ensures mining difficulty adjusts properly
- It is computationally infeasible to derive a private key from a public key (Correct answer)
- It prevents double-spending by making transactions unique
- It allows faster SHA-256 computation on GPUs
Correct answer: It is computationally infeasible to derive a private key from a public key
The DLP hardness on elliptic curves means computing the private key from the public key is computationally infeasible with current technology.
Question 5: What does 'deterministic' mean when describing a cryptographic hash function?
- The function uses a random seed to vary output
- The same input always produces the same output (Correct answer)
- The output length varies based on input
- The function can be reversed with the same algorithm
Correct answer: The same input always produces the same output
Determinism means identical inputs always yield identical hash outputs, which is essential for consensus across blockchain nodes.
Question 6: In HD (Hierarchical Deterministic) wallets, what role does HMAC-SHA512 play?
- It encrypts the wallet's seed phrase for storage
- It derives child key pairs from a parent key and a chain code (Correct answer)
- It generates the random seed for the wallet
- It hashes transaction data before signing
Correct answer: It derives child key pairs from a parent key and a chain code
HD wallets use HMAC-SHA512 with a parent key and chain code as inputs to deterministically derive child private and public keys.
Question 7: What is 'salting' in the context of password hashing for cryptocurrency wallet security?
- Adding extra characters to lengthen a private key
- Appending a random value to a password before hashing to prevent rainbow table attacks (Correct answer)
- Encrypting the hash output with a secondary key
- Splitting a wallet seed across multiple hash functions
Correct answer: Appending a random value to a password before hashing to prevent rainbow table attacks
A salt is a unique random value added to each password before hashing, ensuring identical passwords produce different hashes and defeating precomputed lookup tables.
Cryptocurrency Cryptography and Hashing 4
What is the 'birthday attack' vulnerability in hashing, and why does it matter for blockchain?