CBCP Cryptography 5 â Questions and Answers
Question 1: What is the purpose of a Verifiable Random Function (VRF) in blockchain consensus protocols?
- To encrypt validator communication channels end-to-end
- To produce a pseudorandom output with a cryptographic proof that the output was computed correctly (Correct answer)
- To aggregate signatures from multiple validators into one compact signature
- To derive hierarchical keys for staking reward distribution
Correct answer: To produce a pseudorandom output with a cryptographic proof that the output was computed correctly
A VRF generates a random output along with a proof that anyone can verify was computed correctly from the given input and private key, enabling unpredictable but verifiable leader election.
Question 2: What does it mean for a cryptographic protocol to be 'computationally secure' versus 'information-theoretically secure'?
- Computationally secure protocols use faster algorithms; information-theoretically secure protocols use slower ones
- Computationally secure protocols are safe assuming bounded adversary compute power; information-theoretically secure ones are safe against unlimited compute (Correct answer)
- Computationally secure means formally proven; information-theoretically secure means heuristically validated
- Computationally secure requires hardware support; information-theoretically secure runs in software only
Correct answer: Computationally secure protocols are safe assuming bounded adversary compute power; information-theoretically secure ones are safe against unlimited compute
Information-theoretic security (like a one-time pad) is unbreakable regardless of compute power, while computational security assumes adversaries cannot solve hard mathematical problems within feasible time.
Question 3: In a ring signature scheme used by privacy coins like Monero, what is concealed?
- The transaction amount only
- The identity of the actual signer among a group of possible signers (Correct answer)
- The recipient's address using stealth key derivation
- The signature algorithm used to prevent fingerprinting
Correct answer: The identity of the actual signer among a group of possible signers
Ring signatures allow a signer to produce a signature on behalf of a group, making it cryptographically indistinguishable which group member actually signed.
Question 4: What is the role of a hash pointer in a blockchain data structure?
- It compresses the block header to reduce storage
- It links each block to the previous one by including both the hash and location of prior data, enabling tamper detection (Correct answer)
- It encrypts the pointer to prevent unauthorized traversal
- It maps transaction IDs to their physical disk locations
Correct answer: It links each block to the previous one by including both the hash and location of prior data, enabling tamper detection
A hash pointer stores both the address of previous data and its cryptographic hash, so any tampering with historical data invalidates all subsequent hash pointers.
Question 5: What does 'homomorphic encryption' allow that conventional encryption does not?
- Decryption without knowledge of the private key using a re-encryption key
- Computation on encrypted data such that the result, when decrypted, matches the result of the same computation on plaintext (Correct answer)
- Encryption of data at variable security levels depending on sensitivity
- Splitting a ciphertext among multiple parties who must cooperate to decrypt
Correct answer: Computation on encrypted data such that the result, when decrypted, matches the result of the same computation on plaintext
Homomorphic encryption lets a third party (e.g., a blockchain node) perform operations on ciphertexts and return an encrypted result without ever seeing the plaintext.
Question 6: What is the cryptographic weakness exploited by a 'length extension attack' against hash functions?
- The hash output leaks partial key material when the input exceeds block size
- For Merkle-DamgĂ„rd constructions, knowing H(m) lets an attacker compute H(mâextra) without knowing m (Correct answer)
- The compression function is reversible for inputs shorter than one block
- The IV (initialization vector) is reused across all hash computations
Correct answer: For Merkle-DamgĂ„rd constructions, knowing H(m) lets an attacker compute H(mâextra) without knowing m
Merkle-DamgĂ„rd hash functions (like SHA-1 and SHA-256) are vulnerable to length extension: given H(m), an attacker can compute H(mâpaddingâsuffix) for any chosen suffix.
Question 7: What is the primary advantage of BLS (Boneh-Lynn-Shacham) signatures over ECDSA in blockchain validator systems?
- BLS signatures are shorter than ECDSA signatures for equivalent security
- Multiple BLS signatures can be aggregated into a single compact signature, reducing bandwidth and verification cost (Correct answer)
- BLS does not require a random nonce, eliminating a class of implementation bugs
- BLS signatures can be computed faster than ECDSA on standard hardware
Correct answer: Multiple BLS signatures can be aggregated into a single compact signature, reducing bandwidth and verification cost
BLS signature aggregation allows thousands of validator signatures to be combined into one signature of constant size, dramatically reducing the data validators must exchange and nodes must verify.
CBCP Cryptography 5
What is the purpose of a Verifiable Random Function (VRF) in blockchain consensus protocols?