CBCP Cryptography and Security 5 — Questions and Answers
Question 1: What is a 51% attack's relationship to cryptographic security in proof-of-work blockchains?
- It breaks the SHA-256 hash function directly
- It overcomes cryptographic difficulty by controlling majority hash rate, enabling double-spend reorgs (Correct answer)
- It compromises private keys of honest miners
- It exploits a flaw in ECDSA signature verification
Correct answer: It overcomes cryptographic difficulty by controlling majority hash rate, enabling double-spend reorgs
A 51% attack doesn't break cryptography; it uses majority hash power to rewrite recent chain history, enabling double-spending by reversing confirmed transactions.
Question 2: Which post-quantum cryptographic algorithm family does NIST recommend as a primary standard for digital signatures?
- NTRU lattice schemes
- CRYSTALS-Dilithium (lattice-based) (Correct answer)
- McEliece code-based signatures
- SPHINCS+ hash-based signatures
Correct answer: CRYSTALS-Dilithium (lattice-based)
NIST standardized CRYSTALS-Dilithium (ML-DSA) as its primary post-quantum digital signature standard due to its strong security and performance.
Question 3: What is the purpose of key splitting using Shamir's Secret Sharing in blockchain custody?
- To encrypt private keys with AES before storage
- To divide a private key into n shares such that any t shares reconstruct it, with fewer than t revealing nothing (Correct answer)
- To generate multiple independent private keys from one seed
- To share a public key across multiple blockchains simultaneously
Correct answer: To divide a private key into n shares such that any t shares reconstruct it, with fewer than t revealing nothing
Shamir's Secret Sharing splits a key into n shares with a t-of-n threshold; any t shares reconstruct the secret, but t-1 shares reveal zero information.
Question 4: In the context of blockchain smart contracts, what is a reentrancy attack and which cryptographic control prevents it?
- An attack where validators re-sign blocks; prevented by BLS signatures
- An attack where a malicious contract recursively calls back before state updates; prevented by check-effects-interactions pattern and mutex locks (Correct answer)
- An attack exploiting weak randomness; prevented by VRF integration
- An attack forging transaction signatures; prevented by Schnorr aggregation
Correct answer: An attack where a malicious contract recursively calls back before state updates; prevented by check-effects-interactions pattern and mutex locks
Reentrancy occurs when an external contract calls back into the vulnerable contract before its state is updated; the check-effects-interactions pattern and mutex locks prevent this.
Question 5: What is the difference between a public blockchain's transparency model and zero-knowledge proof-enhanced privacy?
- Public blockchains encrypt all data; ZKPs decrypt it for authorized parties
- Public blockchains expose all transaction data; ZKPs allow proving transaction validity without revealing the underlying data (Correct answer)
- ZKPs replace consensus mechanisms in private blockchains
- Public blockchains use ZKPs by default; privacy chains disable them
Correct answer: Public blockchains expose all transaction data; ZKPs allow proving transaction validity without revealing the underlying data
Public chains are transparent by default; ZKPs (as in Zcash's zk-SNARKs) let nodes verify that rules are followed (balances don't go negative) without seeing amounts or addresses.
Question 6: What is a cryptographic accumulator and how is it used in blockchain scalability?
- A hardware module that accelerates hash computation for miners
- A data structure that proves set membership without revealing the full set, used in stateless clients and rollups (Correct answer)
- A method to accumulate mining rewards before payout
- A technique to batch multiple blocks into one using AES encryption
Correct answer: A data structure that proves set membership without revealing the full set, used in stateless clients and rollups
Cryptographic accumulators (e.g., RSA or Merkle-based) allow compact proofs of membership or non-membership, enabling stateless blockchain clients that don't store full state.
Question 7: Which of the following best describes the security model of a hardware security module (HSM) in enterprise blockchain deployments?
- HSMs store public keys in tamper-evident memory and sign externally
- HSMs perform cryptographic operations internally so private keys never leave the secure boundary (Correct answer)
- HSMs are cloud-based key management services requiring internet access
- HSMs replace blockchain nodes by handling all consensus logic
Correct answer: HSMs perform cryptographic operations internally so private keys never leave the secure boundary
HSMs keep private keys within a tamper-resistant physical boundary, performing signing operations internally so keys are never exposed to the host system.
CBCP Cryptography and Security 5
What is a 51% attack's relationship to cryptographic security in proof-of-work blockchains?