CBCP Cryptography and Security 2 — Questions and Answers
Question 1: Which elliptic curve is used by Bitcoin for its digital signature scheme?
- P-256
- secp256k1 (Correct answer)
- Curve25519
- P-384
Correct answer: secp256k1
Bitcoin uses the secp256k1 elliptic curve, defined by SECG, for its ECDSA signature scheme.
Question 2: What is the primary purpose of a Merkle tree in blockchain cryptography?
- Encrypting transaction data end-to-end
- Providing efficient and tamper-evident summarization of transaction sets (Correct answer)
- Generating key pairs for wallet addresses
- Storing private keys securely on-chain
Correct answer: Providing efficient and tamper-evident summarization of transaction sets
Merkle trees allow efficient, tamper-evident verification of large transaction sets by hashing pairs of hashes up to a single root.
Question 3: In a threshold signature scheme (TSS), what does a (t, n) configuration mean?
- n total keys exist, any t of them must sign to produce a valid signature (Correct answer)
- t keys are public and n keys are private
- Only n-t signers are required to approve
- t is the total number of transactions and n is the block size
Correct answer: n total keys exist, any t of them must sign to produce a valid signature
A (t, n) TSS means there are n shares distributed, and any t of those shareholders must cooperate to produce a valid signature.
Question 4: What cryptographic property ensures that a blockchain transaction cannot be denied by its sender?
- Confidentiality
- Integrity
- Non-repudiation (Correct answer)
- Availability
Correct answer: Non-repudiation
Non-repudiation, achieved via digital signatures, ensures a sender cannot later deny having signed and sent a transaction.
Question 5: Which attack exploits the birthday paradox to find collisions in hash functions?
- Preimage attack
- Brute-force attack
- Birthday attack (Correct answer)
- Length extension attack
Correct answer: Birthday attack
A birthday attack leverages the birthday paradox to find two inputs that hash to the same output far faster than brute force.
Question 6: What does 'key derivation' accomplish in a hierarchical deterministic (HD) wallet?
- It generates a mnemonic seed phrase from a private key
- It derives multiple child key pairs from a single master seed deterministically (Correct answer)
- It encrypts all keys using AES-256
- It combines multiple seeds into one using XOR
Correct answer: It derives multiple child key pairs from a single master seed deterministically
HD wallets use key derivation (BIP-32) to generate a tree of child key pairs from one master seed, enabling backup with a single phrase.
Question 7: What is a length extension attack, and which hash functions are vulnerable?
- An attack that appends data to a plaintext; AES is vulnerable
- An attack that computes H(m || extra) from H(m) without knowing m; MD5 and SHA-1 are vulnerable (Correct answer)
- An attack that extends the key length; only RSA is vulnerable
- An attack that increases block size; SHA-3 is vulnerable
Correct answer: An attack that computes H(m || extra) from H(m) without knowing m; MD5 and SHA-1 are vulnerable
Length extension attacks allow computing H(m || extra) from H(m) alone using Merkle-Damgård construction; MD5, SHA-1, and SHA-2 are vulnerable.
CBCP Cryptography and Security 2
Which elliptic curve is used by Bitcoin for its digital signature scheme?