Blockchain Cryptography Fundamentals Flashcards

Which of the following post-quantum cryptographic algorithm families is a NIST-standardized candidate primarily used for key encapsulation and digital signatures?

Answer: Lattice-based cryptography (e.g., CRYSTALS-Kyber, CRYSTALS-Dilithium)

NIST standardized lattice-based algorithms CRYSTALS-Kyber (key encapsulation) and CRYSTALS-Dilithium (signatures) as post-quantum replacements resistant to Shor's algorithm.

Grover's quantum algorithm poses a threat to symmetric cryptography by reducing the effective security of an n-bit key to approximately:

Answer: n/2 bits

Grover's algorithm provides a quadratic speedup for brute-force search, effectively halving the bit security of symmetric keys, so AES-256 retains ~128 bits of quantum security.

In the context of smart contract security, which cryptographic function does Solidity's `ecrecover` perform?

Answer: Recovers the Ethereum address (public key) from a message hash and ECDSA signature

ecrecover takes a hashed message and an ECDSA signature (v, r, s) and returns the Ethereum address of the signer, enabling on-chain signature verification.

A timing side-channel attack against a cryptographic implementation exploits:

Answer: Variations in execution time that leak information about secret values

Timing attacks measure how long cryptographic operations take; non-constant-time implementations can leak secret key bits through measurable timing differences.

What is a stealth address in privacy-focused blockchains, and which cryptographic operation is it based on?

Answer: A one-time address per transaction derived via Diffie-Hellman key exchange between sender and recipient

Stealth addresses use ECDH between the sender's ephemeral key and the recipient's public key to generate a one-time address per transaction, unlinkable to the recipient's public identity.

Which of the following correctly describes the role of a verifiable random function (VRF) in blockchain protocols like Algorand?

Answer: It produces a publicly verifiable pseudorandom output tied to a private key, used for unpredictable but provable leader election

VRFs let a node produce a random output along with a proof that the output was correctly computed from their private key, enabling fair and verifiable leader selection without a trusted third party.

In a threshold signature scheme (t-of-n), what happens cryptographically if fewer than t key shares attempt to reconstruct the private key?

Answer: They obtain no information about the private key due to the information-theoretic security of secret sharing

In Shamir's Secret Sharing (the basis of most threshold schemes), fewer than t shares reveal zero information about the secret due to the polynomial construction used.