Securing Blockchain Transactions with Cryptographic Techniques BFC Practice Questions (Page 7)

Questions 31–35

1. 31application · medium

   A developer is implementing a blockchain address generator. They plan to use a hash of the public key to create the address. Which additional step is essential to prevent a specific type of attack where an attacker could find a different public key that hashes to the same address?

   • AUse a checksum in the address to detect errors.
   • BUse a cryptographic hash with a large output size, such as 256 bits, and include a version byte.
   • CEncrypt the public key before hashing.
   • DUse the private key directly as the address.
   Reveal Solution Discussion Select an answer first
2. 32expert · hard

   A blockchain platform uses EdDSA (Ed25519) for transaction signatures. A security audit reveals that the random number generator used to create signing keys is flawed, producing a limited set of possible keys. Which attack is most likely to succeed, and what is the best mitigation?

   • AAn attacker could enumerate the limited key space and derive private keys from public keys; the mitigation is to use a cryptographically secure random number generator (CSPRNG) for key generation.
   • BAn attacker could replay signatures; the mitigation is to add a nonce to each transaction.
   • CAn attacker could perform a length-extension attack on the signature; the mitigation is to use HMAC instead of EdDSA.
   • DAn attacker could perform a collision attack on the hash function; the mitigation is to use a stronger hash function.
   Reveal Solution Discussion Select an answer first
4. 33expert · hard

   A developer is implementing a transaction signing process for a high-security blockchain application. They need to ensure that the private key is never exposed to the application layer, but they also need to support multiple users on the same server. Which architecture best meets these requirements?

   • AUse a remote signing service that holds the private keys in a hardware security module (HSM) and exposes a signing API that requires user authentication.
   • BGenerate a new private key for each user and store it in a file on the server with restrictive permissions.
   • CUse a single private key for all users and sign transactions on behalf of users.
   • DStore the private keys in an encrypted database and decrypt them in the application when signing.
   Reveal Solution Discussion Select an answer first
5. 34foundation · easy

   Which property of a cryptographic hash function makes it suitable for linking blocks in a blockchain?

   • ADeterminism – the same input always produces the same hash output
   • BEncryption – the hash output hides the data from unauthorized users
   • CKey-dependence – the hash output changes based on a secret key
   • DReversibility – the original data can be recovered from the hash
   Reveal Solution Discussion Select an answer first
6. 35expert · hard

   A proof-of-work blockchain is experiencing a 51% attack. The attacker is using a large amount of hashing power to rewrite transaction history. Which cryptographic property of the blockchain makes this attack difficult to execute?

   • AThe use of digital signatures to authenticate each transaction.
   • BThe avalanche effect of the hash function, which makes each block's hash unpredictable.
   • CThe Merkle root in each block, which commits to all transactions.
   • DThe computational difficulty of finding a nonce that produces a hash below the target.
   Reveal Solution Discussion Select an answer first