Asymmetric Cryptography and Key Algorithms BDC Practice Questions (Page 5)

Questions 21–25

1. 21application · easy

   A developer is designing a secure messaging system for a corporate environment. Users need to send confidential messages to each other, and the system must ensure that only the intended recipient can read a message. The system uses asymmetric encryption for the message content. Which key should the sender use to encrypt the message?

   • AThe recipient's public key
   • BThe sender's private key
   • CThe recipient's private key
   • DA symmetric key generated from the sender's password
   Reveal Solution Discussion Select an answer first
2. 22expert · hard

   A blockchain network uses a certificate authority (CA) to issue key pairs to nodes. A node's private key is compromised, and the attacker has used it to sign several malicious transactions. The network needs to prevent further damage while maintaining the ability to verify the node's legitimate past transactions. What is the best course of action?

   • AIgnore the compromise and continue using the same key pair, but increase monitoring.
   • BChange the node's private key in the CA database and update all past transactions with the new key.
   • CDelete the node's public key from the network and reissue a new key pair.
   • DRevoke the node's certificate and add the compromised public key to a revocation list, but keep the public key available for verifying historical signatures.
   Reveal Solution Discussion Select an answer first
4. 23expert · hard

   A blockchain developer is implementing a multi-signature wallet that requires 2-of-3 signatures to authorize a transaction. The wallet uses ECDSA signatures. Each of the three key holders has a distinct key pair. A transaction is constructed and signed by two of the three holders. The third holder later wants to verify that the transaction was signed by the other two without seeing their private keys. What should the third holder do?

   • AVerify each signature independently using the corresponding public keys of the two signers.
   • BCombine the two signatures into a single signature and verify it with a combined public key.
   • CDecrypt the transaction with the two signers' private keys.
   • DHash the transaction and compare it to the signatures.
   Reveal Solution Discussion Select an answer first
5. 24expert · hard

   A blockchain developer is implementing a decentralized identity system where users prove ownership of their identity by signing a challenge. The system must prevent replay attacks, where an attacker captures a valid signature and replays it later. Which approach should be used?

   • AUse the same challenge for every authentication request.
   • BInclude a timestamp and a nonce in the challenge, and have the verifier reject challenges older than a certain age.
   • CHash the signature and store it on the blockchain.
   • DEncrypt the signature with the verifier's public key.
   Reveal Solution Discussion Select an answer first
6. 25application · medium

   A developer wants to prove that a message was sent by a specific person without encrypting the message content. Which technique should be used?

   • AHash the message and share the hash.
   • BSign the message with the sender's private key.
   • CEncrypt the message with the recipient's public key.
   • DEncrypt the message with the sender's public key.
   Reveal Solution Discussion Select an answer first