Practical Cryptography GXPN Practice Questions (Page 5)

Questions 21–25

1. 21expert · hard

   A penetration tester is evaluating a system that uses a static Diffie-Hellman key pair for all sessions. The tester captures traffic and later compromises the server's long-term private key. Which sessions can the tester decrypt?

   • AAll past sessions that used the static DH key
   • BOnly the current session
   • COnly sessions that used ephemeral DH keys
   • DNo sessions, because DH is secure
   Reveal Solution Discussion Select an answer first
2. 22expert · hard

   A penetration tester is assessing a custom protocol that uses AES-CBC with PKCS#7 padding. The tester has an oracle that reveals whether padding is valid. The tester wants to decrypt a captured ciphertext block. Which attack should the tester perform?

   • APadding oracle attack to recover plaintext byte by byte
   • BBirthday attack to find a collision in the ciphertext
   • CDowngrade attack to force a weaker cipher
   • DLength extension attack on the ciphertext
   Reveal Solution Discussion Select an answer first
4. 23expert · hard

   A penetration tester is evaluating a web application that uses a custom encryption scheme. The application encrypts each user's session token with AES-128-CBC using a fixed key and a random IV per session. The tester discovers that the application does not authenticate the ciphertext. Which attack is most directly enabled by this flaw?

   • AKey-reuse attack
   • BPadding oracle attack
   • CBirthday attack on the IV
   • DDowngrade attack
   Reveal Solution Discussion Select an answer first
5. 24expert · hard

   A security architect is designing a system where IoT devices must authenticate firmware updates. The devices have limited computational power and storage. The architect needs to choose a cryptographic scheme that minimizes resource usage while providing authenticity and integrity. Which option is the best choice?

   • AUse HMAC-SHA256 with a shared secret embedded in each device.
   • BUse AES-ECB to encrypt the firmware and rely on decryption success as authentication.
   • CUse ECDSA with a 256-bit curve for signing and verification.
   • DUse RSA with a 4096-bit key for signing.
   Reveal Solution Discussion Select an answer first
6. 25application · medium

   A penetration tester is writing a Python script to demonstrate a man-in-the-middle attack on a Diffie-Hellman key exchange. The script intercepts the public values exchanged between two parties and substitutes its own. To complete the attack, the script must derive the shared secret with each party. Which additional step is required?

   • APerform a brute-force search of the prime modulus to derive the private key from the intercepted public value.
   • BUse the intercepted public value directly as the shared secret to impersonate both parties.
   • CGenerate a private key and compute the shared secret using the intercepted public value and the attacker's private key.
   • DDecrypt the intercepted public values using the victim's private key to recover the original shared secret.
   Reveal Solution Discussion Select an answer first