Cryptographic Attacks CASENET Practice Questions (Page 8)

Questions 36–40

1. 36application · medium

   A .NET application uses a padding oracle vulnerability in its CBC-mode decryption routine. An attacker can send modified ciphertexts to an error endpoint that reveals whether padding is valid. The team must fix the vulnerability without breaking existing encrypted data. Which remediation should they implement?

   • ASwitch to authenticated encryption such as AES-GCM and update the decryption routine to verify the tag before any padding check.
   • BUse ECB mode instead of CBC to avoid the padding oracle.
   • CAdd a random IV to each encryption and keep the current CBC decryption logic.
   • DKeep CBC mode but return a generic error message for both padding and authentication failures.
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2. 37foundation · easy

   A .NET application uses CBC-mode encryption and returns detailed error messages when padding is invalid. Which countermeasure would best mitigate a padding oracle attack?

   • AUse authenticated encryption (e.g., AES-GCM)
   • BUse a shorter key to reduce attack surface
   • CUse ECB mode instead of CBC
   • DRemove all error messages from the application
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4. 38expert · hard

   A .NET application uses a custom protocol that encrypts messages with AES-CBC and then applies HMAC-SHA256 to the ciphertext. The developer implemented the HMAC key as the same AES key. A cryptanalyst points out that this is a key-reuse flaw. What is the most secure alternative that still provides both confidentiality and integrity?

   • AUse AES-GCM, which internally uses separate keys for encryption and authentication.
   • BUse AES-ECB with the same HMAC key.
   • CUse a single key for both encryption and HMAC but prepend a domain-separation constant.
   • DUse AES-CBC with a different HMAC key derived from the AES key using a KDF.
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5. 39foundation · easy

   A penetration tester captures a set of password hashes from a .NET web application. The tester wants to recover the plaintext passwords by precomputing hash chains and storing them in a large lookup table. Which type of cryptographic attack is the tester performing?

   • ARainbow table attack
   • BDictionary attack
   • CBrute force attack
   • DSide-channel attack
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6. 40application · medium

   A .NET application uses a hard-coded AES key in the binary. The key was extracted by an attacker, and the application now needs to be fixed. Which solution provides the strongest protection for the key?

   • AUse Azure Key Vault to store the key and retrieve it at runtime
   • BMove the key to a configuration file with restricted permissions
   • CUse a key derived from the machine's MAC address
   • DObfuscate the key in the binary using string encryption
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