Symmetric Encryption and Defensive Coding CASENET Practice Questions (Page 3)

Questions 11–15

1. 11expert · medium

   A security audit of a .NET application that uses AES-CBC with PKCS7 padding reveals a padding oracle vulnerability. The application returns different error messages for valid and invalid padding. The team needs to fix this without changing the encryption algorithm. What is the most effective mitigation?

   • ASwitch to ECB mode to avoid padding.
   • BUse a MAC (e.g., HMAC) over the ciphertext and verify it before decryption, returning a generic error on failure.
   • CCatch the padding exception and return a generic error message.
   • DUse a random IV for each encryption.
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2. 12foundation · easy

   Which block cipher mode of operation provides both confidentiality and authentication?

   • ACTR
   • BECB
   • CCBC
   • DGCM
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4. 13application · medium

   A developer is implementing encryption for a high-throughput API that transmits small JSON payloads. The team needs confidentiality and integrity, and wants to avoid the overhead of a separate HMAC. Which mode of operation should the developer choose?

   • AAES-GCM
   • BAES-CTR with a fixed nonce
   • CAES-ECB
   • DAES-CBC with PKCS7 padding
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5. 14application · medium

   A developer is implementing AES-GCM in .NET and needs to ensure that the nonce is unique for each encryption. The application will encrypt at most 1 million messages per key. Which nonce generation strategy is most appropriate?

   • AUse a 96-bit counter starting at 0 and incrementing for each message
   • BUse a nonce derived from the message hash
   • CUse a random 96-bit nonce for each message
   • DUse a timestamp-based nonce
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6. 15expert · hard

   A developer is implementing AES-CBC encryption and needs to ensure that the ciphertext is not vulnerable to padding oracle attacks. The application currently uses a custom decryption routine that returns different error messages for invalid padding versus invalid MAC. Which change is most important?

   • AUse a fixed padding scheme like zero padding
   • BUse a random IV for each encryption
   • CUse an authenticated encryption mode like GCM, or apply encrypt-then-MAC and verify the MAC before any padding validation
   • DUse a uniform error message for all decryption failures
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