Cryptographic Attacks CASENET Practice Questions (Page 4)

Questions 16–20

1. 16expert · hard

   A .NET application encrypts sensitive data with AES-CBC and uses a separate HMAC for integrity. The security team is concerned about padding oracle attacks. The current implementation validates the HMAC before decryption, but the application returns different error messages for invalid HMAC and invalid padding. Which change is most effective?

   • AUse a single generic error message for all decryption failures
   • BKeep the current design but ensure the HMAC key is different from the encryption key
   • CSwitch to AES-GCM, which provides authenticated encryption and eliminates the padding oracle
   • DUse AES-CBC with a random IV and PKCS7 padding, and validate padding before HMAC
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2. 17application · medium

   A security engineer is reviewing a .NET application that uses a custom encryption algorithm developed in-house. The algorithm has not been publicly reviewed, and the key is only 64 bits. Which cryptographic attack is the algorithm most vulnerable to, and what is the recommended action?

   • ABrute-force attack; replace the custom algorithm with a standard algorithm like AES-256
   • BSide-channel attack; add noise to the encryption timing
   • CRainbow table attack; add a salt to the encryption process
   • DDictionary attack; increase the key size to 128 bits
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4. 18expert · hard

   A .NET application uses a custom encryption routine that encrypts data with AES-CBC and then signs the ciphertext with HMAC-SHA256. The developer uses the same key for both AES and HMAC. A security audit requires the use of authenticated encryption. The team must choose between AES-GCM and the current encrypt-then-MAC approach with separate keys. Which statement is true?

   • AThe encrypt-then-MAC approach is faster than AES-GCM in all scenarios.
   • BAES-GCM is vulnerable to padding oracle attacks.
   • CThe encrypt-then-MAC approach with separate keys is always more secure than AES-GCM.
   • DAES-GCM is generally preferred because it is a standardized authenticated encryption mode that avoids common implementation pitfalls.
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5. 19application · medium

   A security audit reveals that a .NET application's login endpoint responds with 'Invalid username' when the username is unknown and 'Invalid password' when the username exists. An attacker uses this behavior to enumerate valid usernames and then performs a targeted password guessing attack. Which countermeasure is most effective against this attack chain?

   • AUse a longer password hash with a higher iteration count.
   • BSwitch to a time-based one-time password (TOTP) for all users.
   • CReturn the same generic error message for both invalid username and invalid password, and implement account lockout after a small number of failed attempts.
   • DAdd a CAPTCHA only after the first failed login attempt.
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6. 20expert · hard

   A .NET application uses RSA to encrypt session keys that are exchanged between a client and server. The RSA key pair is generated on the server and the private key is stored in a .NET certificate store. The application is deployed in a multi-tenant environment where each tenant has its own database. A security requirement states that a compromise of one tenant's data must not expose other tenants' data. Which key management strategy should be implemented?

   • AUse a single RSA key pair for all tenants but store it in a secure key vault.
   • BStore the RSA private key in each tenant's database.
   • CUse the same RSA key pair but encrypt each tenant's data with a different symmetric key.
   • DGenerate a unique RSA key pair for each tenant and store the private keys in a hardware security module (HSM) with per-tenant access policies.
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