Cryptographic Attacks CASENET Practice Questions (Page 3)

Questions 11–15

1. 11expert · hard

   A .NET application uses a custom encryption library that implements AES-CBC with PKCS#7 padding. The library's decryption method returns different exceptions for valid and invalid padding. An attacker exploits this to decrypt data without knowing the key. The team must fix the vulnerability while maintaining backward compatibility with existing ciphertexts. Which approach is most appropriate?

   • AModify the decryption method to return a generic error for both padding and other failures, and use a MAC to authenticate ciphertexts before decryption.
   • BUse a random IV for each encryption and keep the current decryption logic.
   • CSwitch to AES-GCM and re-encrypt all existing data.
   • DRemove padding by using a stream cipher like RC4.
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2. 12expert · hard

   A .NET application uses RSA-OAEP for encrypting small payloads. The RSA key pair is generated once and stored in a .NET certificate store. The application is deployed across multiple servers behind a load balancer. A security review notes that the private key is identical on all servers. Which risk does this configuration introduce, and what is the best mitigation?

   • ARSA-OAEP is vulnerable to padding oracle attacks; the mitigation is to switch to RSA-PKCS1v1.5.
   • BThe load balancer cannot distribute traffic because each server has the same key; the mitigation is to generate a unique key per server.
   • CThe private key should be stored in the application's web.config file for easier management.
   • DIf one server is compromised, the attacker can decrypt all traffic; the mitigation is to use a centralized key management service or HSM to store the private key.
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4. 13application · medium

   A .NET web application stores password hashes in a database. The hashes are computed using MD5 without a salt. An attacker steals the database and successfully recovers many plaintext passwords. Which combination of weaknesses contributed to this compromise?

   • AThe application did not use a pepper.
   • BMD5 is a fast hash and the lack of salt allows the use of precomputed rainbow tables.
   • CThe database was not encrypted at rest.
   • DMD5 produces a 128-bit hash, which is too short to store.
   Reveal Solution Discussion Select an answer first
5. 14application · medium

   An organization's .NET application stores encrypted credit card numbers. A security audit found that the encryption key is hard-coded in the source code and the same key has been used for five years. Which cryptographic weakness is the primary concern, and what should be done?

   • ASide-channel attack; add code obfuscation to hide the key
   • BFlawed implementation; use ECB mode instead of CBC
   • CWeak algorithm; switch from AES to a proprietary cipher
   • DImproper key management; move the key to a hardware security module (HSM) or Azure Key Vault and implement key rotation
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6. 15application · medium

   A development team is implementing encryption for sensitive documents in an ASP.NET Core application. They plan to use DES with a 56-bit key derived from a user-supplied passphrase using a single round of MD5. A security architect reviews the design. Which change should the architect require?

   • AKeep DES but increase the key size to 128 bits by padding the key with zeros.
   • BUse AES-256 with a key derived from the passphrase using PBKDF2 with a random salt and high iteration count.
   • CUse RC4 with a 256-bit key derived from the passphrase using MD5.
   • DUse 3DES with a key derived from the passphrase using a single SHA-1 hash.
   Reveal Solution Discussion Select an answer first