Cryptographic Attacks CASENET Practice Questions (Page 1)

Questions 1–5

1. 1application · medium

   A .NET application uses a static salt for all password hashes. An attacker has obtained the database and a precomputed table of hashes for common passwords using that same salt. Which attack is the attacker most likely to use, and what is the best defense?

   • ARainbow table attack; use a unique random salt for each user
   • BSide-channel attack; add random delays to the login process
   • CDictionary attack; use a longer password policy
   • DBrute-force attack; increase the hash output size
   Reveal Solution Discussion Select an answer first
2. 2application · medium

   A .NET application uses a custom key-derivation function that applies a single SHA-256 hash to a user's password to generate an encryption key. A security review recommends using PBKDF2 instead. What is the primary security benefit of this change?

   • APBKDF2 produces a longer key than SHA-256.
   • BPBKDF2 applies a salt and many iterations, making brute-force and dictionary attacks computationally expensive.
   • CPBKDF2 is a symmetric encryption algorithm.
   • DPBKDF2 automatically rotates keys periodically.
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4. 3expert · hard

   A .NET application uses a custom key derivation function that is not constant-time. The application is used in a high-security environment where attackers can measure power consumption. Which attack is the application most vulnerable to, and which countermeasure is most appropriate?

   • ADictionary attack; use a stronger password policy
   • BBrute-force attack; increase the key length
   • CPadding oracle attack; use authenticated encryption
   • DSide-channel attack; replace the custom KDF with a standard constant-time KDF like PBKDF2 or HKDF
   Reveal Solution Discussion Select an answer first
5. 4application · medium

   A development team is using the .NET `System.Security.Cryptography` namespace. They need to store a small amount of sensitive data (e.g., a connection string) in a config file. Which approach is most secure against cryptographic attacks?

   • AUse Base64 encoding to hide the connection string
   • BUse TripleDES with a key stored in the same config file
   • CUse AES-ECB with a hard-coded key in the source code
   • DEncrypt the connection string with AES-GCM using a key stored in the machine's DPAPI store
   Reveal Solution Discussion Select an answer first
6. 5expert · hard

   A .NET application uses a custom encryption scheme that encrypts data with AES-CBC and then applies a MAC to the ciphertext. The MAC is computed using a key that is derived from the encryption key by XORing it with a constant. A security researcher identifies a potential key-reuse vulnerability. Which of the following is the most secure way to derive separate keys for encryption and MAC?

   • AUse the same key for both encryption and MAC but prepend a different label to each operation.
   • BUse a key derivation function (KDF) such as HKDF to derive two independent keys from a master key.
   • CUse a random key for encryption and a different random key for MAC, and store both in the application configuration.
   • DUse the same key but XOR it with a different constant for the MAC key.
   Reveal Solution Discussion Select an answer first