Modern Cryptographic Attacks ECES Practice Questions (Page 4)

Questions 16–20

1. 16application · medium

   A developer implements RSA encryption using the public exponent e=3 and does not use any padding scheme. What is the most significant vulnerability?

   • AThe private key can be derived from the public key because e is small.
   • BThe ciphertext is longer than the plaintext, causing a denial of service.
   • CSmall-message attacks: if the plaintext is small, the ciphertext may be recovered by taking the cube root.
   • DThe encryption is slower than with a larger exponent.
   Reveal Solution Discussion Select an answer first
2. 17expert · hard

   A security team is designing a new authentication system for a high-security facility. The system must use smart cards with PIN entry. The team is considering two designs: Design A uses a constant-time PIN comparison but is vulnerable to power analysis because the card's processor draws different power when processing different PIN digits. Design B uses a masked AES implementation to equalize power consumption but has a variable-time PIN comparison. The team must choose one design. Which design is the better choice, and why?

   • ADesign A, because timing side-channels are easier to exploit remotely than power analysis, which requires physical access
   • BDesign B, because power analysis is a more severe threat in a physical access scenario, and timing attacks can be mitigated by adding random delays
   • CDesign B, because timing attacks are always more dangerous than power analysis
   • DDesign A, because constant-time comparison is the industry standard and power analysis is not a practical threat
   Reveal Solution Discussion Select an answer first
4. 18foundation · easy

   Which of the following is considered a protocol-level attack surface in a cryptographic system?

   • AAn attacker exploits a vulnerability in the TLS handshake to force the use of an older, weaker version of the protocol.
   • BA flaw in the random number generator causes predictable session keys.
   • CAn attacker observes the power consumption of a smart card during RSA decryption.
   • DAn attacker injects malicious code into the memory of a server to steal private keys.
   Reveal Solution Discussion Select an answer first
5. 19application · medium

   A security team suspects that a cryptographic key is leaking through cache-timing behavior on a shared server. The application uses AES with a software implementation. Which mitigation is most appropriate?

   • AUse a larger AES key size, such as AES-256, to make cache attacks harder.
   • BUse an AES-NI hardware instruction set if available, or a constant-time software implementation.
   • CFlush the CPU cache before each AES operation.
   • DRun the AES operation in a higher-priority thread to reduce timing noise.
   Reveal Solution Discussion Select an answer first
6. 20application · medium

   A developer is implementing a file encryption feature. The code uses the same AES key and IV for every file encrypted. A security reviewer identifies this as a critical flaw. Which attack is the system most vulnerable to, and what is the correct fix?

   • AReplay attack; add a timestamp to the ciphertext
   • BCiphertext collision and pattern analysis; use a unique IV for each encryption and a unique key per file or derive keys per file
   • CTiming side-channel; implement constant-time encryption
   • DPadding oracle attack; use a different padding scheme
   Reveal Solution Discussion Select an answer first