CREST CREST Cryptography & Data Protection 2 — Questions and Answers
Question 1: Which key exchange protocol allows two parties to establish a shared secret over an insecure channel without transmitting the secret?
- RSA key exchange
- Diffie-Hellman (Correct answer)
- AES key wrapping
- Certificate pinning
Correct answer: Diffie-Hellman
Diffie-Hellman key exchange enables two parties to derive a shared secret using public values without ever transmitting the secret itself.
Question 2: What type of attack exploits weak random number generation to predict or recover cryptographic keys?
- Padding oracle attack
- Side-channel attack
- Entropy attack (Correct answer)
- Brute-force attack
Correct answer: Entropy attack
An entropy attack exploits predictable or low-entropy random number generators to recover or predict cryptographic key material.
Question 3: Which attack on block ciphers exploits predictable CBC-mode padding to decrypt ciphertext without knowing the key?
- Meet-in-the-middle attack
- Padding oracle attack (Correct answer)
- Birthday attack
- Replay attack
Correct answer: Padding oracle attack
A padding oracle attack leverages server responses about invalid padding in CBC mode to iteratively decrypt ciphertext byte by byte.
Question 4: In the context of PKI, what does a Certificate Revocation List (CRL) contain?
- Expired certificates awaiting renewal
- Serial numbers of revoked certificates (Correct answer)
- Trusted root CA public keys
- Certificate signing requests
Correct answer: Serial numbers of revoked certificates
A CRL is a list published by a CA containing the serial numbers of certificates that have been revoked before their expiry date.
Question 5: What is the primary purpose of a Hardware Security Module (HSM) in a cryptographic system?
- Compressing encrypted files
- Generating and protecting cryptographic keys in tamper-resistant hardware (Correct answer)
- Scanning for weak ciphers on a network
- Storing hashed passwords in a database
Correct answer: Generating and protecting cryptographic keys in tamper-resistant hardware
An HSM is a dedicated hardware device that generates, stores, and manages cryptographic keys in a tamper-resistant environment.
Question 6: Which cipher mode of operation produces a unique ciphertext even when the same plaintext is encrypted with the same key multiple times?
- ECB (Electronic Codebook)
- CBC with a fixed IV
- CTR (Counter) with a unique nonce (Correct answer)
- DES single encryption
Correct answer: CTR (Counter) with a unique nonce
CTR mode with a unique nonce (or counter) produces different ciphertext for identical plaintexts, unlike ECB which deterministically maps identical blocks.
CREST CREST Cryptography & Data Protection 2
Which key exchange protocol allows two parties to establish a shared secret over an insecure channel without transmitting the secret?