CISSP Cryptography 4 — Questions and Answers
Question 1: Which attack exploits the mathematical relationship between related RSA keys when the same plaintext is encrypted with multiple public keys sharing the same exponent?
- Timing attack
- Håstad's broadcast attack (Correct answer)
- Meet-in-the-middle attack
- Side-channel attack
Correct answer: Håstad's broadcast attack
Håstad's broadcast attack uses the Chinese Remainder Theorem to recover plaintext when the same message is encrypted with multiple RSA public keys using a small exponent like e=3.
Question 2: What is the role of a Key Derivation Function (KDF) in cryptographic systems?
- To generate random prime numbers for RSA
- To derive cryptographic keys from a master secret or password (Correct answer)
- To distribute public keys to all users
- To verify the integrity of a private key
Correct answer: To derive cryptographic keys from a master secret or password
A KDF derives one or more cryptographic keys from a source secret (like a password or master key), often adding salt and iterations to resist brute force.
Question 3: In a hybrid encryption scheme, what is the typical role of asymmetric cryptography?
- Encrypting the bulk of the data for efficiency
- Encrypting the symmetric session key used for data encryption (Correct answer)
- Replacing symmetric keys entirely
- Generating hash values for integrity checks
Correct answer: Encrypting the symmetric session key used for data encryption
Hybrid schemes use fast symmetric encryption for bulk data and asymmetric encryption only to securely transmit the symmetric session key.
Question 4: Which of the following is an example of a stream cipher?
- AES
- 3DES
- RC4 (Correct answer)
- Blowfish
Correct answer: RC4
RC4 is a stream cipher that generates a pseudorandom keystream XORed with plaintext one byte at a time, unlike block ciphers like AES and Blowfish.
Question 5: What cryptographic concept does the Diffie-Hellman problem rely on for its security?
- The difficulty of factoring large prime numbers
- The difficulty of computing discrete logarithms (Correct answer)
- The difficulty of solving elliptic curve equations
- The difficulty of reversing hash functions
Correct answer: The difficulty of computing discrete logarithms
Diffie-Hellman security relies on the computational difficulty of the discrete logarithm problem: given g^x mod p, finding x is computationally infeasible.
Question 6: When using AES in GCM mode, what additional security property does it provide beyond encryption?
- Key authentication
- Authenticated encryption with associated data (AEAD) (Correct answer)
- Quantum resistance
- Non-repudiation
Correct answer: Authenticated encryption with associated data (AEAD)
AES-GCM provides AEAD, combining encryption with an authentication tag that detects any tampering with the ciphertext or associated unencrypted data.
Question 7: What is the purpose of key escrow in an enterprise cryptographic environment?
- To increase encryption speed by caching keys
- To allow authorized recovery of encrypted data if the original key is lost (Correct answer)
- To generate new keys automatically upon expiration
- To distribute keys to all employees simultaneously
Correct answer: To allow authorized recovery of encrypted data if the original key is lost
Key escrow stores copies of cryptographic keys with a trusted third party so encrypted data can be recovered if the original key holder is unavailable.
CISSP Cryptography 4
Which attack exploits the mathematical relationship between related RSA keys when the same plaintext is encrypted with multiple public keys sharing the same exponent?