Cryptography and Network Security(BCSE309L)
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In a zero-knowledge proof system, the verifier: a) Learns the secret value but cannot prove it to others. b) Receives a digitally signed copy of the secret. c) Computes the secret using a one-way function. d)
A security engineer notices that an employee is bypassing an access control system by exploiting a weakness in modular arithmetic calculations. The system verifies passwords by checking whether H(P)≡K mod 101, where H(P) is a hash of the password and K is a stored value. The engineer suspects that the employee is leveraging a property of numbers to derive passwords quickly.
What is the most likely vulnerability being exploited?
a) Small prime modulus makes it easy to compute modular inverses. b) Fermat’s theorem can be used to derive the hash function’s properties. c) The modulus is prime, allowing easy calculation of discrete logarithms. d) The system is using a weak form of modular exponentiation that leaks information.
Which of the following scenarios could cause an RSA digital signature to fail? a) The signer uses a random number for each signature. b) The verifier does not have the correct public key. c) The private key is longer than the message. d) The message is not encrypted before signing.
A hacker intercepts and replays a digitally signed financial transaction to transfer funds multiple times, even though the signature is valid.
Which best practice would prevent this?
a) Encrypting the digital signature before sending it b) Using c) Using a d) Storing all signatures in a public database to detect duplicates
The birthday paradox affects cryptographic hash functions because: b) It increases the probability of finding collisions. c) It guarantees two inputs will have the same hash. d) It allows us to generate hashes faster.
A 64-bit plaintext is encrypted with DES. After 3 rounds, how many bits are affected?
a) 16
b) 32
c) 48
d) 64
A government agency signs legal documents using RSA digital signatures. One day, they notice that different documents have identical digital signatures even though the contents are different. The RSA keypair is valid, and the documents were signed independently.
What is the most probable reason for this security failure?
a) The private key was compromised, allowing signature reuse. b) The same random value was used during the signature generation process. c) The hash function used before signing has collisions. d) The agency mistakenly encrypted the documents instead of signing them.
Bob connects to a public Wi-Fi at a coffee shop and uses a website that claims to support end-to-end encrypted messaging using Diffie-Hellman key exchange. However, an attacker sitting nearby is able to intercept and read Bob’s messages. Later, Bob realizes that his device never received a security warning.
Which of the following is the most likely
a) The attacker modified Bob’s public key and inserted their own during the key exchange. b) Bob’s device cached an older session key, allowing replay attacks. c) The attacker performed a chosen-ciphertext attack on the encrypted messages. d) The key exchange used a prime number that was too small, making it easy to compute discrete logarithms.
A company implements an IPsec VPN to secure remote employee communications. After a penetration test, the security team finds that an attacker can still see employees' visited websites and read metadata about the connections. However, the actual payloads remain encrypted.
What misconfiguration is most likely causing this issue?
a) The company used IPsec in Transport Mode instead of Tunnel Mode. b) The VPN did not authenticate users properly before establishing encryption. c) The attacker exploited an AES timing attack to leak partial key information. d) The company used an outdated hashing algorithm in the IKE phase.
Suppose Alice and Bob are using Diffie-Hellman with prime p=23 and generator g=5. If Alice sends A=8 and Bob sends B=19 , what is their shared secret?
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