Unit 5: Digital Signature and Key management - Practice Quiz

1 What is the main purpose of a digital signature?

2 Which property is generally provided by a digital signature but not by ordinary encryption alone?

3 What is usually applied to a message before it is digitally signed?

4 Which key is normally used by the sender to create a digital signature?

5 Which key does a recipient normally use to verify a digital signature?

6 What is a forgery attack on a digital signature?

7 What does a replay attack attempt to do?

8 Which attack involves an attacker obtaining the victim's private signing key?

9 The RSA digital signature scheme is based primarily on the difficulty of which problem?

10 In the basic RSA signature process, which key is used to generate the signature?

11 Which key is used to verify an RSA digital signature?

12 The ElGamal digital signature scheme is based on the difficulty of the:

13 Which value should normally be unique and secret for each ElGamal signature?

14 The Schnoor digital signature scheme is commonly described as being based on which mathematical problem?

15 What type of value is typically used in the commitment step of a Schnoor signature?

16 Which standard is specifically associated with digital signatures based on the Digital Signature Algorithm?

17 What is the role of a certificate authority in a public-key infrastructure?

18 What is required for two parties to use a symmetric encryption algorithm securely?

19 What is the main function of a key distribution center?

20 What is the main goal of a symmetric key agreement protocol?

21 Alice and Bob share a secret key and use a MAC to authenticate messages. Why does a digital signature provide stronger non-repudiation than the MAC?

22 A company requires a trusted third party to participate in every signed transaction and resolve disputes immediately. Which signature model best satisfies this requirement?

23 Why is a message digest normally signed instead of applying the signature algorithm directly to a large message?

24 A receiver obtains a message, its digital signature, and the sender's certified public key. Which procedure correctly verifies the signature?

25 An attacker records a valid digitally signed funds-transfer request and sends the same request again later. Which measure most directly prevents this replay attack?

26 In DSA-, ElGamal-, or Schnorr-type signatures, what is the main consequence of reusing the same secret nonce for two different messages?

27 In a textbook RSA signature scheme, let , , , and the message representative be . What is the signature ?

28 For textbook RSA, signatures satisfy a multiplicative relation: if signs and signs , then can sign . Which technique is designed to prevent this weakness?

29 For ElGamal signatures, let , , private key , nonce , and . Using and , what is the signature?

30 Why must the ElGamal signing nonce satisfy ?

31 In a Schnorr signature variant, . If the same nonce is used with challenges and , which expression recovers the private key ?

32 A Schnorr scheme uses , , , and . Which verification computation reconstructs ?

33 In the traditional Digital Signature Standard (DSS), which pair identifies the signature algorithm and the hash operation used in the original specification?

34 During DSA verification, the verifier computes , , and . Which final condition indicates a valid signature?

35 A key distribution center gives Alice a session key and a ticket for Bob. The ticket contains the session key encrypted with Bob's long-term key. Why can Alice forward the ticket without reading it?

36 A system encrypts many session keys using one key-encrypting key (KEK). What is the most serious result if the KEK is compromised?

37 Alice and Bob perform Diffie-Hellman with , , Alice's private value , and Bob's private value . What shared value do they derive?

38 Alice and Bob use unauthenticated Diffie-Hellman to establish a symmetric session key. Which improvement most directly protects them from a man-in-the-middle attack?

39 Alice receives a certificate containing Bob's identity and public key. What must Alice verify before trusting the binding?

40 Alice downloads Bob's public key from an unauthenticated website and then compares its fingerprint with Bob over a trusted phone call. What security benefit does the comparison provide?

41 Which security consequence is most characteristic of an arbitrated digital signature system when compared with a direct digital signature system?

42 Alice and Bob share a MAC key, while Alice also owns a certified digital-signature key. Why can a valid MAC generally not provide the same nonrepudiation property as Alice's digital signature?

43 A protocol signs JSON documents, but different implementations may reorder object members or encode numbers such as , , and differently. Which measure most directly prevents signature failures or representation-based attacks?

44 A sender requires confidentiality during transport and wants the recipient to retain a signature on the plaintext that can later be verified by a third party. Which construction best satisfies both goals?

45 Two ECDSA signatures use the same nonce and therefore the same . For message representatives , the signatures are and , where . Assuming the required inverses exist, which formulas recover the nonce and private key?

46 In textbook RSA verification, a pair is accepted whenever , with no hash or structured encoding. How can an attacker create an existential forgery without obtaining any signatures?

47 An RSA-CRT signer produces a correct signature and, during a fault, a signature that is correct modulo one prime factor of but incorrect modulo the other. Which computation is most likely to factor ?

48 What is the principal cryptographic effect of the random salt in RSA-PSS?

49 In ElGamal signatures, let , generator , private key , message representative , and nonce . Using and , which signature is produced?

50 Why must an ElGamal signing nonce satisfy ?

51 Consider a Schnorr variant with private key , nonce , challenge , and response . If the same nonce is used for distinct challenges , which expression recovers ?

52 In a Fiat–Shamir Schnorr signature, why is the challenge normally computed as , where , rather than as alone?

53 In classic DSA with a -bit subgroup order and a SHA-256 message digest, how is the integer used in the signing equation normally derived from the digest?

54 A DSA implementation replaces random nonces with deterministic nonces derived from the private key and message digest, as in RFC 6979. Which statement best describes the resulting security improvement?

55 A KDC distributes a session key by sending Alice a copy encrypted under her long-term key and Bob a copy encrypted under . An attacker later compromises and has recorded Alice's old KDC messages. What is the main consequence?

56 A storage system encrypts each large object under a random data-encryption key and wraps those keys under a key-encryption key. What is the major operational advantage when the key-encryption key is rotated?

57 Alice and Bob share a high-entropy PSK and run ephemeral Diffie–Hellman, authenticating the complete transcript with MACs under keys derived from the PSK and DH secret. If the PSK is compromised only after the session and both ephemeral exponents were erased, what property can the session key retain?

58 What additional assurance does explicit key confirmation provide beyond implicit key authentication in a symmetric key-agreement protocol?

59 A server certificate has a valid signature chain to a trusted root and is within its validity period. However, an intermediate CA certificate contains a critical name constraint excluding the server's DNS namespace. What should a conforming path validator do?

60 A client uses trust on first use by storing the public-key fingerprint observed during its first connection and rejecting later changes. Which risk remains inherent in this method?