# Computer Networks Questions & Answers – Public Key Cryptography

This set of Computer Networks Multiple Choice Questions & Answers (MCQs) focuses on “Public Key Cryptography”.

1. What are the two keys that are used in asymmetric key cryptography?
a) Secret key and private key
b) Private key and public key
c) Public key and secured key
d) Secured key and private key

Explanation: In asymmetric key cryptography, each user needs to have 2 keys; one is public and another one is private key. Public key is used for encryption, private is used for decryption.

2. Who are three discoverers of the RSA algorithm in public key cryptography?
a) Rivest, shamir, Aldleman
c) Robert, shamuel, alice

Explanation: RSA algorithm is a public key algorithm, which was discovered at M.I.T in 1978. It is very strong but the only disadvantage is, it requires more key length in order to provide more security.

3. What is the first public key cryptography algorithm?
a) RSA algorithm
b) AES algorithm
c) 3DES algorithm
d) Knapsack algorithm

Explanation: Knapsack is the first public key algorithm, that was not that much secured and now it is not in use. Knapsack is the founding algorithm for further algorithms in public key cryptography.

4. Which of the following is not a property of public key cryptography?
a) Uses different keys
b) Every user has a unique private key
c) Public key can be same for number of users
d) Cannot deduce private key from public key

Explanation: There will be a unique public key for every user, who publishes it. Public key prevents spoofing, by acting as the receiver. Sender uses the public key of the receiver to encrypt the message and the receiver will decrypt the message with his secret key.

5. What are the types in schemes of public key encryption?
a) RSA, EIGamal, Elliptic curve cryptography
b) Elliptic curve, RSA
c) RSA, EIGamal
d) EIGmal, RSA

Explanation: RSA is the initial one and it achieves security using factoring of huge numbers. EIGamal uses numbers module p to strengthen the algorithm. Elliptic curve scheme uses discrete logarithms to achieve security.
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6. Which of the following is correct about RSA scheme in public key cryptography?
a) It is efficient for decryption
b) Lengthy keys are required for more security
c) It is not efficient for encryption
d) It is not popular

Explanation: RSA schemes need at least 1024 bits in a key for providing good level of security. Symmetric key cryptography uses 128 bits. The more the key length, more the security in RSA, this is why RSA slows down the system.

7. Which of the following is an asymmetric key cryptography algorithm?
a) DES
b) AES
c) RC4
d) RSA

Explanation: RSA is named with the first letters of the discoverers (Rivest, Shamir, Aldleman). It uses lengthy keys for security and uses the concept of factoring large numbers to be secured.

8. Which of the following doesn’t match as a property of elliptic curve cryptography?
a) Used over RSA
b) Uses elliptic curve theory
c) Key length is larger than RSA
d) Uses discrete logarithms

Explanation: Elliptic curve cryptography is much faster and efficient than RSA algorithm. And keys are smaller in size, which are generated by elliptic curve equation.

9. Which of the following is the disadvantage of asymmetric cryptography?
a) No need of key distribution
b) Security
c) Using digital signatures
d) Used to decrypt small messages

Explanation: Public key cryptography cannot deal well with bulk messages. It uses very lengthy keys compared to symmetric key cryptography; this makes the system slow to proceed with bulk messages.

10. A sender encrypts a message using recipient’s public key in public key cryptography?
a) True
b) False

Explanation: A sender can make use of pubic directory to acquire the public key of the recipient. After acquiring the public key of the receiver, sender will encrypt the message using receiver public key. If sender encrypts the message with his own secret key, then only the sender can decrypt it.

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