Question

Bob is trying to send an encrypted message to Alice using the Asymmetric Key approach. Which key will Bob use to encrypt the message for Alice?

Alice's Private Key

Bob's Public Key

Alice's Public Key

Bob's Private Key

Alice wants to digitally sign a message so that Bob can be assured that the message came from Alice and has not been changed in transit. Which key must Alice use to encrypt the message digest?

Bob's Public Key

Bob's Private Key

Alice's Private Key

Alice's Public Key

Bob received a digitally signed message from Alice (i.e. message was signed by Alice). Which key should Bob use to decrypt the message digest (hash) in order to verify that the message indeed came from Alice?

Bob's Private Key

Alice's Private Key

Bob's Public Key

Alice's Public Key

Alice and Bob wish to securely communicate using the public key (asymmetric key) system. What's the minimum number of keys they would need added together (i.e. total number of keys needed for Alice and Bob combined)?

2

1

3

4

There are 5 participants in a Symmetric Key system and they all wish to communicate with each other in a secure fashion using Symmetric Keys. What's the minimum number of Symmetric Keys needed for this scenario?

5

10

20

1

A fundamental problem/security concern with Public Key (Asymmetric Key) based systems is verifying the association between an entity and that entity's Public Key. This problem is addressed by:

Digital Certificates

Signature Verification

Nonrepudiation

Zero-knowledge proof

Alice and Bob met in person and shared a secret cryptographic key. They will use the same key both for encryption and for decryption. Which type of cryptographic system are they using?

Symmetric Key Cryptography

Asymmetric Key Cryptography

Cryptographic Hash Functions

Public Key Cryptography

Bob sends an encrypted message to Alice using the Asymmetric Key approach. Which key will Alice use to decrypt the message from Bob?

Alice's Public Key

Bob's Private Key

Bob's Public Key

Alice's Private Key

Choose the best answer: Cryptographic techniques (e.g., encryption, hashing) help with the following information security objective(s):

Confidentiality

Integrity

Authentication and Non-Repudiation

All of the above

Choose the best answer: In information security, Cryptography (when used properly) can help protect:

Data at rest (over time)

Data in transit (over distance)

Both data at rest and data in transit

None of the above

Answer 1

Dear Learner,

Here is the solution

1. Bob is trying to send an encrypted message to Alice using the Asymmetric Key approach. Which key will Bob use to encrypt the message for Alice?

Alice's Public Key ; Since in asymmetric key cryptography, the sender only knows the public key of the recipient, he/she will use the latter's public key.

2.Alice wants to digitally sign a message so that Bob can be assured that the message came from Alice and has not been changed in transit. Which key must Alice use to encrypt the message digest?

Alice's Private Key ; For the authentication of the message that it came from a particular sender, Alice would use her own private key.

3.Bob received a digitally signed message from Alice (i.e. message was signed by Alice). Which key should Bob use to decrypt the message digest (hash) in order to verify that the message indeed came from Alice?

Alice's public key; if the key was signed by Alice then only Bob can verify that the message came from her.

4.Alice and Bob wish to securely communicate using the public key (asymmetric key) system. What's the minimum number of keys they would need added together (i.e. total number of keys needed for Alice and Bob combined)?

4; Alice would need to have the public key of Bob, and private key of herself. Same goes for Bob as well. So, total 4 keys would be required.

5.There are 5 participants in a Symmetric Key system and they all wish to communicate with each other in a secure fashion using Symmetric Keys. What's the minimum number of Symmetric Keys needed for this scenario?

10; since all the participants will need to have the keys of all the others, the total number of keys required in a network of n participants is given by ⁿC₂ = n*(n-1)/2, hence 5*4/2 = 10.

6.Bob sends an encrypted message to Alice using the Asymmetric Key approach. Which key will Alice use to decrypt the message from Bob?

Alice's private key ; Since in the asymmetric key approach, encryption is done using recipient's public key. Decryption would be done by the recipient's own private key.

7.Choose the best answer: Cryptographic techniques (e.g., encryption, hashing) help with the following information security objective(s):

All of the above; those are the four main goals of cryptography.

8.Choose the best answer: In information security, Cryptography (when used properly) can help protect:

Both data at rest and data in transit​​ ​​​​​; For protecting data in transit, we protect the data using different protocols (symmetric or asymmetric approaches) to protect the contents of data in transit. For protecting data at rest, one can simply protect the data by encrypting sensitive data prior to storing it and/or choose to encrypt the storage disk itself (using password).