Question

Part 4: Explain in a couple of paragraphs how public key encryption can be used to implement a digital signature. Be sure you are very clear on when a private key is used and when a public key is used.

Part 5: Generally, a digital signature involves encrypting a cryptographic hash, or digest, generated from the message. Explain why we do not encrypt the message itself. You can answer this question in one sentence.

Part 6: For each of the following scenarios below, tell what type of encryption is most appropriate and in a sentence or two explain the reasoning for your choice.
1. Alice wants to send a confidential message to Bill, whom she has never met and who lives in a distant country.
2. Charlie wants to be sure that no one but he can see the financial and medical records he has stored on his computer.
3. David needs a way to check that large computer files stored on corporate servers have not been modified.
4. Eddard uses a "cloud" backup service; he wants to be sure the operators of the service cannot read his files.
5. Frank needs to send a message to George. The message need not be confidential, but George must be assured that it actually came from Frank.

Answer 1

Part 4:

In digital signature, user digitally signed the data or encrypted the data using his private key while receiver verifies the signature using user's public key.

Part 5:

Possible reason for encrypting hash function rather than message itself is message is very large in size in comparison of hash key.

Part 6:

a.) In that case, Alice can public key encryption method. He can use Bill's public key to encrypt the message and send to him. We know message can be decrypted using Bill's private key and since only Bill has access to its private key, only he can decrypt the message.

b.) Charlie can use one-key encryption method as only he has access to that key then no one other than himself can see the actual data.

c.) In order to preserve integrity of the data, David can use hash function to convert the whole data into a message digest.

d.) In order to ensure that message is came from the intended person, Frank can digitally sign the message using his private key and at the receiving end George can verify it using Frank's public key.