James F. Kurose, Keith W. Ross

Problems

Security In Computer Networks

6

9780132856201

18

Suppose
Alice wants to send an e-mail to Bob. Bob has a public-private key
pair (K_{B}^{+}
,K_{B}^{–
}), and Alice has
Bob’s certificate. But Alice does not have a public, private key
pair. Alice and Bob (and the entire world) share the same hash
function H(.).

a. In this situation, is it possible to design a scheme so that Bob can verify that Alice created the message? If so, show how with a block diagram for Alice and Bob.

b. Is it possible to design a scheme that provides confidentiality for sending the message from Alice to Bob? If so, show how with a block diagram for Alice and Bob.

a. In this situation, is it possible to design a scheme so that Bob can verify that Alice created the message? If so, show how with a block diagram for Alice and Bob.

- No, it is not possible
- Without setting up a public-private-key pair, or a pre-shared secret, there is no way for Bob to verify that Alice was or had created the message.

b. Is it possible to design a scheme that provides confidentiality for sending the message from (4 points)

- Yes, it is possible.
- The most straightforward way to provide confidentiality is for Alice to encrypt the message with symmetric key technology (such as DES or AES) and for Bob to decrypt the message on receipt (p706)
- Alice encrypts her message with Bob’s public key, and she sends the encrypted message to Bob’s e-mail address. When Bob receives the message, he simply decrypts it with his private key (p706)