Section 10.10. Exercises | Computer and Communication Networks (paperback)

10.10. Exercises

1.	Assume that in round 4 of a DES process for a message, L ₄ = 4de5635d (in hex), R ₄ = 3412a90e (in hex) and k ₄ = be1142 7e6ac2 (in hex). Find R ₅ and L ₅ .
2.	Use DES to encrypt a 64-bit message, including all 1s with a 56-bit secret key consisting of 0101...01. Assuming a 1-bit rotation in the key process, find the outputs of the first round.
3.	Check your system to find out about its encryption utility. If a DES algorithm is used: Use an experiment to measure the speed of encryption and decryption. Repeat part (a), using different keys.
4.	Write a computer program to find the ciphertext for a message, using DES to encrypt a 64-bit message, including all 0s with a 56-bit secret key consisting of 1010...10. Assume that a 1-bit rotation in the key process.
5.	In the RSA encryption algorithm, show how either of Equations (10.6) and (10.7) is concluded from the other one.
6.	For an RSA encryption of a 4-bit message 1010, we choose a = 5, b = 11, and x = 3. Find the public and the private keys for this security action, and show the ciphertext.
7.	Apply RSA and do the following. Encrypt a = 5, b = 11, x = 7, and m = 13. Find the corresponding y . Decrypt the ciphertext
8.	Normally the speed of the RSA encryption algorithm is much lower than secret-key encryption algorithms. To solve this issue, we can combine RSA with a secret-key encryption algorithm, such as AES. Assume that user 1 chooses an AES key of 256 bits and encrypts this key with the user 2's RSA public key and also encrypts the message with the AES secret key. Prove that this AES key is too small to encrypt securely with RSA. Provide an example of a public key that can be recovered by an attacker.
9.	Consider again the combination of the two encryption methods discussed in the previous problem. What solutions would be feasible to overcome the mentioned vulnerability of this method?
10.	In the Diffie-Hillman key-exchange protocol, prove that the two keys k ₁ and k ₂ are equal.
11.	Computer simulation project . Write a computer program to simulate the operation of packet filtering in firewalls. Your program must include the following features. The firewall sets up to allow in only HTTP requests to a specific server. An outside intruder attempts to send packets with a forged internal address. All packets from a given remote server must be kept out.