# 网络安全代写｜ECS726U Security and Authentication Open Book Exam

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## Question 1

(a) What is cryptography?

(b) Explain what is an active security attack and a passive security attack. Give an
example of an active and a passive attack.

(c) Caesar has learnt that his encryption code is very easy to break. To increase the
security of his code he creates a double–Caesar code, where he encrypts two times
using the following keys: Key one, a! d, b! e, and so on. Second Key, a!x, b! y,
and so on. Is there a security problem with this encryption mechanism? Explain.

(d) If a bit error occurs in the transmission of a ciphertext character in an 8-bit Cipher
Feedback (CFB) mode with a 64 bit shift register, how far does the error propagate
when deciphering the plaintext?

## Question 2

(a) In modern encryption algorithms what is more important: (i) To keep the encryption
algorithm secret or (ii) to keep the encryption-key secret. Why?

(b) Bob and Alice want to exchange secret/confidential information using an RSA algo
rithm. Explain how they would do this?

(c) The video-conference application “Haste” promotes its software as a secure com
munication channel as it uses encryption where the mode of operation is using an
Electronic Code Book. Explain this mode of operation. Explain if it is secure or not.

(d) Double DES can be attacked using the meet-in-the-middle attack. Explain what this
attack is trying to achieve and how it is carried out.

## Question 3

(a) To operate, PGP uses four kinds of encryption keys. Describe the uses of these keys.

(b) Explain the difference between a short term and a long term key. Explain how they
are used.

(c) Describe a major problem related to the use of certificates for key distribution in
public–key encryption.

(d) Supposedly Diffie-Hellman is a secure mechanism to exchange keys, however it is
prone to the man–in–the–middle attack. Explain how this attack is carried out.

## Question 4

(a) In IPSec, what is a security association?

(b) Give three different attacks that a firewall cannot protect against.

(c) How can a Trojan Horse attack be stopped? Explain.

(d) An international company wants to implement the following security services between
different parts of the company. The company wants all messages in the LANs are
authenticated. Any message that goes out of a LAN to another LAN has to be
confidential between the sender and recipient and provide some protection to traffic
analysis. Also, it has to be authenticated between the LAN gateways. How would you
implement this services using IPSec? Explain.

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