Network Security Lab 6

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Department of Computer Science and Computer Engineering

NSW Assignment Stage 06 (Prac 06)
Individual Submission: Hard copy/ Soft copy shown to the tutor/lecturer
Deadline: End of this lab session
Worth: 1% for lab work + 0.5% BONUS for at least 1 hour attendance


Objectives:
(1) To understand and learn more about computer and IT security based on lecture
notes and internet sources.
Exercise 1
1. Explain the ways by which the cryptographic systems are categorized.
Ans:
Mainly in three ways:
a. type of encryption operations used
i. substitution (for example, in case of alphabet as well twenty replace t
by z and w by a y)
ii. transposition ,changing the position of the data or the character
iii. product, multiply data with secret code.
b. number of keys used
i. single-key or private(same key for encryption and decryption)
ii. two-key or public (different key for encryption and decryption)
c. way in which plaintext is processed
i. block (take a known sized block of data to encrypt)
ii. stream (encryption is performed on each byte or word sequentially)



2. Explain the following:
i. Plain text: Original message that needed to be sent.
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ii. Cipher text: When the plain text is converted in to a coded message using the
encryption operations.
iii. Cipher: Cipher Algorithm is used to execute the cryptographic systems. Key
iv. Encipher-The procedure of converting plain text to cipher text.
v. Decipher: recovering/ retrieving the plain text from cipher.
vi. Cryptography: study of encryption principles/methods.
vii. Cryptanalysis: (code breaking) - study of principles/ methods of deciphering cipher
text without knowing key.



3. Explain the Symmetric Cipher Model.
Detail the five ingredients of the symmetric cipher model, shown in Stallings Figure

Plaintext - original message


Encryption algorithm – performs substitutions/transformations on plaintext


Secret key – control exact substitutions/transformations used in encryption
algorithm

ciphertext - scrambled message


Decryption algorithm – inverse of encryption algorithm


Recover Plaintext - original message

Its requirements
 a strong encryption algorithm
 a secret key known only to sender / receiver




Transmitting end
Receiving end
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Exercise 2
1. Explain Feistel Cipher Structure for implementing symmetric encryption with the
help of a diagram.
Feistel Cipher Structure : used for Multiple encryption


Design elements of Feistel Cipher
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The exact realization of a Feistel network depends on the choice of the following parameters
& design features:
• Block size
• key size - increasing size improves security, makes exhaustive key searching harder,
but may slow cipher
• number of rounds - increasing number improves security
• Sub-key generation algorithm - greater complexity can make analysis harder, but
slows cipher
• Round function - greater complexity can make analysis harder, but slows cipher fast
• Software en/decryption - more recent concern for practical use
• Ease of analysis - for easier validation & testing of strength