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Cryptography: Stream Cipher vs Block Cipher, Security Concepts, and Feistel Cipher Network, Lecture notes of Network security

University of Engineering & ManagementNetwork security

A complete detailed walkthrough of the various Network Security and Cryptography concepts.

Typology: Lecture notes

2021/2022

Available from 11/26/2022

rajarshi-banerjee
rajarshi-banerjee 🇮🇳

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Cryptography and Network Security
Module 2
Very short, Short & Long Type Questions Solution Model
1. Compare stream cipher with block cipher with example.
Stream cipher: Processes the input stream continuously and producing
one element at a time. Example: caeser cipher.
Block cipher: Processes the input one block of elements at a time
producing an output block for each input block. Example: DES.
2. Differentiate unconditionally secured and computationally secured .
An Encryption algorithm is unconditionally secured means, the condition is if the
cipher
text generated by the encryption scheme doesn’t contain enough
information to determine
corresponding plaintext.
Encryption is computationally secured means,
1. The cost of breaking the cipher exceed the value of enough information.
2. Time required to break the cipher exceed the useful
lifetime of information.
3. Define
Diffusion &
Cnfusion.
Diffusion:
It means each plaintext digits affect the values of many ciphertext digits
which is
equivalent to each ciphertext digit is affected by many plaintext digits. It
can be achieved by performing permutation on the data. It is the
relationship between the plaintext and ciphertext.
Confusion:
It can be achieved by substitution algorithm. It is the relationship between
cipher text and
key.
4. What are the design parameters of Feistel cipher network?
*Block size
*Key size
*Number of Rounds
1
pf3
pf4
pf5
pf8
pf9
pfa

Partial preview of the text

Download Cryptography: Stream Cipher vs Block Cipher, Security Concepts, and Feistel Cipher Network and more Lecture notes Network security in PDF only on Docsity!

Cryptography and Network Security

Module 2 Very short, Short & Long Type Questions Solution Model

  1. Compare stream cipher with block cipher with example. Stream cipher: Processes the input stream continuously and producing one element at a time. Example: caeser cipher. Block cipher: Processes the input one block of elements at a time producing an output block for each input block. Example: DES.
  2. Differentiate unconditionally secured and computationally secured. An Encryption algorithm is unconditionally secured means, the condition is if the cipher text generated by the encryption scheme doesn’t contain enough information to determine corresponding plaintext. Encryption is computationally secured means,
  3. The cost of breaking the cipher exceed the value of enough information.
  4. Time required to break the cipher exceed the useful lifetime of information.
  5. Define Diffusion & Cnfusion. Diffusion: It means each plaintext digits affect the values of many ciphertext digits which is equivalent to each ciphertext digit is affected by many plaintext digits. It can be achieved by performing permutation on the data. It is the relationship between the plaintext and ciphertext. Confusion: It can be achieved by substitution algorithm. It is the relationship between cipher text and key.
  6. What are the design parameters of Feistel cipher network? *Block size *Key size *Number of Rounds

*Sub key generation algorithm *Round function *Fast software Encryption/Decryption *Ease of analysis

  1. Define Product cipher. It means two or more basic cipher are combined and it produce the resultant cipher is called the product cipher.
  2. Explain Avalanche effect. A desirable property of any encryption algorithm is that a small change in either the plaintext or the key produce a significant change in the ciphertext. In particular, a change in one bit of the plaintext or one bit of the key should produce a change in manybits of the ciphertext. If the change is small, this might provider a way to reduce the size of the plaintext or key space to be searched.
  3. Give the five modes of operation of Block cipher. i. Electronic Codebook(ECB) ii. Cipher Block Chaining(CBC) iii. Cipher Feedback(CFB) iv. Output Feedback(OFB) v. Counter(CTR)
  4. State advantages of counter mode. *Hardware Efficiency * Software Efficiency *Preprocessing * Random Access
  • Provable Security * Simplicity.
  1. Define Multiple Encryption.
  1. What is traffic Padding? What is its purpose? Traffic padding produces ciphertext output continuously, even in the absence of the plain text. A continuous random data stream is generated. When plain text is available, it is encrypted and transmitted. When input plaintext is not present, random data are encrypted and transmitted. This makes it impossible to for an attacker to distinguish between true dataflow.
  2. List the evaluation criteria defined by NIST for AES? The evaluation criteria for AES is as follows: 1.Security
  3. Cost
  4. Algorithm and implementation characteristics
  5. What is Triple Encryption? How many keys are used in triple encryption? Triple Encryption is a technique in which encryption algorithm is performed three times using three keys.
  6. List the schemes for the distribution of public keys. Public announcement Publicly available directory Public key authority Public-key certificates
  1. Drawback of 3-DES. Algorithm is sluggish in software The number of rounds in thrice as that of DES 3DES uses 64 bit block size To have higher efficiency and security a larger block size is needed.
  2. List out an evaluation criteria for round 2. General security Software implementation Hardware implementation Attacks Encryption Vs Decryption Key ability-Ability to change keys quickly with minimum of resources. Versatility and Flexibility Instruction level parallelism.
  3. List out the attacks to RSA. Brute force - Trying all possible private keys. Mathematical attacks - The approaches to factor the product of two prime numbers. Timing attack - Depends on the running time of the decryption algorithm.
  4. Prove that 3 is a primitive root of 7. That is, if is a primitive root of the prime number, then the numbers a mod p, a2 mod p,… ap-1 mod p 3 mod 7, 9 mod 7, 27 mod 7, …. 656 mod 7 3, 2, 6,…..5.
  5. Write any one technique of attacking RSA. The most widely used public-key cryptosystem is RSA. The difficulty of attacking RSA is based on the difficulty of finding the prime factors of a composite number. THE FACTORING PROBLEM: We can identify three approaches to attacking RSA mathematically.
  6. Factor n into its two prime factors .This enables calculation of Ø(n) =(p-1)x(q-1) ,which in turn enables determination of d ≡e-1 (mod Ø(n)).
  7. Determine Ø(n) directly, without first determining p and q. Again, this enables determination of d ≡e-1 (mod Ø(n)).
  8. Determine d directly, without first determining Ø(n).

ciphertext.

  1. What is DES? Data Encryption Standard (DES) is a widely-used method of data encryption using a private (secret) key. DES applies a 56-bit key to each 64-bit block of data. The process can run in several modes and involves 16 rounds or operations.
  2. Compare the symmetric and asymmetric key cryptography. Symmetric Encryption uses a single secret key that needs to be shared among the people who needs to receive the message while Asymmetric encryption uses a pair of public key, and a private key to encrypt and decrypt messages when communicating.
  3. Symmetric Encryption is an age old technique while asymmetric Encryption is relatively new.
  4. Asymmetric Encryption was introduced to complement the inherent problem of the need to share the key in symmetric encryption model eliminating the need to share the key by using a pair of public-private keys.
  5. What are the disadvantages of double DES? The following are the disadvantages of double DES
  6. Reduction to a single stage. \
  7. Meet in the middle attacks.
  8. Double DES is less secure than triple DES.
  9. Double DES is within brute force attack.
    1. What is message authentication? It is a procedure that verifies whether the received message comes from assigned source has not been altered. It uses message authentication codes, hash algorithms to authenticate the message.
    2. Define the classes of message authentication function. Message encryption: The entire cipher text would be used for authentication. Message Authentication Code: It is a function of message and secret key produce a fixed length value. Hash function: Some function that map a message of any length to fixed length which serves as authentication.
    3. What are the requirements for message authentication? The requirements for message authentication are

i. Disclosure: Release of message contents to any person or process not processing the appropriate cryptographic key ii. Traffic Analysis: Discovery of the pattern of traffic between parties. In a connection oriented application, the frequency and duration of connections could be determined. In either a connection oriented or connectionless environment, the number and length of messages between parties could be determined. iii. Masquerade: Insertion of messages into the network from a fraudulent source. This includes the creation of messages by an opponent that are purported to come from an authorized entity. Also included are fraudulent acknowledgements of message receipt or no receipt by someone other than the message recipient. iv. Content modification: Changes to the contents of a message , including insertion, deletion, transposition, and modification. v. Sequence modification: Any modification to a sequence of messages between parties, including insertion, deletion, and modification. vi. Timing modification: Delay or replay of messages. In a connection oriented application, an entire sessio or sequence of mes ges could be a replay of some previous valid session, or individual messages in the sequence could be delayed or replayed. In connectionless application, an individual message could be delayed or replayed. vii. Source repudiation: Denial of transmission of message by source. viii. Destination repudiation: Denial of receipt of message by destination.

  1. What you meant by hash function? Hash function accept a variable size message M as input and produces a fixed size hash code H(M) called as message digest as output. It is the variation on the message authentication code.
  2. Differentiate MAC and Hash function? MAC: In Message Authentication Code, the secret key shared by sender and receiver. The MAC is appended to the message at the source at a time which the message is assumed or known to be correct. Hash Function: The hash value is appended to the message at the source at time when the message

⦁ (i) Using SDES decrypt the string 10100010 using the key 0111111101. Show all calculations and result. (ii) Using SDES decrypt the string 10101010 using the key0111111101. Show all calculations and result.