CS8792 – Cryptography and Network Security VII Semester CSE

DHANALAKSHMI COLLEGE OF ENGINEERING

Tambaram, Chennai

Department of Computer Science and Engineering

CS8792 – Cryptography and Network Security

Year / Sem : IV / VII
2 Marks Q & A

Dept. of CSE Dhanalakshmi College of Engineering 1

CS8792 – Cryptography and Network Security VII Semester CSE

UNIT – II
SYMMETRIC KEY CRYPTOGRAPHY

Mathematics of Symmetric key Cryptography – Algebraic structures – Modular arithmetic – Euclid’s

algorithm – Congruence and matrices – Groups, Rings, Fields – Finite fields – Symmetric Key Ciphers
– SDES – Block cipher principle of DES – Strength of DES – Differential and linear cryptanalysis –
Block cipher design principles – Block cipher mode of operation – Evaluation criteria of AES –
Advanced Encryption Standard – RC4 – Key distribution

PART – A
1. List the parameters (block size, key size and number of rounds) for AES versions. (A/M−18)
S. No. AES version / Block size Key size Number of Rounds
1 AES – 128 11 10
2 AES – 192 13 12
3 AES – 256 15 14

2. Define – Field and Finite Field

A field is defined as a set of elements on which two arithmetic operations (addition and multiplication)
have been defined and has the properties of ordinary arithmetic such as closure, associativity,
commutativity, distributivity and having both additive and multiplicative inverses.
A finite field is a field with a finite number of elements. It can be shown that the order of a finite field
(number of elements in the field) must be a power of a prime p n, where is a positive integer. Finite fields
are important in several areas of cryptography.

3. What is meant by Avalanche Effect?

Avalanche effect is a characteristic of an encryption algorithm in which a small change in the plaintext
or key gives rise to a large change in the cipher text. For a hash code, the avalanche effect is a
characteristic in which a small change in the message gives rise to a large change in the message digest.

4. What is DES?
Data Encryption Standard (DES) is a symmetric key block cipher published by the National Institute of
Standards and Technology (NIST). The algorithm used in DES is referred as Data Encryption
Algorithm (DEA). For DES, data are encrypted in 64-bit blocks using a 56-bit key. The algorithm
transforms 64-bit input in a series of steps into a 64-bit output.

5. What are the disadvantages of double DES?

Disadvantages of double DES
1) Reduction to a single stage
2) Meet in the middle attacks
3) Double DES is less secure than triple DES

Dept. of CSE Dhanalakshmi College of Engineering 2

 CS8792 – Cryptography and Network Security VII Semester CSE

4) Double DES is within brute force attack

6. What is triple DES? Give its strength. (M/J−14)
Triple DES involves repeating the DES algorithm three times on the plaintext using two or three
different keys to produce the cipher text.
Strength of DES
Triple DES provides a relatively simple method of increasing key size of DES to protect against
various attacks, without the need to design a completely new block cipher algorithm.

7. List the parameters of AES.

Parameters of AES
1) Key size (words / bytes / bits)
2) Plaintext Block Size (words /bytes /bits)
3) Number of Rounds
4) Round Key Size (words / bytes /bits)
5) Expanded Key Size (words / bytes)

8. Distinguish between DES and AES.

(A/M−17)
S. No. DES AES
1 DES stands for Data Encryption Standard. AES stands for Advanced Encryption Standard.
Key length can be of 128 – bits, 192 – bits and
2 Key length is 56 bits in DES.
256 – bits.
DES involves 16 rounds of identical Number of rounds depends on key length:
3
operations. 10 (128 – bits), 12 (192 – bits) and 14 (256 – bits).
DES can be broken easily because it has AES is more secure than DES and it is the de facto
4
known vulnerabilities. world standard.

9. List the different evaluation criteria of AES.

Different evaluation criteria of AES
1) Security – Effort requires to cryptanalyzing an algorithm
2) Cost – Refers to high computational efficiency
3) Algorithm and implementation characteristics – Refers to flexibility, simplicity and suitability

10. Mention the role of discrete logarithms in cryptography.

Discrete logarithms are fundamental to a number of public-key algorithms, including Diffie-Hellman key
exchange and digital signature algorithm. Also many theorems in cryptography are based on discrete
logarithms and its applications.

11. What is group in modern algebra?

A group G is a set of elements with a binary operation, that associates to each order pair (a, b) of elements in
G. Each element in G must satisfy the following axioms.

Dept. of CSE Dhanalakshmi College of Engineering 3

 CS8792 – Cryptography and Network Security VII Semester CSE

1) Closure
2) Associative
3) Identity element
4) Inverse element
5) Commutative

12. What is ring in modern algebra?

A ring R is a set of elements with two binary operations, addition and multiplication, such that for all a, b, c in
R must satisfy the following axioms.
1) Closure
2) Associative
3) Identity element
4) Inverse element
5) Commutative
6) Closure under multiplication
7) Associativity of multiplication and commutativity of multiplication
8) Distributive laws

13. What is S-Box? How is it used in DES?

Substitution box (S-Box) is a basic component of symmetric key algorithms which performs substitution. S-
boxes are basically non linear substitution tables where either the output is smaller than the input or vice versa.
DES uses 8 S-boxes, each with a 6-bit input and a 4-bit output.

14. What is S-AES?

S-AES stands for Simplified Advanced Encryption Standard. It has similar properties and structure to AES
with much smaller parameters. The S-AES encryption algorithm takes a 16-bit block of plain text as input and
a 16-bit key and produces a 16-bit block of cipher text as output. The S-AES decryption algorithm takes a 16-
bit block of cipher text as input and the same 16-bit key and produces a original 16-bit block of plaintext as
output.

15. When two numbers are said to be relatively prime?

Two numbers are said to be relatively prime, if they have no prime factors in common and their only common
divisor is 1.
Dept. of CSE Dhanalakshmi College of Engineering 4
 CS8792 – Cryptography and Network Security VII Semester CSE

Example: 5, 7 and 11, 13 are relatively prime, because their only common divisor is 1.

16. What is modular arithmetic?

A modular arithmetic is a kind of integer arithmetic that reduces all numbers to one of a fixed set [0 ... n-1] for
some number n. Any integer outside this range is reduced to one in this range by taking the remainder after
division by n. Many symmetric and asymmetric algorithms are used this modular arithmetic to produce the
output.

17. Mention the RC4 algorithm.

RC4 (Rivest Cipher 4) is a stream cipher and variable length key algorithm. The algorithm is based on the use
of a random permutation. This algorithm encrypts one byte at a time and a key input is pseudorandom bit
generator that produces a stream 8-bit number that is unpredictable. The output of the generator is called key-
stream, is combined one byte at a time with the plaintext stream cipher using X-OR operation.

18. List the uses of RC4.

Uses of RC4
1) RC4 is used in Secure Socket Layer / Transport Layer Security (SSL/TLS) standards to define the
communication between web browsers and servers.
2) It is used in Wired Equivalent Privacy Protocol (WEP) that is the part of the IEEE 802.11 Wireless
LAN standard.
3) It is used in Wi-Fi Protected Access protocol (WPA) that is also the part of the IEEE 802.11 Wireless
LAN standard.

19. What is man-in-the-middle attack?

Man-in-the-middle attack is a form of active wiretapping attack in which the attacker intercepts and selectively
modifies communicated data in order to masquerade as one or more of the entities involved in a
communication.

20. What is meet-in-the-middle attack?

The meet-in-the-middle attack is one of the types of known plaintext attacks. The intruder has to know some
parts of plaintext and their cipher texts. Using meet-in-the-middle attacks it is possible to break ciphers, which
have two or more secret keys for multiple encryption using the same algorithm.
C = Eb (kb, Ea(ka, P)) and P = Da (ka, Db (kb, C)), where C is a cipher text, P is a plain text, E is an algorithm
for encryption, D is an algorithm for decryption and ka and kb are two secret keys

Dept. of CSE Dhanalakshmi College of Engineering 5