Name of the College : Noorul Islam College of Engineering
University : Anna University
Degree : B.E
Department : Computer Science and Engineering
Subject Code/Name : IT 1352 – Cryptography & Network Security
Document Type : Question Bank
Website : niceindia.com
Download :https://www.pdfquestion.in/uploads/ni…k_Security.pdf
NICE Cryptography & Network Security Question Paper
UNIT I
1. What is cryptology? :
Cryptology is the study of cryptography and cryptanalysis.
2. What is the difference between an unconditionally secure cipher and a computationally secure cipher? :
An unconditionally secure cipher is a scheme such that if the cipher text generated by the scheme does not contain enough information to determine uniquely the corresponding plain text, no matter how much cipher text is available.
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A computationally secure scheme is such that the cost of breaking the cipher exceeds the value of the encrypted information and the time required to break the cipher exceeds the useful lifetime of the information.
3. Briefly define the Caesar cipher.
The Caesar cipher involves replacing each letter of the alphabet with the letter standing three places further down the alphabet.
For example :
Plain : meet me after the toga party
Cipher : PHHW PH DIWHU WKH WRJD SDUWB
4. Briefly define the monoalphabetic cipher? :
A monoalphabetic cipher maps from a plain alphabet to cipher alphabet. Here a single cipher alphabet is used per message.
5. Briefly define the playfair cipher.
The best-known multiple-letter encryption cipher is the playfair, which treats diagrams in the plain text as single units and translates these units into cipher text diagrams.
6. What are the two problems with one-time pad? :
1.It makes the problem of making large quantities of random keys.
2.It also makes the problem of key distribution and protection.
7. What is a transposition cipher? :
Transposition cipher is a cipher, which is achieved by performing some sort of permutation on the plaintext letters.
8. What are the two basic functions used in encryption algorithms? :
The two basic functions used in encryption algorithms are
** Substitution
** Transposition
9. How many keys are required for two people to communicate via a cipher? :
If both sender and receiver use the same key, the system is referred to as symmetric, single key, secret key, or conventional encryption. If the sender and receiver each use a different key, the system is referred to as asymmetric, two-key, or public-key encryption.
10. What is the difference between a block cipher and a stream cipher? :
A block cipher processes the input one block of elements at a time, producing an output block for each input block.
A stream cipher processes the input elements continuously, producing output one element at a time, as it goes along.
11. What are the two approaches to attacking a cipher? :
The two approaches to attack a cipher are :
** Cryptanalysis
** Brute-force attack
12. What is Steganography? :
This conceals the existence of the message.
13. Why is it important to study feistel cipher? :
This cipher can be used to approximate the simple substitution cipher by utilizing the concept of a product cipher, which is the performing of two or more basic ciphers in sequence in such a way that the final result or product is cryptographically stronger than any of the component ciphers.
14. Why is it not practical to use an arbitrary reversible substitution cipher? :
An arbitrary reversible cipher for a large block size is not practical, however, from an implementation and performance point of view. Here the mapping itself is the key.
15. What is the difference between diffusion and confusion? :
In diffusion, the statistical structure of the plain text is dissipated into long-range statistics of the cipher text. This is achieved by permutation. In confusion, the relationship between the statistics of the cipher text and the value of the encryption key is made complex. It is achieved by substitution.
16. Which parameters and design choices determine the actual algorithm of a feistel cipher? :
** Block size
** Key size
** Number of rounds
** Sub key generation algorithm
** Round functions
** Fast software encryption or decryption
** Ease of analysis
17. What is the purpose of the S-boxes in DES? :
Each row of a S-box defines a general reversible substitution. It consists of a set of eight S-boxes, each of which accepts 6 bits as input and produces 4 bits as output.
18. Explain the avalanche effect.
It is that a small change in either the plaintext or the key should produce a significant change in the cipher text.
A change in one of the bit of the plaintext or one bit of the key should produce a change in many bits of the cipher text.
19. What is the difference between a mono alphabetic cipher and a poly alphabetic
cipher? :
Mono alphabetic cipher : Here a single cipher alphabet is used.
Poly alphabetic cipher : Here a set of related mono alphabetic substitution rules is used.
20. List the types of cryptanalytic attacks.
** Cipher text only
** Known plaintext
** Chosen plaintext
** Chosen cipher text
** Chosen text
UNIT II
1. What is the purpose of the State array?
A single 128-bit block is depicted as a square matrix of bytes. This block is copied into the State array, which is modified at each stage of encryption or decryption. After the final stage, State is copied to an output matrix.
2. How is the S-box constructed?
The S-box is constructed in the following fashion:
Initialize the S-box with the byte values in ascending sequence row by row. The first row contains {00}, {01}, {02}, ……….., {0F}; the second row contains {10},{11},etc; and so on. Thus, the value of the byte at row x, column y is {x y}. Map each byte in the S-box to its multiplicative inverse in the finite field GF (28); the value {00} is mapped to itself. Consider that each byte in the S-box consists of 8 bits labeled (b7,b6,b5,b4,b3,b2,b1,b0).Apply the following transformation to each bit of each byte in the S-box.
3. Briefly describe Sub Bytes.
Sub byte uses an S-box to perform a byte-by-byte substitution of the block. The left most 4 bits of the byte are used as row value and the rightmost 4 bits are used as a column value. These row and column values serve as indexes into the S-box to select a unique 8- bit value.
4. What is the difference between differential and linear cryptanalysis?
In differential cryptanalysis, it breaks the DES in less 255 complexities. In cryptanalysis, it finds the DES key given 247 plaintexts.
5. Define product cipher.
Product cipher performs two or more basic ciphers in sequence in such a way that the final result or product is cryptologically stronger than any of the component ciphers.