CH 3

Question 1

Define Cryptosystem.

Answer 1

it is an implementation of cryptographic techniques and their accompanying infrastructure to provide information security services.

Question 2

A cryptosystem is also referred to as a ….. .

Answer 2

cipher system

Question 3

What are the basic components of a Cryptosystem ?

Answer 3

• Plaintext
• Encryption Algorithm
• Ciphertext
• Decryption Algorithm
• Encryption Key
• Decryption Key

Question 4

Define Plaintext.

Answer 4

It is the data to be protected during transmission.

Question 5

Define Encryption Algorithm.

Answer 5

It is a mathematical process that produces a
ciphertext for any given plaintext and encryption key. It is a cryptographic algorithm that takes plaintext and an encryption key as input and produces a ciphertext.

Question 6

Define Ciphertext.

Answer 6

It is the scrambled version of the plaintext produced by the encryption algorithm using a specific the encryption key. The ciphertext is not guarded. It flows on public channel. It can be intercepted or compromised by anyone who has access to the communication channel.

Question 7

Define Decryption Algorithm.

Answer 7

It is a mathematical process, that produces a unique plaintext for any given ciphertext and decryption key. It is a cryptographic algorithm that takes a ciphertext and a decryption key as input, and outputs a plaintext. The decryption algorithm essentially reverses the encryption algorithm and is thus closely related to it.

Question 8

Define Encryption Key.

Answer 8

It is a value that is known to the sender. The sender inputs the encryption key into the encryption algorithm along with the plaintext in order to compute the ciphertext.

Question 9

Define Decryption Key.

Answer 9

It is a value that is known to the receiver. The decryption key is related to the encryption key, but is not always identical to it. The receiver inputs the decryption key into the decryption algorithm along with the ciphertext in order to compute the plaintext.

Question 10

For a given cryptosystem, a collection of all possible decryption keys is called a ….. .

Answer 10

Key space.

Question 11

An ….. is an unauthorized entity who attempts to

determine the plaintext.

Answer 11

Interceptor (an attacker)

Question 12

An interceptor can see the ciphertext and may know the decryption algorithm. He, however, must never know the ….. .

Answer 12

Decryption key

Question 13

What are the 2 types of cryptosystems based on the manner in which encryption-decryption is carried out in the system ?

Answer 13

• Symmetric Key Encryption

- Asymmetric Key Encryption

Question 14

The encryption process where same keys are used for encrypting and decrypting the information is known as ….. .

Answer 14

Symmetric Key Encryption

Question 15

Symmetric cryptosystems are also sometimes referred to as ….. .

Answer 15

secret key

cryptosystems

Question 16

A few well-known examples of symmetric key encryption methods are:

Answer 16

• Digital Encryption Standard (DES)
• Triple-DES (3DES)
• IDEA
• BLOWFISH.

Question 17

What are The salient features of cryptosystem based on symmetric key encryption ?

Answer 17

• Persons using symmetric key encryption must share a common key prior to exchange of information.
• Keys are recommended to be changed regularly to prevent any attack on the system.
• A robust mechanism needs to exist to exchange the key between the communicating parties.
• As keys are required to be changed regularly, this mechanism becomes expensive and cumbersome.
• In a group of n people, to enable two-party
  communication between any two persons, the number of keys required for group is n × (n – 1)/2 .
• Length of Key (number of bits) in this encryption is smaller and hence, process of encryption-decryption is faster than asymmetric key encryption.
• Processing power of computer system required to run symmetric algorithm is less.

Question 18

What are the two restrictive challenges of employing symmetric key cryptography ?

Answer 18

• Key establishment: Before any communication, both the sender and the receiver need to agree on a secret symmetric key.
• Trust Issue: Since the sender and the receiver use the same symmetric key, there is an implicit requirement that the sender and the receiver ‘trust’ each other.

Question 19

What is Asymmetric Key Encryption ?

Answer 19

The encryption process where different keys are used for encrypting and decrypting the information is known as Asymmetric Key Encryption. Though the keys are different, they are mathematically related and hence, retrieving the plaintext by decrypting ciphertext is feasible.

Question 20

What are The salient features of Asymmetric Key Encryption ?

Answer 20

• Every user in this system needs to have a pair of dissimilar keys, private key and public key. These keys are mathematically related – when one key is used for encryption, the other can decrypt the ciphertext back to the original plaintext.
• Length of Keys (number of bits) in this encryption is large and hence, the process of encryption-decryption is slower than symmetric key encryption.
• Processing power of computer system required to run asymmetric algorithm is higher.

Question 21

What are the challenges of the Public Key Cryptosystem ?

Answer 21

• The user needs to trust that the public key that he is using in communications with a person really is the public key of that person and has not been spoofed by a malicious third party. This is usually accomplished through a Public Key Infrastructure (PKI) consisting a
  trusted third party. The most common method of making the verified public keys available is to embed them in a certificate which is digitally signed by the trusted third party.

Question 22

What is the relation between the encryption key and decryption key in symmetric cryptosystems ?

Answer 22

In this case they are the same and they are symmetric.

Question 23

What is the relation between the encryption key and decryption key in public key cryptosystems ?

Answer 23

in this case they are different , but mathematically

related. the encryption key is public while the decryption key is private.

Question 24

Due to the advantages and disadvantage of both the systems, ….. are often used in the practical information
security systems.

Answer 24

both symmetric key and public key cryptosystems.