CS Free MCQ Bank

[ Option A ]

AES (Advanced Encryption Standard) is a symmetric key encryption algorithm, meaning the same secret key is used for both encryption and decryption. 
When secure communication is required between two persons, they must share a unique secret key. If every pair of users uses a different key, then the total number of keys required equals the number of unique pairs that can be formed among the users. For n persons, the number of such pairs is n(n−1)/2.
For 16 persons, the required keys are 16*15/2=120. Therefore, 120 unique keys are needed so that each pair of persons can communicate securely using AES.

COMPARISON WITH ASYMMETRIC ENCRYPTION:

FOR 16 USERS:

• AES (Symmetric) : 16*15/2 = 120 Keys.
• RSA (Asymmetric) : 2*16 = 32 Keys.

[ Option B ]

The primary advantage of setting up a DMZ (Demilitarized Zone) using a firewall is to create multiple zones of trust within a network, which enhances security. A DMZ acts as a buffer zone between the public Internet and the internal private network, isolating public-facing servers (such as web, email, and DNS servers) from sensitive internal resources. This setup allows external users to access services in the DMZ while preventing direct access to the internal network.

[ Option D ]

[ Option B ]

The SHA-2 (Secure Hash Algorithm 2) family is a set of cryptographic hash functions designed to provide secure hashing for data integrity and digital signatures. 
SHA-2 includes variants such as SHA-224, SHA-256, SHA-384, and SHA-512, where the number in the name indicates the length of the hash output in bits.
SHA-256, the most commonly used variant, produces a 256-bit (32-Byte) hash value.

[ Option C ]

Data Encryption Standard (DES) is a symmetric key block cipher widely used for secure data transmission. 

• In DES, the input block size is 64 bits, meaning the plaintext is divided into 64-bit blocks before encryption. 
• The key size is 56 bits, although the original key is 64 bits long, 8 bits are used for parity and do not contribute to the actual encryption. 
• The output block size is also 64 bits, so each 64-bit plaintext block is transformed into a 64-bit ciphertext block. 

This design allows secure encryption and decryption using the same 56-bit key.
 

[ Option D ]

In cryptography, when a readable message (Plain Text) is converted into an unreadable form using an encryption algorithm and a key, the result is called Cipher Text.

[ Option A ]

By default, IIS (Internet Information Services) allows users to access web content without requiring credentials. This is called Anonymous Authentication, where the server uses a built-in account to handle requests from unauthenticated users.

[ Option B ]

Data Encryption Standard (DES) is a Symmetric Key block cipher algorithm that uses the same secret key for both encryption and decryption.

• In DES, the input block size is 64 bits, meaning the plaintext is divided into 64-bit blocks before encryption. 
• The key size is 56 bits, although the original key is 64 bits long, 8 bits are used for parity and do not contribute to the actual encryption. 
• The output block size is also 64 bits, so each 64-bit plaintext block is transformed into a 64-bit ciphertext block. 

The encryption process in DES consists of a series of operations known as rounds. In DES total 16 rounds are performed, making the encryption much stronger and harder to break through Brute-Force or Statistical Attacks.

[ Option D ]

A Firewall is a network security system that monitors and controls incoming and outgoing network traffic based on predefined security rules. Its main purpose is to protect a computer network from unauthorized access through the network.
Network Scanning is a technique used by attackers to identify open ports, active hosts, and running services in a network to find vulnerabilities. A properly configured firewall can block suspicious requests, restrict unauthorized access to ports, and prevent scanning attempts, thereby protecting the system from such attacks.

• Dumpster Diving involves retrieving sensitive information from discarded materials such as printed documents.
• Physical Theft refers to stealing hardware like computers or storage devices.
• Shoulder Surfing occurs when someone observes a user entering passwords or confidential information.

[ Option A ]

Given, Plaintext = SACKGAUL, but it says read from right to left, so we reverse it: LUAGKCAS. Now, the positions are:

[ Option D ]

Fabrication refers to creating fake messages or data, so authentication mechanisms are used to detect and prevent this.
Modification means altering messages during transmission. Ensuring message integrity detects such tampering.
Interception refers to unauthorized access or eavesdropping. Confidentiality (encryption) protects the message content.

[ Option A ]

Secure Hash Algorithm-1 (SHA-1) produces a message digest of 160 bits (20 bytes). SHA-1 takes an input of any length (less than 264 bits) and outputs a fixed-length hash value of 160 bits. This hash value, called the message digest, is used for data integrity verification, digital signatures, and other cryptographic applications.

[ Option D ]

A Honeypot is a security mechanism used in Intrusion Detection Systems (IDS) to attract attackers and study their behavior. It is designed to look like a real system containing valuable data or services, but it is actually a Decoy that isolates and monitors malicious activities. 

Honeypots help in diverting attackers’ attention away from critical systems and collecting information about their methods, tools, and intentions. 
However, Legitimate Users are not supposed to know about or access honeypots, as their purpose is purely for monitoring and deception.

[ Option B ]

In Information Security, different mechanisms are used to protect data and system resources. 

• Non-Repudiation ensures that a sender or receiver cannot deny having participated in a communication.
• Data Integrity ensures that the data received is exactly the same as the data sent by the authorized entity without any modification.
• Data Confidentiality means protecting data from unauthorized disclosure.
• Access Control is a security mechanism that prevents unauthorized users from accessing or using system resources.

[ Option C ]

Symmetric key algorithms use the same key for encryption and decryption. RC4, Blowfish, and DES (Data Encryption Standard) are examples of symmetric key cryptography algorithms.
Diffie-Hellman is a key exchange algorithm and is part of asymmetric key cryptography, used to securely exchange cryptographic keys over a public channel. It is not an encryption algorithm itself.

[ Option A ]

In the RSA cryptosystem, two prime numbers p and q are chosen. Here p=11 and q=13. First compute n=p*q = 11*13=143. Next compute Euler’s totient function ϕ(n)=(p−1)(q−1) = 10*12 = 120.
In RSA, the public key e and private key d must satisfy the condition e*d ≡1(modϕ(n)). Given d=7, we need a value of e such that 7e≡1 (mod 120). The multiplicative inverse of 7 modulo 120 is 103, since 7*103=721 and 721 mod 120 = 1.
Therefore, the value of the public key e is 103.

[ Option D ]

A Proxy Firewall filter works at the Application Layer of the OSI model. It acts as an intermediary between a client and a server, intercepting all requests and responses.

[ Option A ]

A plain text message can be hidden or protected in two main ways.
Steganography is a technique used to hide secret messages or data within another file, such as an image, audio, or video, in such a way that its existence is not apparent to others. 
For example, a confidential message can be hidden inside the pixel values (modifying the Least Significant Bits (LSBs) of the Pixels) of an image file without noticeably changing the image itself.

Cryptography transforms plaintext into ciphertext via encryption, rendering the content unintelligible to outsiders without the decryption key.

[ Option A ]

In RSA encryption, we are given P = 7, Q = 17, public key e = 5, and plaintext M = 10.
First, calculate n = P × Q = 7 × 17 = 119 and φ(n) = (P - 1) × (Q - 1) = 6 × 16 = 96.
The public key pair is therefore (5, 119). Using the encryption formula C = M^e mod n, we get C = 10⁵ mod 119 = 40.

[ Option C ]

Phishing is a cyber-attack technique in which attackers trick or persuade users into revealing sensitive information such as passwords, banking details, credit card numbers, or personal data.
The attacker typically sends fake emails, messages, or creates fraudulent websites that appear to come from trusted organizations like banks or social media platforms.
When the user believes the message is legitimate and enters their information, the attacker collects that data.

[ Option B ]

A Message Digest is generated using a cryptographic hash function such as SHA or MD5, which converts a message of any length into a fixed-length hash value.
The important property of a message digest is that it is computed without using any secret key, it only depends on the input message. It is mainly used to Verify Data Integrity, meaning it helps detect whether the message has been modified.

[ Option B ]

Public Key Infrastructure (PKI) is a framework used to manage digital certificates and public-key encryption. It provides mechanisms for authentication, data integrity, confidentiality, and secure communication over networks.
The X.509 is an international standard that defines the format of digital certificates used in PKI. These certificates contain information such as the public key, certificate holder’s identity, issuing authority, and validity period, and are issued by a Certificate Authority (CA).

• X.25 is a packet-switched network protocol.
• IEEE 802.11 defines standards for wireless LAN (Wi-Fi).
• IEEE 802.3 defines standards for Ethernet networks.

[ Option A ]

A key of n bits means the key is composed of n binary digits, and each bit can take two possible values: 0 or 1. Since each bit has two possibilities and the bits are independent, the total number of different combinations that can be formed is 2 multiplied n times, which equals  2ⁿ. Therefore, an n-bit key can generate 2ⁿ distinct encryption keys.
For example, if the key length is 3 bits, the possible keys are, 000, 001, 010, 011, 100, 101, 110, 111 = 8 Keys = 2³.