1. Intro to Crypto

Question 1

What is the main purpose of applying cryptography to secure systems?

Answer 1

To protect data from unauthorized access and ensure confidentiality, integrity, and authenticity

This includes using various cryptographic algorithms and principles.

Question 2

What are the two main types of encryption?

Answer 2

Symmetric and Asymmetric encryption

Symmetric uses a single key, while Asymmetric uses a pair of public and private keys.

Question 3

Define symmetric encryption.

Answer 3

An encryption method where the sender and receiver share the same secret key for data encryption and decryption.

Question 4

What is a ciphertext?

Answer 4

The encrypted message that is not human-readable.

Question 5

What are the two types of symmetric encryption algorithms?

Answer 5

• Stream Ciphers
• Block Ciphers

Question 6

What is a stream cipher?

Answer 6

Encrypts data bit-by-bit or byte-by-byte in a continuous stream.

Question 7

What is a block cipher?

Answer 7

Encrypts each block of data independently using the same key.

Question 8

Define asymmetric encryption.

Answer 8

An encryption method that uses two different keys (public and private) for encrypting and decrypting messages.

Question 9

What is the key difference between symmetric and asymmetric encryption?

Answer 9

Symmetric uses a single key, while asymmetric uses a pair of keys (public for encryption, private for decryption).

Question 10

List the fundamental features (primitives) of cryptography.

Answer 10

• Confidentiality
• Integrity
• Authentication

Question 11

What does confidentiality in cryptography ensure?

Answer 11

Keeps information secret from adversaries; achieved through encryption techniques.

Question 12

What is cryptanalysis?

Answer 12

The study of breaching cryptographic security systems to gain access to encrypted messages.

Question 13

Differentiate between cryptanalysis and brute-force attacks.

Answer 13

• Cryptanalysis: Analytical approach exploiting weaknesses in encryption.
• Brute-force: Systematic approach testing every possible key combination.

Question 14

What is the objective of a brute-force attack?

Answer 14

To output a human-readable plaintext that identifies the underlying secret key.

Question 15

What is the processing complexity of a brute-force attack for a 15 character password containing letters and digits?

Answer 15

2^90

Explanation:

• Step 1: determine Character set size
  
  • Uppercase letters: 26 characters
  • Lowercase letters: 26 characters
  • Digits: 10 characters
  Total character set size = 26+26+10=62
• Step 2: Calculate the total combinations
  
  • For a password of length L, the total combinations are:
    
    • 62^L
    • Here, L=15 , so the total combinations are: 62^{15}
• Step 3: Approximate using Log
  
  • Use log base 2
  • log2 (62) = log10 (62) / log10 (2) = 5.95
• 62^15 = (2^5.95) ^ 15 = (2 ^ 5.95 * 15) = 2 ^ 89.25 =⇒ 2^90

Question 16

Name the four types of cryptanalytic attacks based on information known.

Answer 16

• Ciphertext-only
• Known-plaintext
• Chosen-Plaintext
• Chosen-ciphertext

Question 17

What is the key assumption of Kerckhoffs’s principle?

Answer 17

The key must be kept secret, even if everything about the algorithm is public knowledge.

Question 18

What is unconditional security in cryptography?

Answer 18

A cryptographic algorithm is unconditionally secure if the ciphertext lacks information to uniquely determine the corresponding plaintext.

Question 19

What is computational security?

Answer 19

Ciphertext/algorithm is difficult to decrypt using current or future resources, but not theoretically impossible.

Question 20

What are the three complexities to consider when analyzing attack feasibility?

Answer 20

• Processing Complexity
• Data Complexity
• Storage Complexity

Question 21

What does processing complexity measure?

Answer 21

The time needed to perform the attack.

Question 22

What does data complexity refer to?

Answer 22

The amount of data needed as input to the attack.

Question 23

What is storage complexity?

Answer 23

The amount of memory needed to execute the attack.

Question 24

What is a shift cipher?

Answer 24

A method that replaces each character of the plaintext with a corresponding character of the ciphertext using a key.

Question 25

What does the key in a shift cipher represent?

Answer 25

A number between 0 and 25 that indicates how many positions to shift each character.

Question 26

What is the total number of unique characters in the English language considered in a shift cipher?

Answer 26

26 unique characters (a-z).

Question 27

How is encryption performed in a shift cipher?

Answer 27

By shifting each plaintext character forward by k places, where k is the key.

Question 28

What happens to characters when the sum exceeds 25 during encryption?

Answer 28

Characters wrap around to the start of the alphabet using the modulo operation.

Question 29

What is the process of decryption in a shift cipher?

Answer 29

Shift the ciphertext characters backward by the same number of k places.

Question 30

What is a mono-alphabetic cipher?

Answer 30

A cipher where each plaintext character is replaced by a corresponding character of ciphertext, maintaining a 1-1 mapping.

Question 31

What are the two approaches for cryptanalysis of a shift cipher?

Answer 31

* Brute Force * Statistical Attack

Question 32

What is the brute force approach to breaking a shift cipher?

Answer 32

Sequentially trying each key until a readable decrypted message is found.

Question 33

What does the statistical attack exploit in a shift cipher?

Answer 33

The frequency of characters in the plaintext language to infer the decryption key.

Question 34

What is the Index of Coincidence (IoC)?

Answer 34

The probability that two randomly selected characters in a text string are equal.

Question 35

How is the IoC for the English language calculated?

Answer 35

By squaring the probabilities of each letter and summing the squares.

Question 36

What is the approximate IoC value for English?

Answer 36

0.065.

Question 37

What is the significance of calculating the IoC in cryptanalysis?

Answer 37

It helps identify the key used to generate a given ciphertext.

Question 38

What is the first step in applying IoC in a statistical attack?

Answer 38

Compute the frequency vector q for the ciphertext.

Question 39

What does wrapping around refer to in a shift cipher?

Answer 39

Returning to the start of the alphabet after exceeding the character set limit.

Question 40

Fill in the blank: The sum of the probabilities squared in the IoC calculation for a sufficient large plaintext is represented by _______.

Answer 40

I(p,p) = {displaystylesum_{i=0}^{25}{p_i}^2}

Question 41

True or False: In a shift cipher, each plaintext character can map to multiple ciphertext characters.

Answer 41

False.

Question 42

What is the result of the encryption of plaintext character 'p' with key k?

Answer 42

E(k, p) = (p + k) mod 26.

Question 43

What does the statistical attack on a shift cipher require for each possible key value?

Answer 43

Compute the IoC between the frequency vector p and a shifted version of vector q.

Question 44

What is the result of applying a shift of j positions to the frequency vector q?

Answer 44

It simulates undoing the encryption with the shift cipher.

Question 45

What is the key to be found when the IoC is closest to 0.065?

Answer 45

The value of j that maximizes the computed IoC.

Question 46

What is the formula for the IoC when analyzing the ciphertext?

Answer 46

I_j = I (p,q_{(i + j)}: mod: 26) = {displaystylesum_{i=0}^{25}{p_i} : . :q(i+j)}: mod :20.