Crypto - 1. Historical Principles

Question 1

What is the purpose of cryptography?

Answer 1

Cryptography ensures the confidentiality and integrity of messages or transmission channels.

Question 2

Define encryption and decryption in cryptography.

Answer 2

Encryption is transforming plaintext to ciphertext, while decryption is reversing ciphertext to plaintext.

Question 3

What is the difference between symmetric and asymmetric cryptography?

Answer 3

Symmetric cryptography uses the same key for encryption and decryption; asymmetric cryptography uses a public key for encryption and a private key for decryption.

Question 4

Fill in the blank: In symmetric cryptography, the same key is used for both ______ and ______.

Answer 4

Encryption and decryption.

Question 5

What is the primary function of authentication in cryptography?

Answer 5

Authentication appends a tag to a message for verification to ensure its origin or integrity.

Question 6

What is a Shift Cipher?

Answer 6

A cipher that shifts each letter in the plaintext by a fixed number of positions, such as the Caesar cipher.

Question 7

Explain mono-alphabetic substitution.

Answer 7

Mono-alphabetic substitution replaces each letter uniquely, and is vulnerable to frequency analysis as ciphertext letter frequencies match plaintext frequencies.

Question 8

How does a Vigenère cipher work?

Answer 8

A Vigenère cipher uses a keyword to shift each letter in the plaintext by different amounts, making it a poly-alphabetic substitution cipher.

Question 9

What was the Enigma machine used for?

Answer 9

The Enigma machine was a complex cipher device used in WWII for secure communication, famously decrypted by Alan Turing’s team.

Question 10

What is ‘perfect secrecy’ in cryptography?

Answer 10

Perfect secrecy means that ciphertext reveals nothing about plaintext, even if an adversary has unlimited computational power.

Question 11

Why does perfect secrecy require long keys?

Answer 11

For perfect secrecy, the entropy of the key must be at least as high as the entropy of the plaintext, meaning the key must be as long as the message.

Question 12

What is the One-Time Pad (OTP) and its key requirement?

Answer 12

The OTP is a cipher that provides perfect secrecy with a key as long as the message, which must never be reused.

Question 13

What is the difference between the One-Time Pad and Vigenère cipher?

Answer 13

The OTP requires a key as long as the plaintext and is perfectly secure, while the Vigenère cipher is shorter-keyed and relatively easy to break.

Question 14

What does computational security assume about attackers?

Answer 14

Computational security assumes that attackers have limited computing power and time, allowing only a small chance of breaking the encryption.

Question 15

Define security strength in cryptography.

Answer 15

Security strength measures how hard it is for an adversary to break encryption, often quantified in bits (e.g., 128-bit security).

Question 16

Fill in the blank: The principle that ‘only the key should be secret’ is known as ______.

Answer 16

Kerckhoffs’ Principle.

Question 17

What is Kerckhoffs’ Principle?

Answer 17

A principle stating that a cryptographic system should remain secure even if everything about the system, except the key, is public knowledge.

Question 18

What is a ciphertext-only attack?

Answer 18

An attack where the adversary has access only to ciphertexts and no information about the plaintext or key.

Question 19

What is an IND-CPA secure scheme?

Answer 19

A scheme is IND-CPA secure if it withstands chosen plaintext attacks where an adversary has access to chosen plaintexts and corresponding ciphertexts.

Question 20

What is semantic security?

Answer 20

Semantic security ensures an adversary cannot gain useful information about the plaintext from the ciphertext alone.

Question 21

How does a randomized encryption scheme enhance security?

Answer 21

Randomized encryption ensures that identical plaintexts produce different ciphertexts, improving semantic security.

Question 22

What is a nonce in cryptography?

Answer 22

A nonce is a unique, single-use value used in encryption to prevent repeated ciphertexts for the same plaintext.

Question 23

What is IND-CCA security?

Answer 23

A cryptographic scheme is IND-CCA secure if it resists chosen plaintext and ciphertext attacks, making it stronger against adversarial queries.

Question 24

Define existential unforgeability under chosen message attacks (EU-CMA).

Answer 24

A property of authentication schemes where an adversary cannot produce a valid message-tag pair that has not been previously queried.

Question 25

What is the purpose of authentication in cryptography?

Answer 25

Authentication verifies message integrity and origin, ensuring that a message has not been altered and is from the legitimate sender.

Question 26

Fill in the blank: The principle of ensuring the origin and integrity of a message is known as ______.

Answer 26

Authentication.

Question 27

What type of attack is ‘random tag guessing’?

Answer 27

A universal forgery attack where an adversary attempts to guess valid message-tag pairs without prior knowledge.

Question 28

What are implementation attacks in cryptography?

Answer 28

Attacks exploiting flaws in cryptographic implementations, such as coding errors or side-channel attacks.

Question 29

What are human attacks in cryptography?

Answer 29

Attacks targeting user behavior, such as poor key management or social engineering, rather than the cryptographic system itself.

Question 30

Explain the concept of computational complexity in breaking cryptographic systems.

Answer 30

Computational complexity refers to the required time and resources for an attacker to break encryption, often estimated based on known hardware capabilities.

Question 31

Describe the difference between online and offline complexity in cryptographic attacks.

Answer 31

Online complexity refers to data required from active interactions, while offline complexity refers to computational time needed without real-time data.

Question 32

Fill in the blank: An encryption scheme’s ______ strength measures how hard it is for attackers to break it.

Answer 32

Security.

Question 33

Why is semantic security important in encryption schemes?

Answer 33

Semantic security prevents adversaries from deducing information about the plaintext from observing the ciphertext, crucial for data confidentiality.