Crypto - 2. Secret

Question 1

What is a secret key used for in cryptography?

Answer 1

A secret key is used to encrypt and decrypt data securely between parties.

Question 2

Name three methods for distributing a secret key.

Answer 2

Hand-to-hand, smart card, or a key establishment scheme.

Question 3

What is a stream cipher?

Answer 3

A stream cipher generates a keystream that is XORed with plaintext to encrypt or with ciphertext to decrypt.

Question 4

How does a Linear Feedback Shift Register (LFSR) work in cryptography?

Answer 4

An LFSR uses a feedback polynomial to generate a keystream, common in many stream ciphers.

Question 5

Fill in the blank: The ______ cipher is a well-known stream cipher that has faced vulnerabilities.

Answer 5

RC4

Question 6

What is the main difference between a block cipher and a stream cipher?

Answer 6

A block cipher encrypts fixed-size blocks of data, while a stream cipher encrypts data bit by bit using a keystream.

Question 7

Define a block cipher.

Answer 7

A block cipher is a mapping that encrypts fixed-size data blocks using a secret key, with a reversible function for decryption.

Question 8

What does a pseudo-random permutation (PRP) in block ciphers ensure?

Answer 8

PRP ensures the block cipher output is indistinguishable from a random permutation without the key.

Question 9

Fill in the blank: The ______ cipher was one of the first standardized block ciphers, based on a Feistel network structure.

Answer 9

DES

Question 10

Explain Triple-DES and its key strength.

Answer 10

Triple-DES applies DES encryption three times with different keys, resulting in a 112-bit or 168-bit key strength.

Question 11

What was the goal of the Advanced Encryption Standard (AES) competition?

Answer 11

To find a secure block cipher to replace DES, with 128-bit block size and key sizes of 128, 192, and 256 bits.

Question 12

Who developed the Rijndael cipher, later standardized as AES?

Answer 12

Joan Daemen and Vincent Rijmen.

Question 13

What field is used for finite field arithmetic in AES?

Answer 13

AES uses the Galois Field GF(2^8) for its arithmetic operations.

Question 14

Fill in the blank: The four main transformations in AES are _______, _______, _______, and _______.

Answer 14

SubBytes, ShiftRows, MixColumns, and AddRoundKey.

Question 15

What is Electronic Codebook (ECB) mode?

Answer 15

ECB mode encrypts each plaintext block independently, which can reveal patterns in the data.

Question 16

Why is ECB mode considered weak?

Answer 16

Because identical plaintext blocks produce identical ciphertext blocks, potentially revealing data patterns.

Question 17

What is Cipher Block Chaining (CBC) mode?

Answer 17

CBC mode XORs each plaintext block with the previous ciphertext block before encryption, creating block dependencies.

Question 18

How does Counter Mode (CTR) work in encryption?

Answer 18

CTR generates a keystream by encrypting sequential counters and XORs it with the plaintext.

Question 19

Fill in the blank: The ______ mode combines a block cipher with a message authentication scheme, ensuring both confidentiality and integrity.

Answer 19

Authenticated encryption mode (e.g., GCM or CCM)

Question 20

Define the Sponge Construction in cryptography.

Answer 20

A method for generating cryptographic hashes or keystreams by absorbing input and squeezing out the output.

Question 21

What is the role of the SpongeWrap construction?

Answer 21

It provides authenticated encryption by using the sponge construction for both keystream generation and MAC.

Question 22

Explain CBC-MAC.

Answer 22

A message authentication code created by encrypting each block of a message with CBC mode and using the final block as the MAC.

Question 23

What is a cryptographic permutation?

Answer 23

A bijective function that maps an input to an output in a reversible way, often used in cryptographic algorithms.

Question 24

What is a pseudo-random function (PRF)?

Answer 24

A function that provides unpredictable output, used in MACs and encryption for secure message authentication.

Question 25

Explain incremental MACs and their use case.

Answer 25

Incremental MACs provide authentication for large data streams, allowing for progressive MAC calculation.

Question 26

Why is peer review critical for cryptographic primitives?

Answer 26

To ensure robustness against new attacks and maintain confidence in security.

Question 27

What is a keyed sponge function?

Answer 27

A sponge function that incorporates a secret key, used to create a MAC or a stream cipher.

Question 28

Fill in the blank: AES operates with a block size of ______ bits and key sizes of _______, _______, or _______ bits.

Answer 28

128, 128, 192, or 256.

Question 29

Name two advantages of block ciphers over stream ciphers.

Answer 29

They are suitable for fixed-size data encryption and allow secure modes of operation for data integrity.

Question 30

What are the four main transformations in AES?

Answer 30

SubBytes, ShiftRows, MixColumns, and AddRoundKey.

Question 31

Fill in the blank: In Cipher Block Chaining (CBC) mode, each plaintext block is XORed with the ______ before encryption.

Answer 31

Previous ciphertext block.

Question 32

Define Counter Mode (CTR).

Answer 32

CTR mode encrypts sequential counter values to generate a keystream, which is then XORed with the plaintext.

Question 33

What is the role of the Galois Field in AES?

Answer 33

It enables secure finite field arithmetic necessary for transformations like MixColumns.

Question 34

Why is the Sponge construction considered flexible?

Answer 34

It can absorb any length of input and generate a secure keystream or hash of arbitrary length.

Question 35

What is the main advantage of authenticated encryption?

Answer 35

It provides both data confidentiality and integrity using encryption and authentication in one process.

Question 36

Explain the concept of a nonce in cryptography.

Answer 36

A unique, one-time value used in encryption to prevent identical ciphertexts for the same plaintext.