Crypto - 2A. Symmetric Design

Question 1

What is the purpose of bit transposition in cryptography?

Answer 1

To reorder bits within a set according to a defined permutation without changing individual bit values.

Question 2

Fill in the blank: A bit transposition is a type of ______ but not vice-versa.

Answer 2

Permutation

Question 3

How does a bit transposition differ from a general permutation?

Answer 3

A bit transposition only reorders bits, while a permutation can map any bit position to any value, making it more flexible.

Question 4

Give an example of bit transposition in cryptographic algorithms.

Answer 4

DES uses Initial Permutation (IP) and specific transpositions within its rounds.

Question 5

What is the purpose of ShiftRows in AES?

Answer 5

ShiftRows is a bit transposition operation in AES that shifts rows to provide diffusion.

Question 6

Define linearity in cryptographic functions.

Answer 6

A function is linear if it satisfies f(x ⊕ y) = f(x) ⊕ f(y) for all inputs x and y.

Question 7

Fill in the blank: A function is affine if it is linear with an additional ______ term.

Answer 7

Constant

Question 8

Explain why linear functions are not ideal for encryption.

Answer 8

Linear functions produce predictable patterns, making them insecure for encryption without additional complexity.

Question 9

What is non-linearity in cryptographic functions?

Answer 9

Non-linearity means that a function does not follow linear or affine patterns, which helps increase security by making outputs less predictable.

Question 10

Provide an example of a non-linear function in cryptography.

Answer 10

The AND function, f(a, b) = a ⋅ b, is non-linear as it does not satisfy f(x ⊕ y) = f(x) ⊕ f(y).

Question 11

Define diffusion in cryptographic terms.

Answer 11

Diffusion ensures that one input bit affects multiple output bits, spreading information and increasing security.

Question 12

Fill in the blank: Diffusion is achieved when an input bit influences ______ output bits.

Answer 12

Multiple

Question 13

Why is diffusion important in encryption?

Answer 13

Diffusion helps scatter information across the output, making it harder to deduce the input from the output.

Question 14

Explain the bend-mix-notch-shuffle approach in symmetric cryptography.

Answer 14

A sequence that applies non-linearity (bend), linear mixing (mix), symmetry-breaking (notch), and bit reordering (shuffle) to strengthen encryption.

Question 15

Who developed the bend-mix-notch-shuffle approach?

Answer 15

Joan Daemen

Question 16

What is the purpose of the ‘bend’ step in the bend-mix-notch-shuffle sequence?

Answer 16

To apply a non-linear mapping that introduces initial complexity in the transformation.

Question 17

Fill in the blank: The ‘mix’ step in the bend-mix-notch-shuffle sequence ensures ______ across bits.

Answer 17

Diffusion

Question 18

What does the ‘notch’ step achieve in the bend-mix-notch-shuffle process?

Answer 18

The notch step breaks symmetry, reducing predictability in the function.

Question 19

What is the function of the ‘shuffle’ step in symmetric encryption design?

Answer 19

The shuffle step reorders bits, ensuring that diffusion spreads globally across the output.

Question 20

Why are linear, affine, and non-linear functions combined in symmetric cryptography?

Answer 20

To balance simplicity and complexity, providing both efficiency and security in encryption functions.

Question 21

How does the KECCAK-f algorithm use bit transposition?

Answer 21

KECCAK-f employs the π (pi) bit transposition and ρ (rho) rotation to achieve secure mixing of bits.

Question 22

Fill in the blank: In symmetric cryptography, ______ helps each bit in the output depend on all input bits.

Answer 22

Global Dependency

Question 23

Why are multiple rounds used in symmetric encryption?

Answer 23

Multiple rounds increase complexity, making it harder for attackers to analyze or predict outputs.