3.4.1 Symmetric Cryptography

Question 1

How many keys are used in symmetric cryptography?

Answer 1

Only one, since the same key KA,B is used to encrypt and to decrypt the message.

Question 2

If you have the key and the encrypted message, how can you decrypt?

Answer 2

D=(key, E(key, P)) = P, where P is the plain text.

Question 3

Cite some symmetric cryptographic protocols.

Answer 3

DES, 3DES, AES, RC4.

Question 4

What is a stream cipher?

Answer 4

In stream ciphers, one bit or byte is encrypted at a time. It is based on pseudorandom bit sequence (emulating one-time pad).

Question 5

Cite examples of stream ciphers.

Answer 5

RC4, SEAL.

Question 6

What is a block cipher?

Answer 6

In block ciphers, blocks of equal length are encrypted at a time. Typically size are power of 2. Usually they have more diffusion than stream ciphers.

Question 7

Cite examples of block ciphers.

Answer 7

DES, 3DES, AES, IDEA, Blowfish, CAST-n

Question 8

What is more common: stream or block ciphers?

Answer 8

Block ciphers, because they are commonly considered more efficient.

Question 9

Which standard is currently being used for block ciphers?

Answer 9

Advanced Encryption Standard (AES)

Question 10

Which algorithm is used for Advanced Encryption Standard (AES)?

Answer 10

Rijndael.

Question 11

What is shannon’s substitution-permutation network? In which cryptographic primitives is it based on?

Answer 11

Shannon’s substitution-permutation network is modern substitution-transposition product cipher. It is based on two cryptographic operations: substitution (S-box) and permutation (P-box)

Question 12

Which important characteristics does Shannon’s S-P Networks provide?

Answer 12

They provide confusion and diffusion of message.

Question 13

What does diffusion in the context of cipher means?

Answer 13

Diffusion is the propriety to dissipate statistical structure of a plaintext over bulk of ciphertext.

Question 14

What does confusion in the context of cipher means?

Answer 14

Confusion is the propriety in which makes the relationship between ciphertext and key as complex as possible.

Question 15

What is cornerstone of modern symmetric cryptography?

Answer 15

Shannon’s S-P networks.

Question 16

What is the main advantage of the Feistel model compared to a substitution-permutation network?

Answer 16

One advantage of the Feistel model compared to a substitution-permutation network is that the round function “F” does not have to be invertible.

Question 17

How to Encrypt using a Feistel model?

Answer 17

Split the plaintext block into two equal pieces, (L_0, R_0)
For each round i =0,1,…,n, compute
L_{i+1} = R_i,
R_{i+1}= L_i XOR F(R_i, K_i).

Then the ciphertext is (R_{n+1}, L_{n+1}).

Question 18

How to decrypt using a Feistel model?

Answer 18

Decryption of a ciphertext (R_{n+1}, L_{n+1}) is accomplished by computing for i=n,n-1,…,0

R_{i} = L_{i+1},
L_{i} = R_{i+1}XOR F(L_{i+1}, K_{i}).

Then (L_0,R_0) is the plaintext again.

Question 19

What happens if one or more of those parameters are increased using Feistel cipher: Block size, Key size, number of rounds, subkey generation. round function?

Answer 19

Improved security but they may slow cipher.

Question 20

Is it secure to use only substitution to generate a cipher?

Answer 20

No, because of language characteristics.

Question 21

Is it secure to use only transposition to generate a cipher?

Answer 21

No, because of language characteristics.

Question 22

Explain what happens if the following operations are being used:

1. two substitutions
2. two transpositions
3. substitution followed by a transposition

Answer 22

1. make a more complex substitution
2. make a more complex transposition
3. makes a new much harder cipher

Question 23

What is the bridge between classical to modern ciphers?

Answer 23

The use of transposition associated with substitution.

Question 24

What is the avalanche effect?

Answer 24

It is the key desirable property of encryption algorithm in which a change of one input or key bit results in changing approximately half of the output bits - highly non-linear, chaotic behavior.

Question 25

Which symmetric cryptographic protocol exhibits a strong avalanche effect?

Answer 25

Data Encryption Standard (DES).

Question 26

Which kind of key is considered weak for Data Encryption Standard (DES) security?

Answer 26

• Keys that generate subkeys with either all 0’s or all 1’s (four in total)
• Keys that generate only two different subkeys (six pair of keys) -> those are considered semiweak keys.
• Keys that generate only four different subkeys (48 keys) -> those are considered possible weak keys.

As whole, 64 keys out of ~7.2 x10^16 are considered weak.

Question 27

Algebraic structure: is DES closed?

Answer 27

No, DES is not closed. It means that it is possible to use double encryption, since it is not true that for every K1 and K2 there is a K3 such as E(K2, E(K1,M)) = E(K3,M). Since this is not true, double encryption might be used.

Question 28

Algebraic structure: is DES pure?

Answer 28

No, DES is not pure. By not being pure, DES allows to use triple encryption to increase the key length. To be pure, for every K1, K2, K3 there would be a K4 such that E(K3, E(K2, E(K1, M))) = E(K4, M). Since DES is not pure, it is possible to use 3-DES.