Flashcards in Symmetric Cryptography Deck (28):

1

## How many keys are used in symmetric cryptography?

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

2

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

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

3

## Cite some symmetric cryptographic protocols.

### DES, 3DES, AES, RC4.

4

## What is a stream cipher?

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

5

## Cite examples of stream ciphers.

### RC4, SEAL.

6

## What is a block cipher?

### 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.

7

## Cite examples of block ciphers.

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

8

## What is more common: stream or block ciphers?

### Block ciphers, because they are commonly considered more efficient.

9

## Which standard is currently being used for block ciphers?

### Advanced Encryption Standard (AES)

10

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

### Rijndael.

11

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

### 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)

12

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

### They provide confusion and diffusion of message.

13

## What does diffusion in the context of cipher means?

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

14

## What does confusion in the context of cipher means?

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

15

## What is cornerstone of modern symmetric cryptography?

### Shannon's S-P networks.

16

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

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

17

## How to Encrypt using a Feistel model?

###
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}).

18

## How to decrypt using a Feistel model?

###
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.

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?

### Improved security but they may slow cipher.

20

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

### No, because of language characteristics.

21

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

### No, because of language characteristics.

22

##
Explain what happens if the following operations are being used:

1. two substitutions

2. two transpositions

3. substitution followed by a transposition

###
1. make a more complex substitution

2. make a more complex transposition

3. makes a new much harder cipher

23

## What is the bridge between classical to modern ciphers?

### The use of transposition associated with substitution.

24

## What is the avalanche effect?

### 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.

25

## Which symmetric cryptographic protocol exhibits a strong avalanche effect?

### Data Encryption Standard (DES).

26

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

###
- 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.

27

## Algebraic structure: is DES closed?

### 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.

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