Flashcards in Symmetric Encryption Deck (23):

1

## What technique is used to cause "Confusion"?

### substitution

2

## Is substitution sufficient encryption

### No

3

## Why is "Diffusion" important to encryption

### It defeats cryptoanalysis to remove the frequency of common letters/words in a message

4

## What technique is used to cause "Diffusion"

### permutation

5

## Are substitution and diffusion sufficient for encryption?

### No. Need also rounds,.. need repeated rounds of substitution and diffusions

6

## Why are multiple rounds used for block ciphers? and encryption in general?

### Because only 1 round does not sufficiently substitute and permutate every single bit.

7

## How long is the actual Key in DES?

### 56 bits

8

## How is decryption in DES done?

### Same steps of operations as in encryption except that per-round keys are used in reverse order

9

## How to express DES round operations in an algebraic expressions? (hint: there are 2 expressions)

###
L(n+1) = Rn

R(n+1) = Ln XOR Mangler(Rn,Kn)

10

## What is the Feistel structure? (hint: it's DES's decryption structure)

### The characteristic that the only difference between encryption and decryption is the key schedule

11

## In a DES round, what performs the bulk of the processing?

### Mangler function

12

## What are the 2 shortcomings of DES

###
1. key space is too small, only 2^56 different keys which can be overcome by brute force (main reason)

2. S-box design criteria was a secret and not openly shared for improvement

13

## How is triple DES different from DES

###
3 different sets of 56-bit keys could be used with 3 encryption/decryption rounds

Advantage w triple DES is that it supports multiple key lengths up to 168-bits (56*3)

Drawback is that triple DES is not efficient for running DES 3x

14

## AES possible key lengths are:

### 128, 192, or 256 bits

15

## What conceptual steps are in each of multiple rounds in AES?

###
1. Substitute bytes (S-Box table)

2. Shift rows (permutation)

3. Mix columns (substitute bytes in colmn as fxn of bytes in colmn)

4. XOR round-based key

16

## Is AES reversible

### Yes

17

##
To encode very large messages, ECB could be used.

What happens when the message doesn't evenly fit into the plaintext blocks?

How do we keep from very making an entry for every possible plaintext block?

###
1. the last block is padded to fill out the block size

2. use an entry only for the text being ciphered

Same key is used for all plaintext blocks

18

## What's the shortcoming of ECB (Encryption Code Book)?

### for the same plaintext block, we get the same ciphered block because the same key is used for all blocks

19

## What would attackers do to exploit this weakness?

### Attackers could substitute known cipher blocks to plaintext blocks to conduct cryptoanalysis

20

## What's the most common way to encrypt a large message?

###
CBC - Cipher Block Chaining

The ciphered block is used and XORed with encryption of the next block

21

## How is a CBC encrypted message decrypted?

### Going backwards from the last to 1st ciphered block, decrypt, and to decrypt the 1st ciphered block, the receiver must also have the Initialization vector (that is initially XORed w the first plaintext message)

22

## List cipher algorithms that protect confidentiality

### AES, DES, CBC, ECB

23