Chapter 3 Symmetric Key Crypto

Question 1

what are the two sub ciphers in symmetric key crypto?

Answer 1

stream cipher and block cipher

Question 2

what is a stream cipher?

Answer 2

based on a one time pad
the key is short
key is stretched into a long KEYSTREAM
keystream is used just like a one time pad

Question 3

what are the sub contents of block cipher

Answer 3

confusion and diffusion

Question 4

what is a block cipher?

Answer 4

based on codebook concept
each key yields a different codebook
employs both confusion and diffusion

Question 5

what are the two sub contents in Stream ciphers?

Answer 5

A5/1 algorithm and RC4

Question 6

what is A5/1 algorithm

Answer 6

it is based on shift registers, used in GSM mobile phone system

Question 7

what is RC4

Answer 7

based on a changing lookup table

Question 8

for A5/1

Answer 8

uses 3 shift registers
x: 19 bits
y: 22 bits
z: 23 bits
and at each step you take the major
and use XOR
efficient in HARDWARE

Question 9

what is RC4

Answer 9

self modifying look up table
permutation of byte values 0.1.///255
swap elements in current lookup table 
select a keystream byte from table
efficient in SOFTWARE

Question 10

Famous man in stream cipher

Answer 10

shamir the death of stream ciphers

Question 11

what is a round function

Answer 11

ciphertext is obtained from plaintext by iterating a round function

Question 12

what is the input of the round function

Answer 12

input consists of key and output of previous round

Question 13

what is a Feistel cipher

Answer 13

a type of block cipher
splitting the plain text left and right halves
F round function
K subkey
XOR

Question 14

what is DES

Answer 14

Data Encryption Standard

NSA secretly involved

Question 15

Stream ciphers..

Answer 15

idea of one time pad,we trade provable security for relatively small and manageable key

Question 16

stream ciphers is confusion or diffusion

Answer 16

confusion only

Question 17

block ciphers…

Answer 17

classic code book ciphers

key determines the code book

Question 18

block ciphers is confusion or diffusion

Answer 18

employ both confusion and diffusion

Question 19

DES is

Answer 19

block cipher standards

Question 20

what is a stream cipher

Answer 20

it takes a key K of n bits in length and stretches it into a long keystream
This keystream is then XORed with the plaintext P to produce cipher text C

Question 21

What is ECB

Answer 21

Electronic Codebook

Question 22

what is Electronic Codebook

Answer 22

each block encrypted independently
identical plaintexts are encrypted similarly
no chaining, no error propagation
does not hide data patterns
alice doesn't like it

Question 23

what is CBC

Answer 23

Cipher block Chaining

Question 24

what is cipher block chaining

Answer 24

allows random access to cipher text
changing IV or the first plaintext block results in different cipher text
Error propagation
iv need not be secret but its integrity should be protected

Question 25

what is CTR

Answer 25

counter

Question 26

What is the CTR counter cipher

Answer 26

no chaining dependencies

no error propagation

Question 27

what is Feistel cipher

Answer 27

The input is broken into two equal size blocks, Left and Right, which are then repeatedly cycled through the algorithm. each cycle, a hash function F is applied to the right block and the key, and the result of the hash is XOR-ed into the left block. The blocks are then swapped. The XOR-ed result becomes the new right block and the unaltered right block becomes the left block. the process is then repeated a number of times

Question 28

what is DES

Answer 28

Data Encryption Standard

Question 29

what is Data encryption Standard

algorithm?

Answer 29

it is a Feistel type substitution permeation
16 cycle Feistel system
56 key permuted into
16 rounds
48 sub keys one for each cycle
L and R blocks are 32 bits each yielding an overall block of 64 bits
S-boxes 32 bit data block and one 48subkeys as input
output 32 bits of output

Question 30

what is 3DES

Answer 30

Triple DES

Question 31

what is the Triple DES?

Answer 31

it was developed to address the obvious flaws in DES. it extends the key size of DES by applying the algorithm three times with three different keys. so NO brute force

Question 32

what is AES

Answer 32

Advanced Encryption Standard

Question 33

who is AES by

Answer 33

by Rijndael