Chapter 3 Symmetric Key Crypto Flashcards
what are the two sub ciphers in symmetric key crypto?
stream cipher and block cipher
what is a stream cipher?
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
what are the sub contents of block cipher
confusion and diffusion
what is a block cipher?
based on codebook concept
each key yields a different codebook
employs both confusion and diffusion
what are the two sub contents in Stream ciphers?
A5/1 algorithm and RC4
what is A5/1 algorithm
it is based on shift registers, used in GSM mobile phone system
what is RC4
based on a changing lookup table
for A5/1
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
what is RC4
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
Famous man in stream cipher
shamir the death of stream ciphers
what is a round function
ciphertext is obtained from plaintext by iterating a round function
what is the input of the round function
input consists of key and output of previous round
what is a Feistel cipher
a type of block cipher splitting the plain text left and right halves F round function K subkey XOR
what is DES
Data Encryption Standard
NSA secretly involved
Stream ciphers..
idea of one time pad,we trade provable security for relatively small and manageable key
stream ciphers is confusion or diffusion
confusion only
block ciphers…
classic code book ciphers
key determines the code book
block ciphers is confusion or diffusion
employ both confusion and diffusion
DES is
block cipher standards
what is a stream cipher
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
What is ECB
Electronic Codebook
what is Electronic Codebook
each block encrypted independently identical plaintexts are encrypted similarly no chaining, no error propagation does not hide data patterns alice doesn't like it
what is CBC
Cipher block Chaining
what is cipher block chaining
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
what is CTR
counter
What is the CTR counter cipher
no chaining dependencies
no error propagation
what is Feistel cipher
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
what is DES
Data Encryption Standard
what is Data encryption Standard
algorithm?
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
what is 3DES
Triple DES
what is the Triple DES?
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
what is AES
Advanced Encryption Standard
who is AES by
by Rijndael
