Chapter 6 - Crypto

Question 1

Confusion

Answer 1

Occurs when relationship between the plain text and the key is so complicated that an attacker can’t merely continue altering the plaintext and analyzing the resulting ciphertext to determine the key.

Question 2

Diffusion

Answer 2

Occurs when a change in the plaintext results in multiple changes spread throughout the ciphertext.

Question 3

Crypto Math AND

Answer 3

uses ^. Only true (1) when both X and Y are true (1).

Question 4

Crypto Math OR

Answer 4

uses down carrot. Only false when both are false.

Question 5

Crypto Math NOT

Answer 5

uses -. Only used on one variable at a time. X=0, -X=1

Question 6

Crypto Math Exclusive OR (XOR)

Answer 6

Only returns true value when only one of the input values is true. Uses plus in a circle.

Question 7

One way function

Answer 7

math function that easily produces output values for each possible combination of inputs but makes it impossible to retrieve input values.

Question 8

Transposition Ciphers

Answer 8

Use encryption algorithm to rearrange letters of a plaintext message. Ex: apple=elppa.

Question 9

Substitution Ciphers

Answer 9

• Use the encryption algorithm to replace each character or bit of the plaintext message with a different character. (Caesar cipher).
• More sophisticated versions use multiple alphabets. (Vigenere cipher) looks like a crossword puzzle

Question 10

One Time Pads

Answer 10

• Use a different substitution alphabet for each letter of the plaintext message. Usually written as a very long series of numbers to be plugged into the function.
• Known as Vernam ciphers
• Must meet these requirements:
○ Pad must be randomly generated.
○ Must be physically protected against disclosure
○ Must be used once.
○ Key must be at least as long as the message to be encrypted.
○ VENONA = soviets used a pattern in key generation.
○ Hard to distribute keys, messages should be short due to key length.

Question 11

Running Key Ciphers

Answer 11

AKA book cipher. Agree to use the same book

Question 12

Block Ciphers

Answer 12

• Operate on chunks of a message and apply the encryption algorithm to an entire message block at the same time.
• Transposition is block cipher
• Most modern encryption algorithms implement block cipher

Question 13

Stream Ciphers

Answer 13

Operate on one character or bit of a message at a time.

Ceasar cipher, one time pad

Question 14

AES Block & Key Size

Answer 14

128

128,192,256

Question 15

Rijndael Block & Key Size

Answer 15

Variable

128,192, 256

Question 16

Blowfish (often used in SSH) Block & Key Size

Answer 16

64

32-448

Question 17

DES Block & Key Size

Answer 17

64

56

Question 18

IDEA (used in PGP) Block & Key Size

Answer 18

64

128

Question 19

RC2 Block & Key Size

Answer 19

64

128

Question 20

RC4 Block & Key Size

Answer 20

Streaming

128

Question 21

RC5 Block & Key Size

Answer 21

32,64,128

0-2040

Question 22

Skipjack Block & Key Size

Answer 22

64

80

Question 23

3DES Block & Key Size

Answer 23

64

112 or 168

Question 24

Twofish

Answer 24

128

1-256

Question 25

DES Electronic Codebook Mode

Answer 25

Least secure, encrypts each 64 bit bock with same key.
Cryptanalysis could break blocks
Impractical for anything but very small amounts of data (keys and parameters to initiate)

Question 26

DES Cipher Block Chaining Mode

Answer 26

Each block of ciphertext XORed with the block of ciphertext immediately preceding it before encrypting with DES.
Implements an IV (initialization vector) and XORs it with the first block of the message.
Corruption will flow if one block is corrupted.

Question 27

DES Cipher Feedback Mode

Answer 27

Streaming cipher version of CBC.
Operates against data produced in real time.
Instead of breaking message into blocks, it uses memory buffers of the same block size. When buffer fills, it’s encrypted and sent.
Uses IV and chaining.

Question 28

DES Output Feedback Mode

Answer 28

Same as CBC, but instead of XORing encrypted version of previous block of ciphertext, it XORs the plain text with a seed value.
IV used to create seed value.
Future seeds values derived by running DES algorithm on previous seed value.
Advantage: no chaining, errors do not propagate

Question 29

DES Counter Mode

Answer 29

Uses stream cipher, but instead of creating seed value from each previous encrypted seed values, it uses a simple counter that increments each operation.
Errors do not propagate.

Question 30

Triple DES

Answer 30

Four versions:

1. Encrypts plaintext 3 times using 3 different keys: k1, k2, k3. Known as DES-EEE3 mode.
   a. E(K1,(k2,E(k3,P)))
   b. Effective key length of 168 bits.
2. Uses 3 keys, but replaces second encryption with decryption. DES-EDE3
   a. E(k1, D(k2, E(k3,P)))
3. Uses 2 keys (DES-EEE2)
   a. E(K1,E(K2,E(K1,P)))
   b. 112 bits
4. Uses 2 keys with decryption (DES-EDE2)
   a. E(K1,D(K2,E(K1,P)))
   b. 112 bits

Question 31

IDEA

Answer 31

• Developed because DES too short key length.
• Operates on 64-bit blocks of plaintext.
• Begins operation with 128 bit key.
• Broken up into series of 52 16-bit subkeys.

Question 32

Blowfish

Answer 32

Operates on 64-bit block of text.
Uses variable-length keys from 32 to 448 bits.
Much faster than IDEA and DES.

Question 33

Skipjack

Answer 33

• Approved for use by gov in FIPS 185, the Escrowed Encryption Standard (EES).
  
  • 64 bit block of text.
  • 80 bit key.
  • Provides cryptographic routines supporting the Clipper and Capstone encryption chips.
  • NIST and Dept of Treasury each hold part of the key.
  • Not used at large because of mistrust of government key escrow.

Question 34

AES

Answer 34

• Only allows processing of 128-bit blocks, Rjindael allows use of block size equal to key length.
○ 128-bit keys require 10 rounds of encryption
○ 192-bit keys require 12 rounds of encryption
256-bit keys require 14 rounds of encryption

Question 35

Twofish

Answer 35

• AES finalist
• Block sipher
• Operates on 128-bit blocks
• Keys up to 256 bits in length
• Uses:
○ Prewhitening - XORing plaintext with a separate subkey before first round of encryption
○ Postwhitening - uses similar operation after 16th round of encryption