Lec 3: Basic Cryptography I

Question 1

What is cryptography?

Answer 1

Greek for “secret writing”

Question 2

What do you do with Cryptography?

Answer 2

1. Confidentiality - obscure a message from eavesdroppers
2. Integrity - assure recipient that a message was not altered
3. Authentication - verify that a message was sent by a certain identity
4. Non-repudiation - convince a 3rd party that what was said is accurate

Question 3

What is a cryptographer?

Answer 3

invents cryptosystems

Question 4

What is a cryptanalyst?

Answer 4

breaks cryptosystems

Question 5

What is cryptology?

Answer 5

the study of crypto systems

Question 6

What is a cipher?

Answer 6

mechanical way of encrypting text

Question 7

What is code?

Answer 7

semantic translation - e.g. “eat breakfast tomorrow” = “attack on Thursday”

Question 8

What do we mean when we say a cryptosystem is “secure”?

Answer 8

1- if enemy intercepts ciphertext, cannot recover plaintext

Question 9

What is the issue in making cryptosystem’s secure precise?

Answer 9

1, What else might your enemy know?

-The kind of encryption function you are using?

— Kerckhoffs’ principle: assume everything except key is public knowledge

• Some plaintext-ciphertext pairs from last year?
• Ciphertext for plaintext the enemy selected?
• Some information about how you choose keys?
  2. What do we mean by “cannot recover plaintext” ?
• Ciphertext contains no information about plaintext, or
• No efficient computation could make a reasonable guess

Question 10

What are the kinds of cryptographic analysis?

Answer 10

Question 11

What is the caesar cipher? What are the advantages and disadvantages?

Answer 11

Question 12

What is a monoalphabetic ciphers?

Answer 12

Question 13

What is the cryptanalysis of monoalphabetic ciphers?

Answer 13

Question 14

What is the order of frequency of single letters in English (the first 5)?

Answer 14

Question 15

How to break monoalphabetic ciphers?

Answer 15

Question 16

What are the most used digrams and trigrams?

Answer 16

Question 17

What is the problem with monoalphabetic ciphers?

Answer 17

Question 18

How does polyalphabetic substitutions?

Answer 18

Question 19

What is the Vigenère Tableau?

Answer 19

1. Multiple substitutions
   - Can choose “complimentary” ciphers so that the frequency distribution flattens out
   - More generally: more substitutions means flatter distribution
2. Vigenère Tableau
   - Invented by Blaise de Vigenère for the court of Henry III of France (c. 1500’s)
   - Collection of 26 permutations
   - Usually thought of as a 26 x 26 grid
   - Key is a word

Question 20

What is the weakness of polyalphabetic substituion ciphers?

Answer 20

Question 21

What is that Kasiski method?

Answer 21

Question 22

What is variance?

Answer 22

Question 23

How do you estimate variance from frequency?

Answer 23

Question 24

What is the index of coincidence?

Answer 24

Question 25

What is index of coincidence used for?

Answer 25

Question 26

How was polyalphabetics cracked circa 1900?

Answer 26

Question 27

What is Enigma?

Answer 27

Question 28

What are perfect substitution ciphers?

Answer 28

Question 29

What are one-time pads?

Answer 29

Question 30

What are the problems with perfect substitution?

Answer 30

Question 31

What are perfect ciphers like in practice?

Answer 31

Question 32

What assumption do we make about attackers on ciphers?

Answer 32

Question 33

What are key crypto-primitives?

Answer 33

Encryption, one-way hash functions, digital signatures

Question 34

What determines who knows what keys?

Answer 34

shared key vs public key

Question 35

What is the granularity of operation?

Answer 35

block ciphers vs stream ciphers

Question 36

What terms in the cryptosystem describe the difficulty of breaking?

Answer 36

empirical hardness vs computational complexity

Question 37

How does shared key cryptography work?

Answer 37

1. sender & receiver use the same key 2. key must remain private (i.e. secret) 3. also called symmetric or secret key cryptography 4. Examples: DES, AES ...

Question 38

Shared key notation

Answer 38

Question 39

Show secure channel in shared key notation

Answer 39

Question 40

Stream ciphers vs block ciphers

Answer 40

Question 41

What are the design goals of the typical stream cipher: RC4?

Answer 41

Question 42

Where has the typical stream cipher RC4 been used?

Answer 42

Question 43

How does the RC4 algorithm work?

Answer 43

Question 44

How random is "random" bitstream?

Answer 44

Goal: given part of the output stream, would like it to be impossible to distinguish from a random string? What does random mean here? - pseudorandom can still derive some of the output stream

Question 45

What is the history and problems of RC4 security? How do we handle them?

Answer 45

Question 46

How do block ciphers work?

Answer 46

Question 47

What are Feistel networks?

Answer 47

Question 48

How do Feistel networks work?

Answer 48

● What if plaintext \< blocksize? - Padding ● What if plaintext \> blocksize? - Need to encrypt multiple blocks - But how? Block cipher modes

Question 49

What are the different block cipher modes?

Answer 49

Question 50

How does the Electronic Code Book (ECB) work?

Answer 50

Question 51

How does the Cipher Block Chaining (CBC) work?

Answer 51

Question 52

What is the value of CBC?

Answer 52

Question 53

What was the issue with the Deibold voting machine?

Answer 53

● Initialization vector (IV) in CBC is supposed to be chosen randomly ● Code from Deibold voting machine DesCBCEncrypt((des\_c\_block\*)tmp, des\_c\_block\*)record.m\_Data, totalSize, DESKEY, NULL, DES\_ENCRYPT) ● Consequence? could make it do anything

Question 54

What are the general problems with Shared Key Crypto?

Answer 54

Question 55

What is the goals of message integrity?

Answer 55

Question 56

How do hash algorithms work and what are they used for?

Answer 56

Question 57

What are modification detection codes?

Answer 57

Question 58

What is desirable uniformity?

Answer 58

Question 59

Hash functions for cryptographic use fall in one or both of the following classes. What are those classes?

Answer 59

Question 60

How do iterated hash functions work?

Answer 60

Question 61

What are Message Authentication Codes?

Answer 61