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CSE 127: Intro to Security > Lec 3: Basic Cryptography I > Flashcards

Lec 3: Basic Cryptography I Flashcards

1
Q

What is cryptography?

A

Greek for “secret writing”

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2
Q

What do you do with Cryptography?

A
  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
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3
Q

What is a cryptographer?

A

invents cryptosystems

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4
Q

What is a cryptanalyst?

A

breaks cryptosystems

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5
Q

What is cryptology?

A

the study of crypto systems

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6
Q

What is a cipher?

A

mechanical way of encrypting text

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7
Q

What is code?

A

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

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8
Q

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

A

1- if enemy intercepts ciphertext, cannot recover plaintext

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9
Q

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

A

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
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10
Q

What are the kinds of cryptographic analysis?

A
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11
Q

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

A
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12
Q

What is a monoalphabetic ciphers?

A
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13
Q

What is the cryptanalysis of monoalphabetic ciphers?

A
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14
Q

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

A
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15
Q

How to break monoalphabetic ciphers?

A
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16
Q

What are the most used digrams and trigrams?

A
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17
Q

What is the problem with monoalphabetic ciphers?

A
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18
Q

How does polyalphabetic substitutions?

A
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19
Q

What is the Vigenère Tableau?

A
  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
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20
Q

What is the weakness of polyalphabetic substituion ciphers?

A
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21
Q

What is that Kasiski method?

A
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22
Q

What is variance?

A
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23
Q

How do you estimate variance from frequency?

A
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24
Q

What is the index of coincidence?

A
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25
Q

What is index of coincidence used for?

A
26
Q

How was polyalphabetics cracked circa 1900?

A
27
Q

What is Enigma?

A
28
Q

What are perfect substitution ciphers?

A
29
Q

What are one-time pads?

A
30
Q

What are the problems with perfect substitution?

A
31
Q

What are perfect ciphers like in practice?

A
32
Q

What assumption do we make about attackers on ciphers?

A
33
Q

What are key crypto-primitives?

A

Encryption, one-way hash functions, digital signatures

34
Q

What determines who knows what keys?

A

shared key vs public key

35
Q

What is the granularity of operation?

A

block ciphers vs stream ciphers

36
Q

What terms in the cryptosystem describe the difficulty of breaking?

A

empirical hardness vs computational complexity

37
Q

How does shared key cryptography work?

A
  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 …
38
Q

Shared key notation

A
39
Q

Show secure channel in shared key notation

A
40
Q

Stream ciphers vs block ciphers

A
41
Q

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

A
42
Q

Where has the typical stream cipher RC4 been used?

A
43
Q

How does the RC4 algorithm work?

A
44
Q

How random is “random” bitstream?

A

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
45
Q

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

A
46
Q

How do block ciphers work?

A
47
Q

What are Feistel networks?

A
48
Q

How do Feistel networks work?

A

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

49
Q

What are the different block cipher modes?

A
50
Q

How does the Electronic Code Book (ECB) work?

A
51
Q

How does the Cipher Block Chaining (CBC) work?

A
52
Q

What is the value of CBC?

A
53
Q

What was the issue with the Deibold voting machine?

A

● 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

54
Q

What are the general problems with Shared Key Crypto?

A
55
Q

What is the goals of message integrity?

A
56
Q

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

A
57
Q

What are modification detection codes?

A
58
Q

What is desirable uniformity?

A
59
Q

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

A
60
Q

How do iterated hash functions work?

A
61
Q

What are Message Authentication Codes?

A

CSE 127: Intro to Security (19 decks)

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