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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
What is index of coincidence used for?
26
How was polyalphabetics cracked circa 1900?
27
What is Enigma?
28
What are perfect substitution ciphers?
29
What are one-time pads?
30
What are the problems with perfect substitution?
31
What are perfect ciphers like in practice?
32
What assumption do we make about attackers on ciphers?
33
What are key crypto-primitives?
Encryption, one-way hash functions, digital signatures
34
What determines who knows what keys?
shared key vs public key
35
What is the granularity of operation?
block ciphers vs stream ciphers
36
What terms in the cryptosystem describe the difficulty of breaking?
empirical hardness vs computational complexity
37
How does shared key cryptography work?
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
Shared key notation
39
Show secure channel in shared key notation
40
Stream ciphers vs block ciphers
41
What are the design goals of the typical stream cipher: RC4?
42
Where has the typical stream cipher RC4 been used?
43
How does the RC4 algorithm work?
44
How random is "random" bitstream?
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
What is the history and problems of RC4 security? How do we handle them?
46
How do block ciphers work?
47
What are Feistel networks?
48
How do Feistel networks work?
● What if plaintext \< blocksize? - Padding ● What if plaintext \> blocksize? - Need to encrypt multiple blocks - But how? Block cipher modes
49
What are the different block cipher modes?
50
How does the Electronic Code Book (ECB) work?
51
How does the Cipher Block Chaining (CBC) work?
52
What is the value of CBC?
53
What was the issue with the Deibold voting machine?
● 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
What are the general problems with Shared Key Crypto?
55
What is the goals of message integrity?
56
How do hash algorithms work and what are they used for?
57
What are modification detection codes?
58
What is desirable uniformity?
59
Hash functions for cryptographic use fall in one or both of the following classes. What are those classes?
60
How do iterated hash functions work?
61
What are Message Authentication Codes?

CSE 127: Intro to Security (19 decks)

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