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Secret Key Cryptography > Chapter 4 - Cryptographers Toolbox > Flashcards

Chapter 4 - Cryptographers Toolbox Flashcards

1
Q

How is the strength of a cipher measured?

A
  • In bits
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2
Q

What is the limiting factor in determining the strength of a cipher?

A
  • The size of the key
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3
Q

What is substitution?

A
  • One unit is substituted for another unit in a text
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4
Q

In the context of substitution what is a plain text unit ?

A
  • single letters, pairs of letters, or blocks
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5
Q

In the context of substitution what are cipher text units?

A
  • single letters, blocks of letters, letter-digit combinations
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6
Q

In the context of substitution where computer cryptography is used used what are units?

A
  • bits, bytes or blocks of bits and bytes of any length
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7
Q

In the context of substitution when using plain text units what makes the cipher stronger?

A
  • making the frequencies of letters more uniform
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8
Q

What can be used in solving ciphers?

A
  • The difference of letter frequencies
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9
Q

What is homophonic substitution?

A
  • Using extra pairs to provide additional substitutes for common letters
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10
Q

What is homophonic and letter pair substitution use for?

A
  • Prevents an opponent from using frequency analysis
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11
Q

What is huffman coding?

A
  • Based on a letter frequency table the most frequent letters get short codes while rarer letters get longer codes
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12
Q

What is the prefix property in huffman coding?

A
  • No code is a prefix of any longer code
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13
Q

What is mixed huffman coding?

A
  • Based on the frequency of a letter arbitrarily choose number of bits to represent the letter
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14
Q

What is transposition?

A
  • Changing the orders of characters in a message
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15
Q

What is it called when you change the order of characters in a message ?

A
  • Transposition
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16
Q

What is the simplest method of transposition

A
  • Route transposition
17
Q

What is route transposition

A
  • letters of a message are written into a rectangle in one order and read out in a different order
18
Q

What is it called when letters of a message are written into a rectangle in one order and read out in a different order

A
  • Route transposition
19
Q

What is Fractionation?

A
  • Division of characters into smaller units
20
Q

What is a random number generator?

A
  • Anything the produces a sequence of numbers in some given range
21
Q

What is something that produces a sequence of number in some given range?

A
  • a random number generator
22
Q

What is randomness a property of?

A
  • its a property of a sequence or of the generator
23
Q

What is randomness not a property of?

A
  • individual numbers in a sequence
24
Q

What can a generator be?

A
  • It might be some physical process
25
Q

What are the limitations of physical processes of a generator?

A
  • Most are not fast enough for cryptographic purposes
26
Q

What are random numbers, generated by a math function or computer program, called?

A
  • They are called pseudorandom numbers
27
Q

Why are pseudorandom numbers considered weaker then random numbers?

A
  • If an opponent determines a portion of the random sequence they may be able to calculate the preceding and following numbers and be able to read the message
28
Q

What is a key difference between pseudorandom and random sequences?

A
  • random sequences never repeat. Pseudorandom sequences eventually repeat
29
Q

What is a ‘period’ of a sequence?

A
  • The number of terms before the sequence repeats
30
Q

What property of a pseudo random sequence determines the strength of a cipher ?

A
  • the period of the sequence. The longer the period the stronger the cipher
31
Q

What is important to remember about numbers in a random sequence?

A
  • the numbers in the sequence aren’t necessarily equally probable
32
Q

What is random number generator called when it produces a sequence of numbers with equal probabilities?

A
  • A uniform distribution
33
Q

What is another way to say ‘uniform distribution’?

A
  • equiprobable
34
Q

What is the term for performing one encipherment and then another encipherment?

A
  • Composing the two encipherments
35
Q

What is the term for performing one encipherment and then another encipherment?

A
  • Composing the two encipherments
36
Q

When two encipherments are composed what is happening?

A
  • one encipherment is being preformed and then another encipherment is performed on top of it
37
Q

What is it called when when the first encryption uses a code then a cipher is applied?

A
  • Super encipherment
38
Q

What is the most common form of super encipherment?

A
  • non carrying addition
39
Q

What is another term for non carrying addition?

A
  • addition modulo 10

Secret Key Cryptography (4 decks)

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