Chapter 4 - Cryptographers Toolbox

Question 1

How is the strength of a cipher measured?

Answer 1

• In bits

Question 2

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

Answer 2

• The size of the key

Question 3

What is substitution?

Answer 3

• One unit is substituted for another unit in a text

Question 4

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

Answer 4

• single letters, pairs of letters, or blocks

Question 5

In the context of substitution what are cipher text units?

Answer 5

• single letters, blocks of letters, letter-digit combinations

Question 6

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

Answer 6

• bits, bytes or blocks of bits and bytes of any length

Question 7

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

Answer 7

• making the frequencies of letters more uniform

Question 8

What can be used in solving ciphers?

Answer 8

• The difference of letter frequencies

Question 9

What is homophonic substitution?

Answer 9

• Using extra pairs to provide additional substitutes for common letters

Question 10

What is homophonic and letter pair substitution use for?

Answer 10

• Prevents an opponent from using frequency analysis

Question 11

What is huffman coding?

Answer 11

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

Question 12

What is the prefix property in huffman coding?

Answer 12

• No code is a prefix of any longer code

Question 13

What is mixed huffman coding?

Answer 13

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

Question 14

What is transposition?

Answer 14

• Changing the orders of characters in a message

Question 15

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

Answer 15

• Transposition

Question 16

What is the simplest method of transposition

Answer 16

• Route transposition

Question 17

What is route transposition

Answer 17

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

Question 18

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

Answer 18

• Route transposition

Question 19

What is Fractionation?

Answer 19

• Division of characters into smaller units

Question 20

What is a random number generator?

Answer 20

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

Question 21

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

Answer 21

• a random number generator

Question 22

What is randomness a property of?

Answer 22

• its a property of a sequence or of the generator

Question 23

What is randomness not a property of?

Answer 23

• individual numbers in a sequence

Question 24

What can a generator be?

Answer 24

• It might be some physical process

Question 25

What are the limitations of physical processes of a generator?

Answer 25

* Most are not fast enough for cryptographic purposes

Question 26

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

Answer 26

* They are called pseudorandom numbers

Question 27

Why are pseudorandom numbers considered weaker then random numbers?

Answer 27

* 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

Question 28

What is a key difference between pseudorandom and random sequences?

Answer 28

* random sequences never repeat. Pseudorandom sequences eventually repeat

Question 29

What is a ‘period’ of a sequence?

Answer 29

* The number of terms before the sequence repeats

Question 30

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

Answer 30

* the period of the sequence. The longer the period the stronger the cipher

Question 31

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

Answer 31

* the numbers in the sequence aren't necessarily equally probable

Question 32

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

Answer 32

* A uniform distribution

Question 33

What is another way to say ‘uniform distribution’?

Answer 33

* equiprobable

Question 34

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

Answer 34

* Composing the two encipherments

Question 35

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

Answer 35

* Composing the two encipherments

Question 36

When two encipherments are composed what is happening?

Answer 36

* one encipherment is being preformed and then another encipherment is performed on top of it

Question 37

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

Answer 37

* Super encipherment

Question 38

What is the most common form of super encipherment?

Answer 38

* non carrying addition

Question 39

What is another term for non carrying addition?

Answer 39

* addition modulo 10