9. Applied Cryptography

Question 1

What are classic cryptosystems studied in applied cryptography?

Answer 1

Substitution, transposition, codebooks, and one-time pad.

Question 2

What is a simple substitution cipher?

Answer 2

A cipher that replaces each plaintext letter with another letter.

Question 3

What is a Caesar cipher?

Answer 3

A substitution cipher using a fixed shift.

Question 4

How many keys does a Caesar cipher have?

Answer 4

26 possible keys.

Question 5

Why is Caesar cipher insecure?

Answer 5

Small keyspace allows brute force attacks.

Question 6

What is brute force attack?

Answer 6

Trying all possible keys until the correct one is found.

Question 7

What is exhaustive key search?

Answer 7

Systematically testing every possible key.

Question 8

What is keyspace?

Answer 8

The set of all possible keys.

Question 9

Why must keyspace be large?

Answer 9

To make brute force infeasible.

Question 10

Is large keyspace sufficient for security?

Answer 10

No, ciphers can still be broken by clever attacks.

Question 11

What is frequency analysis?

Answer 11

Using letter frequency statistics to break ciphers.

Question 12

Why are substitution ciphers vulnerable?

Answer 12

They preserve letter frequency patterns.

Question 13

What is language redundancy?

Answer 13

Natural languages contain predictable patterns.

Question 14

How does redundancy help attackers?

Answer 14

It enables statistical cryptanalysis.

Question 15

What is a monoalphabetic cipher?

Answer 15

Uses one substitution alphabet.

Question 16

What is a polyalphabetic cipher?

Answer 16

Uses multiple substitution alphabets.

Question 17

What is the Vigenère cipher?

Answer 17

A polyalphabetic substitution cipher using a repeating key.

Question 18

What is the period in Vigenère?

Answer 18

The length of the key.

Question 19

Why is Vigenère more secure than Caesar?

Answer 19

It obscures letter frequencies.

Question 20

Is Vigenère unbreakable?

Answer 20

No, it can be broken using statistics.

Question 21

What is the Kasiski test?

Answer 21

A method to find Vigenère key length.

Question 22

How does Kasiski test work?

Answer 22

Finds repeated sequences and computes distances.

Question 23

What is Index of Coincidence?

Answer 23

Probability two random letters are the same.

Question 24

How is IC used?

Answer 24

To estimate key length.

Question 25

How is Vigenère broken after finding key length?

Answer 25

By treating it as multiple Caesar ciphers.

Question 26

What is unconditional security?

Answer 26

Security even with unlimited computing power.

Question 27

What cipher is unconditionally secure?

Answer 27

One-time pad.

Question 28

What is computational security?

Answer 28

Security against feasible computation.

Question 29

Give examples of computationally secure systems.

Answer 29

AES, DES, RSA.

Question 30

What is a secure cryptosystem (definition)?

Answer 30

No shortcut attack better than brute force.

Question 31

What is a shortcut attack?

Answer 31

An attack faster than exhaustive search.

Question 32

What are Shannon’s conditions for classical cipher security?

Answer 32

Key length ≥ message length and key used once.

Question 33

What is Kerckhoffs’ principle?

Answer 33

Algorithm is public, key is secret.

Question 34

What is ciphertext-only attack?

Answer 34

Attacker knows only ciphertext.

Question 35

What is known-plaintext attack?

Answer 35

Attacker knows some plaintext-ciphertext pairs.

Question 36

What is chosen-plaintext attack?

Answer 36

Attacker chooses plaintexts to encrypt.

Question 37

What is adaptive chosen-plaintext attack?

Answer 37

Next plaintext depends on previous output.

Question 38

What is chosen-ciphertext attack?

Answer 38

Attacker decrypts chosen ciphertexts.

Question 39

What is forward search attack?

Answer 39

Encrypting guessed plaintexts in public key systems.

Question 40

What is a transposition cipher?

Answer 40

A cipher that rearranges letters.

Question 41

Does transposition change letters?

Answer 41

No, it only rearranges them.

Question 42

What is the rail fence cipher?

Answer 42

A simple transposition cipher.

Question 43

Why are transposition ciphers detectable?

Answer 43

They preserve letter frequencies.

Question 44

What is anagramming?

Answer 44

Rearranging ciphertext to recover plaintext.

Question 45

What is double transposition cipher?

Answer 45

Two consecutive transpositions.

Question 46

Why is double transposition stronger?

Answer 46

It spreads plaintext structure.

Question 47

What modern concept resembles double transposition?

Answer 47

Diffusion in block ciphers.

Question 48

What is a codebook cipher?

Answer 48

A cipher mapping words to codewords.

Question 49

Why are codebooks complex substitutions?

Answer 49

They substitute whole words or phrases.

Question 50

What historical example used codebooks?

Answer 50

Zimmermann telegram.

Question 51

What is the one-time pad (OTP)?

Answer 51

A cipher using random key as long as message.

Question 52

How does OTP encryption work?

Answer 52

XOR plaintext with key.

Question 53

Why is OTP provably secure?

Answer 53

Ciphertext gives no info about plaintext.

Question 54

What are OTP requirements?

Answer 54

Random key, same length, used once, secret.

Question 55

Why is OTP impractical?

Answer 55

Key distribution and length.

Question 56

What is depth in OTP?

Answer 56

Reusing the same key.

Question 57

Why is key reuse dangerous in OTP?

Answer 57

XOR cancels key and reveals patterns.

Question 58

What happens when two OTP messages use same key?

Answer 58

C1⊕C2 = P1⊕P2.

Question 59

Why does OTP in depth break security?

Answer 59

It leaks information.

Question 60

What was the VENONA project?

Answer 60

Breaking reused Soviet one-time pads.

Question 61

Why was VENONA successful?

Answer 61

Keys were reused.

Question 62

What is information-theoretic security?

Answer 62

Another name for unconditional security.

Question 63

What does 'secure until broken' mean?

Answer 63

Security is based on no known attacks.

Question 64

Why must cryptosystems be publicly analyzed?

Answer 64

To detect design flaws.

Question 65

Why is redundancy bad for security?

Answer 65

It leaks structure.

Question 66

What is diffusion in cryptography?

Answer 66

Spreading plaintext statistics.

Question 67

What is the main lesson from classical ciphers?

Answer 67

Design must resist statistical attacks.

Question 68

What is the role of statistics in cryptanalysis?

Answer 68

Reveal hidden patterns.

Question 69

Why are modern ciphers complex?

Answer 69

To resist known attacks.

Question 70

What is the main security lesson from substitution ciphers?

Answer 70

Large keyspace alone is insufficient.