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Unit 4: Cryptography - Practice Quiz

MTH381 — Number Theory And Cryptography 60 Questions
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1 What is the primary goal of cryptography?

introduction Easy
A. Securing communication so that only intended parties can read it
B. Detecting hardware faults in networks
C. Increasing the speed of data transmission
D. Compressing data to save storage space

2 The process of converting plaintext into an unreadable form is called:

introduction Easy
A. Encryption
B. Decryption
C. Authentication
D. Compression

3 The Caesar cipher, used by Julius Caesar, is an example of which type of cipher?

history and overview of cryptography Easy
A. Hash function
B. Block cipher
C. Shift cipher
D. Public-key cipher

4 The readable, original message before encryption is known as:

history and overview of cryptography Easy
A. Keytext
B. Cryptotext
C. Ciphertext
D. Plaintext

5 Classical cryptography primarily relies on which two basic techniques?

classical cryptography Easy
A. Hashing and salting
B. Sampling and filtering
C. Compression and encoding
D. Substitution and transposition

6 In classical cryptography, a cipher that rearranges the positions of characters without changing them is called a:

classical cryptography Easy
A. Product cipher
B. Stream cipher
C. Transposition cipher
D. Substitution cipher

7 The shift cipher encryption of a letter with key (mod 26) is given by:

shift cipher Easy
A.
B.
C.
D.

8 Using a shift cipher with key , the letter A (value 0) encrypts to:

shift cipher Easy
A. D
B. B
C. E
D. C

9 How many possible keys does a shift cipher on the English alphabet have?

shift cipher Easy
A. 25
B. 26
C. 13
D. 52

10 The affine cipher encryption function has the form:

affine cipher Easy
A.
B.
C.
D.

11 For the affine cipher to be decryptable, the key value must satisfy which condition?

affine cipher Easy
A. must be even
B. must be prime
C.
D.

12 In a general monoalphabetic substitution cipher, each plaintext letter is:

substitution cipher Easy
A. Doubled in the ciphertext
B. Removed and re-inserted elsewhere
C. Replaced by a fixed corresponding ciphertext letter
D. Shifted by a random amount each time

13 How many possible keys does a general monoalphabetic substitution cipher on 26 letters have?

substitution cipher Easy
A.
B.
C.
D.

14 Which attack is commonly used to break simple substitution ciphers?

substitution cipher Easy
A. Timing attack
B. Brute-force factoring
C. Man-in-the-middle attack
D. Frequency analysis

15 The Vigenère cipher is best described as a:

vigenere cipher Easy
A. Pure transposition cipher
B. Public-key cipher
C. Monoalphabetic substitution cipher
D. Polyalphabetic substitution cipher

16 In the Vigenère cipher, what determines the amount each letter is shifted?

vigenere cipher Easy
A. A single fixed number
B. The message length
C. A random number generator
D. The letters of a repeating keyword

17 The Hill cipher performs encryption using which mathematical operation?

Hill cipher Easy
A. Simple letter shifting
B. Matrix multiplication modulo 26
C. Bitwise XOR
D. String reversal

18 For a Hill cipher key matrix to be usable for decryption, the matrix must be:

Hill cipher Easy
A. Symmetric
B. Singular
C. Invertible modulo 26
D. Diagonal

19 A permutation (transposition) cipher encrypts a message by:

permutation cipher Easy
A. Rearranging the order of the plaintext characters
B. Adding a numeric key to each letter
C. Multiplying blocks by a matrix
D. Replacing each letter with another symbol

20 A cryptosystem in which the same key is used for both encryption and decryption is called:

cryptosystems Easy
A. Symmetric-key cryptosystem
B. Public-key cryptosystem
C. Asymmetric cryptosystem
D. Hash-based cryptosystem

21 In modern cryptography, the property that ensures a message has not been altered during transmission is known as:

introduction Medium
A. Non-repudiation
B. Integrity
C. Availability
D. Confidentiality

22 The Caesar cipher, historically attributed to Julius Caesar, is a specific instance of which general cipher with a fixed key?

history and overview of cryptography Medium
A. Affine cipher with
B. Vigenere cipher with key CCC
C. Shift cipher with
D. Hill cipher of order

23 Classical ciphers such as the shift, affine, and substitution ciphers are all classified as symmetric because:

classical cryptography Medium
A. The same key is used for encryption and decryption
B. They rely on two mathematically related public keys
C. The plaintext and ciphertext are always identical in length only for palindromes
D. They only work on symmetric matrices

24 Using a shift cipher with key , the plaintext CAT encrypts to (with ):

shift cipher Medium
A. GDX
B. FDW
C. GEX
D. HEY

25 A message encrypted with a shift cipher yields the ciphertext KHOOR. If the key is , the recovered plaintext is:

shift cipher Medium
A. GDKKN
B. HELLO
C. HULLO
D. WORLD

26 Why is the shift cipher considered cryptographically weak against a brute-force attack on the English alphabet?

shift cipher Medium
A. The key must always be a prime number
B. Only 25 non-trivial keys need to be tried
C. It requires a full key table
D. It uses modular exponentiation which is slow

27 Using the affine cipher , the letter C (value ) encrypts to:

affine cipher Medium
A. S
B. T
C. R
D. N

28 In the affine cipher , which value of is invalid as a key?

affine cipher Medium
A.
B.
C.
D.

29 For the affine encryption , the correct decryption function is:

affine cipher Medium
A.
B.
C.
D.

30 How many distinct valid keys exist for the affine cipher over the 26-letter alphabet?

affine cipher Medium
A.
B.
C.
D.

31 The size of the key space for a general monoalphabetic substitution cipher on the 26-letter alphabet is:

substitution cipher Medium
A.
B.
C.
D.

32 Despite its enormous key space, a monoalphabetic substitution cipher is easily broken primarily by:

substitution cipher Medium
A. Frequency analysis of letters
B. Brute-forcing all keys
C. Computing modular inverses of blocks
D. Factoring the ciphertext length

33 The Vigenere cipher is classified as a polyalphabetic cipher because:

vigenere cipher Medium
A. It substitutes each letter with two ciphertext letters
B. It permutes the positions of letters within blocks
C. It requires an invertible matrix key
D. It uses multiple shift alphabets determined by a repeating keyword

34 Encrypting the plaintext HELLO with the Vigenere keyword KEY produces:

vigenere cipher Medium
A. SIJVS
B. RIKVS
C. RIJVS
D. RIJVR

35 Which technique is used to determine the probable key length of a Vigenere cipher by examining repeated ciphertext sequences?

vigenere cipher Medium
A. Diffie-Hellman exchange
B. Chinese Remainder Theorem
C. Kasiski examination
D. Euclidean algorithm

36 In the Hill cipher, encryption of a block is performed by:

Hill cipher Medium
A. Applying a single fixed shift to each letter
B. Permuting letter positions using a permutation key
C. Adding a repeating key vector modulo 26
D. Multiplying the plaintext vector by a key matrix modulo 26

37 Which of the following matrices is a valid Hill cipher key over ?

Hill cipher Medium
A.
B.
C.
D.

38 A permutation (transposition) cipher achieves security by:

permutation cipher Medium
A. Adding a keyword shift to each letter
B. Rearranging the positions of plaintext letters without changing them
C. Replacing each letter with a different fixed letter
D. Multiplying letter blocks by a matrix

39 A cryptosystem is formally defined as a five-tuple . What does represent?

cryptosystems Medium
A. The set of decryption rules
B. The set of all ciphertexts
C. The set of all plaintexts
D. The finite set of possible keys

40 Kerckhoffs's principle states that the security of a cryptosystem should depend on:

cryptosystems Medium
A. Frequently changing the alphabet ordering
B. Using the longest possible plaintext blocks
C. The secrecy of the key alone, not the algorithm
D. Keeping the encryption algorithm completely secret

41 In an affine cipher over with encryption , a cryptanalyst knows that plaintext 'E' () maps to 'F' () and plaintext 'T' () maps to 'A' (). What is the key pair ?

affine cipher Hard
A.
B.
C.
D.

42 How many valid distinct keys exist for the affine cipher over , and how does this compare to the shift cipher?

affine cipher Hard
A. keys, which is the shift cipher's
B. keys, which is roughly the shift cipher's
C. keys, which is the shift cipher's
D. keys, which is the shift cipher's

43 For a Hill cipher over using key matrix , what is the decryption matrix ?

Hill cipher Hard
A.
B.
C.
D.

44 Which condition must a Hill cipher key matrix over satisfy to be invertible?

Hill cipher Hard
A. must be symmetric with
B. exactly
C. must be a prime number
D.

45 A Vigenère ciphertext has index of coincidence . English plaintext has IC and random text . Using the Friedman estimate, which keyword length is most consistent with this observation?

vigenere cipher Hard
A. A key equal to the message length (one-time-pad-like)
B. A key of length (effectively a shift)
C. A moderately long key (around –)
D. A key of length exactly

46 The Kasiski examination finds repeated trigram distances of , , and in a Vigenère ciphertext. What is the most probable keyword length?

vigenere cipher Hard
A.
B.
C.
D.

47 The shift cipher is a special case of the affine cipher. Which restriction on the affine parameters produces exactly the shift cipher?

shift cipher Hard
A. with arbitrary
B. with arbitrary coprime to
C. with arbitrary in
D. for all valid

48 What is the size of the keyspace for a general monoalphabetic substitution cipher on a -letter alphabet, and why is it still breakable?

substitution cipher Hard
A. ; vulnerable because one letter is fixed
B. ; vulnerable to frequency analysis
C. ; vulnerable to brute force enumeration
D. ; vulnerable to known-plaintext linear algebra

49 In a permutation (transposition) cipher with block length using permutation (position receives the letter from position ), what property is preserved from plaintext to ciphertext?

permutation cipher Hard
A. The total number of distinct letters is halved
B. The letter frequency distribution is unchanged
C. The positions of vowels are unchanged
D. The digram frequencies are unchanged

50 Decrypt the affine ciphertext letter 'R' () given key over . What is the plaintext letter?

affine cipher Hard
A. 'K' ()
B. 'H' ()
C. 'C' ()
D. 'M' ()

51 A cryptosystem is defined as a five-tuple . Which requirement must hold for every key used for encryption?

cryptosystems Hard
A. The encryption rule must be a non-injective map from to
B. Each must map all plaintexts to a single ciphertext
C. and must be disjoint sets
D. There exists with for all

52 Why is the Hill cipher vulnerable to a known-plaintext attack, unlike simple substitution against ciphertext-only frequency analysis?

Hill cipher Hard
A. Each ciphertext block reveals the full key independently
B. The Hill cipher preserves single-letter frequencies exploitable directly
C. Given enough plaintext-ciphertext block pairs, the key matrix is recovered by solving a linear system mod
D. The determinant of the key can be guessed from ciphertext length

53 Which statement best characterizes the difference between confusion and diffusion as applied to classical ciphers?

classical cryptography Hard
A. Substitution ciphers provide confusion; transposition ciphers provide diffusion
B. Both properties are provided equally by the shift cipher alone
C. Substitution ciphers provide diffusion; transposition ciphers provide confusion
D. Diffusion is only achievable using public-key cryptosystems

54 The autokey Vigenère variant appends the plaintext to the key. Compared to the standard repeating-key Vigenère, what is its principal cryptanalytic advantage?

vigenere cipher Hard
A. It removes the periodic key structure that Kasiski and IC methods exploit
B. It guarantees perfect secrecy regardless of key length
C. It preserves single-letter frequencies exactly like a shift cipher
D. It reduces the effective keyspace, making it easier to brute force

55 Assuming English plaintext, a ciphertext-only attack on a shift cipher succeeds because of which principle?

shift cipher Hard
A. Shift ciphers preserve digram order allowing dictionary matching
B. The key is always as in the historical Caesar cipher
C. The most frequent ciphertext letter likely corresponds to plaintext 'E', revealing the shift
D. Every shift produces a unique letter-frequency histogram shape

56 Kerckhoffs's principle, foundational to modern cryptography, asserts which of the following?

history and overview of cryptography Hard
A. A cryptosystem should remain secure even if everything except the key is public
B. Only government-approved ciphers can be considered secure
C. A cryptosystem's algorithm must always be kept secret for security
D. Security depends on using the longest possible ciphertext

57 Why can never be used as the multiplier in an affine cipher over ?

affine cipher Hard
A. exceeds the alphabet size and causes overflow
B. , so has no inverse and the map is not injective
C. maps every letter to itself, producing no encryption
D. is prime, which is disallowed for multipliers

58 A permutation cipher uses block size . How many possible keys exist, and how is decryption performed?

permutation cipher Hard
A. keys; decrypt by re-applying the same permutation
B. keys; decrypt by reversing each block
C. keys; decrypt by cyclic left shift
D. keys; decrypt by applying the inverse permutation

59 Under Shannon's definition of perfect secrecy, which condition must hold for the one-time pad?

cryptosystems Hard
A. The key may be reused if it is truly random
B. The ciphertext must be longer than the plaintext to hide length
C. The plaintext distribution must be uniform for secrecy to hold
D. for all plaintexts and ciphertexts

60 A polyalphabetic substitution cipher differs from a monoalphabetic one primarily because it does which of the following?

substitution cipher Hard
A. Rearranges letter positions without changing their identity
B. Uses multiple substitution alphabets so a plaintext letter can map to different ciphertext letters
C. Substitutes each letter with a fixed unique symbol permanently
D. Encrypts using arithmetic modulo a prime rather than
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