This set of Data Structures & Algorithms Multiple Choice Questions & Answers (MCQs) focuses on “Hill Cipher”.

1. Hill cipher requires prerequisite knowledge of?

a) integration

b) differentiation

c) matrix algebra

d) differential equation

View Answer

Explanation: Hill cipher uses matrix multiplication in order to encrypt the given plain text. So it requires prerequisite knowledge of matrix algebra.

2. Hill cipher is an example of ____________

a) mono-alphabetic cipher

b) substitution cipher

c) transposition cipher

d) additive cipher

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Explanation: Hill cipher is a substitution cipher. It falls under the category of poly alphabetic cipher as it uses multiple substitutions at different positions in order to cipher the plain text.

3. Encryption in hill cipher is done using ______________

a) matrix multiplication

b) a 5×5 table

c) vigenere table

d) matrix inversion

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Explanation: Encryption of plain text in playfair cipher is done using matrix multiplication. Then modulo 26 of the resulting matrix is taken so as to get the ciphered text.

4. What is poly graphic substitution cipher?

a) a substitution based cipher which uses multiple substitutions at different positions

b) a substitution based cipher which uses fixed substitution over entire plain text

c) a substitution based cipher in which substitution is performed over a block of letters

d) a transposition based cipher which uses fixed substitution over entire plain text

View Answer

Explanation: Poly graphic cipher is a type of substitution cipher in which substitution is performed over a block of letters. Hill cipher is an example of poly graphic cipher.

5. Which of the following was the first poly graphic cipher to be able to operate on more than 3 letters at once?

a) autokey cipher

b) hill cipher

c) one time pad cipher

d) playfair cipher

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Explanation: Poly graphic cipher is a type of substitution cipher in which substitution is performed over a block of letters. Hill cipher was the first poly graphic cipher to be able to operate on more than 3 letters at once.

6. Which of the following is hardest to break using frequency analysis?

a) Vigenere cipher

b) Hill cipher

c) Caesar cipher

d) Affine cipher

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Explanation: Out of the given options hill cipher is the hardest cipher to break using frequency analysis. Although it is quite vulnerable to other forms of attack.

7. Hill cipher is harder to crack than playfair cipher.

a) True

b) False

View Answer

Explanation: Both hill cipher and playfair cipher are less vulnerable to frequency analysis. But hill cipher is quite vulnerable to other forms of attack and thus less secure than playfair cipher.

8. What will be the plain text corresponding to cipher text “YGQ“ if hill cipher is used with keyword as “GYBNQKURP”?

a) SECRET

b) WORLD

c) DOLLAR

d) HELLO

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Explanation: To decrypt the message we follow the reverse procedure. We first find the inverse of the keyword matrix then multiply it with cipher matrix.

9. What will be the size of a key matrix if the plain text is “SECRET”?

a) 1×6

b) 5×1

c) 6×1

d) 6×6

View Answer

Explanation: Hill cipher uses a n x n matrix in order to cipher the plain text. In this case n=6 so a 6×6 matrix will be used.

10. A key matrix used for encryption in hill cipher must be?

a) invertible matrix

b) non invertible matrix

c) square matrix

d) rectangular matrix

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Explanation: A key matrix used for encryption in hill cipher must be invertible matrix. Otherwise it will be impossible to decipher the message.

11. What will be the ciphered text if the plain text “SAN” is encrypted using hill cipher with keyword as “GYBNQKURP”?

a) RAJ

b) JAR

c) ARJ

d) AJR

View Answer

Explanation: We first convert the keyword and plain text into their corresponding matrix respectively. Then we multiply those matrix and take modulo 26 of the resulting matrix to get the ciphered text.

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