This set of Cryptography Multiple Choice Questions & Answers (MCQs) focuses on “Block Cipher Systems”.

1. In affine block cipher systems if f(m)=Am + t, what is f(m1+m2) ?

a) f(m1) + f(m2) + t

b) f(m1) + f(m2) + 2t

c) f(m1) + t

d) f(m1) + f(m2)

View Answer

Explanation: In general f(∑(i=1 to n) m_i = ∑(i=1 to n) f(m_i) + tδ_n) where δ_n=0 if n is odd and 1 if n is even.

2. In affine block cipher systems if f(m)=Am + t, what is f(m1+m2+m3) ?

a) f(m1) + f(m2) + f(m3) + t

b) f(m1) + f(m2) + f(m3) +2t

c) f(m1) + f(m2) + f(m3)

d) 2(f(m1) + f(m2) + f(m3))

View Answer

Explanation: In general f(∑(i=1 to n) m_i =∑(i=1 to n) f(m_i ) + tδ_n) where δ_n=0 if n is odd and 1 if n is even.

3. If the block size is ‘s’, how many affine transformations are possible ?

a) 2^{s} (2^{s}-1)(2^{s}-1)(2^{s}-1^{2})………(2^{s}-1^{(s-1)})

b) 2^{s} (2^{s}-1)(2^{s}-2)(2^{s}-2^{2})………(2^{s}-2^{(s-2)})

c) 2^{s}s (2^{s}-1)(2^{s}-2)(2^{s}-2^{2})………(2^{s}-2^{(s-1)})

d) 2^{s} (2^{s}-1)(2^{s}-2)(2^{s}-2^{2})………(2^{s}-2^{(s-3)})

View Answer

Explanation: 2

^{s}(2

^{s}-1)(2

^{s}-2)(2

^{s}-2

^{2})………(2

^{s}-2

^{(s-1)}) is the maximum number of affine transformations possible for a block size ‘s’ matrix.

4. What is the number of possible 3 x 3 affine cipher transformations ?

a) 168

b) 840

c) 1024

d) 1344

View Answer

Explanation: Since ‘A’ cannot have columns of ‘0’s. so there are ‘7’ choices i.e. 001/010/011/100/101/110/111. ‘a1’ is chosen for first column of ‘A.

We have ‘6’ choices for second column, let ‘a2’ be chosen for second column.

The final column can be any 3-tuple except 0, a1, a2, a1+a2. That means any one of the remaining ‘4’ 3-tuples may be chosen for the final column.

(Total number [email protected] possibilities for A)}=k=7×6×4=168

(Number of [email protected] block cipher transformation)}=k×t=8×168 =1344

5. Super-Encipherment using two affine transformations results in another affine transformation.

a) True

b) False

View Answer

Explanation: f(g(m))=A_1 g(m)+c_1

f(g(m))=A_1 (A_2 m+c_2)+c_1 f(g(m))=A_1 A_2 m+A_1 c_2+c_1 f(x)=A_3 m+c_3

where

A_3=A_1 A_2

c_3=A_1 c_2+c_1

This results in another affine transformation, and does not improve the security.

6. If the key is 110100001, the output of the SP network for the plaintext: 101110001 is

a) 110100011

b) 110101110

c) 010110111

d) 011111010

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7. If the key is 110100001 where,

If ki=0, then S_i (x)=((1 1 0 | 0 1 1 | 1 0 0 ))x+((1 1 1))

and If ki=1, then S_i (x)=((0 1 1 | 1 0 1 | 1 0 0))x+((0 1 1))

then the output of the SP network for the plaintext: 101110001 is

a) 010110011

b) 111000011

c) 110110111

d) 010110110

View Answer

8. Confusion hides the relationship between the ciphertext and the plaintext.

a) True

b) False

View Answer

Explanation: Confusion hides the relationship between the ciphertext and the key.

9. The S-Box is used to provide confusion, as it is dependent on the unknown key.

a) True

b) False

View Answer

Explanation: The S-Box is used to provide confusion, as it is dependent on the unknown key.

The P-Box is fixed, and there is no confusion due to it, but it provides diffusion.

a) SP Networks

b) Feistel Cipher

c) Hash Algorithm

d) Hill Cipher

View Answer

Explanation: The figure is the Feistel Cipher Structure.

11. Which of the following slows the cryptographic algorithm –

1) Increase in Number of rounds

2) Decrease in Block size

3) Decrease in Key Size

4) Increase in Sub key Generation

a) 1 and 3

b) 2 and 3

c) 3 and 4

d) 2 and 4

View Answer

Explanation: Increase in any of the above 4 leads to slowing of the cipher algorithm i.e. more computational time will be required.

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