This set of Basic Cryptography questions and answers focuses on “Overview”.

1. In AES, to make the s-box, we apply the transformation –

b’i = bi XOR b(i+4) XOR b(i+5) XOR b(i+6) XOR b(i+7) XOR ci

What is ci in this transformation?

a) ci is the ith bit of byte c with value 0x63

b) ci is the ith bit of byte c with value 0x25

c) ci is the ith bit of byte c with value 0x8F

d) ci is the ith bit of byte c with value 0x8A

View Answer

Explanation: ci is the ith bit of byte c with value 0x63 i.e,

c = 01100011

2. The inverse s-box permutation follows,

b’i = b(i+2) XOR b(i+5) XOR b(i+7) XOR di

Here di is –

a) di is the ith bit of a byte ‘d’ whose hex value is 0x15

b) di is the ith bit of a byte ‘d’ whose hex value is 0x05

c) di is the ith bit of a byte ‘d’ whose hex value is 0x25

d) di is the ith bit of a byte ‘d’ whose hex value is 0x51

View Answer

Explanation: The value of ‘d’ is 0x05.

3. What is the block size in the Simplified AES algorithm?

a) 8 bits

b) 40 bits

c) 16 bits

d) 36 bits

View Answer

Explanation: The block size for the AES algorithm is 16 bits.

4. Which function can be used in AES multiplication –

a) m(x)=x^{7}+x^{4}+x^{3}

b) m(x)=x^{8}+x^{4}+x^{3}+x+1

c) m(x)=x^{8}+x^{3}+x^{2}+x+1

d) m(x)=x^{8}+x^{5}+x^{3}+x

View Answer

Explanation: m(x)=x

^{8}+x

^{4}+x

^{3}+x+1 stands for 100011011 which is an irreducible polynomial. Others are not irreducible polynomials.

5. In the DES algorithm the Round Input is 32 bits, which is expanded to 48 bits via ______________

a) Scaling of the existing bits

b) Duplication of the existing bits

c) Addition of zeros

d) Addition of ones

View Answer

Explanation: The round key is 48 bits. The input is 32 bits. This input is first expanded to 48 bits (permutation plus an expansion), that involves duplication of 16 of the bits.

6. Using Differential Crypt-analysis, the minimum computations required to decipher the DES algorithm is-

a) 2^{56}

b) 2^{43}

c) 2^{55}

d) 2^{47}

View Answer

Explanation: Differential Crypt-analysis requires only 2

^{47}computations to decipher the DES algorithm.

7. Using SDES, the Plaintext for the Ciphertext 00001111, given that the key is 1111111111 is,

a) 01100111

b) 00001010

c) 11111111

d) 01101101

View Answer

Explanation: Perform the SDES Decryption algorithm and compute the cipher text.

8. 12. For p = 11 and q = 17 and choose e=7. Apply RSA algorithm where Cipher message=11 and thus find the plain text.

a) 88

b) 122

c) 143

d) 111

View Answer

Explanation: n = pq = 11 × 19 = 187.

C=M^e mod n ; C=11^23 mod 187 ; C = 88 mod 187.

9. Reduce the following Big-O notation. O[ ax^{7} + 3 x^{3} + sin(x)] =

a) O[ax^{7}].

b) O[sin(x)].

c) O[x^{7}].

d) O[x^{7} + x^{3}].

View Answer

Explanation: O[ ax

^{7}+ 3 x

^{3}+ sin(x)] = O(ax

^{7}) = O(x

^{7}).

10. Reduce the following Big-O notation. O[ e^{n} + an^{10}] =

a) O[ an^{10}].

b) O[ n^{10} ].

c) O[ e^{n} ].

d) O[ e^{n}+ n^{10} ].

View Answer

Explanation: O[ e

^{n}+ an

^{10}] = O[ e

^{n}].

**Sanfoundry Global Education & Learning Series – Cryptography and Network Security.**

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