Name: IDNo:
BPDC, Second Semester - 2019-2020 Ques : 16
/15 Pages : 2
Course No: SS G513 Comprehensive Exam Course Title: Network Security
Date: 02nd June 2020 Weightage: 15%
Total Duration: 3 Hrs
(PART – A) Max. Marks. 15
PART A (MAX) : 45 Min M.E.(SS)
NOTE: ANSWER ALL QUESTIONS IN THE SPACE PROVIDED
(Pl. take care to answer theory questions in full brief and crisp sentences)
(You will have to submit Part A, only after which Part B will be given)
Full credit will be given only if all the intermediate formulas/steps/calculations/values as appropriate
are shown.
1. A webserver is hosted for student fee payment. A student, who got a heavy fine, tries to replace the
corresponding database entry. Which Security Goal is being compromised? Why? [1M]
2. Polyalphabetic cipher makes it better suited than monoalphabetic cipher. Give reasons. [0.5M]
3. Given the S-Box structure, if the 8-bit input to the S-Boxes is (BD)16, what the 4 bit output. [1M]
0 1 2 3 0 1 2 3
0 3 2 0 1 0 2 3 0 1
1 2 1 3 0 1 2 1 3 0
2 1 2 1 3 2 1 2 1 3
3 0 3 3 2 3 0 3 3 2
S-box 1 S-box 2
Input: (BD)16
Output:
4. Electronic Code Book (ECB) takes a plaintext of length l bits and divides into fixed length block of b bits
and uses the same key k to encrypt each block. Why is it usually used only for transmission of single values?
(ie, plaintext having length l <= b bits) [1M]
5. Given the random no generator Xn+1 = aXn (mod 26), where a = 7 and seed = 1, compute the the first 4
random numbers generated. [1M]
6. Using fermat’s theorem find ax(mod 23) where a = 13, x = 394 [1M]
7. What property of the one way function makes it suitable for public key cryptography? What is the one-way
function in RSA based encryption/decryption. [1M]
8. In Elliptic Curve Cryptography (ECC), E27(2,3) represent the equation: [1M]
9. In SHA-512 based hash functions, it’s required to pad the original message with a variable length bit pattern.
Why is this required? A message of unknown length which when padded with a pattern of length of 212 bits
resulted in a total length of the padded message to be 7168 bits. Find out the length of the original message.
Block size used for SHA-512 is 1024. Format of the input message is
original message (max len =2128 bits) Padding (1000...0) Len of original msg (128 bits)
[1M]
10. Can digital signature alone provide non-repudiation. Justify. [1M]
11. Briefly describe a security attack possible in Deffie-Hellman key exchange algorithm. [1M]
P.T.O
1
12. If "n" entities wants to use symmetric key based encryption mechanism, how many keys are required. How
does Key Distribution Center(KDC) center help in reducing the number of keys required for communication
between the entities. [1M]
13. One of the approaches used to implement one-time password is to store the list of passwords in a file.
Mention any one disadvantage of this scheme. [0.5M]
14. In password based authentication mechanism, a random string called the salt is stored along with the
password. How does this help in increased security. [1M]
15. In the tunnel mode of IP Sec protocol, the entire IP packet can be protected. [1M]
16. Briefly describe the role of Security Policy Database (SPD) in IP Sec protocol. [1M]
*****
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BPDC, Dubai Campus - Second Semester, 2019-2020 Ques: 5
Pages: 1
Course No: SS G513 Comprehensive Exam Course Title: Network Security
Date: 2nd June 2020 Weightage: 25%
Duration: 135 Min
(Part –B) Max. Marks. 25
M.E.(SS)
Note: Answer ALL Questions. (Use Text Book Conventions for all problems)
Full credit will be given only if all the intermediate formulas/steps/calculations/values as appropriate are shown.
1. a) Describe the decryption and encryption process in the affine cipher. What is it advantage over
additive and multiplicative cipher. [2M]
b) It is known that Affine cipher was used to encrypt the plain text using the keys K1= 7 (in Z26*) and K2=12
(in Z26), to get the encrypted text C=MFUIDB. Find out the plain text. (Show the steps clearly)
[3M]
2. Simplified stream cipher RC4 is implemented with a state vector S of 8 values, (i.e. the state vector contains
permutations of all 3-bit numbers (0 to 7). The plaintext P vector values are also from 0 to 7 (ie. 3 bits). The
key vector K has a key length of 4 (i.e. it has 4 entries with each entry taking values 0 to 7(ie. 3 bits). ). Given
that K = [4, 7, 0, 3] and P =[6, 4, 0, 1], obtain initial permutation of S and the cipher stream. [5M]
3. a) Consider a digital signature system, when the sender chooses the primes numbers are p =13, q = 37 and
public key e =13. The sender wants to send a message M = to the receiver. Show the sequence of calculations
at the sender side to digitally sign the message and those at the receiver side to verify the same.
[3M]
b) When using the digital signature, usually the encryption is performed on the message digest rather than
the message itself. Justify. [2M]
4. a) Refer to the figure below, used for creating a symmetric session key (KS_AB) between 'A' and c. KA and KB
are the pre-shared secret keys between A,KDC and B,KDC respectively.
Describe how the protocol works clearly stating the reasons. What would be the result of not including the
KS_AB in the box encrypted by KA transmitted from KDC to A, in step 2. Note: A box with the name of key
means that it is encrypted with the key mentioned on the top of the box. [3M]
1
b) In Diffie-Hellman Key agreement protocol between A and B, the publically agreed values of the
prime number p = 23 and generator element g =14. A chooses x =7, and B chooses y =10. Find
the values R1, R2 transferred from A to B, & B to A respectively and shared symmetric key K.
All intermediate values and final key calculations in arriving at the shared symmetric key are to
be shown. [2M]
5. a) A computer system uses hashed password based authentication mechanism. The hashes are computed
using the sum of the ASCII values of the characters in the password mod 31. There are 4 users A,B,C and
D, whose actual passwords happen to be ATOS, uyrt, 1838, 81N3 respectively.
[3M]
i) Describe how the password based authentication works.
ii) Show the contents of the password file (only username and hashed password needs to be shown).
ii) When the user D login to the system, the steps involved in authentication (with appropriate values.)
b) In IPSec Protocol, Security Association Database (SAD) is used to define the communication parameters
as shown below. [2M]
Node A(NA) Node B (NB)
Outbound SA Inbound SA
To Protocol Authentication Encryption From Protocol Authentication Encryption
NB ESP None AES(K1) NA ESP None AES(K1)
Inbound SA Outbound SA
From Protocol Authentication Encryption To Protocol Authentication Encryption
NB AH SHA(K2) NA AH SHA(K2)
i) Describe how the SA tables are used by IP Sec protocols.
ii) When a packet is transmitted from NA to NB, is it possible to ensure confidentiality? Justify
iii) When a packet is transmitted from NB to NA, how is possible to ensure that the message indeed
originated from NB.
♣♣♣♣♣
RC4 Algorithm.
Initially S[i]=i for i = 0,7 To obtain the cipher text
T[] is initalised with key K repeated
i, j = 0;
To obtain the initial Permutation on S while (true) {
j = 0; i = (i + 1) mod 8;
j = (j + S[i]) mod 8;
for i = 0 to 7 do
Swap (S[i], S[j]);
j = (j + S[i] + T[i]) mod 8 t = (S[i] + S[j]) mod 8;
Swap(S[i],S[j]); k = S[t];
end }
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BPDC, Dubai Campus - Second Semester, 2019-2020 Ques: 4
Pages: 1
Course No: SS G513 Test 2(OB) Course Title: Network Security
Date: 14th Apr 2020 M.E.(SS) Weightage: 20%
Duration: 50 Min Max. Marks. 20
Note: Answer ALL Questions. (Use Text Book Conventions for all problems)
Full credit will be given only if all the intermediate formulas/steps/calculations/values as appropriate are shown.
1. Shown below is the Cipher Feedback Mode (CFB) based encryption process. The output Oi of the Encryption
box (Encrypt) is a simple function: Oi = St_P_Box(Ii) ⨁ K. Show the output at the appropriate stages of
encryption process. Assume that Initialization Vector (IV=(BE)16), b = 8, s = 4, K = (DD)16 and plain text to be
encrypted is (P =(FB )16). (Given values are in base-16, Show the intermediate values clearly & the final o/p in base-16)
[5M]
Encrypt Oi = St_P_Box(Ii) ⨁ K (i = I, 2)
Outputs to be shown
I1 =
O1 =
I2 =
O2 =
C1 =
C2 =
2 6 3 1 4 8 5 7
Straight P-Box
2. Given the linear congruential generator for random number generation using the formula 𝑋𝑛=1 = 𝑎𝑋𝑛 (𝑚𝑜𝑑 𝑚),
where m = 13 and X0 = 1, justify whether choosing a value of a = (2) or a = (3) is better, why? [5M]
3. i) Given p = 101 and a =5, using Fermat’s little theorem find a506 ( mod p)
ii) Using Euler’s method find inverse of a (i.e a-1 mod n). Given a = 113 , n = 100 [2M]
For calculation of exponentiation in 3 ii ) use the Fast exponentiation method x^y mod n) [3M]
Note: Show all the formulas used including the calculations and /intermediate steps values as appropriate.
4. Consider an RSA cryptosystem where p = 11, q = 53 and d = 331. For the plain text PT = 371 find the
corresponding encrypted text CT.
Show all the formulas used including the calculations and intermediate steps values as appropriate. (For
exponentiation, you could use direct calculation to get the result. Use of Fast exponentiation method steps
are not necessary). [5M]
♣♣♣♣♣
1
BPDC, Dubai Campus - Second Semester, 2019-2020 Questions: 4
Pages: 1
Course No: SS G513 Test 1(CB) Course Title: Network Security
Date: 25th Feb 2020 ME (SS) Weightage: 20%
Duration: 50 Min Max. Marks. 20
Note: Answer ALL Questions. (Show intermittent steps clearly)
1. a) Distinguish between passive and active attacks in the context of computer network security. Give an
appropriate example for each from a real life scenario. [3M]
b) Ransomware virus, typically encrypts the disk data and blocks access to the data, until the victim pays
some ransom amount to the attacker. As a network administrator what would be your strategy to
safeguard the data, expecting such an attack could happen. Assume that the data is so critical that the
success rate must by near 100%. [2M]
2. a) Show how the Vigenere Cipher text VIXMDBV can be decrypted using the key UAE. [3M]
b) It is required to securely transmit numbers consisting of digits (0-9) and a decimal point. If multiplicative
cipher is employed what would the ideal modulus value. Can key 5 be used for encryption? What would
the key for decryption be? Give the necessary justification and calculations as necessary. [2M]
3. a) One-Time-Pad cryptosystem archives prefect secrecy. Justify? [2M]
b) Distinguish between monoalpabetic cipher and polyalphbetic ciphers, with proper reasons. Give a
representative encryption technique for each of these. [3M]
4. a)For the General structure of S-DES shown below, if the i/p to Round 2 is (AE)16, what is the output of
Round 2 (Assume K2 = (49)16) . (show steps clearly). [3M]
10 bit
cipher
key
0 1 2 3 0 1 2 3
0 3 2 0 1 0 2 3 0 1
1 2 1 3 0 1 2 1 3 0
2 1 2 1 3 2 1 2 1 3
3 0 3 3 2 3 0 3 3 2
S-box 1 S-box 2
Expansion P-Box Straight P-Box
3 4 2 1
2 4 1 3 3 1 4 2
b) Briefly explain what is meant by Diffusion and Confusion, along with its significance in a cryptosystem.
[OR]
b) Briefly explain the need for 2DES and its disadvantage. [2M]
♣♣♣♣♣
1
BITS PILANI, DUBAI CAMPUS Q=1
/5 SECOND SEMESTER 2019 – 2020
M.E(S.S)
Pg = 1
ASSIGNMENT
Course Code: SS G513 Date: 18.04.2020
Course Title: Network Security Max Marks: 5
To be submitted on 20.04.2020, by12:00 PM (UAE TIME) Weightage: 5%
Name: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ID No: . . . . . . . . . . . . . . . . . . . . . . . .Sec / Prog: . . . . . . . . ..
1. Simplified stream cipher RC4 uses a state vector S of 8 values, containing the permutation
of all 3-bit numbers (0 to 7). Since S vector has 3-bit values, the plaintext P vector values
are also from 0 to 7 (ie. 3 bits). The key vector K has a key length of 4 which means it has 4
entries with each entry taking values 0 to 7(ie. 3 bits).
Note: Initially S[i] = i (i= 0, 7), and T[i] = K[i%key length] (i = 0,7).
Given that K = [0, 1, 3, 6] and P = [0, 5, 4, 3], obtain A) initial permutation of S, and
B) stream cipher C = []
i j = 0 S = [0, 1, 2, 3, 4, 5, 6, 7]
0
A) Initial Permutation of S = [1M]
i j S k P[i] C[i]
0 0
B) Stream Cipher C = [4M]
