Cryptography
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Cryptography
2081 Boards
Section A
Answer any two questions.
1
Let us consider the 4 bits key set as {1100, 1010, 0000, 1111, 0101, 1001} and input text as {1011, 1110, 1011, 1000}. Now trace the first full round operation of IDEA algorithm.
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2
What is Message Authentication Code? List the operation of computing digest value in different passes of MD4. Describe about Needhom-Schroeder protocol.
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3
Why do we need discrete logarithm? Illustrate with an example. Consider a Diffie-Hellman scheme with a common prime p = 13 between user A and user B. Suppose public key of A is 10 and public key of B is 8. Now determine their private keys and shared secret key. Select any valid primitive root of 13.
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Section B
Answer any eight questions.
4
Show the encryption of plain text "ALGORITHM" using the key "PSEUDOCODE" using playfair cipher.
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(5 *5 )Key Matrix Construction
Starting with key "PSEUDOCODE":
- Remove duplicate letters → unique sequence: P S E U D O C A
- Fill remaining alphabet (skip J, combine I/J): B F G H I K L M N Q R T V W X Y Z
The 5×5 matrix:
| P | S | E | U | D |
|---|---|---|---|---|
| O | C | A | B | F |
| G | H | I | K | L |
| M | N | Q | R | T |
| V | W | X | Y | Z |
Plaintext Preparation
ALGORITHM = 9 letters (odd count) → pad last letter with X Digraphs: AL · GO · RI · TH · MX
Encryption (Rule Applied to Each Pair)
| Pair | Rule | Positions | Ciphertext |
|---|---|---|---|
| AL | Rectangle | A(1,2)↔L(2,4) → swap cols | OE |
| GO | Rectangle | G(2,0)↔O(1,0)... same col! | MG (shift down) |
| RI | Rectangle | R(3,3)↔I(2,2) → swap cols | QK |
| TH | Rectangle | T(3,4)↔H(2,1) → swap cols | LN |
| MX | Rectangle | M(3,0)↔X(4,2) → swap cols | VQ |
Final Ciphertext: OE MG QK LN VQ
Three Playfair Rules (must memorize for exam):
- Same row → Replace each letter with the one to its right (wrap around)
- Same column → Replace each letter with the one below (wrap around)
- Rectangle → Replace each letter with the one in its own row but the other letter's column
PS : for law 1: yadi last ko letter xa bhane tei row ko pailo element le replace garne (i.e formally ek circular shift)
column lai, chai tei column ko pailo (mathillo) element le replace garne
5
Discuss the working mechanism of kerberos protocol.
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6
What is the use of firewall? How circuit level gateway differs from stateful inspection firewall?
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Firewalls are network security systems that monitor and control incoming and outgoing network traffic based on predetermined security rules. Their primary uses include:
- Preventing unauthorized access: Blocking malicious traffic and hackers.
- Protecting against malware: Filtering out known viruses and other malicious software.
- Controlling network access: Defining which services and applications are allowed to access the network.
- Logging and reporting: Recording network activity for auditing and analysis.
Circuit Level Gateway vs. Stateful Inspection Firewall:
| Feature | Circuit Level Gateway | Stateful Inspection Firewall |
|---|---|---|
| Inspection Level | Operates at the network layer (Layer 3) and transport layer (Layer 4). | Operates at the network, transport, and application layers (Layers 3, 4, and 7). |
| State Tracking | Does not track the state of network connections. Forwards packets based on source/destination IP addresses and ports. | Tracks the state of active connections (e.g., TCP handshakes). Allows return traffic for established connections. |
| Security | Less secure; vulnerable to attacks that exploit application-level vulnerabilities. | More secure; can detect and prevent more sophisticated attacks by analyzing the context of network traffic. |
| Performance | Generally faster due to less processing. | Can be slower due to state tracking and deeper inspection. |
| Complexity | Simpler to configure. | More complex to configure. |
7
What is intrusion ? Explain any two types of intrusion detection system.
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8
Find the multiplicative inverse of polynomial {95} using extended euclidean Algorithm.
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9
What is DoS attack ? Discuss about PKI trust model.
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10
Using Vignere cipher with key = “worlds”, encrypt the plain text “hello everyone”.
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11
Describe the different modes of block cipher.
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12
Write short notes on (any two)
Totient value of any positive integer.
Properties of hash function.
Virus or Worms.
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