Two Mark Questions
1.A Define Stegnography.
- Steganography is a technique for hiding a secret message within a larger one in such a way that others cannot discern the presence or contents of the hidden message.
- While cryptography renders a message unintelligible to outsiders using various text transformations, steganography focuses on concealing the very existence of the message itself.
1.B What are the two basic functions used in encryption algorithms?
- All encryption algorithms are fundamentally based on two basic functions (or building blocks) for transforming plaintext into ciphertext
- Substitution: This is a technique in which each element in the plaintext (such as a bit, letter, or group of bits or letters) is uniquely mapped into or replaced by another element.
- Transposition (or Permutation): This is a technique in which the elements in the plaintext are systematically rearranged or transposed in their positions. No elements are added, deleted, or replaced; only their order is changed.
1.C List the attacks on Block Ciphers.
- Symmetric block ciphers can be subjected to several types of attacks depending on the information available to the cryptanalyst, the mathematical properties of the algorithm, and the physical implementation of the cipher
- The primary attacks identified in your sources include:
- Brute-Force Attacks (Exhaustive Key Search): The attacker tries every possible key on a piece of ciphertext until an intelligible translation into plaintext is obtained.
- Cryptanalytic Attacks: Cryptanalysis relies on exploiting the mathematical or structural characteristics of the encryption algorithm, sometimes combined with statistical properties of the plaintext or sample plaintext-ciphertext pairs.
1.D Define group and ring.
1.E What is a Substitute byte transformation in AES?
1.F Write the difference between public key and private key.
- In asymmetric (public-key) cryptography, a user possesses a paired set of mathematically related keys: a public key and a private key.
- These keys perform complementary cryptographic operations,but they differ fundamentally in their accessibility, confidentiality, and specific roles in securing communications:
- Public Key: This key is made publicly available. It is placed in a public register, directory, or other accessible file, allowing any participant in the network to access it
- Private Key: This key must be kept strictly secret. It is generated locally and is known only to the individual owner who created it; it is never distributed over the network
1.G Define Hash Function.
1.H What are the challenges of creating a sustainable ecosystem?
Five Mark Questions
SECTION I:
2.A) Explain the Play Fair cipher algorithm? Encrypt the message 'MY BALOON' using the key 'MONACRY'.
The Playfair cipher is a symmetric encryption technique that encrypts pairs of letters (digraphs) instead of individual letters. It uses a 5 × 5 matrix containing the letters of the alphabet (I and J are treated as the same letter).
Encryption Steps:
1. Choose a key and remove duplicate letters.
2. Construct a 5 × 5 matrix:
a. Write the unique letters of the key first.
b. Fill the remaining cells with the unused letters of the alphabet.
c. Combine I and J into one cell.
3. Prepare the plaintext:
a. Remove spaces.
b. Split the message into pairs of letters.
c. If both letters in a pair are the same, insert X between them.
d. If one letter remains at the end, append X.
4. Encrypt each pair using these rules:
a. Same row: Replace each letter with the letter to its right (wrap around if necessary).
b. Same column: Replace each letter with the letter below it (wrap around if necessary).
c. Different row and column: Replace each letter with the letter in the same row but in the column of the other letter (rectangle rule).
Here is an Example:
Key: MONARCHY (assuming "MONACRY" is a typo; the standard Playfair key is "MONARCHY")
Step 1: Construct the Matrix
Step 2: Prepare the Plaintext
- Given Plaintext is: MY BALLOON
- After removing spaces, the plaintext becomes: MYBALLOON
- Form digraphs from the plaintext:
LX (insert X because of repeated LL)
Step 3: Encrypt Each Pair
1. MY
Ciphertext Generated: NC IB SU PM NA
Final Ciphertext is: NCIBSUPMNA
2.B) Briefly define the Monoalphabetic Cipher. What is the difference between monoalphabetic cipher and a polyalphabetic cipher?
- A monoalphabetic substitution cipher is an encryption approach in which each plaintext letter always maps to the same ciphertext letter throughout the message.
- The Caesar-shift cipher is one of the major examples of a monoalphabetic cipher, in which every letter shifts on the basis of a numeric key.
- In contrast, a polyalphabetic substitution cipher improves on the monoalphabetic technique by using different monoalphabetic substitutions successively as one proceeds through the plaintext message
The primary differences between the two category of ciphers are as follows:
- The Number of Alphabets and Rules Used: Monoalphabetic ciphers rely on a single mapping rule per message, whereas polyalphabetic ciphers utilize a set of related monoalphabetic substitution rules
- The Role of the Key: In a polyalphabetic cipher, the key determines which specific monoalphabetic rule is chosen for each given character transformation in the sequence
- Character Mapping (One-to-One vs. One-to-Many): In a monoalphabetic cipher, a plaintext letter is always replaced by the same ciphertext letter. In a polyalphabetic cipher (such as the Vigenère cipher), there are multiple possible ciphertext letters for each plaintext letter, depending on the active key character
- Resistance to Letter Frequency Analysis: Monoalphabetic ciphers are relatively easy to break because they fully reflect the letter frequency data of the original plaintext language. Polyalphabetic ciphers are more secure because utilizing multiple substitution alphabets obscures this frequency information, making simple frequency analysis much more difficult
3.A) Explain about Hill cipher. Consider the plaintext “paymoremoney” and use the encryption key K given below to find the cipher text:
- The Hill cipher, developed by the mathematician Lester Hill in 1929, is a prominent example of a multiletter (or polygraphic) substitution cipher
- Unlike traditional substitution ciphers that encrypt letters one by one, the Hill cipher processes blocks of letters simultaneously using concepts from linear algebra and matrix arithmetic modulo 26
How does the Hill Cipher Work?
- To encrypt a message using the Hill cipher, the plaintext is first divided into blocks of a fixed size, m. Each letter in the block is converted into a numerical value from 0 to 25 (a=0,b=1,…,z=25.
- The encryption process takes a block of m plaintext letters (represented as a row vector P) and multiplies it by an m×m encryption key matrix K:
where C is the resulting ciphertext row vector
Decryption Process:Decryption reverses the process by multiplying the ciphertext vector by the multiplicative inverse of the key matrix
Encryption using 3x3 Key Matrix:
4. Convert numbers back to letters:
(17,17,11) translates to "RRL"
By repeating this calculation block-by-block for "mor", "emo", and "ney", the final ciphertext is RRLMWBKASPDH
3.B) Discuss any four substitution techniques of encryption and list their merits and demerits.
SECTION - II:
4.A: Compare and contrast stream ciphers and block ciphers.
4.B: Explain how GCD is calculated with Euclid’s algorithm and calculate the GCD of (270, 192).
(OR)
5.A: Explain the DES algorithm with a neat sketch.
SECTION - III:
6.A: Explain the four tasks performed in each round of the AES Cipher.
6.B: Given p=19, q=23, and e=3, use the RSA algorithm to find n, (n), and d.
(OR)
7.A: Explain how to use the Chinese Remainder Theorem to reconstruct integers from their residues modulo functions.
7.B: Briefly explain the Diffie Hellman key exchange with an example.
SECTION - IV:
8.A: Explain the Secure Hash Algorithm with a neat sketch.
(OR)
9.A: Explain the requirements of hash functions.
9.B: Explain the applications of Cryptographic Hash Functions.
SECTION - V:
10.A: Explain how decentralization in blockchain provides advantages over traditional centralized systems.
10.B: Describe the Bitcoin protocol in detail.
(OR)
11.A: Explain what smart contracts are and how they operate on a blockchain network.
11.B: Compare and contrast Proof-of-Work (PoW), Proof-of-Stake (PoS), and Delegated Proof-of-Stake (DPoS)
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