Cryptography · Unit 1 · 7 hrs
Introduction and Classical Ciphers
Exam-focused notes for Introduction and Classical Ciphers (Cryptography, CSC327): what the TU syllabus asks and how it has actually been tested, with 11 solved past questions from this unit.
What this unit covers
- Security: Computer Security, Information Security, Network Security, CIA Triad, Cryptography, Cryptosystem, Cryptanalysis, Security Threats and Attacks, Security Services, Security Mechanisms
- Classical Cryptosystems
- Substitution Techniques: Ceasar, Monoalphabetic, Playfair, Hill, Polyalphabetic ciphers, One-time pad
- Transposition Techniques: Rail Fence Cipher
- Modern Ciphers: Block vs. Stream Ciphers, Symmetric vs. Asymmetric Ciphers
Substitution Techniques
Given the key "HELLOWORLD", encrypt the plaintext "TURINGTEST" using Play fair cipher. [5]
- Key: HELLOWORLD - Plaintext: TURINGTEST - Cipher: Playfair (5x5 matrix, I/J combined) Key letters, removing duplicates (keep first occurrence): H, E, L, O, W, R, D (HELLOWORLD → H, E, L, [L dup], O, W, [O dup], R, [L dup], D) Fill remaining alphabet (I/J ...
Full solved answer →Decrypt the ciphertext 'HI' using Hill Cipher where the key is. Ciphertext: HI, Key: $$\begin{bmatrix} 5 & 3 \ 4 & 3 \end{bmatrix}$$
- Ciphertext: HI - Key Matrix: $$K = \begin{bmatrix} 5 & 3 \\ 4 & 3 \end{bmatrix}$$ - Alphabet mapping: A=0, B=1, ..., Z=25, modulus 26. --- - H = 7 - I = 8 $$C = \begin{bmatrix} 7 \\ 8 \end{bmatrix}$$ --- $$P = K^{-1} C \pmod{26}$$ --- $$\det(K) = (5)(3) -...
Full solved answer →Show the encryption of plain text "ALGORITHM" using the key "PSEUDOCODE" using playfair cipher. [5]
- Plaintext: ALGORITHM - Key: PSEUDOCODE - Convention: I/J share one cell (standard Playfair) --- Key: P S E U D O C O D E Remove duplicates (keep first occurrence): P, S, E, U, D, O, C (The second O, D, E are dropped.) Fill remaining letters A, B, F, G, H,...
Full solved answer →Using Vigenere cipher with key = “worlds”, encrypt the plain text “hello everyone”. [5]
- Plaintext: hello everyone → letters only: helloeveryone (13 letters) - Key: worlds - Formula: $Ci = (Pi + Ki) \bmod 26$, with A=0, ..., Z=25 Pos 1 2 3 4 5 6 7 8 9 10 11 12 13 ------------------------------------------------ P h e l l o e v e r y o n e K w...
Full solved answer →Show encryption and decryption of "csit" using hill cipher having key, [5]
- Plaintext: "csit" - Cipher method: Hill cipher - Key matrix: NOT provided in the question (the LaTeX/image key is missing). Missing data note: The actual key matrix is not readable in the question. Without the key, the exact ciphertext cannot be uniquely ...
Full solved answer →The message “IMOGUN” was encrypted with a Playfair cipher using keyword “GALOIS”. Decrypt the message. [5]
- Ciphertext: IMOGUN - Keyword: GALOIS - Convention: I/J combined into a single cell. Keyword unique letters: G, A, L, O, I, S. Then fill remaining alphabet (I/J shared, skip already-used letters): Remaining after G,A,L,O,I,S: B, C, D, E, F, H, K, M, N, P, ...
Full solved answer →Decrypt the message 'GVPJ' using Hill cipher taking the key as {Row1 = 3 7, Row2= 5 12}. [5]
- Ciphertext: GVPJ - Key matrix: $K = \begin{pmatrix} 3 & 7 \\ 5 & 12 \end{pmatrix}$ - Letter encoding: A=0, B=1, ..., Z=25 - Decryption: $P = K^{-1} \cdot C \pmod{26}$ --- G V P J ------------ 6 21 15 9 Pairs (column vectors): $$C1 = \begin{pmatrix} 6 \\ 2...
Full solved answer →Differentiate between Symmetric and Asymmetric cipher. Encrypt the message 'HELL' using the key 'FAIL' using Vernam cipher. [5]
- Plaintext: HELL - Key: FAIL - Cipher: Vernam cipher - Letter mapping used: $A=0, B=1, \dots, Z=25$ --- Feature Symmetric Cipher Asymmetric Cipher --------- Keys used Single shared secret key for both encryption and decryption Two keys: a public key and a ...
Full solved answer →Transposition Techniques
How substitution ciphers are different from transposition ciphers? Given a message M="CSIT PROGRAM IS A HOT CAKE", encrypt M using Rail Fence cipher with rail size 3. [5]
Feature Substitution Cipher Transposition Cipher --------- Basic idea Each plaintext character is replaced by another character/symbol Character identity is retained but its position is changed Operation Maps letters to other letters via a key/rule Rearrang...
Full solved answer →Which one is more secure, monoalphabetic cipher or poly alphabetic cipher? Justify. Using rail fence cipher encrypt the text 'LEARNING AND TEACHING ARE DIFFERENT' using 3 as rails. [5]
- Plaintext: LEARNING AND TEACHING ARE DIFFERENT - Number of rails: 3 - Task 1: Compare security of monoalphabetic vs polyalphabetic cipher. - Task 2: Encrypt using rail fence with 3 rails. --- Polyalphabetic cipher is more secure than monoalphabetic cipher...
Full solved answer →Distinguish between stream cipher and block cipher. Encrypt the message WE ARE IN SAME RACE UNTILL OVER LIVE END using Rail fence cipher using 4 as a number of rails. [5]
- Message: WE ARE IN SAME RACE UNTILL OVER LIVE END - Number of rails: 4 - Task: distinguish stream vs block cipher; encrypt using Rail Fence cipher. Let me first extract and verify the letters (spaces removed): W E A R E I N S A M E R A C E U N T I L L O V...
Full solved answer →Make Unit 1 stick
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