SUBSTITUTION TECHNIQUES…
SUBSTITUTION TECHNIQUES
Substitution Technique என்பது Classical Cryptography-ல் பயன்படுத்தப்படும் ஒரு encryption method. இதில்: Plaintext-ல் உள்ள letters / symbols-ஐ வேறு letters / symbols-ஆக replace செய்வது Substitution Technique. Simple: Original Letter ↓ Repla…
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Definition
Substitution Technique என்பது Classical Cryptography-ல் பயன்படுத்தப்படும் ஒரு encryption method. இதில்: Plain……
Explanation
Example
A → D B → E C → F Plaintext: ABC Ciphertext: DEF இங்கு lett……
Simple Explanation
Suppose plaintext: HELLO ஒரு substitution rule: H → K E → H L → O O → R அ……
Explanation
Core idea
Substitution technique-ன் core concept:……
Important
Symbol is changed, position i……
Comparison
Substitution vs transposition
இந்த difference மிகவும் முக்கி……
Explanation
Example
CAT ↓ FDW C → F A → D T → W Transposition Letter remain……
Explanation
Basic model
Let: P = Plaintext C = Ciphertext K = Key E = Encryption D = Decryption Encryption: C = E(K, P)……
Explanation
Types of substitution techniques
Important substitution techniques: Substitution Techniques | |----------------------------- | | | Caesar Mono……
Definition
Caesar Cipher என்பது simplest substitution cipher. ஒ……
Program
Example
Explanation
Alphabet: Plain : A B C D E F G H I J K L……
Real Life Example
Caesar example
Plaintext: HELLO Key: 3 Conversi……
Formula
Caesar formula
Letters are represented as numbers: A = 0 B = 1 C = 2 ... Z = 25 Encryp……
Dry Run
Caesar dry run
Explanation
Wrap around
Suppose: X X value: 23 Key: 3 Then: (23 + 3) m……
Important
Caesar important
Caesar cipher: Single alphabet Fixed shift Simple substitution E……
Definition
Monoalphabetic Substitution Cipher-ல் entire message-க்கும் ஒரு fixed substi……
Real Life Example
Example
Plain alphabet: ABCDEFGHIJKLMNOPQRSTUVWXYZ Cipher alphabet: QWERTYUIOPASD……
Important
Important property
If: A → Q once, then every oc……
Explanation
Example
BANANA If: A → Q then all A's bec……
Explanation
Monoalphabetic key space
26 letters-ஐ 26 different positions-ல் arrange செய்யலாம். Therefore key space: 26! This ……
Explanation
Frequency analysis
English language-ல் சில letters அதிகமாக வரும். Examples: E T A O I N Suppose cipherte……
Explanation
Monoalphabetic weakness
Because: Same plaintext letter ↓ Same ciphertext letter every time. S……
Comparison
Caesar vs monoalphabetic
Important
Caesar cipher itself is a spe……
Definition
Playfair Cipher என்பது single letters-ஐ encrypt செய்யாமல் pair of letters-ஐ enc……
Explanation
Playfair matrix
Playfair cipher பொதுவாக: 5 × 5 matrix பயன்படுத்துகிறது. English alphabet……
Real Life Example
Playfair key example
Keyword: MONARCHY Repeated le……
Explanation
Example
M O N A R C H Y B D E F G I K……
Explanation
Playfair plaintext preparation
Suppose: BALLOON Pairs உருவாக்க வேண்டும். Initially: BA LL OO N ஒரே pair-ல் repeated……
Explanation
Playfair rule 1: same row
If two letters same row-ல் இருந்தால்: ஒவ்வொரு letter-க்கும் right side letter எடுத்துக்கொ……
Explanation
Playfair rule 2: same column
If letters same column: கீழே இருக்கும் letter எடுத்த……
Explanation
Playfair rule 3: rectangle
Letters different row and different column-ல் இருந்தால் rectangle உருவாக்க வேண……
Explanation
Playfair advantage
Monoalphabetic cipher individual letter frequency reveal செய்யும். Playfair: ……
Definition
Hill Cipher என்பது Linear Algebra மற்றும் Matrix Mathematics பயன்படுத……
Explanation
Hill basic idea
Letters convert to numbers: A = 0 B = 1 C = 2 ... Z = 25 Th……
Formula
Hill encryption formula
C = KP mod 26 Where: C = Ciphe……
Real Life Example
Simple hill example
Suppose key matrix: K = [3 3 2 5] Plaintext: HI Values: H = 7 I = 8 Vector: P = [7 8] Multiply: C = KP mod 26……
Formula
P = K⁻¹ C mod 26…
Important
Key matrix must have modular inverse. Condition: gcd(det(K), 26) = 1 This……
Important
Hill important
Hill Cipher: Uses matrices Uses modular arithmetic E……
Definition
Polyalphabetic Cipher என்பது multiple substitution alphabets பயன்படுத்தும் ……
Explanation
Why polyalphabetic?
Monoalphabetic: A → D every time. Therefore frequency visible. Polyalpha……
Comparison
Mono vs poly
Monoalphabetic A → X A → X A → X Po……
Definition
Vigenère Cipher என்பது famous polyalphabetic substi……
Real Life Example
Vigenere example
Plaintext: ATTACKATDAWN Key: LEMON Key plaintext……
Explanation
Letter values
A = 0 B = 1 ... Z = 25 Encryption form……
Explanation
First letter calculation
Plaintext: A = 0 Key: L = 11 ……
Explanation
Second letter
Plaintext: T = 19 Key: E = 4 ……
Explanation
Vigenere result
For the standard example: Plaintext ……
Explanation
Why vigenere stronger than caesar
Caesar: One shift for full message Vigenère: Different shifts based on ……
Explanation
Vigenere weakness
If keyword short and repeatedly used: LEMONLEMONLEMON... pattern repeats. Attacker can analyze:……
Definition
One-Time Pad (OTP) என்பது theoretically perfect secrecy வழங்கக்கூடிய substitution-based encryption technique,……
Formula
One-time pad formula
Encryption: C = (P + K) mod 26 Decryption: P = (C - K) mod 26 Formula Vigenère போல இ……
Real Life Example
Otp example
Plaintext: HELLO Random key: XMCKL Key ex……
Explanation
Otp security
Correctly used one-time pad offers: Perfect secrecy This concept is associated with Claude Shanno……
Explanation
Otp biggest rule
Never reuse the same one-time……
Important
One Key = One Message Only Ke……
Explanation
Substitution technique classification
ஒரு simple conceptual classification: Substitution | |-----------------……
Comparison
Monoalphabetic vs polyalphabetic
Table
Substitution techniques summary table
Explanation
Cryptanalysis of substitution ciphers
Substitution cipher-ஐ attack செய்ய attacker பின்வற்றை study செய்ய……
Explanation
English letter frequency
English-ல் common letters: E T A O I N Common digraphs: TH HE IN ER A……
Important
Important concept: key space
Cipher security-க்கு key space முக்கியம். Caesar 25 useful keys Monoalphabetic 26! possible subst……
Explanation
Confusion
Substitution technique cryptography-ல் Confusion என்ற concept-க்கு தொடர்புடையத……
Explanation
Diffusion
Transposition techniques mainly diffusion concept-ஐ support செய்யும் என்று classical explanation-ல்……
Explanation
Product cipher
Modern block ciphers pure substitution மட்டும் பயன்படுத்துவதில்லை. அவை: Substitution + Permutation / Transpos……
Important
Important difference: substitution vs encoding
Substitution cipher என்பது se……
Explanation
Example
Text → Base64 Base64 encryption அல்ல. Substitution cipher……
Common Mistake
Error 1
"Substitution changes the position of le……
Common Mistake
Error 2
"Transposition changes letters.……
Common Mistake
Error 3
"Caesar cipher is polyalphabetic." W……
Common Mistake
Error 4
"Vigenère uses only one alpha……
Common Mistake
Error 5
"Playfair encrypts single lett……
Common Mistake
Error 6
"Hill cipher does not use mathematics." Wrong……
Remember
C M P H V O C = Caesar M = Monoalphabetic P = Playfair H = Hill V = Vigenère O = On……
TRB Point
TRB / NET / SET examinations-க்கு முக்கிய points: Substitution cipher replaces plaintext symbols with ciphert……
MCQ
Substitution cipher mainly:…
MCQ
Caesar cipher is a:…
MCQ
Caesar cipher encryption form……
MCQ
A monoalphabetic cipher uses:…
MCQ
Monoalphabetic cipher is vuln……
MCQ
Playfair cipher encrypts:…
MCQ
A pair of letters is called:…
MCQ
Playfair cipher commonly uses:…
MCQ
Hill cipher uses:…
MCQ
Hill cipher encryption formul……
MCQ
Vigenère cipher is:…
MCQ
Vigenère cipher primarily use……
MCQ
One-Time Pad key should be:…
MCQ
Which cipher provides perfect……
MCQ
Substitution primarily contri……
TRB Point
Exam short answer
Substitution Technique என்பது plaintext-ல் உள்ள characters-ஐ ஒரு key அல்லது substitution rule அடிப்படையில் வே……
Summary
SUBSTITUTION TECHNIQUES |…
| | | | | | Caesar Mono Playfair Hill Vigenère OTP | | ……
One-Time Pad = Random Key + Same Length + Use Once One-line memory: Substitution Technique = ……
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