Classical ciphers are encryption techniques used historically to conceal information by transforming plaintext into ciphertext using a set of well-defined rules. They are typically symmetric key ciphers, meaning the same key is used for both encryption and decryption. While no longer secure by modern standards, they are foundational to cryptography.

🔐 Types of Classical Ciphers (with Examples)

1. Caesar Cipher (Shift Cipher)

• Description: Each letter in the plaintext is shifted by a fixed number of positions down the alphabet.

• Key: The number of positions to shift.

Example:
Plaintext: HELLO
Shift: +3
Encryption:
H → K, E → H, L → O, L → O, O → R
Ciphertext: KHOOR

Decryption: Reverse the shift (–3): KHOOR → HELLO

2. Atbash Cipher

• Description: A substitution cipher where each letter is replaced by its mirror image in the alphabet. (A ↔ Z, B ↔ Y, ..., M ↔ N)

• Key: None (fixed transformation)

Example:
Plaintext: HELLO
Encryption:
H → S, E → V, L → O, L → O, O → L
Ciphertext: SVOOL

3. Affine Cipher

• Description: Uses mathematical functions to encrypt. Each letter is mapped using:
  E(x) = (ax + b) mod 26
  where x is the letter index (A = 0), a and b are keys, and a must be coprime with 26.

Example:
Key: a = 5, b = 8
Plaintext: HELLO
H (7) → (5×7 + 8) mod 26 = 43 mod 26 = 17 → R
E (4) → (5×4 + 8) = 28 mod 26 = 2 → C
L (11) → (5×11 + 8) = 63 mod 26 = 11 → L
O (14) → (5×14 + 8) = 78 mod 26 = 0 → A
Ciphertext: RCLLA

4. Vigenère Cipher

• Description: A polyalphabetic substitution cipher using a keyword to shift letters.

• Key: A repeating keyword

Example:
Plaintext: ATTACKATDAWN
Key: LEMON
Repeat key to match length: LEMONLEMONLE
Encrypt:
A+L → L, T+E → X, T+M → F, A+O → O, C+N → P, K+L → V, ...
Ciphertext: LXFOPVEFRNHR

5. Playfair Cipher

• Description: A digraph (pair-letter) substitution cipher using a 5x5 matrix of letters from a keyword.

• Key: A 5x5 letter matrix (J is often combined with I)

Example:
Key: MONARCHY
Matrix built from key:

M O N A R

C H Y B D

E F G I/J K

L P Q S T

U V W X Z

Plaintext: HELLO → HE LL OX (pairs padded if needed)
Encrypt using rules:

• Same row → next right

• Same column → next down

• Rectangle → swap columns

Encrypted: HE → DM, LL → SP, OX → ZN
Ciphertext: DMSPZN

6. Rail Fence Cipher

• Description: A transposition cipher where text is written in a zig-zag pattern across multiple "rails" and read line by line.

Example (2-rail):
Plaintext: HELLO WORLD
Write in zig-zag:

H . L . O . W . R . D

. E . L . . O . L .

Read rows: HLOWRD EL OL → Ciphertext: HLOWRDELLOL

7. Columnar Transposition Cipher

• Description: Write plaintext in rows under column headers (keyword letters), then read columns in key order.

Example:
Plaintext: HELLO WORLD
Key: ZEBRAS → Alphabetical order of key letters: E(2), B(1), A(0), etc.

Z E B R A S

H E L L O W

O R L D X X

Alphabetical order: A (4), B (2), E (1), R (3), S (5), Z (0)
Column order based on sorting: A B E R S Z

Indexes:

• A → index 4
• B → index 2
• E → index 1
• R → index 3
• S → index 5
• Z → index 0
• Column A (index 4): O, X → OX
•  Column B (index 2): L, L → LL 
•  Column E (index 1): E, R → ER 
•  Column R (index 3): L, D → LD 
•  Column S (index 5): W, X → WX
•  Column Z (index 0): H, O → HO 
• Ciphertext: Read columns as per sorted order of key. 

✅ Final Ciphertext:

OXLLERLDWXHO