Caesar Cipher

Nerd Cafe

The Caesar Cipher is one of the simplest and most widely known encryption techniques. It shifts each letter in the plaintext by a fixed number of places down the alphabet. Let’s go step by step:

Step 1: Understand the Concept

• Encryption: Each letter in the plaintext is shifted by a fixed number (key). For example, with a shift of 3, A becomes D, B becomes E, and so on.

• Decryption: Reverse the process by shifting letters back by the same key.

Step 2: Choose the Key

The key is the number of positions each letter is shifted. For instance, a key of 3 means each letter in the plaintext will move three steps forward.

Step 3: Define the Alphabet

The standard alphabet is used:

ABCDEFGHIJKLMNOPQRSTUVWXYZ

If you encounter lowercase letters, treat them similarly:

abcdefghijklmnopqrstuvwxyz

Step 4: Encryption Formula

Use the formula:

$$C_{i}=(P_{i}+K)\;\;mod\;\;26$$

Where:

• 𝐶_𝑖: Ciphertext letter (encrypted letter)

• 𝑃_𝑖: Plaintext letter (original letter)

• 𝐾: Key (shift value)

• 26: Total letters in the English alphabet

Step 5: Decryption Formula

Reverse the encryption formula:

$$P_{i}=(C_{i}-K)\;\;mod\;\;26$$

Step 6: Practical Example

• Plaintext: "HELLO"

• Key: 3

Step 6.1: Encryption

Take each letter and shift it by 3:

• H → K

• E → H

• L → O

• L → O

• O → R

Ciphertext: "KHOOR"

Step 6.2: Decryption

Take each letter in the ciphertext and shift it back by 3:

• K → H

• H → E

• O → L

• O → L

• R → O

Decrypted Text: "HELLO"

Step 7: Implementation in Python

def caesar_cipher_encrypt(plaintext, key):
    result = ""
    for char in plaintext:
        if char.isalpha():
            # Handle uppercase and lowercase separately
            start = ord('A') if char.isupper() else ord('a')
            result += chr((ord(char) - start + key) % 26 + start)
        else:
            result += char  # Non-alphabetic characters remain unchanged
    return result

def caesar_cipher_decrypt(ciphertext, key):
    return caesar_cipher_encrypt(ciphertext, -key)  # Reverse the process

# Example usage
plaintext = "HELLO"
key = 3

encrypted_text = caesar_cipher_encrypt(plaintext, key)
print("Encrypted:", encrypted_text)

decrypted_text = caesar_cipher_decrypt(encrypted_text, key)
print("Decrypted:", decrypted_text)

Output

Encrypted: KHOOR
Decrypted: HELLO

Notes

• Non-alphabetic characters are typically not encrypted but can be handled based on specific requirements.

• The Caesar Cipher is not secure for modern cryptography but is excellent for understanding the basics of encryption.