Caesar Cipher

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What is Caesar Cipher?

• One of the simplest and oldest encryption techniques

• Type of substitution cipher where each letter is shifted by a fixed number

• Named after Julius Caesar who used it for military communications

• Also known as shift cipher or Caesar shift

Caesar Cipher Alphabet Example

• Shift = 3

Plain :  a  b  c  d  e
Cipher:  d  e  f  g  h

• Example:

HELLO → KHOOR

Mathematical Representation:

• Encryption:

E(x) = (x + k) mod 26

• Decryption:

D(x) = (x - k) mod 26

• Where x is letter position

(A=0, B=1, ..., Z=25)

Weaknesses:

• Only 25 possible keys (shifts 1-25)

• Extremely vulnerable to brute force attacks

• Frequency analysis can easily break it

• Not suitable for modern secure communications

Python Implementation

1. Encryption Function

def caesar_encrypt(text: str, shift: int) -> str:
    # Define a function that takes a string (text) and an integer (shift)
    # and returns an encrypted string

    result = ""
    # Initialize an empty string to store the encrypted result

    for char in text:
        # Loop through each character in the input text one by one

        if char.isupper():
            # Check if the character is an uppercase letter (A–Z)

            result += chr((ord(char) + shift - 65) % 26 + 65)
            # Convert character to ASCII using ord()
            # Subtract 65 to normalize 'A' to 0
            # Add the shift value
            # Use modulo 26 to wrap around the alphabet
            # Add 65 to return to ASCII range of uppercase letters
            # Convert back to character using chr() and append to result

        elif char.islower():
            # Check if the character is a lowercase letter (a–z)

            result += chr((ord(char) + shift - 97) % 26 + 97)
            # Convert character to ASCII
            # Subtract 97 to normalize 'a' to 0
            # Apply the shift
            # Wrap using modulo 26
            # Add 97 to return to lowercase ASCII range
            # Convert back to character and append

        else:
            # If the character is not a letter (space, number, punctuation)

            result += char
            # Append the character unchanged

    return result
    # Return the fully encrypted string

2. Decryption Function

def caesar_decrypt(text: str, shift: int) -> str:
    return caesar_encrypt(text, -shift)   # Works perfectly in Python

Practical Examples

Example 1: Basic Encryption & Decryption

text = "Hello, Caesar Cipher!"
shift = 3

encrypted = caesar_encrypt(text, shift)
print(encrypted)          # Output: Khoor, Fdhvdu Flskhu!

decrypted = caesar_decrypt(encrypted, shift)
print(decrypted)          # Output: Hello, Caesar Cipher!

Example 2: ROT13 (Very Common Variant)

# ROT13 = Caesar cipher with shift = 13
print(caesar_encrypt("Secret message", 13))
# Output: Frperg zrffntr

# Decrypting ROT13 is the same as encrypting again
print(caesar_encrypt("Frperg zrffntr", 13))
# Output: Secret message

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Keywords

Caesar Cipher, shift cipher, substitution cipher, ROT13, encryption, decryption, modular arithmetic, brute force, frequency analysis, cryptography, classical cipher, Julius Caesar, monoalphabetic, wrap around, key space, ord chr, Python cipher, cipher text, plain text, crypto basics, nerd cafe , نرد کافه

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