Java Cypher Tool

A simple, interactive command-line tool for encrypting and decrypting messages using classic algorithms. Designed for encryption enthusiasts and Java students.

Features

• Encrypt & Decrypt: Choose between two modes.
• Algorithms:
  • ROT13: A classic shift cipher (shift by 13).
  • Atbash: Reverses the alphabet (A -> Z, B -> Y).
  • Caesar Cipher: A historical cipher with a fixed shift of 3.
• Robust Input: handles invalid inputs options and empty strings gracefully.

How to Run

1. Compile:

   javac Main.java CypherTool.java

2. Run:

   java Main

Usage Example

Welcome to the Cypher Tool!

Select operation:
1. Encrypt
2. Decrypt
$> 1

Select cypher:
1. ROT13
2. Atbash
3. Caesar (Shift 3)
$> 1

Enter the message:
$> Hello World!

Encrypted message (ROT13):
Uryyb Jbeyq!

Main Method Flow

The main() method controls the entire program:

1. Display welcome message
2. Loop forever:
   a. Ask user: Encrypt, Decrypt, or Exit?
   b. If Exit → quit program
   c. Ask which cipher to use
   d. Ask for message input
   e. Validate input
   f. Process message (encrypt or decrypt)
   g. Display result
   h. Go back to step 2

Key Concept: The loop allows users to process multiple messages without restarting the program.

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Two Cipher Algorithms Explained

1. ROT13 (Rotate by 13)

What it does: Shifts each letter 13 positions forward in the alphabet.

Visual Example:

Original:  A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
ROT13:     N O P Q R S T U V W X Y Z A B C D E F G H I J K L M

H e l l o  →  U r y y b
(shift 13)

Why ROT13 is special: Applying ROT13 twice gives you the original message back!

"Hello" → ROT13 → "Uryyb" → ROT13 → "Hello"

Code Logic:

(c - 'A' + 13) % 26

• c - 'A' = position in alphabet (0-25)
• + 13 = shift by 13
• % 26 = wrap around using modulo (when we go past Z, wrap to A)

Example: H (position 7) → (7 + 13) % 26 = 20 → U

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Project Structure

• Main.java: Entry point.
• CypherTool.java: Contains all logic and encryption algorithms.
• STUDY_GUIDE.md: Explains the math and logic behind the code.
• pseudocode.txt: High-level logic overview.
• presentation.md: preparation for project presentation to tutors.