# Lab 05 — Xdysgjkd

Objective

Encrypt and decrypt player chat messages in JavaCraft to ensure secure communication.

Introduction

In this lab, you will add cryptography to the JavaCraft chat system. You will implement a cipher in Java, integrate it into the chat logic, and test it by sending encrypted messages between players.

All chat messages in JavaCraft must follow the format:


@[name]: [text]

Your task is to ensure that this format is preserved while also securing messages so they cannot be easily intercepted or read by unintended recipients.

Instructions

1. Log into the Raspberry Pi

Connect your computer to the same Wi-Fi network as the Raspberry Pi.
Open a terminal and log in with SSH:

   ssh piNNNN@piNNNN

Replace NNNN with your group’s assigned number. 3. Enter the password you set in Week 1.

2. Develop a Cipher in Java

Choose a cipher algorithm (see Appendix for options).
Modify your JavaCraft code so that it can:
- Encrypt a given message
- Decrypt an encrypted message
Test your cipher with simple strings before integrating it into JavaCraft.

3. Integrate Cipher into JavaCraft Chat

Open the JavaCraft source file
Modify the code so that:
- All outgoing messages are encrypted before being sent.
- All incoming messages are decrypted before being displayed.
- Messages that fail to decrypt are shown as-is with a warning.
Ensure the @[name]: [text] format is validated after decryption using your DFA/NFA.

4. Sending Encrypted Messages

When sending an encrypted message, use the following format:

cipherNumber,key(optional),encryptedMessage

Examples:

Caesar cipher (shift = 3):

  1,3,KHOOR

Vigenère cipher (key = KEY):

  5,KEY,RIJVS

Atbash cipher (no key):

  2,SVOOL

5. Test Your Encrypted Communication

Recompile and run the modified chat system:
Connect at least two players on the same network.
Exchange at least ten valid encrypted messages.
- Each message must decrypt correctly back into the format @[name]: [text].
Send at least two invalid messages (wrong format or bad decryption) and confirm they are rejected.

6. Validate Security

Confirm that intercepted messages appear as encrypted text.
Verify that only players with the correct cipher and key can decrypt them.
Ensure your DFA/NFA validator checks the message format after decryption.

7. Shut Down the Raspberry Pi

When finished, shut down safely:

sudo shutdown -h now

Wait until the device powers down before unplugging it.

Lab Submission

Show the TA your working chat with integrated encryption.
Submit:
- Modified JavaCraft code
- A log of at least 10 valid encrypted messages and 2 invalid messages

Summary

By completing this lab, you have:

Implemented a cipher in Java
Integrated encryption and decryption into the JavaCraft chat
Preserved the @[name]: [text] format through DFA/NFA validation
Tested secure communication between multiple players

Appendix: Cipher Help

Below is a list of ciphers categorized by difficulty, along with examples and references for further reading.

Easy Ciphers

Caesar Cipher (Cipher #1)
- Difficulty: Easy
- Explanation: Shifts each letter in the plaintext by a fixed number of positions down the alphabet.
- Example:
  - Plaintext: HELLO
  - Shift: 3
  - Ciphertext: KHOOR
- Reference: Caesar Cipher
Atbash Cipher (Cipher #2)
- Difficulty: Easy
- Explanation: Substitutes each letter with its opposite in the alphabet (e.g., A ↔ Z).
- Example:
  - Plaintext: HELLO
  - Ciphertext: SVOOL
- Reference: Atbash Cipher
ROT13 Cipher (Cipher #3)
- Difficulty: Easy
- Explanation: A Caesar cipher with a fixed shift of 13; applying it twice returns the original text.
- Example:
  - Plaintext: HELLO
  - Ciphertext: URYYB
  - Double ROT13: URYYB → HELLO
- Reference: ROT13 Cipher - CyberChef
Rail Fence Cipher (Cipher #4)
- Difficulty: Easy
- Explanation: Arranges text in a zigzag pattern on multiple "rails" and reads it line by line.
- Example:
  - Plaintext: HELLO WORLD
  - Rails: 3
  - Ciphertext: HOLELWRDLO
- Reference: Rail Fence Cipher - Crypto Corner

Medium Ciphers

Vigenère Cipher (Cipher #5)
- Difficulty: Medium
- Explanation: Uses a keyword to determine the shift for each letter.
- Example:
  - Plaintext: HELLO
  - Keyword: KEY
  - Ciphertext: RIJVS
- Reference: Vigenère Cipher
Playfair Cipher (Cipher #6)
- Difficulty: Medium
- Explanation: Encrypts pairs of letters using a 5x5 grid derived from a keyword.
- Example:
  - Plaintext: HELLO
  - Keyword: MONARCHY
  - Ciphertext: CFSUPM
- Reference: Playfair Cipher
Columnar Transposition Cipher (Cipher #7)
- Difficulty: Medium
- Explanation: Writes the plaintext in rows and reads it off column by column based on a keyword.
- Example:
  - Plaintext: HELLOWORLD
  - Keyword: KEY
  - Ciphertext: EORHLODLWL
- Reference: Columnar Transposition
Autokey Cipher (Cipher #8)
- Difficulty: Medium
- Explanation: A variant of the Vigenère cipher where the key is the keyword followed by the plaintext itself.
- Example:
  - Plaintext: HELLO
  - Keyword: KEY
  - Ciphertext: RIJSS
- Reference: Autokey Cipher

Difficult Ciphers

Beaufort Cipher (Cipher #9)
- Difficulty: Difficult
- Explanation: Similar to the Vigenère cipher but uses a different encryption algorithm involving subtraction.
- Example:
  - Plaintext: HELLO
  - Keyword: KEY
  - Ciphertext: DANZQ
- Reference: Beaufort Cipher
Bifid Cipher (Cipher #10)
- Difficulty: Difficult
- Explanation: Combines substitution and transposition using coordinates in a 5x5 grid.
- Example:
  - Plaintext: HELLO
  - Keyword: KEYWORD
  - Ciphertext: fhycz
- Reference: Bifid Cipher
Affine Cipher (Cipher #11)
- Difficulty: Difficult
- Explanation: Applies a mathematical function to each letter's numeric equivalent.
- Example:
  - Plaintext: HELLO
  - Function: 5x + 8 mod 26
  - Ciphertext: RCLLA
- Reference: Affine Cipher

Detailed Explanations

Caesar Cipher (Cipher #1)

Difficulty: Easy
Explanation: Each letter in the plaintext is shifted by a fixed number of positions down the alphabet.
Example:
- Plaintext: HELLO
- Shift: 3
- Encryption Steps:
  1. Convert each letter to its numerical equivalent (A=0, B=1, ..., Z=25).
  2. Apply the shift: (Letter Index + Shift) mod 26.
  3. Convert the result back to a letter.
- Calculation:
  - H (7) → (7 + 3) mod 26 = 10 → K
  - E (4) → (4 + 3) mod 26 = 7 → H
  - L (11) → (11 + 3) mod 26 = 14 → O
  - L (11) → (11 + 3) mod 26 = 14 → O
  - O (14) → (14 + 3) mod 26 = 17 → R
- Ciphertext: KHOOR
Reference: Caesar Cipher - Khan Academy

Atbash Cipher (Cipher #2)

Difficulty: Easy
Explanation: Each letter is replaced by its counterpart from the opposite end of the alphabet.
Corrected Example:
- Plaintext: HELLO
- Encryption Steps:
  1. Create a mapping where A ↔ Z, B ↔ Y, ..., M ↔ N.
  2. Substitute each letter in the plaintext with its counterpart.
- Calculation:
  - H → S
  - E → V
  - L → O
  - L → O
  - O → L
- Ciphertext: SVOOL

Reference: Atbash Cipher

ROT13 Cipher (Cipher #3)

Difficulty: Easy
Explanation: A special case of the Caesar cipher with a shift of 13.
Corrected Example:
- Plaintext: HELLO
- Encryption Steps:
  1. Shift each letter by 13 positions.
  2. Apply modulo 26 to wrap around the alphabet.
- Calculation:
  - H (7) → (7 + 13) mod 26 = 20 → U
  - E (4) → (4 + 13) mod 26 = 17 → R
  - L (11) → (11 + 13) mod 26 = 24 → Y
  - L (11) → (11 + 13) mod 26 = 24 → Y
  - O (14) → (14 + 13) mod 26 = 1 → B
- Ciphertext: URYYB
Reference: ROT13 Cipher

Rail Fence Cipher (Cipher #4)

Difficulty: Easy
Explanation: Arranges the plaintext in a zigzag pattern across multiple rails and reads it off row by row.
Corrected Example:
- Plaintext: HELLOWORLD
- Rails: 3
- Encryption Steps:
  1. Write the message in a zigzag pattern across 3 lines.
  2. Read off each line to get the ciphertext.
- Pattern:
```
H    O      L
 E  L W R D
   L   O
```
- Ciphertext: HOLELWRDLO
Reference: Rail Fence Cipher - GeeksforGeeks

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