Encrypt/Decrypt Program

📑 Table Of Content

1. Overview
2. Code Structure
3. Usage
4. Example
5. Important Notes
6. Build
7. Limitations

📜 Overview

This C program is a unique implementation of the Vigenère cipher, a method for encrypting alphabetic text using a series of different Caesar ciphers, where the shift is determined by the letters of a keyword. Unlike a standard Vigenère cipher, this program introduces a twist: the direction of the shift alternates for each character. This enhancement adds an extra layer of security, making it more resistant to standard cryptographic attacks.

How It Works

In a Caesar cipher, each letter in the plaintext is shifted a set number of positions down the alphabet. For example, a shift of 1 would convert A to B, B to C, and so on. The Vigenère cipher extends this idea by using a keyword to determine the shifts. Each letter of the keyword corresponds to a numerical value (A=1, B=2, etc.), and this value is used for shifting the characters of the plaintext.

Unique Twist

In this program:

• The first letter of the plaintext is shifted right based on the first letter of the keyword.
• The second letter is shifted left based on the second letter of the keyword.
• This pattern continues, alternating the direction of the shift.

The result is a more complex encryption pattern that enhances security.

User Authentication and Keys

The program also features user authentication. Each user is assigned specific languages (acting as keys) from the Dungeons & Dragons universe. Only authenticated users can encrypt or decrypt messages using the languages available to them.

Available Keys:

Key	Language		Key	Language
1	Dwarvish		9	Celestial
2	Elvish		10	Draconic
3	Giant		11	DeepSpeech
4	Gnomish		12	Infernal
5	Goblin		13	Primordial
6	Halfling		14	Sylvan
7	Orc		15	Undercommon
8	Abyssal		16	Druidic

🏗️ Code Structure

File: cipher.c

• getInput(char *prompt, char *buffer, size_t size): Handles user input and checks for buffer overflow.
• clearConsole(): Clears the console screen based on the operating system.
• main(): The main function that manages user authentication, option selection, and message processing.
• isNumeric(const char* str): Checks if a given string is numeric.
• getNumericInput(int *num): Retrieves and validates numeric input from the user.
• authenticateUser(const char* userID): Authenticates the user based on predefined IDs.
• isKeyAvailableForUser(const User *user, int keyIndex): Verifies if a language key is available for the authenticated user.
• TranslateMessage(char* msg, int length, int option, const char* key): Encrypts or decrypts a message using alternating shift directions.
• ShiftChar(char c, int amount, char dir, const char* currentAlphabet): Shifts a character left or right based on the specified direction and amount.

File: cipher.h

• Platform-Specific Commands: Contains commands for clearing the console based on the operating system.
• Constants: Defines ASCII values, alphabet range, and the list of available languages (keys).
• User Data: Predefined user list with unique IDs, usernames, and accessible keys.
• Function Prototypes: Prototypes for functions defined in cipher.c.

🛠️ Usage

The program guides users through a series of interactive prompts:

1. Authentication: Enter your unique user ID to log in. Type quit to exit.
2. Choose Action: Select whether to encrypt (write) or decrypt (read) a message.
3. Language Selection: View the available languages (keys) assigned to you and pick one.
4. Enter Text: Input the text you want to encrypt or decrypt. The program processes your input and displays the result.
5. Repeat or Change Language: Decide whether to continue using the same language or return to the language selection.

📋 Example

Here is a sample interaction with the program:

Enter User ID (or 'quit' to exit): Pim
Enter option (0 to Write, 1 to Read, 2 to Logout): 0
Available languages are:
1 (Dwarvish)
4 (Gnomish)
5 (Goblin)
7 (Orc)
8 (Abyssal)
12 (DeepSpeech)
14 (Sylvan)
16 (Druidic)
Enter language number or -1 to go back: 1
Enter your message: hello world
Translated message: ifmmp xpsme
Do you want to continue in this language? (1 for Yes, 0 for No): 1
Enter your message: goodbye
Translated message: tifnqcf
Do you want to continue in this language? (1 for Yes, 0 for No): 0

After choosing No, the program returns to the language selection menu.

⚠️ Important Notes

• Authentication Required: Only authenticated users can access the encryption and decryption features. Usernames and accessible languages are predefined in the cipher.h file.
• Language Access: Users can only use the languages assigned to them.
• Input Validation: The program validates basic inputs (e.g., expecting numeric inputs where needed) but does not check for input length overflow.
• Character Support: The program operates using ASCII values, and only supports the English alphabet (A-Z, a-z). Special characters, numerals, and letters from other alphabets are not supported.

🔧 Build Instructions

To run the program, you need to compile it using the GNU Compiler Collection (GCC). Follow the steps below:

1. Open Terminal: Use Command Prompt or PowerShell on Windows, or Terminal on macOS/Linux.
2. Navigate to Project Directory: Use the cd command to go to the directory containing cipher.c and cipher.h. For example:
   • Windows: cd C:\Users\YourUsername\Desktop\cipher_project
   • macOS/Linux: cd /Users/YourUsername/Desktop/cipher_project
3. Compile the Program:

$ gcc -o cipher cipher.c

gcc: Invokes the GCC compiler.

-o cipher: Names the compiled output file as "cipher".

cipher.c: The source code file to compile.

4. Run the Program:

$ ./cipher

If there are errors during compilation, check the terminal output for troubleshooting hints.

🚧 Limitations

• Character Restrictions: The program only supports the English alphabet. It cannot process numerals, special characters, or letters from non-English alphabets.
• Hardcoded User Data: User credentials and accessible keys are stored directly in the cipher.h file. This design is not ideal for scalability.
• Lack of Error Handling: There is no comprehensive error handling for input overflow or unexpected input types.
• Simple Shift Logic: The shift values are straightforward, which may not provide sufficient security for real-world encryption needs.