Encrypt a BMP file C# Exercise - C# Programming Course

// Import necessary namespaces for file handling
using System; // Basic input/output operations
using System.IO; // FileStream for file reading and writing

class BMPEncryptDecrypt // Main class for the BMP encryption/decryption program
{
    static void Main(string[] args) // Entry point of the program
    {
        // Check if the correct number of arguments (file name) is provided
        if (args.Length != 1) // If no file argument is provided
        {
            Console.WriteLine("Usage: BMPEncryptDecrypt "); // Show usage instructions
            return; // Exit the program if the arguments are incorrect
        }

        string fileName = args[0]; // Get the file name from the command line argument

        // Check if the file exists
        if (!File.Exists(fileName)) // If the file does not exist
        {
            Console.WriteLine("Error: The file does not exist."); // Inform the user about the missing file
            return; // Exit the program if the file is missing
        }

        try
        {
            // Open the BMP file for both reading and writing using FileStream
            using (FileStream fileStream = new FileStream(fileName, FileMode.Open, FileAccess.ReadWrite)) // Open the file in read-write mode
            {
                byte[] buffer = new byte[2]; // Buffer to store the first two bytes of the BMP file

                // Read the first two bytes to check if they are "BM" (0x42, 0x4D)
                int bytesRead = fileStream.Read(buffer, 0, 2); // Read the first two bytes

                // If the file does not have enough bytes, show an error
                if (bytesRead < 2)
                {
                    Console.WriteLine("Error: The file is too small to be a valid BMP file.");
                    return; // Exit the program if the file is too small
                }

                // Check if the first two bytes are "BM" (0x42, 0x4D)
                if (buffer[0] == 0x42 && buffer[1] == 0x4D) // If the file starts with "BM"
                {
                    Console.WriteLine("Encrypting file..."); // Inform the user that the file will be encrypted

                    // Change the first two bytes from "BM" to "MB" (0x4D, 0x42)
                    buffer[0] = 0x4D; // Change 'B' to 'M'
                    buffer[1] = 0x42; // Change 'M' to 'B'
                    fileStream.Seek(0, SeekOrigin.Begin); // Move the file pointer to the start of the file
                    fileStream.Write(buffer, 0, 2); // Write the modified bytes back to the file
                    Console.WriteLine("File encrypted."); // Notify the user that the encryption is done
                }
                else if (buffer[0] == 0x4D && buffer[1] == 0x42) // If the file starts with "MB"
                {
                    Console.WriteLine("Decrypting file..."); // Inform the user that the file will be decrypted

                    // Change the first two bytes from "MB" to "BM" (0x42, 0x4D)
                    buffer[0] = 0x42; // Change 'M' to 'B'
                    buffer[1] = 0x4D; // Change 'B' to 'M'
                    fileStream.Seek(0, SeekOrigin.Begin); // Move the file pointer to the start of the file
                    fileStream.Write(buffer, 0, 2); // Write the modified bytes back to the file
                    Console.WriteLine("File decrypted."); // Notify the user that the decryption is done
                }
                else
                {
                    Console.WriteLine("Error: The file does not appear to be a valid BMP file."); // Inform the user if the file is not a valid BMP
                }
            }
        }
        catch (Exception ex) // Catch any exceptions that occur during file handling
        {
            Console.WriteLine($"An error occurred: {ex.Message}"); // Display the error message
        }
    }
}