C# – Encrypt Message to Text File – Decrypt Message from Text File

Using the Security.Cryptography class, you can encrypt and decrypt files pretty easily. I was experimenting with taking a plaintext message, encrypting that message into a text file, and then reading and decrypting the message. Something that fascinates me is the encryption key is stored and can be extrapolated from the text file.

Compiled in Microsoft Visual Studio Community 2019.

 

// MrNetTek
// eddiejackson.net/blog
// 11/30/2020
// free for public use
// free to claim as your own

using System;
using System.IO;
using System.Security.Cryptography;

public class EncryptDecrypt
{
    public static void Main(string[] args)
    {
        // encryption key for encryption/decryption 
        byte[] key = { 0x02, 0x03, 0x01, 0x03, 0x03, 0x07, 0x07, 0x08, 0x09, 0x09, 0x11, 0x11, 0x16, 0x17, 0x19, 0x16 };

        // ENCRYPT DATA
        try
        {
            // create file stream
            using FileStream myStream = new FileStream(@"C:\csharp\encrypted.txt", FileMode.OpenOrCreate);

            // configure encryption key.  
            using Aes aes = Aes.Create();
            aes.Key = key;

            // store IV
            byte[] iv = aes.IV;
            myStream.Write(iv, 0, iv.Length);

            // encrypt filestream  
            using CryptoStream cryptStream = new CryptoStream(
                myStream,
                aes.CreateEncryptor(),
                CryptoStreamMode.Write);

            // write to filestream
            using StreamWriter sWriter = new StreamWriter(cryptStream);
            string plainText = "Welcome to the lab of MrNetTek!";
            sWriter.WriteLine(plainText);

            // done 
            Console.WriteLine("---SUCCESSFUL ENCRYPTION---\n");

        }
        catch
        {
            // error  
            Console.WriteLine("---ENCRYPTION FAILED---");
            throw;
        }

        // SHOW ENCRYPTED DATA
        try
        {
            string text = System.IO.File.ReadAllText(@"C:\csharp\encrypted.txt");

            // encrypted data
            Console.WriteLine("Encrypted Data: {0}\n\n", text);

            Console.WriteLine("Press any key to view decrypted data\n");
            Console.ReadKey();
        }
        catch
        {
            throw;
        }

        // DECRYPT DATA
        try
        {
            // create file stream
            using FileStream myStream = new FileStream(@"c:\csharp\encrypted.txt", FileMode.Open);
            
            // create instance
            using Aes aes = Aes.Create();

            // reads IV value
            byte[] iv = new byte[aes.IV.Length];
            myStream.Read(iv, 0, iv.Length);

            // decrypt data
            using CryptoStream cryptStream = new CryptoStream(
               myStream,
               aes.CreateDecryptor(key, iv),
               CryptoStreamMode.Read);

            // read stream
            using StreamReader sReader = new StreamReader(cryptStream);

            // display stream
            Console.WriteLine("\n---SUCCESSFUL DECRYPTION---\n");
            Console.WriteLine("Decrypted data: {0}", sReader.ReadToEnd());
            Console.ReadKey();
        }
        catch
        {
            // error
            Console.WriteLine("---DECRYPTION FAILED---");
            throw;
        }
    }
}

Output