AES-256 Encryption and Decryption

Check the below code

class AesEncryption
{
    // Create secret IV
    static byte[] iv = new byte[16] { 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0 };

    public static string EncryptString(string plainText, byte[] key)
    {
        // Instantiate a new Aes object to perform string symmetric encryption
        Aes encryptor = Aes.Create();

        encryptor.Mode = CipherMode.CBC;

        // Set key and IV
        byte[] aesKey = new byte[32];
        Array.Copy(key, 0, aesKey, 0, 32);
        encryptor.Key = aesKey;
        encryptor.IV = iv;

        // Instantiate a new MemoryStream object to contain the encrypted bytes
        MemoryStream memoryStream = new MemoryStream();

        // Instantiate a new encryptor from our Aes object
        ICryptoTransform aesEncryptor = encryptor.CreateEncryptor();

        // Instantiate a new CryptoStream object to process the data and write it to the 
        // memory stream
        CryptoStream cryptoStream = new CryptoStream(memoryStream, aesEncryptor, CryptoStreamMode.Write);

        // Convert the plainText string into a byte array
        byte[] plainBytes = Encoding.ASCII.GetBytes(plainText);

        // Encrypt the input plaintext string
        cryptoStream.Write(plainBytes, 0, plainBytes.Length);

        // Complete the encryption process
        cryptoStream.FlushFinalBlock();

        // Convert the encrypted data from a MemoryStream to a byte array
        byte[] cipherBytes = memoryStream.ToArray();

        // Close both the MemoryStream and the CryptoStream
        memoryStream.Close();
        cryptoStream.Close();

        // Convert the encrypted byte array to a base64 encoded string
        string cipherText = Convert.ToBase64String(cipherBytes, 0, cipherBytes.Length);

        // Return the encrypted data as a string
        return cipherText;
    }

    public static string DecryptString(string cipherText, byte[] key)
    {
        // Instantiate a new Aes object to perform string symmetric encryption
        Aes encryptor = Aes.Create();

        encryptor.Mode = CipherMode.CBC;

        // Set key and IV
        byte[] aesKey = new byte[32];
        Array.Copy(key, 0, aesKey, 0, 32);
        encryptor.Key = aesKey;
        encryptor.IV = iv;

        // Instantiate a new MemoryStream object to contain the encrypted bytes
        MemoryStream memoryStream = new MemoryStream();

        // Instantiate a new encryptor from our Aes object
        ICryptoTransform aesDecryptor = encryptor.CreateDecryptor();

        // Instantiate a new CryptoStream object to process the data and write it to the 
        // memory stream
        CryptoStream cryptoStream = new CryptoStream(memoryStream, aesDecryptor, CryptoStreamMode.Write);

        // Will contain decrypted plaintext
        string plainText = String.Empty;

        try
        {
            // Convert the ciphertext string into a byte array
            byte[] cipherBytes = Convert.FromBase64String(cipherText);

            // Decrypt the input ciphertext string
            cryptoStream.Write(cipherBytes, 0, cipherBytes.Length);

            // Complete the decryption process
            cryptoStream.FlushFinalBlock();

            // Convert the decrypted data from a MemoryStream to a byte array
            byte[] plainBytes = memoryStream.ToArray();

            // Convert the decrypted byte array to string
            plainText = Encoding.ASCII.GetString(plainBytes, 0, plainBytes.Length);
        }
        finally
        {
            // Close both the MemoryStream and the CryptoStream
            memoryStream.Close();
            cryptoStream.Close();
        }

        // Return the decrypted data as a string
        return plainText;
    }

    public static void HowToUse()
    {
        string SecurityKey = "SecurityKey$%^&%*W";
        Console.WriteLine("Enter a text to encrypt : ");
        string inputValue = Console.ReadLine();
        // Create sha256 hash
        SHA256 mySHA256 = SHA256Managed.Create();
        byte[] key = mySHA256.ComputeHash(Encoding.ASCII.GetBytes(SecurityKey));

        string encrypted = EncryptString(inputValue.ToString(), key);
        string decrypted = DecryptString(encrypted, key);

        Console.Write("\n");
        Console.WriteLine("Encrypted test is: {0}", encrypted);
        Console.WriteLine("Original text is: {0}", decrypted);
    }
}

Hi Piyush Mohan,

Below is one of the good article with practical example: 

• AES Encryption In C#
  • https://www.c-sharpcorner.com/article/aes-encryption-in-c-sharp/

Hope this will help you!