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

public void EncryptTextToFile(string para_text, string para_path, byte[] para_key, byte[] para_iv)
...

public string DecryptTextFromFile(string para_path, byte[] para_key, byte[] para_iv)
...

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

public void EncryptTextToFile(string para_text, string para_path, byte[] para_key, byte[] para_iv)
{
	try
	{
		// Create or open the specified file
		using (FileStream _fStream = File.Open(para_path, FileMode.Create))
		// Create a new DES object
		using (DES _des = DES.Create())
		// Create a DES encryptor from the key and IV
		using (ICryptoTransform _encryptor = _des.CreateEncryptor(para_key, para_iv))
		// Create a CryptoStream using the FileStream and encryptor
		using (var cStream = new CryptoStream(_fStream, _encryptor, CryptoStreamMode.Write))
		{
			// Convert the provided string to a byte array
			byte[] _toEncrypt = Encoding.UTF8.GetBytes(para_text);

			// Write the byte array to the crypto stream
			cStream.Write(_toEncrypt, 0, _toEncrypt.Length);
		}
	}
	catch (CryptographicException ex)
	{
		MessageBox.Show("An encryption to file error occurred: {0}", ex.Message);
		throw;
	}
}

public string DecryptTextFromFile(string para_path, byte[] para_key, byte[] para_iv)
{
	try
	{
		// Open the specified file
		using (FileStream _fStream = File.OpenRead(para_path))
		// Create a new DES object.
		using (DES _des = DES.Create())
		// Create a DES decryptor from the key and IV
		using (ICryptoTransform _decryptor = _des.CreateDecryptor(para_key, para_iv))
		// Create a CryptoStream using the FileStream and decryptor
		using (var _cStream = new CryptoStream(_fStream, _decryptor, CryptoStreamMode.Read))
		// Create a StreamReader to turn the bytes back into text
		using (StreamReader reader = new StreamReader(_cStream, Encoding.UTF8))
		{
			// Read back all of the text from the StreamReader, which receives
			// the decrypted bytes from the CryptoStream, which receives the
			// encrypted bytes from the FileStream.
			return reader.ReadToEnd();
		}
	}
	catch (CryptographicException ex)
	{
		MessageBox.Show("A decryption from file error occurred: {0}", ex.Message);
		throw;
	}
}