From memory, you pass the string to be encrypted/decrypted and the passphrase as a single string, separated by a comma. I think that was because this was also to be used in a DLL for use with Delphi etc, but anyway, the point is that this was the simple means of setting up and tearing down AES in C#. Apologies for lack of comments, easier to just strip than to have to sanitise too much for the web.


using System;
using System.Windows.Forms;
using System.Runtime.InteropServices;
using System.Security.Cryptography;
using System.Text;

namespace BlahLib
{
	public interface IBlahLib
	{
		void TestMethod();

		string Encrypt(string original, string key);
		string Decrypt(string encrypted, string key);
		string GetKey(string passphrase);
	}

    [ClassInterface(ClassInterfaceType.None)]
    public class BlahLib : IBlahLib
    {
        public BlahLib()
        {
        }
        
        public void TestMethod()
        {
            MessageBox.Show("Hello from the BlahLib class");
        }

		public string GetKey(string passphrase)
		{
			RijndaelManaged aesEncryption = new RijndaelManaged();            
			aesEncryption.KeySize = 256;            
			aesEncryption.BlockSize = 128;            
			aesEncryption.Mode = CipherMode.ECB;            
			aesEncryption.Padding = PaddingMode.Zeros;
			aesEncryption.GenerateIV();            

			string ivStr = Convert.ToBase64String(aesEncryption.IV);
			byte[] keyText = ASCIIEncoding.UTF8.GetBytes(passphrase);
			string keyStr = Convert.ToBase64String(keyText);  
			string completeKey = ivStr + "," + keyStr;    
            return Convert.ToBase64String(ASCIIEncoding.UTF8.GetBytes(completeKey));        
		}

		public string Encrypt(string original, string key)
		{
			RijndaelManaged aesEncryption = new RijndaelManaged();
			aesEncryption.KeySize = 256;
			aesEncryption.BlockSize = 128;
			aesEncryption.Mode = CipherMode.ECB;
			aesEncryption.Padding = PaddingMode.Zeros;
			aesEncryption.IV =
				Convert.FromBase64String(ASCIIEncoding.UTF8.GetString(Convert.FromBase64String(key)).Split(',')[0]);
			aesEncryption.Key =
				Convert.FromBase64String(ASCIIEncoding.UTF8.GetString(Convert.FromBase64String(key)).Split(',')[1]);

			byte[] plainText = ASCIIEncoding.UTF8.GetBytes(original);
			ICryptoTransform crypto = aesEncryption.CreateEncryptor();

			byte[] cipherText = crypto.TransformFinalBlock(plainText, 0, plainText.Length);
			return Convert.ToBase64String(cipherText);
		}

		public string Decrypt(string encrypted, string key)
		{
			RijndaelManaged aesEncryption = new RijndaelManaged();
			aesEncryption.KeySize = 256;
			aesEncryption.BlockSize = 128;
			aesEncryption.Mode = CipherMode.ECB;
			aesEncryption.Padding = PaddingMode.Zeros;
			aesEncryption.IV =
				Convert.FromBase64String(ASCIIEncoding.UTF8.GetString(Convert.FromBase64String(key)).Split(',')[0]);
			aesEncryption.Key =
				Convert.FromBase64String(ASCIIEncoding.UTF8.GetString(Convert.FromBase64String(key)).Split(',')[1]);

			ICryptoTransform decrypto = aesEncryption.CreateDecryptor();

			byte[] encryptedText = Convert.FromBase64CharArray(encrypted.ToCharArray(), 0, encrypted.Length);
			return ASCIIEncoding.UTF8.GetString(decrypto.TransformFinalBlock(encryptedText, 0, encryptedText.Length));
		}

		
    }
}