StringEncryption.cs

public static class StringEncryption
	{
		private static readonly byte[] Entropy = Encoding.Unicode.GetBytes("4asd894a9d8as5d4sad4asd48sa9w8d4aw8da1d");

		public static string EncryptString(this SecureString input)
		{
			var encryptedData = ProtectedData.Protect(
				Encoding.Unicode.GetBytes(ToInsecureString(input)),
				Entropy,
				DataProtectionScope.CurrentUser);
			return Convert.ToBase64String(encryptedData);
		}

		public static SecureString DecryptString(this string encryptedData)
		{
			try
			{
				var decryptedData = ProtectedData.Unprotect(
					Convert.FromBase64String(encryptedData),
					Entropy,
					DataProtectionScope.CurrentUser);
				return ToSecureString(Encoding.Unicode.GetString(decryptedData));
			}
			catch
			{
				return new SecureString();
			}
		}

		public static SecureString ToSecureString(this string input)
		{
			var secure = new SecureString();
			foreach (var c in input)
			{
				secure.AppendChar(c);
			}

			secure.MakeReadOnly();
			return secure;
		}

		public static string ToInsecureString(this SecureString input)
		{
			string returnValue;
			var ptr = Marshal.SecureStringToBSTR(input);
			try
			{
				returnValue = Marshal.PtrToStringBSTR(ptr);
			}
			finally
			{
				Marshal.ZeroFreeBSTR(ptr);
			}

			return returnValue;
		}
	}