[Worfkflow] Change Item permission with CodeActivity

Hi, I want change the item permission from my CodeActivity developed form my Workflow 2013. I would not want using the WF2010 into WF2013...

I have created a CodeActivity with no problem and It work, but I not know how I can retrieve the SharePoint context (SPContext. SPWeb, SPList, SPListItem, etc...)

Of course I excluded the passage of a context parameter from my WF2013 to my CodeActivity (see website url or item).

Can you help me?