specifying that the operation parameter should be base64Binary encoded string

[ghader]

I have an operation in my service which is like:

public DataHandler loadData(String key)
{...}
I want to specify that the input parameter of the operation should be a 
base64Binary encoded string. How can I do this? I think that I should add 
something to my service.xml but I don't know what to add.
could anyone help?

Thanks