Passing Username & Password to Webservice

Hi Guys,
I have a site that uses a third party webservice, each method you call requires a username and password which is fine but I am unable to find a way of storing these details on the site, as a file/ in web.config in a secure mean. As from what i understand if I encrypt the values in say the web.config I would need to store the encryption key somewhere to decrypt the values before passing them to the third party webservice. And if someone has say ftp access to the site they would have the encrypted values but also the encryption key and could therefore see the username and password.
 
Does anyone have a solution for this or can guide me in the right direction?
 
Thanks
 
Lee

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