Secure username and password

Hi guys,
 After some direction, I am creating a site in asp.net 3.5 and I need to store a single username and password in the web config or in a database record that is used for a third party api but that username ans password need to be stored securly and encrypted but will need to be decrypted to pass to the third party api. and I developing the system cant know the password. any ideas of how you can obtain this?
 
any ideas welcome
thanks
 
Lee

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