I've been looking for a way to hash a given string in C# that uses a predetermined key.

On my adventures through the internet trying to find an example i have seen lots of MD5CryptoServiceProvider examples which seem to use a default key for the machine, but none of them that apply a specific key. I need to have a specific key to encode data as to synchronize it to someone else's server. I hand them a hashed string and an ID number and they use that analyze the data and return a similar set to me. So is there anyway to get md5 to hash via a specific key that would be consistent to both.

I would prefer this to be done in C#, but if its not possible with the libraries can you do so with some web languages like php or asp?

Edit: Misunderstood the scenario I was thrown into and after a little sitting and thinking about why they would have me use a key it appears they want a key appended to the end of the string and hashed. That way the server can appended the key it has along with the data passed to ensure its a valid accessing computer. Anyways... thanks all ^_^

Edit2: As my comment below says, it was the term 'salting' I was oblivious to. Oh the joys of getting thrown into something new with no directions.