The key can be whatever you want as long as it is programmed on both the classmate end and the robot bridge. FIRST recommend using <team number>WPAKEY. So for my team, 3096, we would use 3096WPAKEY.
FIRST details how to set the up in thier "How to Configure Your Radio" document at
http://www.usfirst.org/uploadedFiles...Your_Radio.pdf
In reality, I had a hard time getting the encryption set-up and for shop use I skipped it am running un-encrypted. You bridge will be encrypted by the event staff at your competitions. You will have to take it back off when you get back to the shop to use the robot there.
Hope this helps.