> does the virus need to check in with the backend for every single file for unique keys?

As it's based on CryptoLocker, all the files would be encrypted with the same key. After CryptoLocker was busted by law enforcement, "Fox-IT and fellow firm FireEye introduced an online service which allows infected users to retrieve their private key by uploading a sample file, and then receive a decryption tool" (https://en.wikipedia.org/wiki/Cryptolocker#Takedown_and_reco...)

edit: technically the files are encrypted with different keys but those keys are stored on the victim's machine and encrypted with a single key -- http://www.welivesecurity.com/2013/12/19/cryptolocker-2-0-ne...