Me and my girlfriend are using facetime regularly for talking. I'm worried if anyone can hack our Facetime calls and record our videos? Is it possible? How safe is Facetime?
FaceTime is quite secure. In Feb 2014, Apple published a white paper on the security used in various iOS services, which finally brought to light some of the features that others had previously inferred or guessed at. The section for FaceTime states:
FaceTime is Apple’s video and audio calling service. Similar to iMessage, FaceTime calls also use the Apple Push Notification Service to establish an initial connection to the user’s registered devices. The audio/video contents of FaceTime calls are protected by end-to-end encryption, so no one but the sender and receiver can access them. Apple cannot decrypt the data.
FaceTime uses Internet Connectivity Establishment (ICE) to establish a peer-to-peer connection between devices. Using Session Initiation Protocol (SIP) messages, the devices verify their identity certificates and establish a shared secret for each session. The nonces supplied by each device are combined to salt keys for each of the media channels, which are streamed via Secure Real Time Protocol (SRTP) using AES-256 encryption.
FaceTime calls are encrypted sufficiently for most anyone's needs. FaceTime's encrypted stream acts underneath any other WiFi encryption that might also be present. If you feel that end-to-end data streams encrypted with unique keys for each recipient is safe, then I'd say have fun with FaceTime.
Someone with special skills, tools and time could probably crack it, but you probably don't have a high enough profile to be worth the time/effort.
As far as I have read, you should assume your Facetime traffic is completely unencrypted. There have been assertions by Apple of encryption (the ZDNet article linked above), but they talked about using WPA2. WPA2 has nothing to do with encrypting the voice/video traffic. WPA2 is about making wireless connections to an access point private. Your Facetime traffic is sent to the AP, then to your router, then to your ISP's router then all over the place before it gets to your gf's home network or cellular provider/device. I should note that I have not done a protocol analysis of the Facetime protocol; I'm going by the published statements.
Saying that WPA2 answers the question is like being confident in transporting large sums of money to the bank using an armored truck only between the loading dock and the edge of the parking lot and then putting the money into a Corolla and driving it the rest of the way to its destination.