Level of modem isolation in Galaxy S3 / I9300?
Hi, I am a new user of Replicant/Android.
I've got freshly installed Replicant 4.2 on my Galaxy S3 and I am considering safe options for connectivity. I know for sure that for telephony I won't use GSM, only VPN + SIP. So this means I need only internet and for that I can use either an internal GSM modem or an external modem on a MiFi device (via wifi).
The advantage of external modem would be a great isolation. Of course that secondary device should be considered untrusted/compromised but it's not a problem since I'll use a VPN. The price is not an issue either (25$ for a new 3G device). The only problem is that I would have to carry another device with me, and even the smallest ones are of size of a phone.
So I'm still considering the option of using the internal modem, although I'm worried. It looks like most GSM modems implement a secret universal software communication interface that allows to remotely send silent commands to them, e.g. to turn on the microphone. Of course the modem would have to have hardware connections to be able to directly access e.g. the microphone. I know that the wiki on Galaxy S3 I9300 says the modem doesn't share memory with the CPU but what about e.g. the microphone?
I don't know how Replicant manages the microphone, but if that's an issue for you, Snowden shows how disable built-in cameras and microphones on smartphone so you would use external wired earphones with microphone.
Thanks for your suggestion Xan VFR. This is certainly another option to consider, although a bit heartbreaking given that I bought the phone in a great shape:) On the other hand, from some tutorials on YT, removal of microphones seems to be a rather simple operation in S3.
Also, if using a Bluetooth setup for mic/headphone I would have to make sure that the transmission is encrypted, but current Bluetooth (4.2) is still not secure:
iFixit's Samsung Galaxy S III Repair guides could be useful for you
Thanks for the link, it might be useful in the future.
However, I've decided to use an external modem via wifi. I work at home so the phone lies on my desk most of the time and I can put the second device into a drawer. When I go out I sometimes have a backpack so I can throw it there. And on occasions when I don't have a backpack with me I will have to suffer with my pockets stuffed with devices. Security comes at price:D Also I'm a bit worried about double radiation on my balls...
The microphone should not be accessible by the modem. Audio is routed by the main CPU, specifically the Audio-RIL code does this. So basically the CPU decides if audio should be routed to the modem and then tells the audio codec to forward it. So far, nothing indicates that the modem has some kind of backdoor access to the audio codec, but it's not possible to be 100% certain about this right now. Paul explains the verification difficulties in the linked thread.
thanks for the link and additional informations!