tjackvatten2 I’ve come across many articles and also forums in Swedish suggesting that Swedish police has access to backdoors on mobile communications through agreements with mobile the operators.
This is true for law enforcement in general in any country. This is also true for network traffic going out through your ISP on your home broadband connection and so on. Everything outside of your own device is potentially and very likely wiretapped. Your phone provider or ISP will likely also keep detailed logs of all your activity.
What they can see: They can listen to your regular phone conversations. They can read your sent and received SMS/MMS. They can see what websites you are visiting, unless you use a VPN to hide this from them. They can see what messaging apps and other internet based apps you are using, unless you use a VPN to hide this from them.
tjackvatten2 According to what I've found, it's technically possible for the police to see all data in real-time, including decrypt signal messages
No. They can not decrypt any communication at all, certainly not end-to-end encryption like Signal. The law enforcement in any country is just another hacker in this scenario. The encryption algorithms we have are way way enough to protect against even the strongest hackers out there. It is basically never the encryption that fails. Many encryption algorithms have mathematical proofs that they are unbreakable, meaning it is guaranteed no one ever can break them.
tjackvatten2 Also you don't even need to be suspected of a crime it just enough if the police is interested and want to check you. They don't need any approval from a judge.
I am uncertain about this. Your phone conversations, SMS and MMS messages, and what sites you visit if not using a VPN is basically public knowledge from a technical point of view. There are no privacy protections at all there. On the other hand, these kind of data traffics have traditionally been protected anyway, because they have been considered privacy sensitive.
It might be that an agreement with the phone provider or ISP is enough in this case.
tjackvatten2 Also being associated with someone under investigation may be enough.
This could constitute probable cause, yes, if there is reasons to believe you might be involved too.
tjackvatten2 From what I understand, the cell towers log data which could then be accessed without needing direct access to the phone itself.
True.
tjackvatten2 This raises serious privacy concerns, especially for those of us trying to maintain strong security.
No, it does not. GrapheneOS project have been very clear with that nothing that has with cellular connectivity to do is safe, and that it should be considered totally untrusted for any privacy sensitive task. They have been clear you should use end-to-end encrypted messaging apps such as Signal instead for all your text messages and voice and video calls. They have been very clear that you should use a VPN.
tjackvatten2 Given this situation, would simply removing the SIM card when not actively using cellular networks be a useful protection strategy? Are there better ways to harden GrapheneOS or the device setup to prevent this kind of passive or live surveillance?
The only thing you would hide this way is the exact location of your phone.
tjackvatten2 Should I use a 4G router and just connect the phone on that?
That makes no real difference if you are carrying the 4G router with you, as the only thing you can hide this way is the exact location of your phone.
Again, neither the police or anyone else can break into your phone. In most countries, the police are never allowed to break into any kind of system using exploits, but in the countries where they are allowed to do that, they definitely require a court order, and in that case, they are still just any other hacker, and GrapheneOS protects very well against that.
tjackvatten2 Also, in Swedish this is called "Hemlig Dataavläsning" or just "HDA".
Yes. And it has existed ever since phones were a device with a wire plugged into a phone jack. That law hasn't changed substantially since then.
tjackvatten2 Also, they can not only see everything you are doing live on your phone, they can also use your microphone, camera, location, etc. Is this also possible on a GrapheneOS?
No, they cannot do any of that on any phone, unless they play hacker and find an exploit and break into your device. GrapheneOS protects against hackers very well.