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Since hackers can hack into my mobile phone/computer and use its camera to watch over me,
Q1: Can I know if my mobile phone/computer camera has been hacked?
Q2: If I was hacked, does putting duct tape over camera/webcam really prevent them from watching me? Can they watch me through the screen?
I really appreciate your time.
Good webcams have a built-in feature that turns on the light whenever the camera is active specifically so you can't trick someone into thinking the camera isn't recording when it actually is. If it were done securely, that would be entirely in hardware, so that it can't be bypassed with software. Now, as to whether or not that's actually the case, it may very well be that software (possibly in the form of a firmware update) could bypass this, although that would probably have to be tailor-made for the specific webcam. That said, tape over your camera lens will indeed prevent them from seeing you with the webcam. To get an idea of how much it will help, just turn on a camera application, and cover the lens with what you're thinking of using to see how well it obscures the view.
However, unless you frequently have your webcam pointed at you while you're naked, I would worry more about the data on your computer itself. With this, a hacker can gather all sorts of private information about you. This could be pictures you've taken, documents you've written, saved passwords (or they could possibly install a keylogger to steal your passwords if you don't save them on the device), and lots of other things that could lead to your identity being stolen. They could also potentially install screen capture software to watch what you're doing on your computer. These are, in my opinion, a much bigger threat than someone seeing what my room looks like.
Q1 It depends on the sophistication of the attack. For a user of the device it would be difficult to perform an after-the-fact analysis, however there are examples of software that warn user in real time that some other software is using the camera. One such commercial solution for Mac is Micro Snitch.
But these measures can be disabled by a prepared attacker.
Q2 with your comment:
I want to know if hackers can use screen to do the same thing as camera/webcam.
In very simple words, camera and screen perform opposite functions. Camera turns light waves (image) into electrical signals, screen transforms electrical signals into visible light. The process of conversion (and required devices) is different depending on which way you transform.
Unlike with sound where microphone and speaker can be used to perform the function of the other (to a certain degree), cameras contain lens which focus the light waves on the receptor. A simple computer screen is unable to do it. However it is likely that screens will soon be combined with a camera placed behind, as indicated by the news about Apple or Sony pursuing the idea.
As of today, unless there was a concealed device, the answer to your second question is: hackers are unable to watch you through a regular consumer-grade screen in a computer or a smartphone.
Covering camera with a duct tape either prevents the light from getting onto the sensor or distorts them, effectively thwarting image rendition. Although some light might get through to detect someone presence or lack of thereof, most modern devices have other sensors that would reveal this with better precision.
Duct tape seems to be used by some potentially high profile targets with resources for more sophisticated defence measures (and likely together with other measures).
Can I know if my mobile phone/computer camera has been hacked?
Honestly these days it is not very likely that you will notice a hack unless you are installing from untrusted sources or have rooted (or jailbreak for an Apple device) in which case the sophistication of the malware is not enough to hide all the usual tell-tale signs (slowness, new applications, etc.).
For a targeted hack and in many cases mobile malware, you will almost never know unless you are very closely monitoring network traffic for your devices.
Very often though for webcams, the light will at least come on, but that isn't always true and having a cover is best to be safe, no matter what.
If I was hacked, does putting duct tape over camera/webcam really prevent them from watching me? Can they watch me through the screen?
Yes. Though it may not prevent them from listening to you (as most webcams have microphones embedded in them). Also, again, use a real cover and not tape. This ensures that they cannot even see shadows given your tape, paper, etc. is thin enough.
I'm not entirely sure where you got the idea they can watch you through your screen, because a LCD/LED screen modulates light for output. That being said, an attacker can extract passwords, fingerprints, memory, and install a keylogger which is worse than staring at your person.
Mobile security is not only still an infant in the infosec world, but it has the unfortunate problem of being widely accessible, with many options due to a plethora of manufacturers and a wider variety of users at a price point that makes them easily replaceable. The desktop and server market, while older, still suffer from the expanding scope that is our technological world.
