If I would like to keep my work secret in my computer and I don't use Internet, is it possible for a cracker to steal my data via electric wires or a wireless connection?
First of all, a cracker is considered somebody that reverse engineers software in a way where copy protections or nags screens are removed.
Software cracking (known as "breaking" in the 1980s1) is the modification of software to remove or disable features which are considered undesirable by the person cracking the software, especially copy protection features (including protection against the manipulation of software, serial number, hardware key, date checks and disc check) or software annoyances like nag screens and adware.
Technically it is possible that someone could steal your information and/or data if the attacker resides within your LAN. For example for wire tapping, I use the following hardware:
By placing this device in the network, all packets can be captured. However, this specific hardware is not necessarily required. It is also possible to strip the Ethernet cable and directly tap the RX and TX wires.
From a WLAN perspective, it is possible if someone has access to your WiFi network to put their network card in promiscuous and use Wireshark or a similar product to capture all WiFi traffic. More on that can be found on the Wireshark page: https://wiki.wireshark.org/CaptureSetup/WLAN
Having said that, the likelihood of something happening to you, I consider slim to none. Unless you're really paranoid, disable all your network communication devices (although there are other methods of obtaining information, but this requires very expensive equipment)
Yes, this is possible. It is not an easy task, though, and you are not likely to be a target.
There are series of equipment which are shielded so that such attempts are more difficult. They follow sets of standards called TEMPEST.
Data can not normally be stolen via your power lines. There is no interface for data transmission between a normal computer and the power line. Hardware is made that can provide such an interface, but someone would have to have physical access to your machine, and install that hardware in your computer. Phone line and power line networking
If you have a LAN, either wired or wireless, then someone on your LAN could steal data. If your computer has wi-fi, but you do not use it, you can turn the hardware off. At least, you typically can. There might be some computers with wi-fi and no way to turn it off, but I do not know of any. Note that this is not a trivial effort (stealing stuff). Without internet contact you are unlikely to get malware installed by someone else to make stealing your data possible. LAN traffic can be intercepted, but this is again, not easy.
Keeping a computer off the internet is a very good way to keep things private. Locking that computer out of your LAN is even better. For instance, I still run outdated Windows machines for some old software - but I set the firewall to completely block them from using the network. Unless someone gets physical access to them, they can not be hacked into.
Also, of course, good passwords, and encryption are your friends.
There is a principle called Van Eck phreaking where a receiver is placed near the target machine to collect and transmit the changes in the EMF caused by operating the target device.
No data passes through your power cables, unless you're using HomePlug for your internet connection that is or connecting to another device. It's in theory possible to read your data through tapping your power cable or power supply but it would be so brutally hard to do (next to impossible) that nobody is ever going to try such a thing, literally any other approach would be easier. So it's safe to assume that this won't ever be a worry for the next 100 years at least (possibly for eternity; or at least until power cables and internet cables will be united as one or something like that)
Wireless networking is however ALWAYS hackable either by accessing your router and then using wireshark or something similar to read your packets, or by other more advanced means of tapping your wireless network frequencies technically all unencrypted wireless data is up for grabs since it's literally floating around in the air between your system and your router, and encrypted wireless data while less up for grabs can still be easily hijacked through various means (depends highly on the encryption protocol though how easily, and your router security)
If you are connected to a router or another device with your system, that router or other device which is presumably connected to the internet can be hacked to access your data, if however it is not connected to any other device the only way to hack your data would be physical access, and the only way to keep tabs on that data would be infiltrating the place you keep this device, and secretly insert a wifi or bluetooth device into it and connect it to another device or router for the specific purpose of stealing your data. (Someone would have to want your data REALLY BADLY, and just as badly want to keep it secret from you that they've got your data... not a likely scenario unless you're someone really special who could be a high profile target because of your social status or job)
In other words a lone pc that is not connected to the internet or another device at all is unhackable through remote means unless it has a wireless networking device installed which could be remotely activated (like bluetooth or some types of wifi cards) so for good measure disable any such devices and nobody without physical access can access your data. Encrypt your hard drives, use complicated passwords, etc, etc and the only way into your system besides you doing it yourself would be this.
Why are you asking specifically about electric wires or a wireless connection? If I worked for a government, major company, or organised crime interested in your work, these would be quite at the end of my list to get your things. You should describe what you are actually afraid of.
For example: If I wanted to steal your secret, didn't mind hurting you, didn't mind that you know about the theft, and knew that you used say a Mac with full disk encryption turned on, which is reasonably hard to crack, I'd sneak into your house and knock you out while you are using your computer. Well, I'd send someone in who knows how to do this :-) The point is here stealing the whole computer while it is unlocked so I don't need any passwords to unlock it.
If I didn't want you to know, I'd sneak in and install some hardware that records all keystrokes on your computer, and go from there.
It's all a matter of how badly someone wants your secrets.