The tip-off? If you get on your child's computer and can't see any of their activity. If they always clear their log files and browser history, every time, and use special programs to encrypt files and folders, that's a possible sign. Or if encryption settings on their applications are set to a level stronger than the program's defaults. Any indication that they feel the built-in disk encryption and separate user profile protections aren't enough should have you asking, for what kind of activity?
4. They have multiple accounts you can't access
Many kids have multiple email and social media accounts. That's normal. But if your child has a main email and social media account they don't mind you reading and you come across signs that they have other accounts and log-ons they will not share, make a note of it. It may not be malicious hacking; it could be porn or some other activity you would not approve of (talking to strange adults, buying alcohol, purchasing weapons, etc.). But any sort of absolute privacy should be investigated.
My stepson and his hacking friends had a half-dozen account names. I could see them when I read through the firewall and packet filtering logs. I knew he had them, even when he was denying it. He was surprised to learn that PGP (Pretty Good Privacy) encryption didn't encrypt the whole email. I explained how all email encryption had to allow the email headers to remain in the clear so they could be appropriately routed and handled. After that conversation, all the "secret" accounts disappeared from my future log captures. He didn't stop using them; he just downloaded a new email encryption program, which did perform complete, end-to-end encryption. (Refer to the previous sign about encryption, above.)
5. You find hacking tools on their computer
If you suspect your kid is hacking, take inventory of all the programs and tools you can find on their system. If your kid doesn't think you'll do it or doesn't know you've done it, you might get lucky and they might not be encrypted -- yet. In fact, if you find lots of encrypted files and programs, that's a red flag, too.
Port scanners, vulnerability scanners, credential theft programs, denial-of-service tools, folders of stored malware -- these are strong signs your kid is hacking. If you're not computer-savvy enough to recognize these tools, note the file names and search the internet. If more than one of the unknown programs points back to a hacker (or a computer security defender) website, you probably have a problem.
Why are tools to help defend against hackers a red flag? Isn't that a sign your child wants to become a high-paid computer security consultant when they grow up? Sadly, not usually. I've yet to meet the kid who decided to become a computer security expert before college, unless they'd been defending themselves against other aggressive hackers as a teen.
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