Helping people with computers... one answer at a time.

When using a shared computer it's often easy for others to see your data or history. If you must use a shared computer, you need to protect yourself.

I share a computer and I want to know how to keep my information private.

Ultimately ... you can't. At least not easily, and even then it depends on the data that you're attempting to keep private, and the technical savvy of the individuals that you're attempting to keep it private from.

There's nothing like your own computer. But if you have to share, there are a couple of things that might help. A little.

First, you must realize that anyone with administrative access to the computer can see everything. You can use Windows file permissions to make files accessible to only you, but an administrator could still quite easily access those files using any number of techniques.

So if you're trying to keep things private from the system administrator, normal methods simply won't work.

You need encryption.

It's tempting to consider using the Windows NTFS file system's built-in encryption, which obscures the data to anyone but the Windows login account that owns it, but once again I can envision a potential way for the administrator to be able to login as you and then access the files. Couple that with the fact that the encrypted data is not portable and if the login account is ever mistakenly deleted all the data will be lost, NTFS encryption is just not something I recommend.

I normally recommend TrueCrypt for jobs like this, but unless you have administrative rights it won't work unless you first convince the administrator to install it for you. If you do have admin rights, then TrueCrypt is a perfect solution for securing your data. Only you know the passphrase, you can take your encrypted data to any machine, and you can leave it safely behind without fear of compromise.

There are other virtual drive encryption programs similar to TrueCrypt, but I'd expect them to need the same level of administrative access in order to set up a virtual drive.

So, if you don't have administrative privileges, I'll assume you'll need to encrypt your data some other way, which gets complicated because it's never as seamless as we'd like. You can use tools like WinZip or gpg to encrypt individual files or collections of files. WinZip encryption, as I understand it, can be cracked with a little effort; gpg is more complex to use, but extremely secure. Other solutions exist as well, but the issue is that your encryption must be done manually.

"In the final analysis there's really no substitute for your own computer under your own control."

Another, perhaps much simpler approach is simply not to leave your data on the machine at all. Use a removable USB thumbdrive or external hard disk and take it with you.

No matter how we've secured your data, there are still a couple of other issues we need to worry about.

History - your browser history, your recently used documents list, temporary files and more all remain on the computer in what are often commonly accessible areas, or areas that are at least accessible to the administrator. At a minimum you'll wand to clear your history when your done so as to leave as little trace as possible. You may want to invest in one of the many history wiping programs that do a more thorough job.

Spyware - this is a huge issue on publicly shared computers. You don't know what's installed on the machine, and there may be keystroke loggers and other forms of spyware intercepting whatever it is you're doing. Even with a computer that's only shared between a couple of people, it's one way that person A, with administrative privileges, can spy on person B. For the record, it need not be software. There are also hardware keyloggers that could be installed on the machine.

As you can see there are several issues sharing a computer.

It all really boils down to trust. If you don't trust the people you share a computer with, then you really need to take several extra steps to keep your information secure. And even then, it's not guaranteed.

In the final analysis there's really no substitute for your own computer under your own control.

Article C2963 - March 15, 2007 « »

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Leo Leo A. Notenboom has been playing with computers since he was required to take a programming class in 1976. An 18 year career as a programmer at Microsoft soon followed. After "retiring" in 2001, Leo started Ask Leo! in 2003 as a place for answers to common computer and technical questions. More about Leo.

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3 Comments
carin chapin
October 27, 2009 9:14 AM

How can I protect myself from spyware on a public computer? We use computers at libraries when we are traveling. We do delete our history before logging off. Are there other issues we are not aware of? Thank you

Tony
November 12, 2009 6:29 PM

I just tried going into Properties and marking a file as Hidden. Now I can't figure how to find it again. ??

Janet
March 17, 2010 7:01 AM

Carin-- using a public computer, at a library or internet cafe-- is exactly the situation that Leo is warning you about. You have NO reason to trust the other people who have had access to the machine before you. Unfortunately, Leo is right: if you don't have physical control of the machine, there is NO way to be sure that malicious hardware or software hasn't been installed. There have been criminal convictions of people who installed keystroke loggers on public computers to gain access to bank accounts, steal identities, etc. This is also one of the many ways that e-mail accounts get hijacked on a daily basis. Please don't think this is a hypothetical risk; real people have had their lives messed up by exactly this sort of thing.

If you're using a public computer, you should not access a password-protected financial or business site, or a site which has your private information (to include credit card info) on file. It's fine to use public computers for non-sensitive stuff like reading your hometown newspaper when you're travelling. If you're OK with running the small-but-real risk of having your e-mail account hijacked by spammers, you can use e-mail-- better yet, get a "disposable" travel account with a free provider and abandon it after your trip.

Bottom line, assume that someone is watching everything you do and everything you type when you're on a public computer. If it's harmless, go ahead; if it's private, DON'T DO IT. A better choice: buy a cheap netbook to go with you, get a simple VPN solution to secure it, and then you have control over your situation.

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