In Linux, it is possible to expand an existing filesystem over multiple disk drives. I recently had to do this for a few VPS as part of my day job, since they were running out of disk space and causing some instability with one of our core services.

Here’s how to do it, mainly for my own benefit, for when I inevitably have to do it again and have forgotten how…

  1. First, after you have connected your new drive (either physically or virtually), you need to create a new logical partition on that drive using cfdisk. You can use fdisk -l to find out the drive’s name (e.g. /dev/sdc)
  2. Create a new physical volume for this partition using pvcreate, e.g. pvcreate /dev/sdc5
  3. Extend the existing volume group to include this, e.g. vgextend VOLUMEGROUP /dev/sdc5, where VOLUMEGROUP is the name of the volume group. You can find out what the volume group is called by using the command vgdisplay.
  4. Extend the logical volume to include this extra space, e.g. lvextend -l +100%FREE /dev/VOLUMEGROUP/root
  5. Resize the filesystem. The command for this varies depending on what filesystem is currently installed, so for ext4 this would be resize2fs, and in my case for xfs this is xfs_growfs. E.g. xfs_growfs /dev/VOLUMEGROUP/root

After which, you should be able to type df -h and see increased disk space available to you.

Steganography is the term given to the art of hiding a message, for example in a photograph, in such a way that unless you know it’s there you wouldn’t suspect it was there.

While this is, to some extent, security through obscurity, it can be handy in some situations. Since a cursory look at the files will show something relatively innocuous (holiday snaps for example), an attacker may not notice the presence of the hidden data, and so move on without even attempting to break it.

There are many sophisticated technologies for doing this, however you can do a basic version using fairly standard unix tools.

Preparing your files

The first step is to encrypt your data.

To some extent, this is optional, however should your ruse be rumbled you can be sure that your precious data doesn’t fall into the wrong hands.

gpg -e -u "" -r "" businessplan.doc

Then, you compress the output using Zip. This is important, since unzip will ignore anything it doesn’t recognise as zipped data, which we’ll get onto later.

zip businessplan.doc.gpg

Hiding your file

Hiding your file in an image is relatively straightforward.

cat photo.jpg > myholiday.jpg

What’s happening here is that we’ve combined a photo and your encrypted zip file together into one file (order is important). Your image viewers will only see the first image file, and anyone looking at the directory will just see a (somewhat large) jpeg. If thumbnails are enabled you’ll just see the contents of photo.jpg.

Retrieving your file

To retrieve your file, all your recipient needs to do is run unzip the image file. Unzip will skip over the jpg content with a warning, and then reveal the hidden file. They then need to unencrypted it using their secret key.

unzip myholiday.jpg
gpg -d businessplan.doc.gpg > businessplan.doc

In conclusion

This technique will allow you to hide an encrypted file in a jpeg image, which affords you a certain amount of extra protection. Unless you know a particular image contains encrypted data (or suspect it might and look a little harder) then chances are the presence of the encrypted data won’t be discovered. However, this technique is probably pretty easy to spot if an attacker is looking for it, or performing any kind of data analysis on the file (or even looking at the file size, which could be a give away depending on how much data you’re hiding).

If you are a journalist carrying evidence of war crimes or mass surveillance programs to Brazil, you are likely facing some highly skilled adversaries, so this technique is probably not suitable. But, if you’re a business person who wants to take your new business plan securely across a border without the hassle of possibly being detained and forced to decrypt the file, then this might be more useful.

In any case, I thought it was pretty cool, and I thought I’d share.

Sometime in the next couple of weeks I will be performing a major software upgrade on the server that hosts this blog, as well as a number of services I host on behalf of my clients.

What this means to you

Hopefully nothing.

All being well, there should be no significant downtime and the services hosted by this server will continue uninterrupted.

If you are a client of mine, I will be contacting you directly in the next few days with more details about when the upgrade will be performed and how it might affect you.

I apologise in advance for any possible inconvenience this may cause.