Thou shallst not leave thy disks unencrypted — part 1

When browsing the BdrD about disk encryption, I found that a lot of people seem to encrypt their disks… at least much more than I would have expected. Under Linux, there are four recommended ways  to do it: loop-aes, truecrypt, dm-crypt or LUKS.

I identified 3 main reasons that folks give to justify why bothering about encryption:

• they want to protect their data from thieves if their laptop/HHD/flashdisk … is stolen;
• they want to protect their data from government/secret agencies/police/[anything that represent authority];
• filesystem encryption is more convenient than per-file encryption (with, say, gpg or openssl).

What purely astonished me is that for most people who participate in “security oriented” fora are paranoid for the sake of being paranoid. For instance, I’ve hardly read anything about relative value of data that could be encrypted. Oddly, estimating the value of one information seems to be the thing to wonder about. Being paranoid for the sake of it is expensive, bothersome and useless: would you stop working because chances to die caught by an accident is higher outside than inside your house? Would you stop using oil and barbecue because fried and grilled food is carcinogen?

I’m not telling you you should. My point is to estimate the risk/benefit ratio prior to take any decision.

Why do my photos and/or Powerpoint files look ugly when printed or converted to PDF?

A colleague was swearing at our printer more loudly than usual: ‘‘Believe me or not, he said, but those pictures looked just well on the screen. How does the printer dare charging me for this …’’

                                    
[Illustrative pictures. On the left: what he thought he would have; on the right: what he got.]

Well, my answer did not please him: it’s all his fault. Why? would you ask. Easy:

1. MS Powerpoint as well as Adobe Acrobat Reader both include an anti-aliasing renderer*.
2. Your screen resolution is generally around 72 dpi. As a comparison, printers can generally output at 300+ dpi.
3. You are usually not working in the real (paper) scale, instead the workspace is conveniently fitted to the screen: the 3cm×3cm nice picture you see on your screen could become a 25cm×25cm ugly picture as printed!

How is it important?

1. The anti-aliasing filter blurs the picture a bit: it looks nicer on the screen but is therefore less ‘‘detailed’’ (blurring is a lossy conversion).
2. If you look at a printed image at a suitable distance, 300 dpi will generally be enough not to perceive the individual ink dots. This is good enough for most of the purposes. However, it would not be advisable to go far below that. Please also keep in mind that computer screen image quality look decent even at lower resolution because of the underlying displaying technology: a 72 dpi printed picture does not look good.
3. The default behaviour is to import pictures at the screen resolution, not at the printer resolution. This is all wrong if you want to print your document. The default size of the displayed picture is actually calculated to reach screen resolution: it’s rarely the other way around. Typically, folks just ‘‘import’’ a picture into Powerpoint and since it takes half of the workspace and looks good, they don’t change the size and suppose it will look good as printed.

How can I prevent such behaviours?

1. Disable anti-aliasing to look at the pictures ‘‘as they are’’.
2. Whenever possible (graphs, diagram, etc.), use vector graphics such as SVG, WMF, AI and the like: it will make you PDF/PPT lighter and they do not suffer from resizing (there is no resolution per se, the quality remains identical whatever the size).
3. If vector graphics can’t be used (photos), make sure to keep the pictures resolution above 300/600 dpi. Let’s repeat that again: don’t randomly expand your pictures! Look at the image resolution: the best is to fit your printer resolution.

Oh, and by the way, since it was for a poster, MS Powerpoint was not designed to efficiently create posters. That will be the subject of a new post.

*: note that in some PDF visualization, there is no anti-aliasing filters: even in some versions of Adobe Acrobat Reader, this setting is turned off, that can explain apparent loss of quality during PPT to PDF conversion.

Linux permission issue with recent filesystems on USB

Some days ago, a guy was swearing at people on IRC about “Linux inability” to mount USB devices in rw mode with recent filesystems (ext2/3/4, btrfs, reiserfs, reiser4, xfs, …). No problem with fat though. The answers he got were almost all the same:

Edit /etc/fstab with your favorite editor and add bla bla bla […] and if it doesn’t work, just copy your files as root or come back here to create a udev rule 🙂

Another guy came up with the same problem pestering the channel …
It’s a shame because Linux is just not the one to blame in this case: nobody really understood what it implies to use ext4 on this kind of devices. Here is how to fix this “problem” once for all without further complicated configuration.

A little introduction

Hi folks,

Long time no see ! (expected: this is my first blog, we have never met but I thought you would feel more comfortable this way.)

I’ve decided to create this blog in the Big /dev/random Device (aka BdrD) to gather and share my thoughts and discoveries about computers (mostly) and science (maybe).