Month: October 2006

RTFM, where?

Recently a buddy on msn asked me a linux question, he just started linux so he had some problems getting stuff done.

He downloaded an installer, he said, a .run, but he doesn’t know how to execute it. He tried googling for it and asking on forums, but didn’t get an answer, so he asked me.

I solved his problem, but I still wondered, where you can find that you need to put ‘./’ in front of a file in bash to execute it and where can you find that you probably need to chmod +x the file too if you downloaded it from somewhere, if you are a total newcomer to linux.

The bash tutorial would’ve probably solved it, but do you know that that thing in which you are typing actually is a separate program? Probably not.

I basically learned all this trivial stuff while following the gentoo installation manual, but I guess that’s a bit too much to ask from each new linux user. There should be a good linux introduction that explains this trivial stuff somewhere to which I can redirect new users. Anyone knows one?

Fallacies (1)

I just read this on slashdot:

statistically significant relationship between autism rates and television watching

Offcourse this relation was explained as televisions causing children to become autistic. This is a very annoying fallacy. Why? Because just maybe children with autism like to watch television more than other children.

If there is a relation, and if it’s not coincidental, there are still two ways to explain it as a cause and effect relation — you cant just blindly assume that one of the two is the cause.

Same goes with relations like:

“Children that play violent games are more likely to become criminals” or “Children that are to become criminals are more likely to play games”

Stupid shout culture.

Good and bad CAPTCHA`s

CAPTCHA’s are images which content needs to be written into a textbox by a user to make sure it’s a human instead of some computer script. This is an example of a good CAPTCHA of yahoo:

yahoo53.jpeg

This is an example of a really bad CAPTCHA:
dotmac18.jpeg

What makes a CAPTCHA good, as in hard to solve by a computer? Lets look how a computer would solve a CAPTCHA, there basically are 3 parts:

  1. Remove rubish background.
  2. Remove rubish lines and partition the image into sections, with in each section a letter.
  3. Recognize the letter with a neural network.

Part 1 is very easy in most cases — just filter everything out that isn’t black and isn’t a glyph-ic curve. It gets a bit more difficult if the font and background colors are random, but usually it’s simple to distinguish between a glyph (small, curve-ish, solid color) and a background (solid, usually gradients). Software is way better in this step than humans.

Part 2 is the most difficult part for software. Distorting fonts isn’t that much of a problem, as long as the software can recognize seperate curve-blobs. The real problem comes in when there are red-hering-curves or when several glyphs are connected with curves like in the yahoo CAPTCHA. When the captcha uses undistorted fixed aligned fonts, it isn’t a problem even if you add glyph connecting curves like in the dotmac CAPTCHA, because you only need to add a little bit of code to recognize an authentic glyph curve (small, thin) and then you can predict the position of the other curves. Humans are better in this step than computers.

Part 3 is a bit tedious for software, but usually easier for specifically trained neural networks than for humans.

How to make a good CAPTCHA:

  • Do not add stupid background or differently coloured polygons, they won’t work at all — they will only confuse the human.
  • Do not use a fixed font, size or alignment. Rotate the font a bit, transform it a bit and, most importantly, place them unpredictably.
  • Add glyph like curves the intersect preferably only two glyphs to make them less recognizable. Take care though that you don’t make them too font like, because that’ll prevent the human from recognizing. These extra intersecting curves make CAPTCHA’s strong, because it prevents proper partitioning.
  • Don’t use strange fonts that might seem hard to see, but are easy recognizable. For instance, dotted fonts are very easy to locate when everything else are solid curves.

Update: nice blogpost on breaking captcha’s: http://www.brains-n-brawn.com/default.aspx?vDir=aicaptcha

T60 (2)

After 2 days installing gentoo on my great T60 I’ve accomplished quite a lot so far:

  • WiFi works. 4 seperate components (userland, firmware, kernel module and yet another kernel module) increase complexity a bit. Although the fifth component caused trouble — I forgot to tweak conf.d/net
  • Radeon X1400 and video card worked almost directly after using aticonf — nice! When I upgraded to xorg-server-1.1, though, I got a few crashes when closing X in a not-so-nice manner.
  • Using the synaptics drivers my touchpad now support drag’n’drop, right and middle mouse click and scrolling emulation. Still got to find a way to bind the thinkpad shortcut to disable/enable the point or pad.
  • Finally got the DVD rewriter to work. It seems I overlooked one little stupid generic E-IDE option in the kernel config.
  • Bluetooth and infrared should work — haven’t got the gadgets to try them out
  • Never even saw a PCCard, so didn’t even bother to look into ti
  • Software suspend (hibernation) doens’t work yet, but I definitely be working on that.