PHPBB3 Captcha is super easy
A while back I presented a long-winded algorithm that would crack phpBB3 captchas. However I cracked it a while back and it’s even simpler than I said before. My floodfill routine returns the size of the area it colours in. Soooo… I flood fill background coloured pixels and if it’s a small area we assume it must be part of a letter and keep it. That gives us lots of small segments to join together.
Incidentally we find the background colour by reading the pixels along the top and finding the most regularly occuring colour.
Now we have some small segments we make them touch each other by blurring them and then we force the picture into only two colours. Then using the average density of vertical lines in each letter we rotate them to an approximately correct position. It may throw a few upside down but as long as that letter always comes out that way up the computer doesn’t care.
Now just train Gocr or a neural network or <<insert cunning program here>> to read those letters. Simple. And surprisingly accurate too. We could further improve it with colour checking routines etc but hey, it works.
Monday, May 12th, 2008
