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Re: Captchas was Re: ISP wants to stop outgoing web based spam

  • From: Paul Jakma
  • Date: Wed Aug 16 13:53:27 2006

On Wed, 16 Aug 2006, Simon Waters wrote:

You snipped the bit where I said "It would work for a minority use."

Sorry, don't think that is relevant really - least I have no data on what minority uses are for captchas, nor majority uses or what the difference is.

The reason people use image recognition is it is something (most) humans find very easy, but requires considerable investment of effort (or resource for self training) to teach computers, and readily permits of variations ('click the kitten' being a good example).

Those need vast numbers of "kitten" pictures in order to be immune to dictionary attacks. There's a reason 'captchas' consist of auto-generated images of letters.

You can auto-generate questions too, obviously. With dictionaries of question/answer tuples associated with some template question language.

The tuples can be auto-generated, the strength lies in the variety of the question forms in use across the internet and/or across a site. The questions need not use language, they could be based on ASCII pattern matching, e.g.:


what's the next letter, etc..

Or you could simply test people on their ability to google perhaps? :)

For a demonstration of bashing at ASCII captchas try any good chat bot.

And for image captchas, see:

and there are more. CAPTCHAs are, almost by definition, compelling problems for academia to tackle ;).

The reason no one defeated your text captcha was probably because no one tried, but that won't remain the case if it gets popular. We are locked in another arms race here.

Yes, that applies regardless of the form of the captcha.

Although possibly the mistake is to assume you can distinguish between humans, and computers on the basis of intelligence.

Maybe so.

Paul Jakma	[email protected]	[email protected]	Key ID: 64A2FF6A
The meat is rotten, but the booze is holding out.

Computer translation of "The spirit is willing, but the flesh is weak."