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Answer 1

+1 A:

A CAPTCHA should test whether the entity solving it is human. To my eyes, the problem you are setting looks like it would be fairly trivial to solve algorithmically.

Given that a human can pass the test, then it's certainly possible to write an automated bot which will pass it too. As to whether there is a "back door" which allows access without solving the CAPTCHA, only you can decide that by analysing your source code.

Paul Dixon 2009-07-22 09:08:54

absolutely right, solving that captcha should be a very easy, things like that will be no problem for non-humans, so the only thing you do is annoy users. There are some nice CAPTCHA Algorithms, why not using one of those?

Flo 2009-07-22 09:16:05

This captcha is fairly easy to solve by a computer, but my point is if it's save to implement it this way. A new request means a new number, still POST is sent prior to recieving data (the number), it would be a guess which number to send to the server in the POST-request?

Stefan 2009-07-22 15:12:00

My second paragraph addressed this, in other words, a human operating a typical browser can get past this simple CAPTCHA, so it wouldn't be hard to code a bot to do the same. It just has to behave like a browser. A site user could also use a GreaseMonkey script to fill in teh CAPTCHA for them too.

Paul Dixon 2009-07-22 15:57:43

Answer 2

+1 A:

Traveling Tech Guy 2009-07-22 09:15:22

The trouble with that kitten approach is that you've a 1 in 4 chance of being right. Pretty good odds if success lets you do something like send a spam message.

Paul Dixon 2009-07-22 09:19:48

Also "No one will bother developing an algorithm to analyze photos to that degree" is complete nonsense - computer vision is a very popular and active area of research. There are literally hundreds of computer scientists trying to develop such algorithms. Not for breaking CAPTCHAs, but if they succeed, spammers will certainly make use of the results. The method is viable mainly because it's a hard problem (harder than text recognition) and not yet solvable.

Michael Borgwardt 2009-07-22 09:28:25

Paul - make it 1 in 40 then. Or move the pictures around. Basically, ask a question only a human can answer, rather than confuse humans :)Michael - with some pictures, it's impossible to understand the logic from computer vision. For example: if I ask you "which of these pictures shows men's best friend?" [avoiding the word "dog"], it would require a natural language engine + a computer vision algorithm + neural network driven decision algorithm. If you have the resources to build something like that, you'll make millions selling it, and won't spend time mining emails for spam :)

Traveling Tech Guy 2009-07-22 09:50:48

Answer 3

A:

Converting strings to numbers has already been discussed in another question where many references to the google calculator were given, which does a great job in such conversions, so your approach is not suitable for testing whether your user is human.

As for an alternate solution, I can only link to another great answer.

soulmerge 2009-07-22 09:31:03

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Posting to website using captcha

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