Using WolframAlpha to Hack Text CAPTCHA

Mechanical Turk

Great discussion.

To clarify, there is not a precomputed DB of an enormous number of questions although this would be possible to derive and does occur on a lower level for caching performance purposes. The total count comes from permutation/probability maths based on the question construction algorithm -- when you request a question it generates one which means I can extend the pool quite easily without re-generating a monster cache table.

It is impressive how good Wolfram is at decoding logic, I'll have to have to think about the question construction but I can't make the question too confusing for a real person to solve. As someone mentioned, maybe more abstract questions would be stronger but the difficultly of course lies in generating them. I certainly think logic questions are weaker than a decently obscured/randomised image captcha, but they come with other advantages and work in text-only contexts (e.g. IM-type challenges).

Oh, that was just to make fun of xkcd. I don't think xkcd links are really that good an addition to a discussion here.

Thanks for doing some extraction!

Using your file I've been building up a solver for these questions in Prolog (using DCGs for parsing and simple predicates for the common sense facts and answer calculation). It's nowhere near "done", but it does get 45% of the questions now after only a few hours and 450 lines of code.

The trick to factoring someone else's generative space is to spot the symmetries and build your self a little domain-specific language for explaining those symmetries to your program.

Here's some snippets:

    tomorrow(wednesday,thursday).

    food(butter).

    body_part(arm).
    plenty(arm).
    above_waist(arm).

    ordinal(2) --> lit('2nd').

    question( tomorrow -> Answer) -->
        p(['Tomorrow is ',token(Tomorrow),'. If this is true, what is today?']),
        { tomorrow(Answer,Tomorrow) }.

    question(count_2(P) -> Answer) -->
        p(['The list ',token_list(L),' contains how many ',pred(P),'?']).
        {include(P,L,Goods), length(Goods,Answer)}.

    pred_name('something each person has more than one of',plenty).