Proof of work - an unobtrusive alternative to captchas

Re:Proof of work - an unobtrusive alternative to captchas by underwater on Friday May 06, 2011 @05:29AM (#2520459)

Not necessarily. You store which random number you sent to the client in the session on the server. Once it's been solved you invalidate it.

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Re:Proof of work - an unobtrusive alternative to captchas by tomstuart on Friday May 06, 2011 @05:32AM (#2520462)

How do you know that the response really is the smallest such integer, rather than a precomputed one (e.g. the largest int with a hash beginning "0000")?

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Re:Proof of work - an unobtrusive alternative to captchas by JoachimSchipper on Friday May 06, 2011 @06:14AM (#2520533)

You don't (have to) care about "the smallest", just "any integer with a hash beginning with 0000".

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Re:Proof of work - an unobtrusive alternative to captchas by tomgallard on Friday May 06, 2011 @06:20AM (#2520542)

But once the spamming machine has worked that out once, they can submit the same integer every time.

It would be better if the server sent a request saying send me an integer which has a hash beginning with npqr where n, p q and r are all random characters.

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