[liviu-]

TL;DR from https://www.reddit.com/r/netsec/comments/4dvifg/im_not_a_hum... (though you may be overly-cautious as blackhat.com seems to be the webpage of a computer security conference [1]):

"""

Live attack To obtain an exact measurement of our attack’s accuracy, we run our automated captcha-breaker against reCaptcha. We employ the Clarifai service as it shows the best result amount other services.

Labelled dataset. We created a labelled dataset to exploit the image repetition. We manually labelled 3,000 images collected from challenges, and assigned each image a tag describing the content. We selected the appropriate tags from our hint list. We used pHash for the comparison, as it is very efficient, and allows our system to compare all the images from a challenge to our dataset in 3.3 seconds. We ran our captcha-breaking system against 2,235 captchas, and obtained a 70.78% accuracy. The higher accuracy compared to the simulated experiments is, at least partially, attributed to the image repetition; the history module located 1,515 sample images and 385 candidate images in our labelled dataset.

Average run time. Our attack is very efficient, with an average duration of 19.2 seconds per challenge. The most time consuming phase is running GRIS, consuming phase, as it searches for all the images in Google and processes the results, including the extraction of links that point to higher resolution versions of the images.

"""

[1] https://en.wikipedia.org/wiki/Black_Hat_Briefings