1 / 32

RE: Captchas – Understanding Captcha - Solving services in an economic context

RE: Captchas – Understanding Captcha - Solving services in an economic context. Jade Slayter , Chris Hare, Jay Kim. What is a captcha ?. Captcha stands for Computer Automated Public Turing Test to Tell Computers and Humans apart Created to differentiate between bots and humans Must be

chyna
Download Presentation

RE: Captchas – Understanding Captcha - Solving services in an economic context

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. RE: Captchas – Understanding Captcha- Solving services in an economic context Jade Slayter, Chris Hare, Jay Kim

  2. What is a captcha? • Captcha stands for Computer Automated Public Turing Test to Tell Computers and Humans apart • Created to differentiate between bots and humans • Must be • Solved Easily by humans • Evaluated and generated easily • Hard to be solved by computers

  3. History of the Captcha • The term Was first coined in 2000 • Developed as a way to limit attacks through automated means • Attackers evolve to easily solve captchas • Scripted solvers and paid human solving services • Captchas are one of the few examples where the defenders have the advantage

  4. Automated software solvers • Advantage • Near zero marginal cost • Near infinite capacity • Uses optical character recognition to identify texts • Writing the algorithms is complex • Algorithms often fail • Changed the captchainto a question of commercial viability

  5. Xrumer • Forum spamming tool that solves Captchas • Released 2006 for $540 • Led to an arms race of captcha complexity and algorithm accuracy • Popular forum solutions have adapted so the latest Xrumer cannot crack the captchas • Simple Machine Forums still crackable • In 2009 XRUMER added human solving integration

  6. RecaptchaOCR • Created to solve the specific recaptcha service • Designed for 2008 version of recaptcha • Recaptcha updated in 2009, solver not updated • Tests ran on old and new implementation • ~30% solving accuracy for 2008 version • ~18% solving accuracy for 2009 version • Human accuracy between 75-90%

  7. Economics – Automated • Arms race between solvers and website defenses • Automated solving broken down to • Cost of development • Accuracy • Responsiveness of defending sites • Humans solvers became a cheaper solution

  8. Human Solving Services • Opportunistic Solving • Tricking users to solve CAPTCHAs • Does not play a major role • Paid services • Earliest Symantec blog in September 2006 • “Full time Captcha solver” • Approximately $10/1000

  9. Human solving services cont… (Workflow) CAPTCHA SOLVING WORKFLOW

  10. Human solving services cont… (Paid services - freelance) • After the Symantec blog post, retail solvers became prevalent. • Wages (for freelancers) • In 2007 as high as $10/1000 solved • 2008 went down to $1.50/1000 solved • 2009 $1/1000 solved • 2010 was as low as $0.50/1000

  11. Human solving services cont… (Paid services - retail) • Because freelance work became cheap, retail services were pressured to lower prices • Typethat.biz went from $1/1000 to $0.75/1000 • Some tried to tie services with a product (see picture) • This allowed for wages as high as $7/1000 (Bypass captcha and beatCaptcha) and $20/1000 (image2type) • However others (like GYC) tried to use plugins to reduce wages • According to Mr. E: 50% was profit, 10% to maintenance, the rest split between workers and incentives to partners

  12. Customer Account Creation • Very exclusive • Uses “invite” codes • Live telephone call • “Region-locked” CAPTCHA • Prepayment

  13. The Interface • Generally (though not always) Have an API • ImagetoText requires users to test api first • Others don’t have an api • Authors wrote one using ruby

  14. Pricing • Some places (Antigate and de-captcher) uses bidding system for highest priority • Generally highest bid is about $1/1000 • No (observable) price change in worker side • Implied pure profit for service provider

  15. How well do they work? Accuracy • Human solvers • Of 1,025 CAPTCHAs, 1009 were solved correctly • Seven were unreadable • 6 had ambiguous characters • 3 were ambiguous because of overlap • Captcha-solving services • 86%-89% solved (for most programs)

  16. How well Do they work? Accuracy cont… • Correct solutions dependent on services • Paypal more accurate then youku • Possibly due to being less familiar

  17. How well do they work? Response Time • On average, took about 15-20 seconds. • Response time usually beat the Internet timeout time • Fastest service took just over 9 seconds. Slowest took around a minute

  18. Capacity • Antigate was by far the best • Can solve between 27 – 41 captcha’s per second (maybe even more) • Others were able to be maxed out • Can solve anywhere between 4 per second to 15 per second • Response time and accuracy were dependent on Pacific Time

  19. Measurement Issues • Legality and ethics questioned • Legal, but related cases • Murky ethics, deemed worth the cost • Purchase supported CAPTCHA solving devs • Services not used on intended target sites • Human solved CAPTCHAs done via copies

  20. Captcha Solving Site Comparison • Similar registration • Captcha test • Simple interface given to solve new captchas • Accuracy recorded, accounts banned if inaccurate • Priority given to accurate solvers • Wages vary from 0.5-10 dollars per thousand • Leaderboards per month to encourage competition

  21. Captcha Solving Site Comparison (2) • Captchas with varying questions and answers used to guess countries based on accuracy • Translating numbers to roman numerals • Time zone question • 10% gave clear answer • China and India mostly

  22. Changes in trends • Sites update their captchaimages • Identifying animals and objects • Solving services start off with poor success and rapidly improve

  23. Targeted Sites • Global sites a big target for solving services • Local sites big for services specializing in a langauge • Top five of each site covers majority of the traffic

  24. Discussion and Conclusion • CAPTCHAs are • Simple and easy to solve by humans • “low-impact” quality appeals to sites wary of defense turning away visitors • Easily outsourced to the global unskilled labor market • Do CAPTCHAs actually work?

  25. Discussion and Conclusion cont. • Telling computers and humans apart • Preventing the automated site access • Limiting automated site access • The role of CAPTCHAs today

  26. Telling Computers and Humans Apart • Original purpose of CAPTCHAs was to distinguish humans from machines • To date, no completely general means of solving CAPTCHAs has emerged, nor is creating automated solvers viable as a business model • CAPTCHAs have succeed thus far in this regard

  27. Preventing Automated Site Access • Today, retail price for solving one million CAPTCHAs is as low as $1,000 • CAPTCHAs are an acceptable cost of doing business when measured against the value of gaining access to the protected resource • E-mail spammers using Web mail to send advertisements • Blog spammers seek organic “clicks” and influence result placement of major search engines • CAPTCHAs do not prevent large-scale automated site access.

  28. Limiting Automated Site Access • A CAPTCHA reduces an attacker’s expected profit by the cost of solving the CAPTCHA • If the attacker cannot afford this extra cost, the defense mechanism is successful • For many sites (e.g., low PageRank blogs), CAPTCHAs alone might be sufficient to dissuade abuse

  29. Limiting Automated Site Access cont… • For higher-value sites, they place a utilization constraint on otherwise “free” resources • CAPTCHAs naturally limit site access to those attackers still profitable despite these costs

  30. The Role of CAPTCHAs Today • The profitability of any scam is a function of three factors • The cost of the CAPTCHA-solving • The effectiveness of any secondary defenses (e.g., SMS validation, account shutdowns, additional CAPTCHA screens, etc.) • The efficiency of the attacker’s business model • As the cost of CAPTCHA solving decreases, a site operator must employ secondary defenses more aggressively to maintain a given level of fraud.

  31. The Role of CAPTCHAs Today cont. • Secondary defenses are invariably more expensive both in infrastructure and customer impact when compared to CAPTCHAs • The optimal point for this transition is precisely the point at which the attacker “breaks even” • CAPTCHAs while traditionally viewed as a technological impediment, should be considered more as an economic one • CAPTCHAs continue to put strain on attacker’s business models while minimizing cost and user impact of secondary defenses, but simply work less efficiently over time

  32. Questions?

More Related