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Text of the page (random words):
s wikibooks wikidata item appearance move to sidebar hide from wikipedia the free encyclopedia test to determine whether a user is human this article needs to be updated please help update this article to reflect recent events or newly available information june 2026 this captcha gimpy r c 2005 of smwm obscures its message from computer interpretation by twisting the letters and adding a slight background color gradient a captcha ˈ k æ p tʃ ə kap chə is a type of challenge response turing test used in computing to determine whether the user is human in order to deter bot attacks and spam 1 the term was coined in 2003 by luis von ahn manuel blum nicholas j hopper and john langford 2 it is a contrived acronym for completely automated public turing test to tell computers and humans apart 3 a historically common type of captcha displayed as recaptcha v1 was first invented in 1997 by two groups working in parallel this form of captcha requires entering a sequence of letters or numbers from a distorted image because the test is administered by a computer in contrast to the standard turing test that is administered by a human captchas are sometimes described as reverse turing tests 4 two widely used captcha services are google s recaptcha 5 6 and the independent hcaptcha 7 8 it takes the average person approximately 10 seconds to solve a typical captcha 9 purpose the purpose of captchas is to prevent spam on websites such as promotion spam registration spam and data scraping many websites use captcha effectively to prevent bot raiding captchas are designed so that humans can complete them while most robots cannot 10 newer captchas look at the user s behaviour on the internet to prove that they are a human 11 a normal captcha test only appears if the user acts like a bot such as when they request webpages or click links too fast history since the 1980s 1990s users have wanted to make text illegible to computers 12 the first such people were hackers posting about sensitive topics to internet forums they thought were being automatically monitored on keywords to circumvent such filters they replaced a word with look alike characters hello could become 3 _ _ or 3 0 and others such that a filter could not detect all of them this later became known as leetspeak 13 one of the earliest commercial uses of captchas was in the gausebeck levchin test in 2000 idrive com began to protect its signup page 14 with a captcha and prepared to file a patent 12 in 2001 paypal used such tests as part of a fraud prevention strategy in which they asked humans to retype distorted text that programs have difficulty recognizing 15 paypal co founder and cto max levchin helped commercialize this use altavista implemented an early captcha scheme to protect against fraudulent account registrations 16 17 18 a popular deployment of captcha technology recaptcha was acquired by google in 2009 19 in addition to preventing bot fraud for its users google used recaptcha and captcha technology to digitize the archives of the new york times and books from google books in 2011 20 with the rising application of ai making it feasible to defeat the tests and the appearance of scams disguised as captchas their use risks being outmoded citation needed characteristics captchas are automated requiring little human maintenance or intervention to administer producing benefits in cost and reliability 21 modern text based captchas are designed such that they require the simultaneous use of three separate abilities invariant recognition segmentation and parsing to complete the task 22 invariant recognition refers to the ability to recognize letters despite a large amount of variation in their shapes 23 segmentation is the ability to separate one letter from another made difficult in captchas parsing refers to the ability to understand the captcha holistically in order to correctly identify each character 24 each of these problems poses a significant challenge for a computer even in isolation therefore these three techniques in tandem make captchas difficult for computers to solve 25 whilst primarily used for security reasons captchas can also serve as a benchmark task for artificial intelligence technologies according to an article by ahn blum and langford 26 any program that passes the tests generated by a captcha can be used to solve a hard unsolved ai problem 27 they argue that the advantages of using hard ai problems as a means for security are twofold either the problem goes unsolved and there remains a reliable method for distinguishing humans from computers or the problem is solved and a difficult ai problem is resolved along with it 26 accessibility see also web accessibility many websites require typing a captcha when creating an account to prevent spam this image contains a user trying to type the captcha word sepalbeam to protect against automated spam captchas based on reading text or other visual perception tasks prevent blind or visually impaired users from accessing the protected resource 28 29 because captchas are designed to be unreadable by machines common assistive technology tools such as screen readers cannot interpret them the use of captcha thus excludes a small percentage of users from using significant subsets of such common web based services as paypal gmail orkut yahoo many forum and weblog systems etc 30 in certain jurisdictions site owners could become targets of litigation if they are using captchas that discriminate against certain people with disabilities for example a captcha may make a site incompatible with section 508 in the united states captchas do not have to be visual any hard artificial intelligence problem such as speech recognition can be used as captcha some implementations of captchas permit users to opt for an audio captcha such as recaptcha though a 2011 paper demonstrated a technique for defeating the popular schemes at the time 31 a method of improving captcha to ease the work with it was proposed by protectwebform and named smart captcha 32 developers are advised to combine captcha with javascript since it is hard for most bots to parse and execute javascript a combinatory method which fills the captcha fields and hides both the image and the field from human eyes was proposed 33 one alternative method involves displaying to the user a simple mathematical equation and requiring the user to enter the solution as verification although these are much easier to defeat using software they are suitable for scenarios where graphical imagery is not appropriate and they provide a much higher level of accessibility for blind users than the image based captchas these are sometimes referred to as maptchas m mathematical however these may be difficult for users with a cognitive disorder such as dyscalculia 34 challenges such as a logic puzzle or trivia question can also be used as a captcha there is research into their resistance against countermeasures 35 circumvention two main ways to bypass captcha include using cheap human labor to recognize them and using machine learning to build an automated solver 36 according to former google click fraud czar shuman ghosemajumder there are numerous services which solve captchas automatically 37 machine learning based attacks an example of a recaptcha challenge from 2007 containing the words following finding the waviness and horizontal stroke were added to increase the difficulty of breaking the captcha with a computer program a captcha usually has a text box directly underneath where the user should fill out the text that they see in this case sclt was here there was not a systematic methodology for designing or evaluating early captchas 25 as a result there were many instances in which captchas were of a fixed length and therefore automated tasks could be constructed to successfully make educated guesses about where segmentation should take place other early captchas contained limited sets of words which made the test much easier to game still others example needed made the mistake of relying too heavily on background confusion in the image in each case algorithms were created that were successfully able to complete the task by exploiting these design flaws however light changes to the captcha could thwart them modern captchas like recaptcha rely on present variations of characters that are collapsed together making them hard to segment and they have warded off automated tasks 38 in october 2013 artificial intelligence company vicarious claimed that it had developed a generic captcha solving algorithm that was able to solve modern captchas with character recognition rates of up to 90 39 however luis von ahn a pioneer of early captcha and founder of recaptcha said it s hard for me to be impressed since i see these every few months 50 similar claims to that of vicarious had been made since 2003 40 in august 2014 at usenix woot conference bursztein et al presented the first generic captcha solving algorithm based on reinforcement learning and demonstrated its efficiency against many popular captcha schemas 38 in october 2018 at acm ccs 18 conference ye et al presented a deep learning based attack that could consistently solve all 11 text captcha schemes used by the top 50 popular websites in 2018 an effective captcha solver can be trained using as few as 500 real captchas 41 human labor it is possible to subvert captchas by relaying them to a sweatshop of human operators who are employed to decode captchas a 2005 paper from a w3c working group said that they could verify hundreds per hour 28 in 2010 the university of california at san diego conducted a large scale study of captcha farms the retail price for solving one million captchas was as low as 1 000 42 another technique consists of using a script to re post the target site s captcha as a captcha to the attacker s site which unsuspecting humans visit and solve within a short while for the script to use 43 44 in 2023 researchers prompted chatgpt to trick a taskrabbit worker into solving a captcha by telling the worker it was not a robot and had impaired vision 45 outsourcing to paid services there are multiple internet companies like 2captcha and deathbycaptcha that offer human and machine backed captcha solving services for as low as us 0 50 per 1000 solved captchas 46 these services offer apis and libraries that enable users to integrate captcha circumvention into the tools that captchas were designed to block in the first place 47 insecure implementation howard yeend has identified two implementation issues with poorly designed captcha systems 48 reusing the session id of a known captcha image and captchas residing on shared servers sometimes if part of the software generating the captcha is client side the validation is done on a server but the text that the user is required to identify is rendered on the client side then users can modify the client to display the un rendered text some captcha systems use md5 hashes stored client side which may leave the captcha vulnerable to a brute force attack 49 alternative captchas some researchers have proposed alternatives including image recognition captchas which require users to identify simple objects in the images presented the argument in favor of these schemes is that tasks like object recognition are more complex to perform than text recognition and therefore should be more resilient to machine learning based attacks chew et al published their work in the 7th international information security conference isc 04 proposing three different versions of image recognition captchas and validating the proposal with user studies it is suggested that one of the versions the anomaly captcha is best with 100 of human users being able to pass an anomaly captcha with at least 90 probability in 42 seconds 50 datta et al published their paper in the acm multimedia 05 conference named imagination image generation for internet authentication proposing a systematic way to image recognition captchas images are distorted so image recognition approaches cannot recognize them 51 microsoft jeremy elson john r douceur jon howell and jared saul claim to have developed animal species image recognition for restricting access asirra which ask users to distinguish cats from dogs microsoft had a beta version of this for websites to use 52 they claim asirra is easy for users it can be solved by humans 99 6 of the time in under 30 seconds anecdotally users seemed to find the experience of using asirra much more enjoyable than a text based captcha this solution was described in a 2007 paper to proceedings of 14th acm conference on computer and communications security ccs 53 it was closed in october 2014 54 malicious captcha imitations false captchas are being used to deliver malware threat actors take advantage of verification fatigue to trick people into copying and executing console commands that download and install malicious code such as spyware in order to steal passwords mobile wallet details and other personal information such attempted frauds are reported to succeed more often than others 55 56 57 see also bot prevention defense strategy computing proof of personhood proof of work references the recaptcha project carnegie mellon university cylab www cylab cmu edu archived from the original on 27 october 2017 retrieved 13 january 2017 von ahn luis blum manuel hopper nicholas j langford john may 2003 captcha using hard ai problems for security pdf advances in cryptology eurocrypt 2003 eurocrypt 2003 international conference on the theory and applications of cryptographic techniques lecture notes in computer science vol 2656 pp 294 311 doi 10 1007 3 540 39200 9_18 isbn 978 3 540 14039 9 archived pdf from the original on 4 may 2019 retrieved 30 august 2019 what is captcha google support google inc archived from the original on 6 august 2020 retrieved 9 september 2022 captcha completely automated public turing test to tell computers and humans apart is a mayumi takaya yusuke tsuruta akihiro yamamura 30 september 2013 reverse turing test using touchscreens and captcha pdf journal of wireless mobile networks ubiquitous computing and dependable applications 4 3 41 57 doi 10 22667 jowua 2013 09 31 041 archived pdf from the original on 22 august 2017 what is recaptcha recaptcha help support google com archived from the original on 20 july 2023 retrieved 20 july 2023 sulgrove jonathan 7 july 2022 recaptcha what it is and why you should use it on your website tsts twin state technical services archived from the original on 10 november 2022 retrieved 10 november 2022 websites using hcaptcha trends builtwith com archived from the original on 10 november 2022 retrieved 10 november 2022 hcaptcha about us www hcaptcha com archived from the original on 20 july 2023 retrieved 20 july 2023 bursztein elie bethard steven fabry celine mitchell john c jurafsky dan 2010 how good are humans at solving captchas a large scale evaluation pdf 2...