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data-event-name="pinnable-header.vector-toc.pin">move to sidebar</button> <button class="vector-pinnable-header-toggle-button vector-pinnable-header-unpin-button" data-event-name="pinnable-header.vector-toc.unpin">hide</button> </div> <ul class="vector-toc-contents" id="mw-panel-toc-list"> <li id="toc-mw-content-text" class="vector-toc-list-item vector-toc-level-1"> <a href="#" class="vector-toc-link"> <div class="vector-toc-text">(Top)</div> </a> </li> <li id="toc-Purpose" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#Purpose"> <div class="vector-toc-text"> <span class="vector-toc-numb">1</span> <span>Purpose</span> </div> </a> <ul id="toc-Purpose-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-History" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#History"> <div class="vector-toc-text"> <span class="vector-toc-numb">2</span> <span>History</span> </div> </a> <ul id="toc-History-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Characteristics" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#Characteristics"> <div class="vector-toc-text"> <span class="vector-toc-numb">3</span> <span>Characteristics</span> </div> </a> <ul id="toc-Characteristics-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Accessibility" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#Accessibility"> <div class="vector-toc-text"> <span class="vector-toc-numb">4</span> <span>Accessibility</span> </div> </a> <ul id="toc-Accessibility-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Circumvention" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#Circumvention"> <div class="vector-toc-text"> <span class="vector-toc-numb">5</span> <span>Circumvention</span> </div> </a> <button aria-controls="toc-Circumvention-sublist" class="cdx-button cdx-button--weight-quiet cdx-button--icon-only vector-toc-toggle"> <span class="vector-icon mw-ui-icon-wikimedia-expand"></span> <span>Toggle Circumvention subsection</span> </button> <ul id="toc-Circumvention-sublist" class="vector-toc-list"> <li id="toc-Machine_learning–based_attacks" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Machine_learning–based_attacks"> <div class="vector-toc-text"> <span class="vector-toc-numb">5.1</span> <span>Machine learning–based attacks</span> </div> </a> <ul id="toc-Machine_learning–based_attacks-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Human_labor" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Human_labor"> <div class="vector-toc-text"> <span class="vector-toc-numb">5.2</span> <span>Human labor</span> </div> </a> <ul id="toc-Human_labor-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Outsourcing_to_paid_services" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Outsourcing_to_paid_services"> <div class="vector-toc-text"> <span class="vector-toc-numb">5.3</span> <span>Outsourcing to paid services</span> </div> </a> <ul id="toc-Outsourcing_to_paid_services-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Insecure_implementation" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Insecure_implementation"> <div class="vector-toc-text"> <span class="vector-toc-numb">5.4</span> <span>Insecure implementation</span> </div> </a> <ul id="toc-Insecure_implementation-sublist" class="vector-toc-list"> </ul> </li> </ul> </li> <li id="toc-Alternative_CAPTCHAs" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#Alternative_CAPTCHAs"> <div class="vector-toc-text"> <span class="vector-toc-numb">6</span> <span>Alternative CAPTCHAs</span> </div> </a> <ul id="toc-Alternative_CAPTCHAs-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-See_also" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#See_also"> <div class="vector-toc-text"> <span class="vector-toc-numb">7</span> <span>See also</span> </div> </a> <ul id="toc-See_also-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-References" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#References"> <div class="vector-toc-text"> <span class="vector-toc-numb">8</span> <span>References</span> </div> </a> <ul id="toc-References-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Further_references" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#Further_references"> <div class="vector-toc-text"> <span class="vector-toc-numb">9</span> <span>Further references</span> </div> </a> <ul id="toc-Further_references-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-External_links" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#External_links"> <div class="vector-toc-text"> <span class="vector-toc-numb">10</span> <span>External links</span> </div> </a> <ul id="toc-External_links-sublist" class="vector-toc-list"> </ul> </li> </ul> </div> </div> </nav> </div> </div> <div class="mw-content-container"> <main id="content" class="mw-body"> <header class="mw-body-header vector-page-titlebar"> <nav aria-label="Contents" class="vector-toc-landmark"> <div id="vector-page-titlebar-toc" class="vector-dropdown vector-page-titlebar-toc vector-button-flush-left" > <input type="checkbox" id="vector-page-titlebar-toc-checkbox" role="button" aria-haspopup="true" data-event-name="ui.dropdown-vector-page-titlebar-toc" class="vector-dropdown-checkbox " aria-label="Toggle the table of contents" > <label id="vector-page-titlebar-toc-label" for="vector-page-titlebar-toc-checkbox" class="vector-dropdown-label cdx-button cdx-button--fake-button cdx-button--fake-button--enabled cdx-button--weight-quiet cdx-button--icon-only " aria-hidden="true" ><span class="vector-icon mw-ui-icon-listBullet mw-ui-icon-wikimedia-listBullet"></span> <span class="vector-dropdown-label-text">Toggle the table of contents</span> </label> <div class="vector-dropdown-content"> <div id="vector-page-titlebar-toc-unpinned-container" class="vector-unpinned-container"> </div> </div> </div> </nav> <h1 id="firstHeading" class="firstHeading mw-first-heading"><span class="mw-page-title-main">CAPTCHA</span></h1> <div id="p-lang-btn" class="vector-dropdown mw-portlet mw-portlet-lang" > <input type="checkbox" id="p-lang-btn-checkbox" role="button" aria-haspopup="true" data-event-name="ui.dropdown-p-lang-btn" class="vector-dropdown-checkbox mw-interlanguage-selector" aria-label="Go to an article in another language. Available in 58 languages" > <label id="p-lang-btn-label" for="p-lang-btn-checkbox" class="vector-dropdown-label cdx-button cdx-button--fake-button cdx-button--fake-button--enabled cdx-button--weight-quiet cdx-button--action-progressive mw-portlet-lang-heading-58" aria-hidden="true" ><span class="vector-icon mw-ui-icon-language-progressive mw-ui-icon-wikimedia-language-progressive"></span> <span class="vector-dropdown-label-text">58 languages</span> </label> <div class="vector-dropdown-content"> <div class="vector-menu-content"> <ul class="vector-menu-content-list"> <li class="interlanguage-link interwiki-ar mw-list-item"><a href="https://ar.wikipedia.org/wiki/%D8%AD%D8%B1%D9%88%D9%81_%D8%AA%D8%AD%D9%82%D9%82" title="حروف تحقق – Arabic" lang="ar" hreflang="ar" data-title="حروف تحقق" data-language-autonym="العربية" data-language-local-name="Arabic" class="interlanguage-link-target"><span>العربية</span></a></li><li class="interlanguage-link interwiki-az mw-list-item"><a href="https://az.wikipedia.org/wiki/CAPTCHA" title="CAPTCHA – Azerbaijani" lang="az" hreflang="az" data-title="CAPTCHA" data-language-autonym="Azərbaycanca" data-language-local-name="Azerbaijani" class="interlanguage-link-target"><span>Azərbaycanca</span></a></li><li class="interlanguage-link interwiki-bn mw-list-item"><a href="https://bn.wikipedia.org/wiki/%E0%A6%95%E0%A7%8D%E0%A6%AF%E0%A6%BE%E0%A6%AA%E0%A6%9A%E0%A6%BE" title="ক্যাপচা – Bangla" lang="bn" hreflang="bn" data-title="ক্যাপচা" data-language-autonym="বাংলা" data-language-local-name="Bangla" class="interlanguage-link-target"><span>বাংলা</span></a></li><li class="interlanguage-link interwiki-zh-min-nan mw-list-item"><a href="https://zh-min-nan.wikipedia.org/wiki/Captcha" title="Captcha – Minnan" lang="nan" hreflang="nan" data-title="Captcha" data-language-autonym="閩南語 / Bân-lâm-gú" data-language-local-name="Minnan" class="interlanguage-link-target"><span>閩南語 / Bân-lâm-gú</span></a></li><li class="interlanguage-link interwiki-bg mw-list-item"><a href="https://bg.wikipedia.org/wiki/CAPTCHA" title="CAPTCHA – Bulgarian" lang="bg" hreflang="bg" data-title="CAPTCHA" data-language-autonym="Български" data-language-local-name="Bulgarian" class="interlanguage-link-target"><span>Български</span></a></li><li class="interlanguage-link interwiki-bs mw-list-item"><a href="https://bs.wikipedia.org/wiki/CAPTCHA" title="CAPTCHA – Bosnian" lang="bs" hreflang="bs" data-title="CAPTCHA" data-language-autonym="Bosanski" data-language-local-name="Bosnian" class="interlanguage-link-target"><span>Bosanski</span></a></li><li class="interlanguage-link interwiki-ca mw-list-item"><a href="https://ca.wikipedia.org/wiki/Test_de_CAPTCHA" title="Test de CAPTCHA – Catalan" lang="ca" hreflang="ca" data-title="Test de CAPTCHA" data-language-autonym="Català" data-language-local-name="Catalan" class="interlanguage-link-target"><span>Català</span></a></li><li class="interlanguage-link interwiki-cs mw-list-item"><a href="https://cs.wikipedia.org/wiki/CAPTCHA" title="CAPTCHA – Czech" lang="cs" hreflang="cs" data-title="CAPTCHA" data-language-autonym="Čeština" data-language-local-name="Czech" class="interlanguage-link-target"><span>Čeština</span></a></li><li class="interlanguage-link interwiki-da mw-list-item"><a href="https://da.wikipedia.org/wiki/CAPTCHA" title="CAPTCHA – Danish" lang="da" hreflang="da" data-title="CAPTCHA" data-language-autonym="Dansk" data-language-local-name="Danish" class="interlanguage-link-target"><span>Dansk</span></a></li><li class="interlanguage-link interwiki-de mw-list-item"><a href="https://de.wikipedia.org/wiki/Captcha" title="Captcha – German" lang="de" hreflang="de" data-title="Captcha" data-language-autonym="Deutsch" data-language-local-name="German" class="interlanguage-link-target"><span>Deutsch</span></a></li><li class="interlanguage-link interwiki-et mw-list-item"><a href="https://et.wikipedia.org/wiki/CAPTCHA" title="CAPTCHA – Estonian" lang="et" hreflang="et" data-title="CAPTCHA" data-language-autonym="Eesti" data-language-local-name="Estonian" class="interlanguage-link-target"><span>Eesti</span></a></li><li class="interlanguage-link interwiki-el mw-list-item"><a href="https://el.wikipedia.org/wiki/CAPTCHA" title="CAPTCHA – Greek" lang="el" hreflang="el" data-title="CAPTCHA" data-language-autonym="Ελληνικά" data-language-local-name="Greek" class="interlanguage-link-target"><span>Ελληνικά</span></a></li><li class="interlanguage-link interwiki-es mw-list-item"><a href="https://es.wikipedia.org/wiki/Captcha" title="Captcha – Spanish" lang="es" hreflang="es" data-title="Captcha" data-language-autonym="Español" data-language-local-name="Spanish" class="interlanguage-link-target"><span>Español</span></a></li><li class="interlanguage-link interwiki-eo mw-list-item"><a href="https://eo.wikipedia.org/wiki/CAPTCHA" title="CAPTCHA – Esperanto" lang="eo" hreflang="eo" data-title="CAPTCHA" data-language-autonym="Esperanto" data-language-local-name="Esperanto" class="interlanguage-link-target"><span>Esperanto</span></a></li><li class="interlanguage-link interwiki-eu mw-list-item"><a href="https://eu.wikipedia.org/wiki/Captcha" title="Captcha – Basque" lang="eu" hreflang="eu" data-title="Captcha" data-language-autonym="Euskara" data-language-local-name="Basque" class="interlanguage-link-target"><span>Euskara</span></a></li><li class="interlanguage-link interwiki-fa mw-list-item"><a href="https://fa.wikipedia.org/wiki/%DA%A9%D9%BE%DA%86%D8%A7" title="کپچا – Persian" lang="fa" hreflang="fa" data-title="کپچا" data-language-autonym="فارسی" data-language-local-name="Persian" class="interlanguage-link-target"><span>فارسی</span></a></li><li class="interlanguage-link interwiki-fr mw-list-item"><a href="https://fr.wikipedia.org/wiki/CAPTCHA" title="CAPTCHA – French" lang="fr" hreflang="fr" data-title="CAPTCHA" data-language-autonym="Français" data-language-local-name="French" class="interlanguage-link-target"><span>Français</span></a></li><li class="interlanguage-link interwiki-gl mw-list-item"><a href="https://gl.wikipedia.org/wiki/CAPTCHA" title="CAPTCHA – Galician" lang="gl" hreflang="gl" data-title="CAPTCHA" data-language-autonym="Galego" data-language-local-name="Galician" class="interlanguage-link-target"><span>Galego</span></a></li><li class="interlanguage-link interwiki-ko mw-list-item"><a href="https://ko.wikipedia.org/wiki/CAPTCHA" title="CAPTCHA – Korean" lang="ko" hreflang="ko" data-title="CAPTCHA" data-language-autonym="한국어" data-language-local-name="Korean" class="interlanguage-link-target"><span>한국어</span></a></li><li class="interlanguage-link interwiki-hy mw-list-item"><a href="https://hy.wikipedia.org/wiki/CAPTCHA" title="CAPTCHA – Armenian" lang="hy" hreflang="hy" data-title="CAPTCHA" data-language-autonym="Հայերեն" data-language-local-name="Armenian" class="interlanguage-link-target"><span>Հայերեն</span></a></li><li class="interlanguage-link interwiki-hi mw-list-item"><a href="https://hi.wikipedia.org/wiki/%E0%A4%95%E0%A5%85%E0%A4%AA%E0%A5%8D%E0%A4%9A%E0%A4%BE" title="कॅप्चा – Hindi" lang="hi" hreflang="hi" data-title="कॅप्चा" data-language-autonym="हिन्दी" data-language-local-name="Hindi" class="interlanguage-link-target"><span>हिन्दी</span></a></li><li class="interlanguage-link interwiki-hr mw-list-item"><a href="https://hr.wikipedia.org/wiki/Captcha" title="Captcha – Croatian" lang="hr" hreflang="hr" data-title="Captcha" data-language-autonym="Hrvatski" data-language-local-name="Croatian" class="interlanguage-link-target"><span>Hrvatski</span></a></li><li class="interlanguage-link interwiki-ilo mw-list-item"><a href="https://ilo.wikipedia.org/wiki/CAPTCHA" title="CAPTCHA – Iloko" lang="ilo" hreflang="ilo" data-title="CAPTCHA" data-language-autonym="Ilokano" data-language-local-name="Iloko" class="interlanguage-link-target"><span>Ilokano</span></a></li><li class="interlanguage-link interwiki-id mw-list-item"><a href="https://id.wikipedia.org/wiki/CAPTCHA" title="CAPTCHA – Indonesian" lang="id" hreflang="id" data-title="CAPTCHA" data-language-autonym="Bahasa Indonesia" data-language-local-name="Indonesian" class="interlanguage-link-target"><span>Bahasa Indonesia</span></a></li><li class="interlanguage-link interwiki-is mw-list-item"><a href="https://is.wikipedia.org/wiki/CAPTCHA" title="CAPTCHA – Icelandic" lang="is" hreflang="is" data-title="CAPTCHA" data-language-autonym="Íslenska" data-language-local-name="Icelandic" class="interlanguage-link-target"><span>Íslenska</span></a></li><li class="interlanguage-link interwiki-it mw-list-item"><a href="https://it.wikipedia.org/wiki/CAPTCHA" title="CAPTCHA – Italian" lang="it" hreflang="it" data-title="CAPTCHA" data-language-autonym="Italiano" data-language-local-name="Italian" class="interlanguage-link-target"><span>Italiano</span></a></li><li class="interlanguage-link interwiki-he mw-list-item"><a href="https://he.wikipedia.org/wiki/CAPTCHA" title="CAPTCHA – Hebrew" lang="he" hreflang="he" data-title="CAPTCHA" data-language-autonym="עברית" data-language-local-name="Hebrew" class="interlanguage-link-target"><span>עברית</span></a></li><li class="interlanguage-link interwiki-kk mw-list-item"><a href="https://kk.wikipedia.org/wiki/CAPTCHA" title="CAPTCHA – Kazakh" lang="kk" hreflang="kk" data-title="CAPTCHA" data-language-autonym="Қазақша" data-language-local-name="Kazakh" class="interlanguage-link-target"><span>Қазақша</span></a></li><li class="interlanguage-link interwiki-lv mw-list-item"><a href="https://lv.wikipedia.org/wiki/Cilv%C4%93ktests" title="Cilvēktests – Latvian" lang="lv" hreflang="lv" data-title="Cilvēktests" data-language-autonym="Latviešu" data-language-local-name="Latvian" class="interlanguage-link-target"><span>Latviešu</span></a></li><li class="interlanguage-link interwiki-lt mw-list-item"><a href="https://lt.wikipedia.org/wiki/CAPTCHA" title="CAPTCHA – Lithuanian" lang="lt" hreflang="lt" data-title="CAPTCHA" data-language-autonym="Lietuvių" data-language-local-name="Lithuanian" class="interlanguage-link-target"><span>Lietuvių</span></a></li><li class="interlanguage-link interwiki-hu mw-list-item"><a href="https://hu.wikipedia.org/wiki/Captcha" title="Captcha – Hungarian" lang="hu" hreflang="hu" data-title="Captcha" data-language-autonym="Magyar" data-language-local-name="Hungarian" class="interlanguage-link-target"><span>Magyar</span></a></li><li class="interlanguage-link interwiki-ml mw-list-item"><a href="https://ml.wikipedia.org/wiki/%E0%B4%95%E0%B4%BE%E0%B4%AA%E0%B5%8D%E0%B4%9A%E0%B5%8D%E0%B4%9A" title="കാപ്ച്ച – Malayalam" lang="ml" hreflang="ml" data-title="കാപ്ച്ച" data-language-autonym="മലയാളം" data-language-local-name="Malayalam" class="interlanguage-link-target"><span>മലയാളം</span></a></li><li class="interlanguage-link interwiki-ms mw-list-item"><a href="https://ms.wikipedia.org/wiki/CAPTCHA" title="CAPTCHA – Malay" lang="ms" hreflang="ms" data-title="CAPTCHA" data-language-autonym="Bahasa Melayu" data-language-local-name="Malay" class="interlanguage-link-target"><span>Bahasa Melayu</span></a></li><li class="interlanguage-link interwiki-nl mw-list-item"><a href="https://nl.wikipedia.org/wiki/Captcha" title="Captcha – Dutch" lang="nl" hreflang="nl" data-title="Captcha" data-language-autonym="Nederlands" data-language-local-name="Dutch" class="interlanguage-link-target"><span>Nederlands</span></a></li><li class="interlanguage-link interwiki-ja mw-list-item"><a href="https://ja.wikipedia.org/wiki/CAPTCHA" title="CAPTCHA – Japanese" lang="ja" hreflang="ja" data-title="CAPTCHA" data-language-autonym="日本語" data-language-local-name="Japanese" class="interlanguage-link-target"><span>日本語</span></a></li><li class="interlanguage-link interwiki-no mw-list-item"><a href="https://no.wikipedia.org/wiki/CAPTCHA" title="CAPTCHA – Norwegian Bokmål" lang="nb" hreflang="nb" data-title="CAPTCHA" data-language-autonym="Norsk bokmål" data-language-local-name="Norwegian Bokmål" class="interlanguage-link-target"><span>Norsk bokmål</span></a></li><li class="interlanguage-link interwiki-uz mw-list-item"><a href="https://uz.wikipedia.org/wiki/CAPTCHA" title="CAPTCHA – Uzbek" lang="uz" hreflang="uz" data-title="CAPTCHA" data-language-autonym="Oʻzbekcha / ўзбекча" data-language-local-name="Uzbek" class="interlanguage-link-target"><span>Oʻzbekcha / ўзбекча</span></a></li><li class="interlanguage-link interwiki-pl mw-list-item"><a href="https://pl.wikipedia.org/wiki/CAPTCHA" title="CAPTCHA – Polish" lang="pl" hreflang="pl" data-title="CAPTCHA" data-language-autonym="Polski" data-language-local-name="Polish" class="interlanguage-link-target"><span>Polski</span></a></li><li class="interlanguage-link interwiki-pt mw-list-item"><a href="https://pt.wikipedia.org/wiki/CAPTCHA" title="CAPTCHA – Portuguese" lang="pt" hreflang="pt" data-title="CAPTCHA" data-language-autonym="Português" data-language-local-name="Portuguese" class="interlanguage-link-target"><span>Português</span></a></li><li class="interlanguage-link interwiki-ro mw-list-item"><a href="https://ro.wikipedia.org/wiki/CAPTCHA" title="CAPTCHA – Romanian" lang="ro" hreflang="ro" data-title="CAPTCHA" data-language-autonym="Română" data-language-local-name="Romanian" class="interlanguage-link-target"><span>Română</span></a></li><li class="interlanguage-link interwiki-ru mw-list-item"><a href="https://ru.wikipedia.org/wiki/%D0%9A%D0%B0%D0%BF%D1%87%D0%B0" title="Капча – Russian" lang="ru" hreflang="ru" data-title="Капча" data-language-autonym="Русский" data-language-local-name="Russian" class="interlanguage-link-target"><span>Русский</span></a></li><li class="interlanguage-link interwiki-sa mw-list-item"><a href="https://sa.wikipedia.org/wiki/%E0%A4%B8%E0%A4%82%E0%A4%AD%E0%A5%87%E0%A4%AA%E0%A5%82%E0%A4%B8%E0%A5%8D%E0%A4%B5%E0%A4%B8%E0%A4%BE%E0%A4%9F%E0%A5%8D%E0%A4%AF%E0%A5%82%E0%A4%AA" title="संभेपूस्वसाट्यूप – Sanskrit" lang="sa" hreflang="sa" data-title="संभेपूस्वसाट्यूप" data-language-autonym="संस्कृतम्" data-language-local-name="Sanskrit" class="interlanguage-link-target"><span>संस्कृतम्</span></a></li><li class="interlanguage-link interwiki-scn mw-list-item"><a href="https://scn.wikipedia.org/wiki/CAPTCHA" title="CAPTCHA – Sicilian" lang="scn" hreflang="scn" data-title="CAPTCHA" data-language-autonym="Sicilianu" data-language-local-name="Sicilian" class="interlanguage-link-target"><span>Sicilianu</span></a></li><li class="interlanguage-link interwiki-si mw-list-item"><a href="https://si.wikipedia.org/wiki/%E0%B6%9A%E0%B7%90%E0%B6%B4%E0%B7%8A%E0%B6%A0%E0%B7%8F_%E0%B6%B4%E0%B7%8A%E2%80%8D%E0%B6%BB%E0%B7%84%E0%B7%9A%E0%B6%BD%E0%B7%92%E0%B6%9A%E0%B7%8F" title="කැප්චා ප්‍රහේලිකා – Sinhala" lang="si" hreflang="si" data-title="කැප්චා ප්‍රහේලිකා" data-language-autonym="සිංහල" data-language-local-name="Sinhala" class="interlanguage-link-target"><span>සිංහල</span></a></li><li class="interlanguage-link interwiki-simple mw-list-item"><a href="https://simple.wikipedia.org/wiki/CAPTCHA" title="CAPTCHA – Simple English" lang="en-simple" hreflang="en-simple" data-title="CAPTCHA" data-language-autonym="Simple English" data-language-local-name="Simple English" class="interlanguage-link-target"><span>Simple English</span></a></li><li class="interlanguage-link interwiki-sl mw-list-item"><a href="https://sl.wikipedia.org/wiki/CAPTCHA" title="CAPTCHA – Slovenian" lang="sl" hreflang="sl" data-title="CAPTCHA" data-language-autonym="Slovenščina" data-language-local-name="Slovenian" class="interlanguage-link-target"><span>Slovenščina</span></a></li><li class="interlanguage-link interwiki-sr mw-list-item"><a href="https://sr.wikipedia.org/wiki/%D0%9A%D0%B0%D0%BF%D1%87%D0%B0" title="Капча – Serbian" lang="sr" hreflang="sr" data-title="Капча" data-language-autonym="Српски / srpski" data-language-local-name="Serbian" class="interlanguage-link-target"><span>Српски / srpski</span></a></li><li class="interlanguage-link interwiki-fi mw-list-item"><a href="https://fi.wikipedia.org/wiki/CAPTCHA" title="CAPTCHA – Finnish" lang="fi" hreflang="fi" data-title="CAPTCHA" data-language-autonym="Suomi" data-language-local-name="Finnish" class="interlanguage-link-target"><span>Suomi</span></a></li><li class="interlanguage-link interwiki-sv mw-list-item"><a href="https://sv.wikipedia.org/wiki/Robotfilter" title="Robotfilter – Swedish" lang="sv" hreflang="sv" data-title="Robotfilter" data-language-autonym="Svenska" data-language-local-name="Swedish" class="interlanguage-link-target"><span>Svenska</span></a></li><li class="interlanguage-link interwiki-ta mw-list-item"><a href="https://ta.wikipedia.org/wiki/%E0%AE%95%E0%AE%BE%E0%AE%AA%E0%AF%8D%E0%AE%9A%E0%AE%BE" title="காப்சா – Tamil" lang="ta" hreflang="ta" data-title="காப்சா" data-language-autonym="தமிழ்" data-language-local-name="Tamil" class="interlanguage-link-target"><span>தமிழ்</span></a></li><li class="interlanguage-link interwiki-te mw-list-item"><a href="https://te.wikipedia.org/wiki/%E0%B0%95%E0%B0%BE%E0%B0%AA%E0%B1%8D%E0%B0%9A%E0%B0%BE" title="కాప్చా – Telugu" lang="te" hreflang="te" data-title="కాప్చా" data-language-autonym="తెలుగు" data-language-local-name="Telugu" class="interlanguage-link-target"><span>తెలుగు</span></a></li><li class="interlanguage-link interwiki-th mw-list-item"><a href="https://th.wikipedia.org/wiki/%E0%B9%81%E0%B8%84%E0%B8%9B%E0%B8%95%E0%B9%8C%E0%B8%8A%E0%B8%B2" title="แคปต์ชา – Thai" lang="th" hreflang="th" data-title="แคปต์ชา" data-language-autonym="ไทย" data-language-local-name="Thai" class="interlanguage-link-target"><span>ไทย</span></a></li><li class="interlanguage-link interwiki-tg mw-list-item"><a href="https://tg.wikipedia.org/wiki/%D0%9A%D0%B0%D0%BF%D1%87%D0%B0" title="Капча – Tajik" lang="tg" hreflang="tg" data-title="Капча" data-language-autonym="Тоҷикӣ" data-language-local-name="Tajik" class="interlanguage-link-target"><span>Тоҷикӣ</span></a></li><li class="interlanguage-link interwiki-tr mw-list-item"><a href="https://tr.wikipedia.org/wiki/CAPTCHA" title="CAPTCHA – Turkish" lang="tr" hreflang="tr" data-title="CAPTCHA" data-language-autonym="Türkçe" data-language-local-name="Turkish" class="interlanguage-link-target"><span>Türkçe</span></a></li><li class="interlanguage-link interwiki-uk mw-list-item"><a href="https://uk.wikipedia.org/wiki/CAPTCHA" title="CAPTCHA – Ukrainian" lang="uk" hreflang="uk" data-title="CAPTCHA" data-language-autonym="Українська" data-language-local-name="Ukrainian" class="interlanguage-link-target"><span>Українська</span></a></li><li class="interlanguage-link interwiki-vi mw-list-item"><a href="https://vi.wikipedia.org/wiki/CAPTCHA" title="CAPTCHA – Vietnamese" lang="vi" hreflang="vi" data-title="CAPTCHA" data-language-autonym="Tiếng Việt" data-language-local-name="Vietnamese" class="interlanguage-link-target"><span>Tiếng Việt</span></a></li><li class="interlanguage-link interwiki-wuu mw-list-item"><a href="https://wuu.wikipedia.org/wiki/%E9%AA%8C%E8%AF%81%E7%A0%81" title="验证码 – Wu" lang="wuu" hreflang="wuu" data-title="验证码" data-language-autonym="吴语" data-language-local-name="Wu" class="interlanguage-link-target"><span>吴语</span></a></li><li class="interlanguage-link interwiki-zh mw-list-item"><a href="https://zh.wikipedia.org/wiki/%E9%AA%8C%E8%AF%81%E7%A0%81" title="验证码 – Chinese" lang="zh" hreflang="zh" data-title="验证码" data-language-autonym="中文" data-language-local-name="Chinese" class="interlanguage-link-target"><span>中文</span></a></li> </ul> <div class="after-portlet after-portlet-lang"><span class="wb-langlinks-edit wb-langlinks-link"><a href="https://www.wikidata.org/wiki/Special:EntityPage/Q484598#sitelinks-wikipedia" title="Edit interlanguage links" class="wbc-editpage">Edit links</a></span></div> </div> </div> </div> </header> <div class="vector-page-toolbar"> <div 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You can view its source [e]" accesskey="e"><span>View source</span></a></li><li id="ca-history" class="vector-tab-noicon mw-list-item"><a href="/w/index.php?title=CAPTCHA&action=history" title="Past revisions of this page [h]" accesskey="h"><span>View history</span></a></li> </ul> </div> </div> </nav> <nav class="vector-page-tools-landmark" aria-label="Page tools"> <div id="vector-page-tools-dropdown" class="vector-dropdown vector-page-tools-dropdown" > <input type="checkbox" id="vector-page-tools-dropdown-checkbox" role="button" aria-haspopup="true" data-event-name="ui.dropdown-vector-page-tools-dropdown" class="vector-dropdown-checkbox " aria-label="Tools" > <label id="vector-page-tools-dropdown-label" for="vector-page-tools-dropdown-checkbox" class="vector-dropdown-label cdx-button cdx-button--fake-button cdx-button--fake-button--enabled cdx-button--weight-quiet" aria-hidden="true" ><span class="vector-dropdown-label-text">Tools</span> </label> <div class="vector-dropdown-content"> <div id="vector-page-tools-unpinned-container" class="vector-unpinned-container"> <div id="vector-page-tools" class="vector-page-tools vector-pinnable-element"> <div class="vector-pinnable-header vector-page-tools-pinnable-header vector-pinnable-header-unpinned" data-feature-name="page-tools-pinned" data-pinnable-element-id="vector-page-tools" data-pinned-container-id="vector-page-tools-pinned-container" data-unpinned-container-id="vector-page-tools-unpinned-container" > <div class="vector-pinnable-header-label">Tools</div> <button class="vector-pinnable-header-toggle-button vector-pinnable-header-pin-button" data-event-name="pinnable-header.vector-page-tools.pin">move to sidebar</button> <button class="vector-pinnable-header-toggle-button vector-pinnable-header-unpin-button" data-event-name="pinnable-header.vector-page-tools.unpin">hide</button> </div> <div id="p-cactions" class="vector-menu mw-portlet mw-portlet-cactions emptyPortlet vector-has-collapsible-items" title="More options" > <div class="vector-menu-heading"> Actions </div> <div class="vector-menu-content"> <ul class="vector-menu-content-list"> <li id="ca-more-view" class="selected vector-more-collapsible-item mw-list-item"><a href="/wiki/CAPTCHA"><span>Read</span></a></li><li id="ca-more-viewsource" class="vector-more-collapsible-item mw-list-item"><a href="/w/index.php?title=CAPTCHA&action=edit"><span>View source</span></a></li><li id="ca-more-history" class="vector-more-collapsible-item mw-list-item"><a href="/w/index.php?title=CAPTCHA&action=history"><span>View history</span></a></li> </ul> </div> </div> <div id="p-tb" class="vector-menu mw-portlet mw-portlet-tb" > <div class="vector-menu-heading"> General </div> <div class="vector-menu-content"> <ul class="vector-menu-content-list"> <li id="t-whatlinkshere" class="mw-list-item"><a href="/wiki/Special:WhatLinksHere/CAPTCHA" title="List of all English Wikipedia pages containing links to this page [j]" accesskey="j"><span>What links here</span></a></li><li id="t-recentchangeslinked" class="mw-list-item"><a href="/wiki/Special:RecentChangesLinked/CAPTCHA" rel="nofollow" title="Recent changes in pages linked from this page [k]" accesskey="k"><span>Related changes</span></a></li><li id="t-upload" class="mw-list-item"><a href="/wiki/Wikipedia:File_Upload_Wizard" title="Upload files [u]" accesskey="u"><span>Upload file</span></a></li><li id="t-specialpages" class="mw-list-item"><a href="/wiki/Special:SpecialPages" title="A list of all special pages [q]" accesskey="q"><span>Special pages</span></a></li><li id="t-permalink" class="mw-list-item"><a href="/w/index.php?title=CAPTCHA&oldid=1256528649" title="Permanent link to this revision of this page"><span>Permanent link</span></a></li><li id="t-info" class="mw-list-item"><a href="/w/index.php?title=CAPTCHA&action=info" title="More information about this page"><span>Page information</span></a></li><li id="t-cite" class="mw-list-item"><a href="/w/index.php?title=Special:CiteThisPage&page=CAPTCHA&id=1256528649&wpFormIdentifier=titleform" title="Information on how to cite this page"><span>Cite this page</span></a></li><li id="t-urlshortener" class="mw-list-item"><a href="/w/index.php?title=Special:UrlShortener&url=https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FCAPTCHA"><span>Get shortened URL</span></a></li><li id="t-urlshortener-qrcode" class="mw-list-item"><a href="/w/index.php?title=Special:QrCode&url=https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FCAPTCHA"><span>Download QR code</span></a></li> </ul> </div> </div> <div id="p-coll-print_export" class="vector-menu mw-portlet mw-portlet-coll-print_export" > <div class="vector-menu-heading"> Print/export </div> <div class="vector-menu-content"> <ul class="vector-menu-content-list"> <li id="coll-download-as-rl" class="mw-list-item"><a href="/w/index.php?title=Special:DownloadAsPdf&page=CAPTCHA&action=show-download-screen" title="Download this page as a PDF file"><span>Download as PDF</span></a></li><li id="t-print" class="mw-list-item"><a href="/w/index.php?title=CAPTCHA&printable=yes" title="Printable version of this page [p]" accesskey="p"><span>Printable version</span></a></li> </ul> </div> </div> <div id="p-wikibase-otherprojects" class="vector-menu mw-portlet mw-portlet-wikibase-otherprojects" > <div class="vector-menu-heading"> In other projects </div> <div class="vector-menu-content"> <ul class="vector-menu-content-list"> <li class="wb-otherproject-link wb-otherproject-commons mw-list-item"><a href="https://commons.wikimedia.org/wiki/Category:Captcha" hreflang="en"><span>Wikimedia Commons</span></a></li><li class="wb-otherproject-link wb-otherproject-wikibooks mw-list-item"><a href="https://en.wikibooks.org/wiki/CAPTCHA" hreflang="en"><span>Wikibooks</span></a></li><li id="t-wikibase" class="wb-otherproject-link wb-otherproject-wikibase-dataitem mw-list-item"><a href="https://www.wikidata.org/wiki/Special:EntityPage/Q484598" title="Structured data on this page hosted by Wikidata [g]" accesskey="g"><span>Wikidata item</span></a></li> </ul> </div> </div> </div> </div> </div> </div> </nav> </div> </div> </div> <div class="vector-column-end"> <div class="vector-sticky-pinned-container"> <nav class="vector-page-tools-landmark" aria-label="Page tools"> <div id="vector-page-tools-pinned-container" class="vector-pinned-container"> </div> </nav> <nav class="vector-appearance-landmark" aria-label="Appearance"> <div id="vector-appearance-pinned-container" class="vector-pinned-container"> <div id="vector-appearance" class="vector-appearance vector-pinnable-element"> <div class="vector-pinnable-header vector-appearance-pinnable-header vector-pinnable-header-pinned" data-feature-name="appearance-pinned" data-pinnable-element-id="vector-appearance" data-pinned-container-id="vector-appearance-pinned-container" data-unpinned-container-id="vector-appearance-unpinned-container" > <div class="vector-pinnable-header-label">Appearance</div> <button class="vector-pinnable-header-toggle-button vector-pinnable-header-pin-button" data-event-name="pinnable-header.vector-appearance.pin">move to sidebar</button> <button class="vector-pinnable-header-toggle-button vector-pinnable-header-unpin-button" data-event-name="pinnable-header.vector-appearance.unpin">hide</button> </div> </div> </div> </nav> </div> </div> <div id="bodyContent" class="vector-body" aria-labelledby="firstHeading" data-mw-ve-target-container> <div class="vector-body-before-content"> <div class="mw-indicators"> <div id="mw-indicator-pp-default" class="mw-indicator"><div class="mw-parser-output"><span typeof="mw:File"><a href="/wiki/Wikipedia:Protection_policy#semi" title="This article is semi-protected due to vandalism"><img alt="Page semi-protected" src="//upload.wikimedia.org/wikipedia/en/thumb/1/1b/Semi-protection-shackle.svg/20px-Semi-protection-shackle.svg.png" decoding="async" width="20" height="20" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/en/thumb/1/1b/Semi-protection-shackle.svg/30px-Semi-protection-shackle.svg.png 1.5x, //upload.wikimedia.org/wikipedia/en/thumb/1/1b/Semi-protection-shackle.svg/40px-Semi-protection-shackle.svg.png 2x" data-file-width="512" data-file-height="512" /></a></span></div></div> </div> <div id="siteSub" class="noprint">From Wikipedia, the free encyclopedia</div> </div> <div id="contentSub"><div id="mw-content-subtitle"><span class="mw-redirectedfrom">(Redirected from <a href="/w/index.php?title=CAPTCHAs&redirect=no" class="mw-redirect" title="CAPTCHAs">CAPTCHAs</a>)</span></div></div> <div id="mw-content-text" class="mw-body-content"><div class="mw-content-ltr mw-parser-output" lang="en" dir="ltr"><div class="shortdescription nomobile noexcerpt noprint searchaux" style="display:none">Test to determine whether a user is human</div> <p class="mw-empty-elt"> </p> <style data-mw-deduplicate="TemplateStyles:r1251242444">.mw-parser-output .ambox{border:1px solid #a2a9b1;border-left:10px solid #36c;background-color:#fbfbfb;box-sizing:border-box}.mw-parser-output .ambox+link+.ambox,.mw-parser-output .ambox+link+style+.ambox,.mw-parser-output .ambox+link+link+.ambox,.mw-parser-output .ambox+.mw-empty-elt+link+.ambox,.mw-parser-output .ambox+.mw-empty-elt+link+style+.ambox,.mw-parser-output .ambox+.mw-empty-elt+link+link+.ambox{margin-top:-1px}html body.mediawiki .mw-parser-output .ambox.mbox-small-left{margin:4px 1em 4px 0;overflow:hidden;width:238px;border-collapse:collapse;font-size:88%;line-height:1.25em}.mw-parser-output .ambox-speedy{border-left:10px solid #b32424;background-color:#fee7e6}.mw-parser-output .ambox-delete{border-left:10px solid #b32424}.mw-parser-output .ambox-content{border-left:10px solid #f28500}.mw-parser-output .ambox-style{border-left:10px solid #fc3}.mw-parser-output .ambox-move{border-left:10px solid #9932cc}.mw-parser-output .ambox-protection{border-left:10px solid #a2a9b1}.mw-parser-output .ambox .mbox-text{border:none;padding:0.25em 0.5em;width:100%}.mw-parser-output .ambox .mbox-image{border:none;padding:2px 0 2px 0.5em;text-align:center}.mw-parser-output .ambox .mbox-imageright{border:none;padding:2px 0.5em 2px 0;text-align:center}.mw-parser-output .ambox .mbox-empty-cell{border:none;padding:0;width:1px}.mw-parser-output .ambox .mbox-image-div{width:52px}@media(min-width:720px){.mw-parser-output .ambox{margin:0 10%}}@media print{body.ns-0 .mw-parser-output .ambox{display:none!important}}</style><table class="box-Cleanup_rewrite plainlinks metadata ambox ambox-content" role="presentation"><tbody><tr><td class="mbox-image"><div class="mbox-image-div"><span typeof="mw:File"><a href="/wiki/File:Crystal_Clear_app_kedit.svg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/e/e8/Crystal_Clear_app_kedit.svg/40px-Crystal_Clear_app_kedit.svg.png" decoding="async" width="40" height="40" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/e/e8/Crystal_Clear_app_kedit.svg/60px-Crystal_Clear_app_kedit.svg.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/e/e8/Crystal_Clear_app_kedit.svg/80px-Crystal_Clear_app_kedit.svg.png 2x" data-file-width="128" data-file-height="128" /></a></span></div></td><td class="mbox-text"><div class="mbox-text-span">This article <b>may need to be rewritten</b> to comply with Wikipedia's <a href="/wiki/Wikipedia:Manual_of_Style" title="Wikipedia:Manual of Style">quality standards</a>, as It feels like an essay criticising CAPTCHA.<span class="hide-when-compact"> <a class="external text" href="https://en.wikipedia.org/w/index.php?title=CAPTCHA&action=edit">You can help</a>. The <a href="/wiki/Talk:CAPTCHA" title="Talk:CAPTCHA">talk page</a> may contain suggestions.</span> <span class="date-container"><i>(<span class="date">November 2022</span>)</i></span></div></td></tr></tbody></table> <p class="mw-empty-elt"> </p> <figure class="mw-default-size" typeof="mw:File/Thumb"><a href="/wiki/File:Captcha.jpg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/6/69/Captcha.jpg" decoding="async" width="290" height="80" class="mw-file-element" data-file-width="290" data-file-height="80" /></a><figcaption>This CAPTCHA (<a href="/wiki/ReCAPTCHA_v1" class="mw-redirect" title="ReCAPTCHA v1">reCAPTCHA v1</a>) of "smwm" obscures its message from computer interpretation by twisting the letters and adding a slight background color gradient.</figcaption></figure> <p>A <b>CAPTCHA</b> (<span class="rt-commentedText nowrap"><span class="IPA nopopups noexcerpt" lang="en-fonipa"><a href="/wiki/Help:IPA/English" title="Help:IPA/English">/<span style="border-bottom:1px dotted"><span title="/ˈ/: primary stress follows">ˈ</span><span title="'k' in 'kind'">k</span><span title="/æ/: 'a' in 'bad'">æ</span><span title="'p' in 'pie'">p</span><span title="/./: syllable break">.</span><span title="/tʃ/: 'ch' in 'China'">tʃ</span><span title="/ə/: 'a' in 'about'">ə</span></span>/</a></span></span> <a href="/wiki/Help:Pronunciation_respelling_key" title="Help:Pronunciation respelling key"><i title="English pronunciation respelling"><span style="font-size:90%">KAP</span>-chə</i></a>) is a type of <a href="/wiki/Challenge%E2%80%93response_authentication" title="Challenge–response authentication">challenge–response</a> test used in <a href="/wiki/Computing" title="Computing">computing</a> to determine whether the user is human in order to deter bot attacks and spam.<sup id="cite_ref-1" class="reference"><a href="#cite_note-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> </p><p>The term was coined in 2003 by <a href="/wiki/Luis_von_Ahn" title="Luis von Ahn">Luis von Ahn</a>, <a href="/wiki/Manuel_Blum" title="Manuel Blum">Manuel Blum</a>, Nicholas J. Hopper, and <a href="/wiki/John_Langford_(computer_scientist)" title="John Langford (computer scientist)">John Langford</a>.<sup id="cite_ref-abhl_2-0" class="reference"><a href="#cite_note-abhl-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup> It is a <a href="/wiki/Contrived_acronym" class="mw-redirect" title="Contrived acronym">contrived acronym</a> for "Completely Automated Public <a href="/wiki/Turing_test" title="Turing test">Turing test</a> to tell Computers and Humans Apart."<sup id="cite_ref-3" class="reference"><a href="#cite_note-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup> A historically common type of CAPTCHA (displayed as <a href="/wiki/ReCAPTCHA_v1" class="mw-redirect" title="ReCAPTCHA v1">reCAPTCHA v1</a>) was first invented in 1997 by two groups working in parallel. This form of CAPTCHA requires entering a sequence of letters or numbers in 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 <a href="/wiki/Reverse_Turing_test" title="Reverse Turing test">reverse Turing tests</a>.<sup id="cite_ref-4" class="reference"><a href="#cite_note-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup> </p><p>Two widely used CAPTCHA services are <a href="/wiki/Google" title="Google">Google</a>'s <a href="/wiki/ReCAPTCHA" title="ReCAPTCHA">reCAPTCHA</a><sup id="cite_ref-5" class="reference"><a href="#cite_note-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-6" class="reference"><a href="#cite_note-6"><span class="cite-bracket">[</span>6<span class="cite-bracket">]</span></a></sup> and the independent hCaptcha.<sup id="cite_ref-7" class="reference"><a href="#cite_note-7"><span class="cite-bracket">[</span>7<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-8" class="reference"><a href="#cite_note-8"><span class="cite-bracket">[</span>8<span class="cite-bracket">]</span></a></sup> It takes the average person approximately 10 seconds to solve a typical CAPTCHA.<sup id="cite_ref-9" class="reference"><a href="#cite_note-9"><span class="cite-bracket">[</span>9<span class="cite-bracket">]</span></a></sup> </p> <meta property="mw:PageProp/toc" /> <div class="mw-heading mw-heading2"><h2 id="Purpose">Purpose</h2></div> <p>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.<sup id="cite_ref-10" class="reference"><a href="#cite_note-10"><span class="cite-bracket">[</span>10<span class="cite-bracket">]</span></a></sup> Newer CAPTCHAs look at the user's behaviour on the internet, to prove that they are a human.<sup id="cite_ref-11" class="reference"><a href="#cite_note-11"><span class="cite-bracket">[</span>11<span class="cite-bracket">]</span></a></sup> A normal CAPTCHA test only appears if the user acts like a bot, such as when they request webpages, or click links too fast. </p> <div class="mw-heading mw-heading2"><h2 id="History">History</h2></div> <p>Since the 1980s–1990s, users have wanted to make text illegible to computers.<sup id="cite_ref-:1_12-0" class="reference"><a href="#cite_note-:1-12"><span class="cite-bracket">[</span>12<span class="cite-bracket">]</span></a></sup> The first such people were <a href="/wiki/Hacker_culture" title="Hacker culture">hackers</a>, posting about sensitive topics to <a href="/wiki/Internet_forum" title="Internet forum">Internet forums</a> they thought were being automatically monitored on keywords. To circumvent such filters, they replaced a word with look-alike characters. <i>HELLO</i> could become <span class="nowrap"><code>|-|3|_|_()</code></span> or <span class="nowrap"><code>)-(3££0</code></span>, and others, such that a filter could not detect <i>all</i> of them. This later became known as <a href="/wiki/Leet" title="Leet">leetspeak</a>.<sup id="cite_ref-13" class="reference"><a href="#cite_note-13"><span class="cite-bracket">[</span>13<span class="cite-bracket">]</span></a></sup> </p><p>One of the earliest commercial uses of CAPTCHAs was in the Gausebeck–Levchin test. In 2000, idrive.com began to protect its signup page<sup id="cite_ref-14" class="reference"><a href="#cite_note-14"><span class="cite-bracket">[</span>14<span class="cite-bracket">]</span></a></sup> with a CAPTCHA and prepared to file a patent.<sup id="cite_ref-:1_12-1" class="reference"><a href="#cite_note-:1-12"><span class="cite-bracket">[</span>12<span class="cite-bracket">]</span></a></sup> In 2001, <a href="/wiki/PayPal" title="PayPal">PayPal</a> used such tests as part of a fraud prevention strategy in which they asked humans to "retype distorted text that programs have difficulty recognizing."<sup id="cite_ref-stringham2015_15-0" class="reference"><a href="#cite_note-stringham2015-15"><span class="cite-bracket">[</span>15<span class="cite-bracket">]</span></a></sup> PayPal co founder and CTO <a href="/wiki/Max_Levchin" title="Max Levchin">Max Levchin</a> helped commercialize this use. </p><p>A popular deployment of CAPTCHA technology, <a href="/wiki/ReCAPTCHA" title="ReCAPTCHA">reCAPTCHA</a>, was acquired by Google in 2009.<sup id="cite_ref-16" class="reference"><a href="#cite_note-16"><span class="cite-bracket">[</span>16<span class="cite-bracket">]</span></a></sup> In addition to preventing bot fraud for its users, Google used reCAPTCHA and CAPTCHA technology to digitize the archives of <i><a href="/wiki/The_New_York_Times" title="The New York Times">The New York Times</a></i> and books from Google Books in 2011.<sup id="cite_ref-17" class="reference"><a href="#cite_note-17"><span class="cite-bracket">[</span>17<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading2"><h2 id="Characteristics">Characteristics</h2></div> <p>CAPTCHAs are automated, requiring little human maintenance or intervention to administer, producing benefits in cost and reliability.<sup id="cite_ref-18" class="reference"><a href="#cite_note-18"><span class="cite-bracket">[</span>18<span class="cite-bracket">]</span></a></sup> </p><p>Modern text-based CAPTCHAs are designed such that they require the simultaneous use of three separate abilities—invariant recognition, <a href="/wiki/Image_segmentation" title="Image segmentation">segmentation</a>, and parsing to complete the task.<sup id="cite_ref-19" class="reference"><a href="#cite_note-19"><span class="cite-bracket">[</span>19<span class="cite-bracket">]</span></a></sup> </p> <ul><li>Invariant recognition refers to the ability to recognize letters despite a large amount of variation in their shapes.<sup id="cite_ref-20" class="reference"><a href="#cite_note-20"><span class="cite-bracket">[</span>20<span class="cite-bracket">]</span></a></sup></li> <li>Segmentation is the ability to separate one letter from another, made difficult in CAPTCHAs.</li> <li>Parsing refers to the ability to understand the CAPTCHA holistically, in order to correctly identify each character.<sup id="cite_ref-21" class="reference"><a href="#cite_note-21"><span class="cite-bracket">[</span>21<span class="cite-bracket">]</span></a></sup></li></ul> <p>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.<sup id="cite_ref-bursz_22-0" class="reference"><a href="#cite_note-bursz-22"><span class="cite-bracket">[</span>22<span class="cite-bracket">]</span></a></sup> </p><p>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,<sup id="cite_ref-Ahn2003_23-0" class="reference"><a href="#cite_note-Ahn2003-23"><span class="cite-bracket">[</span>23<span class="cite-bracket">]</span></a></sup> "any program that passes the tests generated by a CAPTCHA can be used to solve a hard unsolved AI problem."<sup id="cite_ref-24" class="reference"><a href="#cite_note-24"><span class="cite-bracket">[</span>24<span class="cite-bracket">]</span></a></sup> They argue that the advantages of using <a href="/wiki/AI-complete" title="AI-complete">hard AI</a> 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.<sup id="cite_ref-Ahn2003_23-1" class="reference"><a href="#cite_note-Ahn2003-23"><span class="cite-bracket">[</span>23<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading2"><h2 id="Accessibility">Accessibility</h2></div> <style data-mw-deduplicate="TemplateStyles:r1236090951">.mw-parser-output .hatnote{font-style:italic}.mw-parser-output div.hatnote{padding-left:1.6em;margin-bottom:0.5em}.mw-parser-output .hatnote i{font-style:normal}.mw-parser-output .hatnote+link+.hatnote{margin-top:-0.5em}@media print{body.ns-0 .mw-parser-output .hatnote{display:none!important}}</style><div role="note" class="hatnote navigation-not-searchable">See also: <a href="/wiki/Web_accessibility" title="Web accessibility">Web accessibility</a></div> <figure class="mw-halign-left" typeof="mw:File/Thumb"><a href="/wiki/File:FancyCaptcha_screenshot.png" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/9/9d/FancyCaptcha_screenshot.png/260px-FancyCaptcha_screenshot.png" decoding="async" width="260" height="144" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/9/9d/FancyCaptcha_screenshot.png/390px-FancyCaptcha_screenshot.png 1.5x, //upload.wikimedia.org/wikipedia/commons/9/9d/FancyCaptcha_screenshot.png 2x" data-file-width="462" data-file-height="255" /></a><figcaption>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.</figcaption></figure> <p>CAPTCHAs based on reading text—or other visual-perception tasks—prevent <a href="/wiki/Blindness" class="mw-redirect" title="Blindness">blind</a> or <a href="/wiki/Visual_impairment" title="Visual impairment">visually impaired</a> users from accessing the protected resource.<sup id="cite_ref-w3c_inaccessibility_25-0" class="reference"><a href="#cite_note-w3c_inaccessibility-25"><span class="cite-bracket">[</span>25<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-26" class="reference"><a href="#cite_note-26"><span class="cite-bracket">[</span>26<span class="cite-bracket">]</span></a></sup> Because CAPTCHAs are designed to be unreadable by machines, common <a href="/wiki/Assistive_technology" title="Assistive technology">assistive technology</a> tools such as <a href="/wiki/Screen_readers" class="mw-redirect" title="Screen readers">screen readers</a> 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.<sup id="cite_ref-27" class="reference"><a href="#cite_note-27"><span class="cite-bracket">[</span>27<span class="cite-bracket">]</span></a></sup> 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 <a href="/wiki/Section_508" class="mw-redirect" title="Section 508">Section 508</a> in the United States. </p><p>CAPTCHAs do not have to be visual. Any hard <a href="/wiki/Artificial_intelligence" title="Artificial intelligence">artificial intelligence</a> problem, such as <a href="/wiki/Speech_recognition" title="Speech recognition">speech recognition</a>, 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.<sup id="cite_ref-28" class="reference"><a href="#cite_note-28"><span class="cite-bracket">[</span>28<span class="cite-bracket">]</span></a></sup> </p><p>A method of improving CAPTCHA to ease the work with it was proposed by ProtectWebForm and named "Smart CAPTCHA".<sup id="cite_ref-29" class="reference"><a href="#cite_note-29"><span class="cite-bracket">[</span>29<span class="cite-bracket">]</span></a></sup> 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.<sup id="cite_ref-30" class="reference"><a href="#cite_note-30"><span class="cite-bracket">[</span>30<span class="cite-bracket">]</span></a></sup> </p><p>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 <a href="/wiki/Dyscalculia" title="Dyscalculia">dyscalculia</a>.<sup id="cite_ref-31" class="reference"><a href="#cite_note-31"><span class="cite-bracket">[</span>31<span class="cite-bracket">]</span></a></sup> </p><p>Challenges such as a logic puzzle, or trivia question can also be used as a CAPTCHA. There is research into their resistance against countermeasures.<sup id="cite_ref-32" class="reference"><a href="#cite_note-32"><span class="cite-bracket">[</span>32<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading2"><h2 id="Circumvention">Circumvention</h2></div> <p>Two main ways to bypass CAPTCHA include using cheap human labor to recognize them, and using <a href="/wiki/Machine_learning" title="Machine learning">machine learning</a> to build an automated solver.<sup id="cite_ref-33" class="reference"><a href="#cite_note-33"><span class="cite-bracket">[</span>33<span class="cite-bracket">]</span></a></sup> According to former Google "<a href="/wiki/Click_fraud" title="Click fraud">click fraud</a> czar" <a href="/wiki/Shuman_Ghosemajumder" title="Shuman Ghosemajumder">Shuman Ghosemajumder</a>, there are numerous services which solve CAPTCHAs automatically.<sup id="cite_ref-ai-security_34-0" class="reference"><a href="#cite_note-ai-security-34"><span class="cite-bracket">[</span>34<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading3"><h3 id="Machine_learning–based_attacks"><span id="Machine_learning.E2.80.93based_attacks"></span>Machine learning–based attacks</h3></div> <figure class="mw-default-size" typeof="mw:File/Thumb"><a href="/wiki/File:Modern-captcha.jpg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/b/b6/Modern-captcha.jpg/220px-Modern-captcha.jpg" decoding="async" width="220" height="42" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/b/b6/Modern-captcha.jpg 1.5x" data-file-width="300" data-file-height="57" /></a><figcaption>An example of a <a href="/wiki/ReCAPTCHA" title="ReCAPTCHA">reCAPTCHA</a> 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.</figcaption></figure> <figure class="mw-default-size" typeof="mw:File/Thumb"><a href="/wiki/File:Captchacat.png" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/4/49/Captchacat.png/220px-Captchacat.png" decoding="async" width="220" height="87" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/4/49/Captchacat.png 1.5x" data-file-width="312" data-file-height="123" /></a><figcaption>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".</figcaption></figure> <p>There was not a systematic methodology for designing or evaluating early CAPTCHAs.<sup id="cite_ref-bursz_22-1" class="reference"><a href="#cite_note-bursz-22"><span class="cite-bracket">[</span>22<span class="cite-bracket">]</span></a></sup> 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<sup class="noprint Inline-Template" style="white-space:nowrap;">[<i><a href="/wiki/Wikipedia:AUDIENCE" class="mw-redirect" title="Wikipedia:AUDIENCE"><span title="An editor has requested that an example be provided. (October 2022)">example needed</span></a></i>]</sup> 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 <a href="/wiki/ReCAPTCHA" title="ReCAPTCHA">reCAPTCHA</a> rely on present variations of characters that are collapsed together, making them hard to segment, and they have warded off automated tasks.<sup id="cite_ref-bursz2_35-0" class="reference"><a href="#cite_note-bursz2-35"><span class="cite-bracket">[</span>35<span class="cite-bracket">]</span></a></sup> </p><p>In October 2013, artificial intelligence company <a href="/wiki/Vicarious_(Company)" class="mw-redirect" title="Vicarious (Company)">Vicarious</a> 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%.<sup id="cite_ref-36" class="reference"><a href="#cite_note-36"><span class="cite-bracket">[</span>36<span class="cite-bracket">]</span></a></sup> However, <a href="/wiki/Luis_von_Ahn" title="Luis von Ahn">Luis von Ahn</a>, 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.<sup id="cite_ref-37" class="reference"><a href="#cite_note-37"><span class="cite-bracket">[</span>37<span class="cite-bracket">]</span></a></sup> </p><p>In August 2014 at Usenix WoOT conference, <a href="/wiki/Elie_Bursztein" title="Elie Bursztein">Bursztein</a> et al. presented the first generic CAPTCHA-solving algorithm based on reinforcement learning and demonstrated its efficiency against many popular CAPTCHA schemas.<sup id="cite_ref-bursz2_35-1" class="reference"><a href="#cite_note-bursz2-35"><span class="cite-bracket">[</span>35<span class="cite-bracket">]</span></a></sup> </p><p>In October 2018 at <a href="/wiki/Association_for_Computing_Machinery" title="Association for Computing Machinery">ACM</a> 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.<sup id="cite_ref-38" class="reference"><a href="#cite_note-38"><span class="cite-bracket">[</span>38<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading3"><h3 id="Human_labor">Human labor</h3></div> <p>It is possible to subvert CAPTCHAs by relaying them to a <a href="/wiki/Sweatshop" title="Sweatshop">sweatshop</a> of human operators who are employed to decode CAPTCHAs. A 2005 paper from a <a href="/wiki/W3C" class="mw-redirect" title="W3C">W3C</a> working group said that they could verify hundreds per hour.<sup id="cite_ref-w3c_inaccessibility_25-1" class="reference"><a href="#cite_note-w3c_inaccessibility-25"><span class="cite-bracket">[</span>25<span class="cite-bracket">]</span></a></sup> In 2010, the <a href="/wiki/University_of_California,_San_Diego" title="University of California, San Diego">University of California at San Diego</a> conducted a large scale study of CAPTCHA farms. The retail price for solving one million CAPTCHAs was as low as $1,000.<sup id="cite_ref-motoyama_39-0" class="reference"><a href="#cite_note-motoyama-39"><span class="cite-bracket">[</span>39<span class="cite-bracket">]</span></a></sup> </p><p>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.<sup id="cite_ref-40" class="reference"><a href="#cite_note-40"><span class="cite-bracket">[</span>40<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-41" class="reference"><a href="#cite_note-41"><span class="cite-bracket">[</span>41<span class="cite-bracket">]</span></a></sup> </p><p>In 2023, <a href="/wiki/ChatGPT" title="ChatGPT">ChatGPT</a> tricked a <a href="/wiki/Taskrabbit" title="Taskrabbit">TaskRabbit</a> worker into solving a CAPTCHA by telling the worker it was not a robot and had impaired vision.<sup id="cite_ref-42" class="reference"><a href="#cite_note-42"><span class="cite-bracket">[</span>42<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading3"><h3 id="Outsourcing_to_paid_services">Outsourcing to paid services</h3></div> <p>There are multiple Internet companies like <i>2Captcha</i> and <i>DeathByCaptcha</i> that offer human and machine backed CAPTCHA solving services for as low as US$0.50 per 1000 solved CAPTCHAs.<sup id="cite_ref-43" class="reference"><a href="#cite_note-43"><span class="cite-bracket">[</span>43<span class="cite-bracket">]</span></a></sup> 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.<sup id="cite_ref-44" class="reference"><a href="#cite_note-44"><span class="cite-bracket">[</span>44<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading3"><h3 id="Insecure_implementation">Insecure implementation</h3></div> <p>Howard Yeend has identified two implementation issues with poorly designed CAPTCHA systems:<sup id="cite_ref-45" class="reference"><a href="#cite_note-45"><span class="cite-bracket">[</span>45<span class="cite-bracket">]</span></a></sup> reusing the session ID of a known CAPTCHA image, and CAPTCHAs residing on shared servers. </p><p>Sometimes, if part of the software generating the CAPTCHA is <a href="/wiki/Client-side" class="mw-redirect" title="Client-side">client-side</a> (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 <a href="/wiki/MD5" title="MD5">MD5</a> hashes stored client-side, which may leave the CAPTCHA vulnerable to a <a href="/wiki/Brute-force_attack" title="Brute-force attack">brute-force attack</a>.<sup id="cite_ref-46" class="reference"><a href="#cite_note-46"><span class="cite-bracket">[</span>46<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading2"><h2 id="Alternative_CAPTCHAs">Alternative CAPTCHAs</h2></div> <p>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. </p><p>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.<sup id="cite_ref-47" class="reference"><a href="#cite_note-47"><span class="cite-bracket">[</span>47<span class="cite-bracket">]</span></a></sup> Datta et al. published their paper in the <a href="/wiki/Association_for_Computing_Machinery" title="Association for Computing Machinery">ACM</a> <a href="/wiki/Multimedia" title="Multimedia">Multimedia</a> '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 recognise them.<sup id="cite_ref-48" class="reference"><a href="#cite_note-48"><span class="cite-bracket">[</span>48<span class="cite-bracket">]</span></a></sup> </p><p>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.<sup id="cite_ref-49" class="reference"><a href="#cite_note-49"><span class="cite-bracket">[</span>49<span class="cite-bracket">]</span></a></sup> 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).<sup id="cite_ref-50" class="reference"><a href="#cite_note-50"><span class="cite-bracket">[</span>50<span class="cite-bracket">]</span></a></sup> It was closed in October 2014.<sup id="cite_ref-51" class="reference"><a href="#cite_note-51"><span class="cite-bracket">[</span>51<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading2"><h2 id="See_also">See also</h2></div> <ul><li><a href="/wiki/Bot_prevention" title="Bot prevention">Bot prevention</a></li> <li><a href="/wiki/Defense_strategy_(computing)" title="Defense strategy (computing)">Defense strategy (computing)</a></li> <li><a href="/wiki/Proof_of_personhood" title="Proof of personhood">Proof of personhood</a></li> <li><a href="/wiki/Proof_of_work" title="Proof of work">Proof of work</a></li> <li><a href="/wiki/ReCAPTCHA" title="ReCAPTCHA">reCAPTCHA</a></li></ul> <div class="mw-heading mw-heading2"><h2 id="References">References</h2></div> <style data-mw-deduplicate="TemplateStyles:r1239543626">.mw-parser-output .reflist{margin-bottom:0.5em;list-style-type:decimal}@media screen{.mw-parser-output .reflist{font-size:90%}}.mw-parser-output .reflist .references{font-size:100%;margin-bottom:0;list-style-type:inherit}.mw-parser-output .reflist-columns-2{column-width:30em}.mw-parser-output .reflist-columns-3{column-width:25em}.mw-parser-output .reflist-columns{margin-top:0.3em}.mw-parser-output .reflist-columns ol{margin-top:0}.mw-parser-output .reflist-columns li{page-break-inside:avoid;break-inside:avoid-column}.mw-parser-output .reflist-upper-alpha{list-style-type:upper-alpha}.mw-parser-output .reflist-upper-roman{list-style-type:upper-roman}.mw-parser-output .reflist-lower-alpha{list-style-type:lower-alpha}.mw-parser-output .reflist-lower-greek{list-style-type:lower-greek}.mw-parser-output .reflist-lower-roman{list-style-type:lower-roman}</style><div class="reflist reflist-columns references-column-width" style="column-width: 30em;"> <ol class="references"> <li id="cite_note-1"><span class="mw-cite-backlink"><b><a href="#cite_ref-1">^</a></b></span> <span class="reference-text"><style data-mw-deduplicate="TemplateStyles:r1238218222">.mw-parser-output cite.citation{font-style:inherit;word-wrap:break-word}.mw-parser-output .citation q{quotes:"\"""\"""'""'"}.mw-parser-output .citation:target{background-color:rgba(0,127,255,0.133)}.mw-parser-output .id-lock-free.id-lock-free a{background:url("//upload.wikimedia.org/wikipedia/commons/6/65/Lock-green.svg")right 0.1em center/9px no-repeat}.mw-parser-output .id-lock-limited.id-lock-limited a,.mw-parser-output .id-lock-registration.id-lock-registration a{background:url("//upload.wikimedia.org/wikipedia/commons/d/d6/Lock-gray-alt-2.svg")right 0.1em center/9px no-repeat}.mw-parser-output .id-lock-subscription.id-lock-subscription a{background:url("//upload.wikimedia.org/wikipedia/commons/a/aa/Lock-red-alt-2.svg")right 0.1em center/9px no-repeat}.mw-parser-output .cs1-ws-icon a{background:url("//upload.wikimedia.org/wikipedia/commons/4/4c/Wikisource-logo.svg")right 0.1em center/12px no-repeat}body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-free a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-limited a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-registration a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-subscription a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .cs1-ws-icon a{background-size:contain;padding:0 1em 0 0}.mw-parser-output .cs1-code{color:inherit;background:inherit;border:none;padding:inherit}.mw-parser-output .cs1-hidden-error{display:none;color:var(--color-error,#d33)}.mw-parser-output .cs1-visible-error{color:var(--color-error,#d33)}.mw-parser-output .cs1-maint{display:none;color:#085;margin-left:0.3em}.mw-parser-output .cs1-kern-left{padding-left:0.2em}.mw-parser-output .cs1-kern-right{padding-right:0.2em}.mw-parser-output .citation .mw-selflink{font-weight:inherit}@media screen{.mw-parser-output .cs1-format{font-size:95%}html.skin-theme-clientpref-night .mw-parser-output .cs1-maint{color:#18911f}}@media screen and (prefers-color-scheme:dark){html.skin-theme-clientpref-os .mw-parser-output .cs1-maint{color:#18911f}}</style><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://web.archive.org/web/20171027203659/https://www.cylab.cmu.edu/partners/success-stories/recaptcha.html">"The reCAPTCHA Project – Carnegie Mellon University CyLab"</a>. <i>www.cylab.cmu.edu</i>. 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Retrieved <span class="nowrap">9 September</span> 2022</span>. <q>CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart) is a [...]</q></cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=unknown&rft.jtitle=Google+Support&rft.atitle=What+is+CAPTCHA%3F&rft_id=https%3A%2F%2Fsupport.google.com%2Fa%2Fanswer%2F1217728&rfr_id=info%3Asid%2Fen.wikipedia.org%3ACAPTCHA" class="Z3988"></span></span> </li> <li id="cite_note-4"><span class="mw-cite-backlink"><b><a href="#cite_ref-4">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFMayumi_TakayaYusuke_TsurutaAkihiro_Yamamura2013" class="citation journal cs1">Mayumi Takaya; Yusuke Tsuruta; Akihiro Yamamura (30 September 2013). <a rel="nofollow" class="external text" href="http://isyou.info/jowua/papers/jowua-v4n3-3.pdf">"Reverse Turing Test using Touchscreens and CAPTCHA"</a> <span class="cs1-format">(PDF)</span>. <i>Journal of Wireless Mobile Networks, Ubiquitous Computing, and Dependable Applications</i>. <b>4</b> (3): 41–57. <a href="/wiki/Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.22667%2FJOWUA.2013.09.31.041">10.22667/JOWUA.2013.09.31.041</a>. <a rel="nofollow" class="external text" href="https://web.archive.org/web/20170822001858/http://isyou.info/jowua/papers/jowua-v4n3-3.pdf">Archived</a> <span class="cs1-format">(PDF)</span> from the original on 22 August 2017.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=article&rft.jtitle=Journal+of+Wireless+Mobile+Networks%2C+Ubiquitous+Computing%2C+and+Dependable+Applications&rft.atitle=Reverse+Turing+Test+using+Touchscreens+and+CAPTCHA&rft.volume=4&rft.issue=3&rft.pages=41-57&rft.date=2013-09-30&rft_id=info%3Adoi%2F10.22667%2FJOWUA.2013.09.31.041&rft.au=Mayumi+Takaya&rft.au=Yusuke+Tsuruta&rft.au=Akihiro+Yamamura&rft_id=http%3A%2F%2Fisyou.info%2Fjowua%2Fpapers%2Fjowua-v4n3-3.pdf&rfr_id=info%3Asid%2Fen.wikipedia.org%3ACAPTCHA" class="Z3988"></span></span> </li> <li id="cite_note-5"><span class="mw-cite-backlink"><b><a href="#cite_ref-5">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://support.google.com/recaptcha/answer/6080904?hl=en">"What is reCAPTCHA? –?reCAPTCHA Help"</a>. <i>support.google.com</i>. <a rel="nofollow" class="external text" href="https://web.archive.org/web/20230720192427/https://support.google.com/recaptcha/answer/6080904?hl=en">Archived</a> from the original on 20 July 2023<span class="reference-accessdate">. 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A Large Scale Evaluation"</a> <span class="cs1-format">(PDF)</span>. <i>2010 IEEE Symposium on Security and Privacy</i>. pp. 399–413. <a href="/wiki/CiteSeerX_(identifier)" class="mw-redirect" title="CiteSeerX (identifier)">CiteSeerX</a> <span class="id-lock-free" title="Freely accessible"><a rel="nofollow" class="external text" href="https://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.164.7848">10.1.1.164.7848</a></span>. <a href="/wiki/Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1109%2FSP.2010.31">10.1109/SP.2010.31</a>. <a href="/wiki/ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <a href="/wiki/Special:BookSources/978-1-4244-6894-2" title="Special:BookSources/978-1-4244-6894-2"><bdi>978-1-4244-6894-2</bdi></a>. <a href="/wiki/S2CID_(identifier)" class="mw-redirect" title="S2CID (identifier)">S2CID</a> <a rel="nofollow" class="external text" href="https://api.semanticscholar.org/CorpusID:14204454">14204454</a>. <a rel="nofollow" class="external text" href="https://web.archive.org/web/20180808033552/https://web.stanford.edu/~jurafsky/burszstein_2010_captcha.pdf">Archived</a> <span class="cs1-format">(PDF)</span> from the original on 8 August 2018<span class="reference-accessdate">. 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Retrieved <span class="nowrap">1 November</span> 2022</span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=unknown&rft.jtitle=Baeldung+on+Computer+Science&rft.atitle=What+is+CAPTCHA+and+How+Does+It+Work%3F&rft.date=2022-06-12&rft.aulast=Stec&rft.aufirst=Albert&rft_id=https%3A%2F%2Fwww.baeldung.com%2Fcs%2Fcaptcha-intro&rfr_id=info%3Asid%2Fen.wikipedia.org%3ACAPTCHA" class="Z3988"></span></span> </li> <li id="cite_note-11"><span class="mw-cite-backlink"><b><a href="#cite_ref-11">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://www.cloudflare.com/learning/bots/how-captchas-work/">"What is a CAPTCHA?"</a>. <i>Cloudflare</i>. 1 November 2022. <a rel="nofollow" class="external text" href="https://web.archive.org/web/20221027061629/https://www.cloudflare.com/learning/bots/how-captchas-work/">Archived</a> from the original on 27 October 2022<span class="reference-accessdate">. Retrieved <span class="nowrap">1 November</span> 2022</span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=unknown&rft.jtitle=Cloudflare&rft.atitle=What+is+a+CAPTCHA%3F&rft.date=2022-11-01&rft_id=https%3A%2F%2Fwww.cloudflare.com%2Flearning%2Fbots%2Fhow-captchas-work%2F&rfr_id=info%3Asid%2Fen.wikipedia.org%3ACAPTCHA" class="Z3988"></span></span> </li> <li id="cite_note-:1-12"><span class="mw-cite-backlink">^ <a href="#cite_ref-:1_12-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-:1_12-1"><sup><i><b>b</b></i></sup></a></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://drive.google.com/open?id=0BzbOLm20p6CrOS1mWEhITGJ4d2s">"idrive turing patent application"</a>. <a rel="nofollow" class="external text" href="https://web.archive.org/web/20230315233244/https://accounts.google.com/v3/signin/identifier?dsh=S956306740%3A1678923164278227&continue=https%3A%2F%2Fdrive.google.com%2Fopen%3Fid%3D0BzbOLm20p6CrOS1mWEhITGJ4d2s&followup=https%3A%2F%2Fdrive.google.com%2Fopen%3Fid%3D0BzbOLm20p6CrOS1mWEhITGJ4d2s&ifkv=AWnogHfWh9qH38C8IGelcYVq9WSJcqP6Q30eP1Bba6t1EcfIlDb1n3eZMtAJSv1IRxdTdxgTsu8r0A&osid=1&passive=1209600&service=wise&flowName=GlifWebSignIn&flowEntry=ServiceLogin">Archived</a> from the original on 15 March 2023<span class="reference-accessdate">. Retrieved <span class="nowrap">19 May</span> 2017</span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=unknown&rft.btitle=idrive+turing+patent+application&rft_id=https%3A%2F%2Fdrive.google.com%2Fopen%3Fid%3D0BzbOLm20p6CrOS1mWEhITGJ4d2s&rfr_id=info%3Asid%2Fen.wikipedia.org%3ACAPTCHA" class="Z3988"></span></span> </li> <li id="cite_note-13"><span class="mw-cite-backlink"><b><a href="#cite_ref-13">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://www.bbc.co.uk/dna/h2g2/A787917">"h2g2 – An Explanation of l33t Speak – Edited Entry"</a>. <i>h2g2</i>. 16 August 2002. <a rel="nofollow" class="external text" href="https://web.archive.org/web/20110906114613/http://www.bbc.co.uk/dna/h2g2/A787917">Archived</a> from the original on 6 September 2011<span class="reference-accessdate">. Retrieved <span class="nowrap">3 June</span> 2015</span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=unknown&rft.jtitle=h2g2&rft.atitle=h2g2+%E2%80%93+An+Explanation+of+l33t+Speak+%E2%80%93+Edited+Entry&rft.date=2002-08-16&rft_id=http%3A%2F%2Fwww.bbc.co.uk%2Fdna%2Fh2g2%2FA787917&rfr_id=info%3Asid%2Fen.wikipedia.org%3ACAPTCHA" class="Z3988"></span></span> </li> <li id="cite_note-14"><span class="mw-cite-backlink"><b><a href="#cite_ref-14">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://drive.google.com/open?id=0BzbOLm20p6CrUE1SSXp5Zjl2MW8">"idrive turing signup page"</a>. <i>Google Drive</i>. <a rel="nofollow" class="external text" href="https://web.archive.org/web/20230315233241/https://accounts.google.com/v3/signin/identifier?dsh=S-569764738%3A1678923161301090&continue=https%3A%2F%2Fdrive.google.com%2Fopen%3Fid%3D0BzbOLm20p6CrUE1SSXp5Zjl2MW8&followup=https%3A%2F%2Fdrive.google.com%2Fopen%3Fid%3D0BzbOLm20p6CrUE1SSXp5Zjl2MW8&ifkv=AWnogHfb-QQLSi-KGh4vgzje6iZGJ1BZZvpaKSlXZLsXVSfSHlafPjo8v6B9qJTV2nuxzahDQYGTtw&osid=1&passive=1209600&service=wise&flowName=GlifWebSignIn&flowEntry=ServiceLogin">Archived</a> from the original on 15 March 2023<span class="reference-accessdate">. Retrieved <span class="nowrap">19 May</span> 2017</span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=unknown&rft.jtitle=Google+Drive&rft.atitle=idrive+turing+signup+page&rft_id=https%3A%2F%2Fdrive.google.com%2Fopen%3Fid%3D0BzbOLm20p6CrUE1SSXp5Zjl2MW8&rfr_id=info%3Asid%2Fen.wikipedia.org%3ACAPTCHA" class="Z3988"></span></span> </li> <li id="cite_note-stringham2015-15"><span class="mw-cite-backlink"><b><a href="#cite_ref-stringham2015_15-0">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFStringham2015" class="citation book cs1">Stringham, Edward P (2015). <i>Private Governance : creating order in economic and social life</i>. <a href="/wiki/Oxford_University_Press" title="Oxford University Press">Oxford University Press</a>. p. 105. <a href="/wiki/ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <a href="/wiki/Special:BookSources/978-0-19-936516-6" title="Special:BookSources/978-0-19-936516-6"><bdi>978-0-19-936516-6</bdi></a>. <a href="/wiki/OCLC_(identifier)" class="mw-redirect" title="OCLC (identifier)">OCLC</a> <a rel="nofollow" class="external text" href="https://search.worldcat.org/oclc/5881934034">5881934034</a>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=book&rft.btitle=Private+Governance+%3A+creating+order+in+economic+and+social+life&rft.pages=105&rft.pub=Oxford+University+Press&rft.date=2015&rft_id=info%3Aoclcnum%2F5881934034&rft.isbn=978-0-19-936516-6&rft.aulast=Stringham&rft.aufirst=Edward+P&rfr_id=info%3Asid%2Fen.wikipedia.org%3ACAPTCHA" class="Z3988"></span></span> </li> <li id="cite_note-16"><span class="mw-cite-backlink"><b><a href="#cite_ref-16">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://googleblog.blogspot.com/2009/09/teaching-computers-to-read-google.html">"Teaching computers to read: Google acquires reCAPTCHA"</a>. <i>Google Official Blog</i>. <a rel="nofollow" class="external text" href="https://web.archive.org/web/20190831195346/https://googleblog.blogspot.com/2009/09/teaching-computers-to-read-google.html">Archived</a> from the original on 31 August 2019<span class="reference-accessdate">. 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Retrieved <span class="nowrap">29 October</span> 2018</span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=article&rft.jtitle=The+New+York+Times&rft.atitle=Deciphering+Old+Texts%2C+One+Woozy%2C+Curvy+Word+at+a+Time&rft.date=2011-03-28&rft.aulast=Gugliotta&rft.aufirst=Guy&rft_id=https%3A%2F%2Fwww.nytimes.com%2F2011%2F03%2F29%2Fscience%2F29recaptcha.html&rfr_id=info%3Asid%2Fen.wikipedia.org%3ACAPTCHA" class="Z3988"></span></span> </li> <li id="cite_note-18"><span class="mw-cite-backlink"><b><a href="#cite_ref-18">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://www.cloudflare.com/learning/bots/how-captchas-work/">"How CAPTCHAs work | What does CAPTCHA mean?"</a>. <i>Cloudflare</i>. <a rel="nofollow" class="external text" href="https://web.archive.org/web/20221027061629/https://www.cloudflare.com/learning/bots/how-captchas-work/">Archived</a> from the original on 27 October 2022<span class="reference-accessdate">. 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Retrieved <span class="nowrap">30 August</span> 2019</span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=bookitem&rft.atitle=CAPTCHA%3A+Using+Hard+AI+Problems+for+Security&rft.btitle=Advances+in+Cryptology%E2%80%94EUROCRYPT+2003&rft.series=Lecture+Notes+in+Computer+Science&rft.pages=294-311&rft.date=2003&rft_id=https%3A%2F%2Fapi.semanticscholar.org%2FCorpusID%3A5658745%23id-name%3DS2CID&rft_id=info%3Adoi%2F10.1007%2F3-540-39200-9_18&rft.isbn=978-3-540-14039-9&rft.aulast=von+Ahn&rft.aufirst=Luis&rft.au=Blum%2C+Manuel&rft.au=Hopper%2C+Nicholas+J.&rft.au=Langford%2C+John&rft_id=https%3A%2F%2Flink.springer.com%2Fcontent%2Fpdf%2F10.1007%2F3-540-39200-9_18.pdf&rfr_id=info%3Asid%2Fen.wikipedia.org%3ACAPTCHA" class="Z3988"></span></span> </li> <li id="cite_note-24"><span class="mw-cite-backlink"><b><a href="#cite_ref-24">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFMoyJonesHarklessPotter2004" class="citation conference cs1">Moy, Gabriel; Jones, Nathan; Harkless, Curt; Potter, Randall (2004). <a rel="nofollow" class="external text" href="https://web.archive.org/web/20200729175253/https://www2.cs.duke.edu/courses/cps296.3/spring07/breaking_captchas.pdf"><i>Distortion estimation techniques in solving visual CAPTCHAs</i></a> <span class="cs1-format">(PDF)</span>. <a href="/wiki/Conference_on_Computer_Vision_and_Pattern_Recognition" title="Conference on Computer Vision and Pattern Recognition">Proceedings of the 2004 IEEE Computer Society Conference on Computer Vision and Pattern Recognition</a>. 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Archived from <a rel="nofollow" class="external text" href="http://www.cs.duke.edu/courses/cps296.3/spring07/breaking_captchas.pdf">the original</a> <span class="cs1-format">(PDF)</span> on 29 July 2020.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=conference&rft.btitle=Distortion+estimation+techniques+in+solving+visual+CAPTCHAs&rft.pages=23-28&rft.pub=IEEE&rft.date=2004&rft_id=info%3Adoi%2F10.1109%2FCVPR.2004.1315140&rft.isbn=978-0-7695-2158-9&rft.aulast=Moy&rft.aufirst=Gabriel&rft.au=Jones%2C+Nathan&rft.au=Harkless%2C+Curt&rft.au=Potter%2C+Randall&rft_id=http%3A%2F%2Fwww.cs.duke.edu%2Fcourses%2Fcps296.3%2Fspring07%2Fbreaking_captchas.pdf&rfr_id=info%3Asid%2Fen.wikipedia.org%3ACAPTCHA" class="Z3988"></span></span> </li> <li id="cite_note-w3c_inaccessibility-25"><span class="mw-cite-backlink">^ <a href="#cite_ref-w3c_inaccessibility_25-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-w3c_inaccessibility_25-1"><sup><i><b>b</b></i></sup></a></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFMay,_Matt2005" class="citation web cs1">May, Matt (23 November 2005). <a rel="nofollow" class="external text" href="http://www.w3.org/TR/turingtest/">"Inaccessibility of CAPTCHA"</a>. <a href="/wiki/W3C" class="mw-redirect" title="W3C">W3C</a>. <a rel="nofollow" class="external text" href="https://web.archive.org/web/20120521023537/http://www.w3.org/TR/turingtest/">Archived</a> from the original on 21 May 2012<span class="reference-accessdate">. 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Infolab.stanford.edu. <a rel="nofollow" class="external text" href="https://web.archive.org/web/20131002170726/http://infolab.stanford.edu/~wangz/project/imsearch/IMAGINATION/ACM05/">Archived</a> from the original on 2 October 2013<span class="reference-accessdate">. Retrieved <span class="nowrap">28 September</span> 2013</span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=unknown&rft.btitle=Imagination+Paper&rft.pub=Infolab.stanford.edu&rft_id=http%3A%2F%2Finfolab.stanford.edu%2F~wangz%2Fproject%2Fimsearch%2FIMAGINATION%2FACM05%2F&rfr_id=info%3Asid%2Fen.wikipedia.org%3ACAPTCHA" class="Z3988"></span></span> </li> <li id="cite_note-49"><span class="mw-cite-backlink"><b><a href="#cite_ref-49">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://web.archive.org/web/20081215032402/http://research.microsoft.com/en-us/um/redmond/projects/asirra/">"Asirra is a human interactive proof that asks users to identify photos of cats and dogs"</a>. <i><a href="/wiki/Microsoft" title="Microsoft">Microsoft</a></i>. Archived from <a rel="nofollow" class="external text" href="https://www.microsoft.com/en-us/research/publication/asirra-a-captcha-that-exploits-interest-aligned-manual-image-categorization/">the original</a> on 15 December 2008.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=unknown&rft.jtitle=Microsoft&rft.atitle=Asirra+is+a+human+interactive+proof+that+asks+users+to+identify+photos+of+cats+and+dogs&rft_id=https%3A%2F%2Fwww.microsoft.com%2Fen-us%2Fresearch%2Fpublication%2Fasirra-a-captcha-that-exploits-interest-aligned-manual-image-categorization%2F&rfr_id=info%3Asid%2Fen.wikipedia.org%3ACAPTCHA" class="Z3988"></span></span> </li> <li id="cite_note-50"><span class="mw-cite-backlink"><b><a href="#cite_ref-50">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFElsonDouceurHowellSaul2007" class="citation conference cs1">Elson, Jeremy; Douceur, John; Howell, Jon; Saul, Jared (October 2007). <a rel="nofollow" class="external text" href="https://www.microsoft.com/en-us/research/publication/asirra-a-captcha-that-exploits-interest-aligned-manual-image-categorization/"><i>Asirra: A CAPTCHA that Exploits Interest-Aligned Manual Image Categorization</i></a>. Proceedings of 14th ACM Conference on Computer and Communications Security. <a href="/wiki/Microsoft" title="Microsoft">Microsoft</a>. <a rel="nofollow" class="external text" href="https://web.archive.org/web/20081215032402/http://research.microsoft.com/en-us/um/redmond/projects/asirra/">Archived</a> from the original on 15 December 2008<span class="reference-accessdate">. Retrieved <span class="nowrap">15 September</span> 2017</span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=conference&rft.btitle=Asirra%3A+A+CAPTCHA+that+Exploits+Interest-Aligned+Manual+Image+Categorization&rft.pub=Microsoft&rft.date=2007-10&rft.aulast=Elson&rft.aufirst=Jeremy&rft.au=Douceur%2C+John&rft.au=Howell%2C+Jon&rft.au=Saul%2C+Jared&rft_id=https%3A%2F%2Fwww.microsoft.com%2Fen-us%2Fresearch%2Fpublication%2Fasirra-a-captcha-that-exploits-interest-aligned-manual-image-categorization%2F&rfr_id=info%3Asid%2Fen.wikipedia.org%3ACAPTCHA" class="Z3988"></span></span> </li> <li id="cite_note-51"><span class="mw-cite-backlink"><b><a href="#cite_ref-51">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://web.archive.org/web/20150207180225/https://research.microsoft.com/en-us/projects/asirra/default.aspx">"After 8 years of operation, Asirra is shutting down effective October 1, 2014. Thank you to all of our users!"</a>. <a href="/wiki/Microsoft" title="Microsoft">Microsoft</a>. Archived from <a rel="nofollow" class="external text" href="https://research.microsoft.com/en-us/projects/asirra/default.aspx">the original</a> on 7 February 2015.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=unknown&rft.btitle=After+8+years+of+operation%2C+Asirra+is+shutting+down+effective+October+1%2C+2014.+Thank+you+to+all+of+our+users%21&rft.pub=Microsoft&rft_id=https%3A%2F%2Fresearch.microsoft.com%2Fen-us%2Fprojects%2Fasirra%2Fdefault.aspx&rfr_id=info%3Asid%2Fen.wikipedia.org%3ACAPTCHA" class="Z3988"></span></span> </li> </ol></div> <div class="mw-heading mw-heading2"><h2 id="Further_references">Further references</h2></div> <ul><li>von Ahn, L; M. Blum and J. Langford. (2004) "<a rel="nofollow" class="external text" href="http://www.cs.cmu.edu/afs/cs/Web/People/aladdin/papers/pdfs/y2004/captcha_cacm.pdf">Telling humans and computers apart (automatically)</a>". <i>Communications of the ACM</i>, <b>47</b>(2):57–60.</li></ul> <div class="mw-heading mw-heading2"><h2 id="External_links">External links</h2></div> <style data-mw-deduplicate="TemplateStyles:r1235681985">.mw-parser-output .side-box{margin:4px 0;box-sizing:border-box;border:1px solid #aaa;font-size:88%;line-height:1.25em;background-color:var(--background-color-interactive-subtle,#f8f9fa);display:flow-root}.mw-parser-output .side-box-abovebelow,.mw-parser-output .side-box-text{padding:0.25em 0.9em}.mw-parser-output .side-box-image{padding:2px 0 2px 0.9em;text-align:center}.mw-parser-output .side-box-imageright{padding:2px 0.9em 2px 0;text-align:center}@media(min-width:500px){.mw-parser-output .side-box-flex{display:flex;align-items:center}.mw-parser-output .side-box-text{flex:1;min-width:0}}@media(min-width:720px){.mw-parser-output .side-box{width:238px}.mw-parser-output .side-box-right{clear:right;float:right;margin-left:1em}.mw-parser-output .side-box-left{margin-right:1em}}</style><style data-mw-deduplicate="TemplateStyles:r1250146164">.mw-parser-output .sister-box .side-box-abovebelow{padding:0.75em 0;text-align:center}.mw-parser-output .sister-box .side-box-abovebelow>b{display:block}.mw-parser-output .sister-box .side-box-text>ul{border-top:1px solid #aaa;padding:0.75em 0;width:217px;margin:0 auto}.mw-parser-output .sister-box .side-box-text>ul>li{min-height:31px}.mw-parser-output .sister-logo{display:inline-block;width:31px;line-height:31px;vertical-align:middle;text-align:center}.mw-parser-output .sister-link{display:inline-block;margin-left:4px;width:182px;vertical-align:middle}@media print{body.ns-0 .mw-parser-output .sistersitebox{display:none!important}}@media screen{html.skin-theme-clientpref-night .mw-parser-output .sistersitebox img[src*="Wiktionary-logo-v2.svg"]{background-color:white}}@media screen and (prefers-color-scheme:dark){html.skin-theme-clientpref-os .mw-parser-output .sistersitebox img[src*="Wiktionary-logo-v2.svg"]{background-color:white}}</style><div role="navigation" aria-labelledby="sister-projects" class="side-box metadata side-box-right sister-box sistersitebox plainlinks"><style data-mw-deduplicate="TemplateStyles:r1126788409">.mw-parser-output .plainlist ol,.mw-parser-output .plainlist ul{line-height:inherit;list-style:none;margin:0;padding:0}.mw-parser-output .plainlist ol li,.mw-parser-output .plainlist ul li{margin-bottom:0}</style> <div class="side-box-abovebelow"> <b>CAPTCHA</b> at Wikipedia's <a href="/wiki/Wikipedia:Wikimedia_sister_projects" title="Wikipedia:Wikimedia sister projects"><span id="sister-projects">sister projects</span></a></div> <div class="side-box-flex"> <div class="side-box-text plainlist"><ul><li><span class="sister-logo"><span class="mw-valign-middle" typeof="mw:File"><span><img alt="" src="//upload.wikimedia.org/wikipedia/en/thumb/0/06/Wiktionary-logo-v2.svg/27px-Wiktionary-logo-v2.svg.png" decoding="async" width="27" height="27" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/en/thumb/0/06/Wiktionary-logo-v2.svg/41px-Wiktionary-logo-v2.svg.png 1.5x, //upload.wikimedia.org/wikipedia/en/thumb/0/06/Wiktionary-logo-v2.svg/54px-Wiktionary-logo-v2.svg.png 2x" data-file-width="391" data-file-height="391" /></span></span></span><span class="sister-link"><a href="https://en.wiktionary.org/wiki/Special:Search/CAPTCHA" class="extiw" title="wikt:Special:Search/CAPTCHA">Definitions</a> from Wiktionary</span></li><li><span class="sister-logo"><span class="mw-valign-middle" typeof="mw:File"><span><img alt="" src="//upload.wikimedia.org/wikipedia/en/thumb/4/4a/Commons-logo.svg/20px-Commons-logo.svg.png" decoding="async" width="20" height="27" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/en/thumb/4/4a/Commons-logo.svg/30px-Commons-logo.svg.png 1.5x, //upload.wikimedia.org/wikipedia/en/thumb/4/4a/Commons-logo.svg/40px-Commons-logo.svg.png 2x" data-file-width="1024" data-file-height="1376" /></span></span></span><span class="sister-link"><a href="https://commons.wikimedia.org/wiki/Category:Captcha" class="extiw" title="c:Category:Captcha">Media</a> from Commons</span></li><li><span class="sister-logo"><span class="mw-valign-middle" typeof="mw:File"><span><img alt="" src="//upload.wikimedia.org/wikipedia/commons/thumb/f/fa/Wikibooks-logo.svg/27px-Wikibooks-logo.svg.png" decoding="async" width="27" height="27" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/f/fa/Wikibooks-logo.svg/41px-Wikibooks-logo.svg.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/f/fa/Wikibooks-logo.svg/54px-Wikibooks-logo.svg.png 2x" data-file-width="300" data-file-height="300" /></span></span></span><span class="sister-link"><a href="https://en.wikibooks.org/wiki/CAPTCHA" class="extiw" title="b:CAPTCHA">Textbooks</a> from Wikibooks</span></li><li><span class="sister-logo"><span class="mw-valign-middle" typeof="mw:File"><span><img alt="" src="//upload.wikimedia.org/wikipedia/commons/thumb/f/ff/Wikidata-logo.svg/27px-Wikidata-logo.svg.png" decoding="async" width="27" height="15" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/f/ff/Wikidata-logo.svg/41px-Wikidata-logo.svg.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/f/ff/Wikidata-logo.svg/54px-Wikidata-logo.svg.png 2x" data-file-width="1050" data-file-height="590" /></span></span></span><span class="sister-link"><a href="https://www.wikidata.org/wiki/Q484598" class="extiw" title="d:Q484598">Data</a> from Wikidata</span></li><li><span class="sister-logo"><span class="mw-valign-middle" typeof="mw:File"><span><img alt="" src="//upload.wikimedia.org/wikipedia/commons/thumb/a/a6/MediaWiki-2020-icon.svg/27px-MediaWiki-2020-icon.svg.png" decoding="async" width="27" height="27" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/a/a6/MediaWiki-2020-icon.svg/41px-MediaWiki-2020-icon.svg.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/a/a6/MediaWiki-2020-icon.svg/54px-MediaWiki-2020-icon.svg.png 2x" data-file-width="100" data-file-height="100" /></span></span></span><span class="sister-link"><a href="https://www.mediawiki.org/wiki/CAPTCHA" class="extiw" title="mw:CAPTCHA">Documentation</a> from MediaWiki</span></li></ul></div></div> </div> <ul><li><a rel="nofollow" class="external text" href="http://www.wisdom.weizmann.ac.il/~naor/PAPERS/human_abs.html">Verification of a human in the loop, or Identification via the Turing Test</a>, Moni Naor, 1996.</li> <li><a rel="nofollow" class="external text" href="http://www.w3.org/TR/turingtest/">Inaccessibility of CAPTCHA: Alternatives to Visual Turing Tests on the Web</a>, a <a href="/wiki/W3C" class="mw-redirect" title="W3C">W3C</a> Working Group Note.</li> <li><a rel="nofollow" class="external text" href="https://web.archive.org/web/20120205201803/http://www2.parc.com/istl/projects/captcha/history.htm">CAPTCHA History</a> from <a href="/wiki/PARC_(company)" title="PARC (company)">PARC</a>.</li> <li><a rel="nofollow" class="external text" href="https://web.archive.org/web/20170915204258/https://pdfs.semanticscholar.org/692a/31f65e29ea3667de46933245f53bda55a65b.pdf">Reverse Engineering CAPTCHAs</a> Abram Hindle, Michael W. Godfrey, Richard C. Holt, 2009-08-24</li></ul> <div class="navbox-styles"><style data-mw-deduplicate="TemplateStyles:r1129693374">.mw-parser-output .hlist dl,.mw-parser-output .hlist ol,.mw-parser-output .hlist ul{margin:0;padding:0}.mw-parser-output .hlist dd,.mw-parser-output .hlist dt,.mw-parser-output .hlist li{margin:0;display:inline}.mw-parser-output .hlist.inline,.mw-parser-output .hlist.inline dl,.mw-parser-output .hlist.inline ol,.mw-parser-output .hlist.inline ul,.mw-parser-output .hlist dl dl,.mw-parser-output .hlist dl ol,.mw-parser-output .hlist dl ul,.mw-parser-output .hlist ol dl,.mw-parser-output .hlist ol ol,.mw-parser-output .hlist ol ul,.mw-parser-output .hlist ul dl,.mw-parser-output .hlist ul ol,.mw-parser-output .hlist ul ul{display:inline}.mw-parser-output .hlist .mw-empty-li{display:none}.mw-parser-output .hlist dt::after{content:": "}.mw-parser-output .hlist dd::after,.mw-parser-output .hlist li::after{content:" · ";font-weight:bold}.mw-parser-output .hlist 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.navbox-even{background-color:#f7f7f7}.mw-parser-output .navbox-odd{background-color:transparent}.mw-parser-output .navbox .hlist td dl,.mw-parser-output .navbox .hlist td ol,.mw-parser-output .navbox .hlist td ul,.mw-parser-output .navbox td.hlist dl,.mw-parser-output .navbox td.hlist ol,.mw-parser-output .navbox td.hlist ul{padding:0.125em 0}.mw-parser-output .navbox .navbar{display:block;font-size:100%}.mw-parser-output .navbox-title .navbar{float:left;text-align:left;margin-right:0.5em}body.skin--responsive .mw-parser-output .navbox-image img{max-width:none!important}@media print{body.ns-0 .mw-parser-output .navbox{display:none!important}}</style></div><div role="navigation" class="navbox" aria-labelledby="CAPTCHAs" style="padding:3px"><table class="nowraplinks hlist mw-collapsible mw-collapsed navbox-inner" style="border-spacing:0;background:transparent;color:inherit"><tbody><tr><th scope="col" class="navbox-title" colspan="2"><link rel="mw-deduplicated-inline-style" 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href="/wiki/ReCAPTCHA" title="ReCAPTCHA">reCAPTCHA</a></li></ul> </div></td></tr><tr><td class="navbox-abovebelow" colspan="2"><div> <ul><li><span class="noviewer" typeof="mw:File"><span title="Category"><img alt="" src="//upload.wikimedia.org/wikipedia/en/thumb/9/96/Symbol_category_class.svg/16px-Symbol_category_class.svg.png" decoding="async" width="16" height="16" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/en/thumb/9/96/Symbol_category_class.svg/23px-Symbol_category_class.svg.png 1.5x, //upload.wikimedia.org/wikipedia/en/thumb/9/96/Symbol_category_class.svg/31px-Symbol_category_class.svg.png 2x" data-file-width="180" data-file-height="185" /></span></span> <b><a href="/wiki/Category:Turing_tests" title="Category:Turing tests">Category</a></b></li> <li><span class="noviewer" typeof="mw:File"><span title="WikiProject"><img alt="" src="//upload.wikimedia.org/wikipedia/commons/thumb/3/37/People_icon.svg/16px-People_icon.svg.png" decoding="async" width="16" 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