In computer security , challenge-response authentication is a family of protocols in which one party presents a question ("challenge") and another party must provide a valid answer ("response") to be authenticated .
62-482: A CAPTCHA ( / ˈ k æ p . tʃ ə / KAP -chə ) is a type of challenge–response test used in computing to determine whether the user is human in order to deter bot attacks and spam. The term was coined in 2003 by Luis von Ahn , Manuel Blum , Nicholas J. Hopper, and John Langford . It is a contrived acronym for "Completely Automated Public Turing test to tell Computers and Humans Apart." A historically common type of CAPTCHA (displayed as reCAPTCHA v1 )
124-621: A braille device . They do this by applying a wide variety of techniques that include, for example, interacting with dedicated accessibility APIs , using various operating system features (like inter-process communication and querying user interface properties), and employing hooking techniques. Microsoft Windows operating systems have included the Microsoft Narrator screen reader since Windows 2000 , though separate products such as Freedom Scientific 's commercially available JAWS screen reader and ZoomText screen magnifier and
186-411: A key derivation function , the challenge value and the secret may be combined to generate an unpredictable encryption key for the session. This is particularly effective against a man-in-the-middle attack, because the attacker will not be able to derive the session key from the challenge without knowing the secret, and therefore will not be able to decrypt the data stream. where This particular example
248-496: A CAPTCHA may make a site incompatible with Section 508 in the United States. CAPTCHAs do not have to be visual. Any hard artificial intelligence problem, such as speech recognition , can be used as CAPTCHA. Some implementations of CAPTCHAs permit users to opt for an audio CAPTCHA, such as reCAPTCHA, though a 2011 paper demonstrated a technique for defeating the popular schemes at the time. A method of improving CAPTCHA to ease
310-548: A beta version of this for websites to use. 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). It was closed in October 2014. Challenge%E2%80%93response authentication The simplest example of
372-420: A challenge-response protocol is password authentication, where the challenge is asking for the password and the valid response is the correct password. An adversary who can eavesdrop on a password authentication can authenticate themselves by reusing the intercepted password. One solution is to issue multiple passwords, each of them marked with an identifier. The verifier can then present an identifier, and
434-671: A fraud prevention strategy in which they asked humans to "retype distorted text that programs have difficulty recognizing." PayPal co founder and CTO Max Levchin helped commercialize this use. A popular deployment of CAPTCHA technology, reCAPTCHA , was acquired by Google in 2009. In addition to preventing bot fraud for its users, Google used reCAPTCHA and CAPTCHA technology to digitize the archives of The New York Times and books from Google Books in 2011. CAPTCHAs are automated, requiring little human maintenance or intervention to administer, producing benefits in cost and reliability. Modern text-based CAPTCHAs are designed such that they require
496-414: A generic CAPTCHA-solving algorithm that was able to solve modern CAPTCHAs with character recognition rates of up to 90%. However, Luis von Ahn , a pioneer of early CAPTCHA and founder of reCAPTCHA, said: "It's hard for me to be impressed since I see these every few months." 50 similar claims to that of Vicarious had been made since 2003. In August 2014 at Usenix WoOT conference, Bursztein et al. presented
558-445: A list of three-letter challenge codes, which the verifier is supposed to choose randomly from, and random three-letter responses to them. For added security, each set of codes is only valid for a particular time period which is ordinarily 24 hours. Another basic challenge-response technique works as follows. Bob is controlling access to some resource, and Alice is seeking entry. Bob issues the challenge "52w72y". Alice must respond with
620-418: A logic puzzle, or trivia question can also be used as a CAPTCHA. There is research into their resistance against countermeasures. Two main ways to bypass CAPTCHA include using cheap human labor to recognize them, and using machine learning to build an automated solver. According to former Google " click fraud czar" Shuman Ghosemajumder , there are numerous services which solve CAPTCHAs automatically. There
682-553: A mental model of web pages displayed on their computer screen. Based on verbosity settings, a screen-reading program informs users of certain formatting changes, such as when a frame or table begins and ends, where graphics have been inserted into the text, or when a list appears in the document. The verbosity settings can also control the level of descriptiveness of elements, such as lists, tables, and regions. For example, JAWS provides low, medium, and high web verbosity preset levels. The high web verbosity level provides more detail about
SECTION 10
#1732863140807744-434: A previous correct response (even if it is not obfuscated by the means of communication) does not allow an adversary to determine the current correct response. Challenge-response protocols are also used in non-cryptographic applications. CAPTCHAs , for example, are meant to allow websites and applications to determine whether an interaction was performed by a genuine user rather than a web scraper or bot . In early CAPTCHAs,
806-662: A prototype of a talking terminal, known as SAID (for Synthetic Audio Interface Driver), for the IBM 3270 terminal . SAID read the ASCII values of the display in a stream and spoke them through a large vocal track synthesizer the size of a suitcase, and it cost around $ 10,000. Dr. Jesse Wright, a blind research mathematician, and Jim Thatcher , formerly his graduate student from the University of Michigan, working as mathematicians for IBM, adapted this as an internal IBM tool for use by blind people. After
868-593: 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. In 2023, ChatGPT tricked a TaskRabbit worker into solving a CAPTCHA by telling the worker it was not a robot and had impaired vision. There are multiple Internet companies like 2Captcha and DeathByCaptcha that offer human and machine backed CAPTCHA solving services for as low as US$ 0.50 per 1000 solved CAPTCHAs. These services offer APIs and libraries that enable users to integrate CAPTCHA circumvention into
930-410: A strong cryptographically secure pseudorandom number generator and cryptographic hash function can generate challenges that are highly unlikely to occur more than once. It is sometimes important not to use time-based nonces, as these can weaken servers in different time zones and servers with inaccurate clocks. It can also be important to use time-based nonces and synchronized clocks if the application
992-480: A word with look-alike characters. HELLO could become |-|3|_|_() or )-(3££0 , and others, such that a filter could not detect all of them. This later became known as leetspeak . One of the earliest commercial uses of CAPTCHAs was in the Gausebeck–Levchin test. In 2000, idrive.com began to protect its signup page with a CAPTCHA and prepared to file a patent. In 2001, PayPal used such tests as part of
1054-466: Is a form of assistive technology ( AT ) that renders text and image content as speech or braille output. Screen readers are essential to people who are blind , and are useful to people who are visually impaired , illiterate , or have a learning disability . Screen readers are software applications that attempt to convey what people with normal eyesight see on a display to their users via non-visual means, like text-to-speech , sound icons, or
1116-413: Is a significant technical challenge; hooking the low-level messages and maintaining an accurate model are both difficult tasks. Operating system and application designers have attempted to address these problems by providing ways for screen readers to access the display contents without having to maintain an off-screen model. These involve the provision of alternative and accessible representations of what
1178-412: Is being displayed on the screen accessed through an API . Existing API s include: Screen readers can query the operating system or application for what is currently being displayed and receive updates when the display changes. For example, a screen reader can be told that the current focus is on a button and the button caption to be communicated to the user. This approach is considerably easier for
1240-446: Is not intrinsically inaccessible. Web browsers, word processors, icons and windows and email programs are just some of the applications used successfully by screen reader users. However, according to some users, using a screen reader is considerably more difficult than using a GUI, and many applications have specific problems resulting from the nature of the application (e.g. animations) or failure to comply with accessibility standards for
1302-449: Is not stored, and it is very difficult to determine a password that matches a given hash. However, this presents a problem for many (but not all) challenge-response algorithms, which require both the client and the server to have a shared secret. Since the password itself is not stored, a challenge-response algorithm will usually have to use the hash of the password as the secret instead of the password itself. In this case, an intruder can use
SECTION 20
#17328631408071364-471: Is possible to subvert CAPTCHAs by relaying them to a sweatshop of human operators who are employed to decode CAPTCHAs. A 2005 paper from a W3C working group said that they could verify hundreds per hour. In 2010, the University of California at San Diego conducted a large scale study of CAPTCHA farms. The retail price for solving one million CAPTCHAs was as low as $ 1,000. Another technique consists of using
1426-466: Is rendered on the client side), then users can modify the client to display the un-rendered text. Some CAPTCHA systems use MD5 hashes stored client-side, which may leave the CAPTCHA vulnerable to a brute-force attack . 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
1488-403: Is that tasks like object recognition are more complex to perform than text recognition and therefore should be more resilient to machine learning based attacks. Chew et al. published their work in the 7th International Information Security Conference, ISC'04, proposing three different versions of image recognition CAPTCHAs, and validating the proposal with user studies. It is suggested that one of
1550-412: Is those who have difficulty reading because of learning disabilities or language barriers. Although functionality remains limited compared to equivalent desktop applications, the major benefit is to increase the accessibility of said websites when viewed on public machines where users do not have permission to install custom software, giving people greater "freedom to roam". This functionality depends on
1612-418: Is vulnerable to a reflection attack . To avoid storage of passwords, some operating systems (e.g. Unix -type) store a hash of the password rather than storing the password itself. During authentication, the system need only verify that the hash of the password entered matches the hash stored in the password database. This makes it more difficult for an intruder to get the passwords, since the password itself
1674-459: Is vulnerable to a delayed message attack. This attack occurs where an attacker copies a transmission whilst blocking it from reaching the destination, allowing them to replay the captured transmission after a delay of their choosing. This is easily accomplished on wireless channels. The time-based nonce can be used to limit the attacker to resending the message but restricted by an expiry time of perhaps less than one second, likely having no effect upon
1736-651: The free and open source screen reader NVDA by NV Access are more popular for that operating system. Apple Inc. 's macOS , iOS , and tvOS include VoiceOver as a built-in screen reader, while Google 's Android provides the Talkback screen reader and its ChromeOS can use ChromeVox. Similarly, Android-based devices from Amazon provide the VoiceView screen reader. There are also free and open source screen readers for Linux and Unix-like systems, such as Speakup and Orca . Around 1978, Al Overby of IBM Raleigh developed
1798-644: The Research Centre for the Education of the Visually Handicapped ( RCEVH ) at the University of Birmingham developed a Screen Reader for the BBC Micro and NEC Portable. With the arrival of graphical user interfaces ( GUI s), the situation became more complicated. A GUI has characters and graphics drawn on the screen at particular positions, and therefore there is no purely textual representation of
1860-401: The actual hash, rather than the password, which makes the stored hashes just as sensitive as the actual passwords. SCRAM is a challenge-response algorithm that avoids this problem. Examples of more sophisticated challenge-response algorithms are: Some people consider a CAPTCHA a kind of challenge-response authentication that blocks spambots . Screen readers A screen reader
1922-439: The application and so mitigating the attack. Mutual authentication is performed using a challenge-response handshake in both directions; the server ensures that the client knows the secret, and the client also ensures that the server knows the secret, which protects against a rogue server impersonating the real server. Challenge-response authentication can help solve the problem of exchanging session keys for encryption. Using
CAPTCHA - Misplaced Pages Continue
1984-433: The challenge is an encrypted integer N , while the response is the encrypted integer N + 1 , proving that the other end was able to decrypt the integer N . A hash function can also be applied to a password and a random challenge value to create a response value. Another variation uses a probabilistic model to provide randomized challenges conditioned on model input. Such encrypted or hashed exchanges do not directly reveal
2046-463: The challenge sent to the user was a distorted image of some text, and the user responded by transcribing the text. The distortion was designed to make automated optical character recognition (OCR) difficult and prevent a computer program from passing as a human. Non-cryptographic authentication was generally adequate in the days before the Internet , when the user could be sure that the system asking for
2108-480: The clear over the communication channel. One way this is done involves using the password as the encryption key to transmit some randomly generated information as the challenge , whereupon the other end must return as its response a similarly encrypted value which is some predetermined function of the originally offered information, thus proving that it was able to decrypt the challenge. For instance, in Kerberos ,
2170-399: The contents of a webpage. Some screen readers can read text in more than one language , provided that the language of the material is encoded in its metadata . Screen reading programs like JAWS , NVDA , and VoiceOver also include language verbosity, which automatically detects verbosity settings related to speech output language. For example, if a user navigated to a website based in
2232-545: The developers of screen readers, but fails when applications do not comply with the accessibility API : for example, Microsoft Word does not comply with the MSAA API , so screen readers must still maintain an off-screen model for Word or find another way to access its contents. One approach is to use available operating system messages and application object models to supplement accessibility API s. Screen readers can be assumed to be able to access all display content that
2294-444: The early IBM Personal Computer (PC) was released in 1981, Thatcher and Wright developed a software equivalent to SAID, called PC-SAID, or Personal Computer Synthetic Audio Interface Driver . This was renamed and released in 1984 as IBM Screen Reader, which became the proprietary eponym for that general class of assistive technology. In early operating systems , such as MS-DOS , which employed command-line interfaces ( CLI s),
2356-409: The exchanged data and retransmits it at a later time to fool one end into thinking it has authenticated a new connection attempt from the other. Authentication protocols usually employ a cryptographic nonce as the challenge to ensure that every challenge-response sequence is unique. This protects against Eavesdropping with a subsequent replay attack . If it is impractical to implement a true nonce,
2418-426: The first generic CAPTCHA-solving algorithm based on reinforcement learning and demonstrated its efficiency against many popular CAPTCHA schemas. In October 2018 at ACM CCS'18 conference, Ye et al. presented a deep learning-based attack that could consistently solve all 11 text captcha schemes used by the top-50 popular websites in 2018. An effective CAPTCHA solver can be trained using as few as 500 real CAPTCHAs. It
2480-439: The graphical contents of the display. Screen readers were therefore forced to employ new low-level techniques, gathering messages from the operating system and using these to build up an "off-screen model", a representation of the display in which the required text content is stored. For example, the operating system might send messages to draw a command button and its caption. These messages are intercepted and used to construct
2542-417: The independent hCaptcha. It takes the average person approximately 10 seconds to solve a typical CAPTCHA. 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. Newer CAPTCHAs look at the user's behaviour on
CAPTCHA - Misplaced Pages Continue
2604-443: The internet, to prove that they are a human. A normal CAPTCHA test only appears if the user acts like a bot, such as when they request webpages, or click links too fast. Since the 1980s–1990s, users have wanted to make text illegible to computers. The first such people were hackers , posting about sensitive topics to Internet forums they thought were being automatically monitored on keywords. To circumvent such filters, they replaced
2666-517: The mistake of relying too heavily on background confusion in the image. In each case, algorithms were created that were successfully able to complete the task by exploiting these design flaws. However, light changes to the CAPTCHA could thwart them. Modern CAPTCHAs like reCAPTCHA rely on present variations of characters that are collapsed together, making them hard to segment, and they have warded off automated tasks. In October 2013, artificial intelligence company Vicarious claimed that it had developed
2728-402: The off-screen model. The user can switch between controls (such as buttons) available on the screen and the captions and control contents will be read aloud and/or shown on a refreshable braille display . Screen readers can also communicate information on menus, controls, and other visual constructs to permit blind users to interact with these constructs. However, maintaining an off-screen model
2790-413: The one string of characters which "fits" the challenge Bob issued. The "fit" is determined by an algorithm defined in advance, and known by both Bob and Alice. The correct response might be as simple as "63x83z", with the algorithm changing each character of the challenge using a Caesar cipher . In reality, the algorithm would be much more complex. Bob issues a different challenge each time, and thus knowing
2852-410: The password to an eavesdropper. However, they may supply enough information to allow an eavesdropper to deduce what the password is, using a dictionary attack or brute-force attack . The use of information which is randomly generated on each exchange (and where the response is different from the challenge) guards against the possibility of a replay attack , where a malicious intermediary simply records
2914-427: The password was really the system they were trying to access, and that nobody was likely to be eavesdropping on the communication channel . To address the insecure channel problem, a more sophisticated approach is necessary. Many cryptographic solutions involve two-way authentication; both the user and the system must verify that they know the shared secret (the password), without the secret ever being transmitted in
2976-664: The phone and does not require special programs or devices on the user side. Virtual assistants can sometimes read out written documents (textual web content, PDF documents, e-mails etc.) The best-known examples are Apple's Siri , Google Assistant , and Amazon Alexa . A relatively new development in the field is web-based applications like Spoken-Web that act as web portals, managing content like news updates, weather, science and business articles for visually-impaired or blind computer users. Other examples are ReadSpeaker or BrowseAloud that add text-to-speech functionality to web content. The primary audience for such applications
3038-442: The platform (e.g. Microsoft Word and Active Accessibility). Some programs and applications have voicing technology built in alongside their primary functionality. These programs are termed self-voicing and can be a form of assistive technology if they are designed to remove the need to use a screen reader. Some telephone services allow users to interact with the internet remotely. For example, TeleTender can read web pages over
3100-535: The protected resource. Because CAPTCHAs are designed to be unreadable by machines, common assistive technology tools such as screen readers cannot interpret them. The use of CAPTCHA thus excludes a small percentage of users from using significant subsets of such common Web-based services as PayPal, Gmail, Orkut, Yahoo!, many forum and weblog systems, etc. In certain jurisdictions, site owners could become targets of litigation if they are using CAPTCHAs that discriminate against certain people with disabilities. For example,
3162-518: The prover must respond with the correct password for that identifier. Assuming that the passwords are chosen independently, an adversary who intercepts one challenge-response message pair has no clues to help with a different challenge at a different time. For example, when other communications security methods are unavailable, the U.S. military uses the AKAC-1553 TRIAD numeral cipher to authenticate and encrypt some communications. TRIAD includes
SECTION 50
#17328631408073224-450: The quality of the software but also on a logical structure of the text. Use of headings, punctuation, presence of alternate attributes for images, etc. is crucial for a good vocalization. Also a web site may have a nice look because of the use of appropriate two dimensional positioning with CSS but its standard linearization, for example, by suppressing any CSS and Javascript in the browser may not be comprehensible. Most screen readers allow
3286-407: The screen display consisted of characters mapping directly to a screen buffer in memory and a cursor position. Input was by keyboard. All this information could therefore be obtained from the system either by hooking the flow of information around the system and reading the screen buffer or by using a standard hardware output socket and communicating the results to the user. In the 1980s,
3348-511: The simultaneous use of three separate abilities—invariant recognition, segmentation , and parsing to complete the task. 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. 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, "any program that passes
3410-494: The tests generated by a CAPTCHA can be used to solve a hard unsolved AI problem." They argue that the advantages of using hard AI problems as a means for security are twofold. Either the problem goes unsolved and there remains a reliable method for distinguishing humans from computers, or the problem is solved and a difficult AI problem is resolved along with it. CAPTCHAs based on reading text—or other visual-perception tasks—prevent blind or visually impaired users from accessing
3472-414: The tools that CAPTCHAs were designed to block in the first place. Howard Yeend has identified two implementation issues with poorly designed CAPTCHA systems: reusing the session ID of a known CAPTCHA image, and CAPTCHAs residing on shared servers. Sometimes, if part of the software generating the CAPTCHA is client-side (the validation is done on a server but the text that the user is required to identify
3534-460: The user to enter the solution as verification. Although these are much easier to defeat using software, they are suitable for scenarios where graphical imagery is not appropriate, and they provide a much higher level of accessibility for blind users than the image-based CAPTCHAs. These are sometimes referred to as MAPTCHAs (M = "mathematical"). However, these may be difficult for users with a cognitive disorder, such as dyscalculia . Challenges such as
3596-603: The user to select whether most punctuation is announced or silently ignored. Some screen readers can be tailored to a particular application through scripting . One advantage of scripting is that it allows customizations to be shared among users, increasing accessibility for all. JAWS enjoys an active script-sharing community, for example. Verbosity is a feature of screen reading software that supports vision-impaired computer users. Speech verbosity controls enable users to choose how much speech feedback they wish to hear. Specifically, verbosity settings allow users to construct
3658-705: 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. Datta et al. published their paper in the ACM Multimedia '05 Conference, named IMAGINATION (IMAge Generation for INternet AuthenticaTION), proposing a systematic way to image recognition CAPTCHAs. Images are distorted so image recognition approaches cannot recognise them. 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
3720-420: The work with it was proposed by ProtectWebForm and named "Smart CAPTCHA". 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. One alternative method involves displaying to the user a simple mathematical equation and requiring
3782-402: 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 reverse Turing tests . Two widely used CAPTCHA services are Google 's reCAPTCHA and
SECTION 60
#17328631408073844-402: Was not a systematic methodology for designing or evaluating early CAPTCHAs. 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 made
#806193