The appearance of large public IM services based on XMPP Core  and XMPP IM  makes it desirable to implement protocols that discourage the sending of large quantities of instant messaging spam (a.k.a. "spim") or, in general, abusive traffic. Abusive stanzas could be generated by XMPP clients connected to legitimate servers or by XMPP servers with virtual clients, where the malicious entities are hosted on networks of "zombie" machines. Such abusive stanzas could take many forms; a full taxonomy is outside the scope of this document.
One technique developed to combat abusive messages and behavior via non-XMPP technologies requires humans to be differentiated from bots using a "Completely Automated Public Turing Test to Tell Computers and Humans Apart" or CAPTCHA (see <http://www.captcha.net/>). These challenge techniques are easily adapted to discourage XMPP abuse. The very occasional inconvenience of responding to a CAPTCHA (e.g., when creating an IM account or sending a message to a new correspondent) is small and perfectly acceptable -- especially when compared to the countless robot-generated interruptions people might otherwise have to filter every day.
An alternative technique to CAPTCHAs requires Desktop PC clients to undertake a Hashcash  challenge. These are completely transparent to PC users. They require clients to perform specified CPU-intensive work, making it difficult to send large amounts of abusive traffic.
Both CAPTCHAs and hashcash have been criticized regarding their effectiveness (or lack thereof). Therefore, the challenge protocol specified herein provides a great deal of flexibility, so that challenges can include CAPTCHAs, hashcash, word puzzles, so-called kitten authentication, and any other mechanism that may be developed in the future.
The generic challenge protocol described in this document is designed for incorporation into protocols such as In-Band Registration (XEP-0077) , Multi-User Chat (XEP-0045) , Privacy Lists (XEP-0016) , and SPIM-Blocking Control (XEP-0159) .
The CAPTCHAs in most common use today are Optical Character Recognition (OCR) challenges where an image containing deformed text is presented and the human enters the characters they can read. However, if OCR software advances more rapidly than the techniques used to disguise text from Artificial Intelligence (AI) then very different CAPTCHAs will need to be deployed. This protocol must be extensible enough to allow the incorporation of CAPTCHA techniques that may not have been envisaged.
Several common CAPTCHA techniques present major problems to users with disabilities (see Inaccessibility of Visually-Oriented Anti-Robot Tests ). Clients running in constrained environments may not be able to perform some challenges (e.g., due to the absence of audio output or a lack of CPU performance). This protocol must allow clients to be offered a choice from a variety of challenges.
A "triggering stanza" is an XMPP <message/>, <presence/>, or <iq/> stanza that is deemed abusive by the receiving entity (e.g., a client) or an intermediate router (e.g., a server). The entity that generates a triggering stanza is called a "sender".
Upon receiving a triggering stanza, an entity MAY send a "challenge stanza". An entity MUST NOT send a challenge stanza under any other circumstances. The entity that generates the challenge stanza is called the "challenger".
The challenge stanza consists of an XMPP <message/> stanza containing a data form for the sender to fill out, formatted according to Data Forms (XEP-0004) , optionally along with a <body/> and other elements. The following rules apply to the challenge stanza.
The sender's client SHOULD ignore the challenge stanza in either of the following cases:
Otherwise, if the challenger provided a URL using Out-of-Band Data, then the sender's client MAY present the URL to the sender, instead of responding to the CAPTCHA form, in any of the following cases:
Otherwise, the sender's client MUST respond to the challenge.
The sender's client MUST respond with a <not-acceptable/> error in any of the following cases:
Otherwise, it MUST select one challenge according to the sender's preferences and submit the sender's response form to the challenger.
The challenger SHOULD send a <service-unavailable/> error to the sender if:
Note: This error MAY be sent even in cases where the challenge became unnecessary while the challenger was waiting for the response.
After receiving a correct response to its challenge, the challenger SHOULD inform the sender that it was successful.
However, if the sender submits an incorrect response the challenger SHOULD send it a <not-acceptable/> error with type "cancel": 
The challenger MAY demand responses to more than one of the challenges it is offering; this is done by including an 'answers' <field/> element in the form, which specifies how many answers the sender needs to include. The challenger also MAY require responses to particular challenges; this is done by including a <required/> element in the compulsory fields.
If the sender finds the request acceptable, it MUST answer all challenges that include a <required/> element. If the total number of answers was specified and it is greater than the number of <required/> elements then the sender MUST also answer one or more of the challenges without a <required/> element. In the example above, the sender should respond to the 'qa' challenge and one of the other challenges ('ocr', 'audio_recog' or 'SHA-256').
The challenger MAY decide the sender has passed a challenge even if the responses are not all perfectly correct.
This section shows how challenges SHOULD be combined with the existing In-Band Registration protocol according to the rules defined in the Extensibility section of XEP-0077.
Note: The <challenge/> wrapper element is not included, because XEP-0077 specifies that data forms shall be contained as the direct children of the <query/> element.
Note that the CAPTCHA form MUST be inside the <query/> element, and the server's challenge ID is specified within the form:
The server MAY include an <instructions/> element and a URL using Out-of-Band Data (e.g., a web page) in the <query/> element (see example above). In-Band Registration recommends that the challenger SHOULD submit the completed x:data form, however if it does not understand the form, then it MAY present the instructions and the included URL to the user instead of providing the required information in-band.
A service that hosts multi-user chat rooms in accordance with XEP-0045 MAY challenge unknown entities that seek to join such rooms or that send messages in such rooms.
Entities MUST address the needs of disabled people and CPU-constrained clients by offering senders a reasonable choice of different types of challenges.
Desktop clients running on modern PCs will typically be configured to automatically perform a specified 'SHA-256' Hashcash challenge (see below) whenever it is below a certain level of difficulty, with the result that many people may not even notice challenges most of the time. However, people using CPU-constrained clients (e.g. Web or mobile clients) would notice the performance hit. They might prefer to take a CAPTCHA challenge instead. 
Visually disabled people using a CPU-constrained client could configure their client to always present them with an audio CAPTCHA challenge.
Most of the challenges below are language sensitive. However, the evaluation of the OCR and Hashcash responses does not depend on the language the sender is using.
Challenge types are distinguished by the 'var' attribute of each <field/> element. Several types of challenges are described below. More challenges MAY be documented elsewhere and registered with the XMPP Registrar (see Field Standardization).
The SHA-256 Hashcash challenge is transparent to average PC users. It is indicated when the value of the 'var' attribute is 'SHA-256'. It forces clients to perform CPU-intensive work, making it difficult to send large amounts of abusive traffic. This significantly reduces abusive traffic, but alone it will not completely stop abusive stanzas from being sent through large collections of 'zombie' computers. 
The challenger MUST set the 'label' attribute of the <field/> element to a hexadecimal random number containing a configured number of bits (e.g., 220 ≤ label < 221).
To pass the test, the sender MUST return a text string that starts with the JID the sender sent the first stanza to (i.e., the stanza that triggered the challenge). The least significant bits of the SHA-256 hash (see SHA ) of the string MUST equal the hexadecimal value specified by the challenger (in the 'label' attribute of the <field/> element). For example, if the 'label' attribute is the 20-bit value 'e03d7' then the following string would be correct:
Note: When configuring the number of bits to be specified by a challenger in the 'label' attribute values, administrators MUST balance the need to make mass abuse as difficult as possible, with the inconvenience that may be caused to the users of less powerful computers. (Most clients will be challenged only very occasionally, so the consumption of 70% of a typical desktop CPU for 4 seconds might be considered appropriate.) Administrators SHOULD increment the configured number of bits from time to time to match increases in the performance of typical desktop PCs. If an administrator notices that abusive robots never attempt the Hashcash challenge, then he SHOULD consider reducing the number of bits, to avoid inconveniencing people unnecessarily.
Note: It may be profitable to send abusive stanzas even if less than one percent of CAPTCHA responses are successful. The effectiveness of a CAPTCHA challenge needs to be close to perfect, unless it is used in combination with other anti-abuse techniques.
If a media type is specified (see table below) then the <field/> element MUST contain a <media/> element that includes a <uri/> element of that type. Clients that support the CAPTCHA type MUST be able to play or render the specified MIME-types (see table below). They MAY also support other formats. 
The 'type' attribute of the <field/> element SHOULD be 'text-single', 'text-private', or 'text-multi' (if no 'type' is specified, the default is 'text-single').  The response MUST be provided in the language specified by the 'xml:lang' attribute of the challenge stanza.
|'var'||Name||Media type||MIME-type||Suggested generic instructions *|
|audio_recog||Audio Recognition||audio||audio/x-wav||Describe the sound you hear|
|ocr **||Optical Character Recognition||image||image/jpeg||Enter the text you see|
|picture_q||Picture Question||image||image/jpeg||Answer the question you see|
|picture_recog||Picture Recognition||image||image/jpeg||Identify the picture|
|qa||Text Question and Answer||-||-||-|
|speech_q||Speech Question||audio||audio/x-wav||Answer the question you hear|
|speech_recog||Speech Recognition||audio||audio/x-wav||Enter the words you hear|
|video_q||Video Question||video||video/mpeg||Answer the question in the video|
|video_recog||Video Recognition||video||video/mpeg||Identify the video|
* See the Internationalization Considerations section of this document.
** The image portrays random characters that humans can read but OCR software cannot.  To pass the challenge, the sender must simply type the characters. The correct answer SHOULD NOT depend on the language specified by the 'xml:lang' attribute of the challenge stanza.
A challenger MAY provide a text question in the <body/> element of a challenge stanza for clients that do not support CAPTCHA forms. Entities that cannot serve Out-of-Band Data URLs MAY use this option to challenge legacy clients.
Note: Robots always attempt the easiest challenge they are offered. So the question MUST be at least as difficult for a robot as the CAPTCHA form.
Note: Even if it provides a text question in the <body/> element, a challenger MUST always provide a CAPTCHA form.
Legacy clients respond to the challenger using a <message/> stanza (not an <iq/>).
The challenger SHOULD treat the stanza as a normal message (instead of as a response to its challenge) if the legacy client either takes too long to submit it or has already responded to the challenge. The challenger MAY treat the response as a normal message even in cases where the challenge became unnecessary while the challenger was waiting for the response.
Otherwise the challenger MUST report the result of the challenge to the legacy client using a <message/> stanza (not an <iq/>).
It is RECOMMENDED that entities employ other techniques to combat abusive stanzas in addition to those described in this document (e.g., see XEP-0161 and Best Practices to Discourage Denial of Service Attacks (XEP-0205) ).
It is expected that this protocol will be an important and successful tool for discouraging abusive traffic. However, much of its success is dependent on the quality of the CAPTCHAs and other puzzles employed by a particular implementation.
The administrator of an application that functions as a challenger SHOULD discontinue the use of CAPTCHA forms under the following circumstances:
Each form field SHOULD include a 'label' attribute. If the sender did not include an 'xml:lang' attribute, then the challenger may not know the correct language for the labels. Therefore, depending on user preferences, the client that receives a challenge MAY present generic but localized text instead of label text that would not be understood by the user. Suggested generic text (to be suitably localized) is provided by Table 1 in the CAPTCHAs section of this document.
The use of CAPTCHAs is not a panacea, and should be combined with other anti-abuse mechanisms, such as those described in XEP-0161 and XEP-0205. For example, the task of finding solutions to CAPTCHAs and other computational puzzles is becoming easier for computer programs, and in any case can be farmed out to third parties. Therefore challengers should limit the number of triggering stanzas (e.g., registration attempts, subscription requests, or chatroom joins) allowed per JabberID or IP address during any given time period, and may simply refuse repeated stanzas by terminating an XML stream with a <policy-violation/> stream error or returning a <not-acceptable/> stanza error as appropriate. In addition, a challenger should feel free to deploy additional anti-abuse mechanisms as needed.
This document requires no interaction with the Internet Assigned Numbers Authority (IANA) .
The XMPP Registrar registers following FORM_TYPE. Additional fields might be defined in future submissions.
The XMPP Registrar registers the following fields for the existing jabber:iq:register FORM_TYPE. Additional fields might be defined in future submissions.
Another protocol could allow users to edit the challenges their server will make on their behalf. For example, the number of SHA-256 bits, a personal or original question and answer, a picture, a video, or a sound recording. Of course Aunt Tillie would typically use this feature only if she was plagued by abusive traffic.
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The Extensible Messaging and Presence Protocol (XMPP) is defined in the XMPP Core (RFC 6120) and XMPP IM (RFC 6121) specifications contributed by the XMPP Standards Foundation to the Internet Standards Process, which is managed by the Internet Engineering Task Force in accordance with RFC 2026. Any protocol defined in this document has been developed outside the Internet Standards Process and is to be understood as an extension to XMPP rather than as an evolution, development, or modification of XMPP itself.
The primary venue for discussion of XMPP Extension Protocols is the <firstname.lastname@example.org> discussion list.
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The following requirements keywords as used in this document are to be interpreted as described in RFC 2119: "MUST", "SHALL", "REQUIRED"; "MUST NOT", "SHALL NOT"; "SHOULD", "RECOMMENDED"; "SHOULD NOT", "NOT RECOMMENDED"; "MAY", "OPTIONAL".
11. Inclusion of a CAPTCHA form not only makes it possible to flexibly support or require a large number of challenge types, but also enables constrained clients to respond to challenges (e.g., mobile phone clients that cannot present web pages, or clients on XMPP-only networks).
15. Otherwise the user's presence would be disclosed, or a robot might dupe the user into providing answers to other people's challenges!
16. If the challenger is a client then it SHOULD be careful not to leak information about the presence of the sender and reply to potentially bogus challenge responses with exactly the same XML that its server would send if the sender were offline.
17. If a large proportion of the responses a server is receiving from another IP are incorrect then it SHOULD inform the administrator of the other server using the protocol specified in Abuse Reporting (XEP-0161)  or Abuse Reporting (XEP-0236) . It SHOULD also automatically block all stanzas from the abusive user, users, server or IP.
20. A CPU-constrained client could ask a faster computer (e.g., its server) to perform a Hashcash challenge for it.
21. The hope is that the extra CPU usage will often be noticed by the owners of the zombie machines, who will be more likely to fix them.
22. Secure Hash Standard: Federal Information Processing Standards Publication 180-2 <http://csrc.nist.gov/publications/fips/fips180-2/fips186-2withchangenotice.pdf>.
23. Audio CAPTCHAs typically require challengers to provide at least the 'audio/x-wav' MIME-type (with the PCM codec) because more efficient patent-free formats are often not supported by constrained clients. It is RECOMMENDED that challengers provide more compact formats (like Ogg Speex or MP3) too.
24. The 'boolean' and 'list-single' field types would make it trivial for a robot to provide a correct response at least some of the time.
27. The Internet Assigned Numbers Authority (IANA) is the central coordinator for the assignment of unique parameter values for Internet protocols, such as port numbers and URI schemes. For further information, see <http://www.iana.org/>.
28. The XMPP Registrar maintains a list of reserved protocol namespaces as well as registries of parameters used in the context of XMPP extension protocols approved by the XMPP Standards Foundation. For further information, see <https://xmpp.org/registrar/>.
Note: Older versions of this specification might be available at http://xmpp.org/extensions/attic/
Use the correct FORM_TYPE for extended IBR, and use a proper MUC join as an example. Also fix typos.
Per a vote of the XMPP Council, advanced status to Draft; concurrently, the XMPP Registrar issued the urn:xmpp:captcha namespace; also changed element name from challenge to captcha in order to match the namespace name.
Further modified examples to track changes to XEP-0231.
Modified examples to track changes to XEP-0231; to avoid confusion with cid URIs, changed name of challenge ID form field from cid to challenge.
Specified that cid field is required; more clearly defined structure of challenge stanza; added several security considerations; corrected several errors in the text and examples.
Moved text regarding labels to new internationalization considerations section; removed necessity that ID of IQ-set shall match ID of challenge, since this is not consistent with existing usage that IDs are generated by the sender of an IQ.
Generalized text to cover abuse rather than just spim; modified temporary namespace to adhere to XMPP Registrar procedures; added use case for joining multi-user chat rooms.
Moved media element definition to XEP-0221; consistently used the terms sender and challenger rather than client and entity; modified provisional namespaces to adhere to XMPP Registrar policies; completed editorial review.
Added reference to SPIM Reporting.
Required WAV instead of MP3 for audio CAPTCHAs; minor corrections.
Added more CAPTCHA types and the legacy question and answer section.