%ents; ]>
Robot Challenges This document specifies an XMPP protocol extension that entities may use to discover whether the sender of an XML stanza is a human user or a robot. &LEGALNOTICE; 0158 Experimental Standards Track Standards JIG XMPP Core XMPP IM XEP-0004 XEP-0066 None None challenge &ianpaterson; 0.5 2006-10-30 ip Added reference to SPIM Reporting 0.4 2005-09-30 ip Required WAV instead of MP3 for audio CAPTCHAs; minor corrections 0.3 2005-09-28 ip Added more CAPTCHA types and the legacy question and answer section 0.2 2005-09-14 ip Minor fixes 0.1 2005-09-14 ip Initial version. 0.0.1 2005-09-07 ip First draft.

The appearance of large public IM services based on &rfc3920; and &rfc3921; makes it desirable to implement protocols that discourage the sending of large quantities of IM SPAM (SPIM) from XMPP clients (connected to legitimate servers) and XMPP servers (with virtual clients) -- all hosted on networks of 'zombie' machines. SPIM is defined here as any type of unsolicited XMPP stanza sent by a 'robot' and delivered to a human, including messages and subscription requests. SPIM has the potential to disrupt people even more than SPAM, because each message interrupts the receiver (humans typically filter SPAM in batch mode).

Several of the most effective techniques developed to combat SPAM require 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 SPIM. The very occasional inconvenience of responding to a CAPTCHA (for example, 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 Hashcash <http://hashcash.org/>. 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 SPIM.

The generic challenge protocol described in this document is designed for incorporation into other protocols like &xep0077; and &xep0159;.

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 &w3turingtest;). Clients running in constrained environments may not be able to perform some challenges (for example, 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.

This protocol requires multimedia to be included within &xep0004;. This section defines a new namespace ('http://jabber.org/protocols/media') for that purpose. The root element for the namespace is <media/>. It MUST be contained within a <field/> element qualified by the 'jabber:x:data' namespace.

If the media is an image or video then the <media/> element SHOULD include 'width' and 'height' attributes specifying the recommended display size of the media in pixels.

The <media/> element SHOULD contain at least one <uri/> element to specify the out-of-band location of the media data. Constrained execution environments prevent some clients rendering media unless it has been received out-of-band (e.g., Web clients). The <uri/> element MUST contain a URI that indicates the location.

The <media/> element MAY also contain one or more <data/> elements for distributing the media in-band. The <data/> element MUST contain the Base64 encoded (in accordance with Section 3 of &rfc3548;) media data. The encoded data SHOULD NOT be larger than 8 KB. Note that if a stanza contains more than one <data/> element then the sending entity MUST take care not to trigger karma limits.

Each <uri/> or <data/> element MUST include a 'type' atribute that specifies the MIME type (see &rfc2045;) of the media.

http://www.victim.com/challenges/speech.wav?F3A6292C http://www.victim.com/challenges/speech.ogg?F3A6292C http://www.victim.com/challenges/speech.mp3?F3A6292C ** Base64 encoded audio ** ]]>

An entity (client or server) MAY send a challenge to a client immediately after receiving a stanza from the client. An entity MUST NOT send challenges under any other circumstances. Hereafter, the entity that sends a challenge is called a "challenger".

Love pills - 75% OFF http://www.spimmer.com/lovepills.html ]]>

Each of the challenge form's <field/> elements (see Data Forms) that are not hidden MAY contain a different challenge and any media required for the challenge. The hidden 'from' field MUST contain the value of the 'to' attribute of the client's stanza that triggered the challenge. If the stanza from the client included an 'id' attribute then the hidden 'sid' field MUST be set to that value. The 'xml:lang' attribute of the challenge stanza SHOULD be the same as the one received from the client. The hidden 'FORM_TYPE' field MUST have a value of "http://jabber.org/protocol/challenge" in accordance with &xep0068;.

The challenger SHOULD include an explanation (in the &BODY; element) for clients that do not support this protocol. The challenger MAY also include a URL (typically a Web page with instructions) using &xep0066; as an alternative for clients that do not support the challenge form. Note: even if it provides a URL, a challenger MUST always provide a challenge form. A constrained client, like a mobile phone, cannot present a Web page to its user.

Your messages to innocent@victim.com are being blocked. Visit the Web page to unblock them. http://www.victim.com/challenge.html?F3A6292C http://jabber.org/protocol/challenge innocent@victim.com spam1 http://www.victim.com/challenges/ocr.jpeg?F3A6292C ** Base64 encoded image ** http://www.victim.com/challenges/picture.jpeg?F3A6292C ** Base64 encoded image ** http://www.victim.com/challenges/speech.wav?F3A6292C http://www.victim.com/challenges/speech.ogg?F3A6292C http://www.victim.com/challenges/video.mpeg?F3A6292C ]]>

The client SHOULD ignore the challenge stanza in either of the following cases:

  • If it has not recently sent (in the last two minutes for example) a stanza to the JID specified in the 'from' field of the form with the 'id' specified in the 'sid' field (or with no 'id' if no 'sid' field is included). Otherwise the user's presence would be disclosed, or a SPIM robot might dupe the user into providing answers to other people's challenges!
  • If the 'from' attribute of the challenge stanza does not match the 'from' field of the form. (If the values are different, then they still match if the bare JIDs are the same, or if the 'from' attribute is the domain of the other JID.)

Otherwise, if the challenger provided a URL using Out-of-Band Data, then the client MAY present the URL to its user, instead of responding to the challenge form, in any of the following cases:

  • if it does not understand the challenge form
  • if it does not support all of the required challenges (see Multiple Challenges)
  • if it does not support enough of the challenges (see Multiple Challenges)

Otherwise, the client MUST respond to the challenger, preserving the 'id' attribute of the challenge stanza.

The client MUST respond with a ¬acceptable; error in any of the following cases:

  • if it does not support all of the required challenges (see Multiple Challenges)
  • if it does not support enough of the challenges (see Multiple Challenges)
  • if its user declines the challenge
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Otherwise, it MUST select one challenge according to the user's preferences and submit the user's response form to the challenger.

http://jabber.org/protocol/challenge innocent@victim.com spam1 7nHL3 ]]>

The challenger SHOULD send a &unavailable; error to the client if:

  • The challenger did not send the specified challenge. If the challenger is a client then it SHOULD be careful not to leak information about the presence of its user and reply to potentially bogus challenge responses with exactly the same XML that its server would send if its user were offline.
  • The client already submitted its response to this challenge.
  • The client took too long to submit its response.

Note: This error MAY be sent even in cases where the challenge became unnecessary while the challenger was waiting for the response.

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After receiving a correct response to its challenge, the challenger SHOULD inform the client that it was successful.

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However, if the client submits an incorrect response the challenger SHOULD send it a ¬acceptable; error with type "cancel": 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 to be defined in a forthcoming proposal on SPIM reporting). It SHOULD also automatically block all stanzas from the abusive user, users, server or IP.

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The enitity may demand responses to more than one of the challenges it is offering by including an 'answers' <field/> element in the form. It may require responses to particular challenges by including <required/> elements in the compulsory fields.

Your messages to innocent@victim.com are being blocked. To unblock them, ask innocent@victim.com to send you a message. http://jabber.org/protocol/challenge innocent@victim.com spam2 2 http://www.victim.com/challenges/ocr.jpeg?F3A6292C http://www.victim.com/challenges/audio.wav?F3A6292C ]]>

If the client 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 client MUST also answer one or more of the challenges without a <required/> element. In the example above, the client should respond to the 'qa' challenge and one of the other challenges ('ocr', 'audio_recog' or 'SHA-256').

http://jabber.org/protocol/challenge innocent@victim.com spam2 2 divad innocent@victim.com2450F06C173B05E3 ]]>

The challenger MAY decide the client has passed a challenge even if the responses are not all perfectly correct.

This section shows how challenges SHOULD be combined with the existing registration protocol according to the rules defined in the Extensibility section of In-Band Registration. Note: The <challenge/> wrapper element is not required.

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Note that the challenge form MUST be inside the &QUERY; element, and the server's challenge ID is specified within the form:

http://jabber.org/protocol/challenge F3A6292C 3 http://www.victim.com/challenges/ocr.jpeg?F3A6292C To register, visit http://www.victim.com/register.html http://www.victim.com/register.html ]]>

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.

http://jabber.org/protocol/challenge F3A6292C 3 7nHL3 bill Calliope ]]>

Entities MUST address the needs of disabled people and CPU-constrained clients by offering people 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. A CPU-constrained client could ask a faster computer (its server, for example) to perform a Hashcash challenge for it.

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 client 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 ®ISTRAR; (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 SPIM. This significantly reduces SPIM, but alone it will not completely stop SPIM being sent through large collections of 'zombie' computers. 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.

The challenger MUST set the 'label' attribute of the <field/> element to a hexadecimal random number containing a configured number of bits (220 ≤ label < 221, for example).

To pass the test, the client MUST return a text string that starts with the JID the client sent the first stanza to (i.e., the stanza that triggered the challenge). The least significant bits of the SHA-256 hash (see &nistfips180-2;) 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:

innocent@victim.com2450F06C173B05E3

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 SPIM 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 SPIM robots never attempt the Hashcash challenge, then he SHOULD consider reducing the number of bits, to avoid inconveniencing people unnecessarily.

For those CAPTCHA types where generic instructions are possible (see table below) then the <field/> element SHOULD NOT include a 'label' attribute (the client MUST present generic instructions to its user in the language of its user interface). Otherwise the 'label' attribute SHOULD ask a specific question in the language indicated by the 'xml:lang' attribute of the challenge stanza.

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. 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.

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 'boolean' and 'list-single' field types would make it trivial for a robot to provide a correct response at least some of the time. The response MUST be provided in the language specified by the 'xml:lang' attribute of the challenge stanza.

'var' Name Media type MIME-type 'label' Example generic instructions
ocr* Optical Character Recognition image image/jpeg No Enter the code you see
picture_recog Picture Recognition image image/jpeg No Describe the picture
video_recog Video Recognition video video/mpeg No Describe the video
speech_recog Speech Recognition audio audio/x-wav No Enter the words you hear
audio_recog Audio Recognition audio audio/x-wav No Describe the sound you hear
picture_q Picture Question image image/jpeg No Answer the question you see
video_q Video Question video video/mpeg No Answer the question in the video
speech_q Speech Question audio audio/x-wav No Answer the question you hear
qa Text Question and Answer - - Yes** -

* The image portrays random characters that humans can read but OCR software cannot. See PWNtcha <http://sam.zoy.org/pwntcha/> for some example OCR CAPTCHA images. To pass the challenge, the user must simply type the characters. The correct answer SHOULD NOT depend on the language specified by the 'xml:lang' attribute of the challenge stanza.

** To pass the challenge, the user must type the answer to the question in the 'label' attribute.

Note: It may be profitable to send SPIM 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-SPIM techniques.

An challenger MAY provide a text question in the &BODY; element of a challenge stanza for clients that do not support challenge 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 challenge form.

Note: Even if it provides a text question in the &BODY; element, a challenger MUST always provide a challenge form.

Your messages to me are being blocked. To unblock them, reply with the color of a stop light followed by 'F3A6292C'. http://jabber.org/protocol/challenge innocent@victim.com spam1 ]]>

Legacy clients respond to the challenger using a &MESSAGE; stanza (not an &IQ;).

red F3A6292C ]]>

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;).

Your message was delivered. Your messages to me are no longer being blocked. ]]> Your message to me was not delivered. ]]>

It is RECOMMENDED that entities employ other techniques to combat SPIM in addition to those described in this document. For example see &xep0161;.

The expectation is that this protocol will be an important and successful tool for discouraging SPIM. However, much of its success is dependent on the quality of the CAPTCHAs employed by a particular implementation.

The administrator of a challenger MUST discontinue the use of Robot Challenges under the following circumstances:

This document introduces no security considerations above and beyond those described in RFC 3920 and RFC 3921.

This document requires no interaction with &IANA;.

Upon approval of this document, the ®ISTRAR; shall register the following protocol namespaces:

  • http://jabber.org/protocol/challenge
  • http://jabber.org/protocol/media

Upon approval of this document, the XMPP Registrar shall register the following new FORM_TYPE. Additional fields will be defined in future submissions.

http://jabber.org/protocol/challenge XEP-0158 forms enabling robot challenges ]]>

Upon approval of this document, the XMPP Registrar shall register the following new fields for the existing jabber:iq:register FORM_TYPE. Additional fields will be defined in future submissions.

jabber:iq:register XEP-0077 ]]> ]]> ]]>

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 only use this feature if she was plagued by SPIM.