SIPPING                                                    H. Tschofenig
Internet-Draft                                    Nokia Siemens Networks
Intended status: Standards Track                             E. Leppanen
Expires: August 28, 2008                                      Individual
                                                            S. Niccolini
                                                                     NEC
                                                         M. Arumaithurai
                                                University of Goettingen
                                                       February 25, 2008


  Completely Automated Public Turing Test to Tell Computers and Humans
             Apart (CAPTCHA) based Robot Challenges for SIP
                draft-tschofenig-sipping-captcha-01.txt

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on August 28, 2008.

Copyright Notice

   Copyright (C) The IETF Trust (2008).

Abstract

   A common approach to deal with unwanted communication attempts is to
   rely on some form of authorization policies, typically whitelists.



Tschofenig, et al.       Expires August 28, 2008                [Page 1]

Internet-Draft   CAPTCHA based Robot Challenges for SIP    February 2008


   In order to populate the entries in such an access control list it is
   helpful to have a way to challenge the entity willing to engage in a
   conversation (unless they are already pre-authorized).  One reason
   why this is desired is to deal with robots that are aggressively
   distributing messages.

   This document describes how "Completely Automated Public Turing Test
   to Tell Computers and Humans Apart" (CAPTCHA) tests, which require
   human interaction, are applied to SIP.










































Tschofenig, et al.       Expires August 28, 2008                [Page 2]

Internet-Draft   CAPTCHA based Robot Challenges for SIP    February 2008


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  UAC, UAS and Proxy Behavior  . . . . . . . . . . . . . . . . .  5
     3.1.  Operation of a SIP Proxy or SIP UAS  . . . . . . . . . . .  5
     3.2.  Operation of UAC . . . . . . . . . . . . . . . . . . . . .  5
   4.  Description of the CAPTCHA XML Doument . . . . . . . . . . . .  6
     4.1.  Structure of XML-Encoded CAPTCHA Challenge . . . . . . . .  6
     4.2.  MIME Type for CAPTCHA Challenge Document . . . . . . . . .  6
     4.3.  The <challenge> Root Element . . . . . . . . . . . . . . .  6
     4.4.  The <media> Element  . . . . . . . . . . . . . . . . . . .  6
     4.5.  The <uri> element  . . . . . . . . . . . . . . . . . . . .  7
     4.6.  The <data> element . . . . . . . . . . . . . . . . . . . .  7
     4.7.  Values . . . . . . . . . . . . . . . . . . . . . . . . . .  7
   5.  Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . .  8
   6.  Example  . . . . . . . . . . . . . . . . . . . . . . . . . . .  9
   7.  XML Schema . . . . . . . . . . . . . . . . . . . . . . . . . . 10
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 12
     9.1.  Captcha Header . . . . . . . . . . . . . . . . . . . . . . 12
     9.2.  4xx Response . . . . . . . . . . . . . . . . . . . . . . . 12
     9.3.  Namespace  . . . . . . . . . . . . . . . . . . . . . . . . 13
     9.4.  Content-Type registration for
           'application/captcha-challenge+xml'  . . . . . . . . . . . 13
     9.5.  CAPTCHA Schema Registration  . . . . . . . . . . . . . . . 15
   10. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 15
   11. Alternative Solution Approaches  . . . . . . . . . . . . . . . 15
     11.1. Challenge by Proxy . . . . . . . . . . . . . . . . . . . . 15
       11.1.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . 15
       11.1.2.  Operation of Proxy when it issues a challenge
                directly  . . . . . . . . . . . . . . . . . . . . . . 17
       11.1.3.  Operation of UAC on receiving a CAPTCHA challenge
                from the SIP  . . . . . . . . . . . . . . . . . . . . 17
     11.2. SIP request redirected by the SIP Proxy  . . . . . . . . . 17
       11.2.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . 17
       11.2.2.  Operation of Proxy when it redirects the INVITE
                to a CAPTCHA UA . . . . . . . . . . . . . . . . . . . 20
       11.2.3.  Operation of UAC when it recieves a challenge
                from a CAPTCHA UA . . . . . . . . . . . . . . . . . . 20
     11.3. SIP Application Interaction Framework  . . . . . . . . . . 20
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
     12.1. Normative references . . . . . . . . . . . . . . . . . . . 21
     12.2. Informative references . . . . . . . . . . . . . . . . . . 21
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22
   Intellectual Property and Copyright Statements . . . . . . . . . . 24





Tschofenig, et al.       Expires August 28, 2008                [Page 3]

Internet-Draft   CAPTCHA based Robot Challenges for SIP    February 2008


1.  Introduction

   The problem of unwanted communication is an imminent challenge and
   only the combination of several techniques can provide some degree of
   protection.  [RFC5039] provides four recommendations that should to
   be considered for an overall solution, namely,

   o  Strong Identity
   o  White Lists
   o  Solve the Introduction Problem
   o  Don't Wait Until its Too Late.

   The human interaction required challenges are mainly used for solving
   the introduction problem targeting to handle requests from user
   agents with whom the recipient do not have former relations.  For
   example, the challenge is initiated towards user agents that are not
   yet white or black-listed, or based on some other criteria.

   The [I-D.tschofenig-sipping-framework-spit-reduction] provides a
   framework for dealing with unwanted communication.  The policy
   contains rules that are applied to requests if the conditions of a
   given rule matche.  The actions of the matching rules are executed
   and one of the actions could be to provide a challenge that must be
   soved by a human before the request is forwarded to the called party
   triggering the corresponding user interface notifications to the
   user.

   There are different techniques already developed for challenging user
   agents.  "Completely Automated Public Turing Test to Tell Computes
   and Humans Apart" (CAPTCHA) [captcha] typically provides a human a
   task either to recognize something or a question to be answered using
   different media types.  [Inaccessibility-of-CAPTCHA] provides
   alternatives to visual test for allowing systems to test for human
   users while preserving access by users with disabilities.  Hashcash
   challenge [hashcash] requires user agents to perform CPU-intensive
   computational puzzles making it difficult to send large amounts of
   requests.  The hashcash concept has been proposed for usage with SIP
   in [I-D.jennings-sip-hashcash].

   Using CAPTCHA techniques for SIP communication requires a mechanism
   for enabling user interaction to be associated with SIP requests.
   When a proxy or user agent server (UAS) server receives a SIP request
   that needs to be challenged, the proxy or UAS sends a challenge to
   the originator of the SIP request before continue handling of the
   request.  After getting the answer to the challenge from the user,
   the user agent client (UAC) needs to provide the answer back towards
   the UAS in order to get the initial request passed to the recipient.




Tschofenig, et al.       Expires August 28, 2008                [Page 4]

Internet-Draft   CAPTCHA based Robot Challenges for SIP    February 2008


   The challenge should offer multiple choices for the UACs to select
   depending on the capabilities of the device where the UAC is running.
   Also, the UAC should be able to authenticate and authorize the source
   of challenge.  The UAC may receive the challenge via a URL or as
   direct media compoment(s).

   The main goal is to support SIP dialog creating request such as SIP
   INVITE, but ideally the solution should also cover non-dialog
   creating requests, e.g., SIP MESSAGE.

   Note that this document presents several different solution
   approaches, see Section 11.  The solution presented in the main part
   of the document is aligned with the work done with [XEP-0158] on
   CAPTCHAs for XMPP.


2.  Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
   and "OPTIONAL" are to be interpreted as described in RFC 2119
   [RFC2119] and indicate requirement levels for compliant
   implementations.

   This document makes also use of the vocabulary defined in RFC 3261
   [RFC3261].


3.  UAC, UAS and Proxy Behavior

3.1.  Operation of a SIP Proxy or SIP UAS

   When a SIP proxy or a SIP UAS receives a SIP request from a UAC, its
   authorization engine may apply the policy to the SIP request, as, for
   example, defined in [RFC5025].  This authorization policy execution
   may result in the need for the proxy (or the UAS) to generate a
   challenge to the UAC, the proxy (or the UAS) can send the challenge
   directly, can send a URI of the challenge, or can redirect the
   request to a special CAPTCHA UA.

3.2.  Operation of UAC

   The UAC either receives a CAPTCHA challenge or a URI of the
   challenge.  The UAC is expected to solve the CAPTCHA puzzle and send
   the answer back to the SIP proxy server or to send a token to
   indicate that it has successfully solved the puzzle.





Tschofenig, et al.       Expires August 28, 2008                [Page 5]

Internet-Draft   CAPTCHA based Robot Challenges for SIP    February 2008


4.  Description of the CAPTCHA XML Doument

   This section describes the content of the CAPTCHA XML document.  The
   XML schema for it can be found in Section 7.

4.1.  Structure of XML-Encoded CAPTCHA Challenge

   A CAPTCHA challenge is an XML document [XML] that MUST be well-formed
   and MUST be valid according to the schema defined in this document,
   including extension schemas available to the validater and applicable
   to the XML document.  The XML documents MUST be based on XML 1.0 and
   MUST be encoded using UTF-8.

   The namespace identifier for elements defined by this specification
   is a URN [RFC2141], using the namespace identifier 'ietf' defined by
   [RFC2648] and extended by [RFC3688].  This URN is:
   urn:ietf:params:xml:ns:captcha.

4.2.  MIME Type for CAPTCHA Challenge Document

   The MIME type for the XML document is 'application/
   capcha-challenge+xml'.

4.3.  The <challenge> Root Element

   The root element of the XML document is <challenge>.

   The <challenge> element contains the namespace definition mentioned
   in Section 4.1.  It also contains a mandatory 'id' attribute for
   correlating the challenge and the answer, and the 'min-tests'
   attribute with the default value set to 1.  With the 'min-tests'
   attribute, it is possible to define the minimum amount of tests that
   need to be solved.

   The <challenge> element MUST have at least one child element.  This
   document defines the <media> element as a child element.  The
   <challenge> element may contain one or more <media> elements.

   The <challenge> element may also be extended by XML elements or
   attributes defined with other namespaces.

4.4.  The <media> Element

   The <media> element contains one child element.  This document
   defines the <uri> and <data> elements as child elements for allowing
   the CAPTCHA challenge be provided directly as content or as a
   reference to an external content.




Tschofenig, et al.       Expires August 28, 2008                [Page 6]

Internet-Draft   CAPTCHA based Robot Challenges for SIP    February 2008


   The <media> element contains a mandatory 'var' attribute indicating
   the type of the challenge (see values from the 'var' column of
   Figure 1).  It may also contain optional 'width' and 'height'
   attributes for providing the size of the content.  In addition, the
   element may contain an 'instr' attribute which purpose is to provide
   instructions related to the challenge (see the 'example generic
   instruction' column from Figure 1).  The required tests can be
   indicated by setting the value of the 'required' attribute to 'true'.

   The <media> element may also be extended by XML elements or
   attributes defined with other namespaces.

4.5.  The <uri> element

   The <uri> element contains a mandatory 'type' attribute indicating
   the MIME type of the challenge.  See values from the 'MIME type'
   column of Figure 1.  The value of the <uri> element is a URL where
   the challenge can be fetched.

   The <uri> element may also be extended by XML attributes defined with
   other namespaces.

4.6.  The <data> element

   The <data> element contains a mandatory 'type' attribute indicating
   the MIME type of the challenge.  See typical values from the 'MIME
   type' column of Figure 1.

   The value of the <data> element is the content of the challenge.

   The <data> element may also be extended by XML attributes defined
   with other namespaces.

4.7.  Values

   The following table copied from [XEP-0158] presents typical values
   for the CAPTCHA challenge.  The 'var' column lists values for the
   'var' attribute of the <media> element.  The 'MIME type' column
   contains values of the corresponding 'type' attribute of the <uri> or
   <data> elements.











Tschofenig, et al.       Expires August 28, 2008                [Page 7]

Internet-Draft   CAPTCHA based Robot Challenges for SIP    February 2008


+---------------+-------------+-------+--------+------------------------+
| 'var'         | Name        | Media | MIME   | Example generic        |
|               |             | type  | type   | instructions           |
+---------------+-------------+-------+--------+------------------------+
| ocr*          | Optical Char| image | image/ | Enter the code         |
|               | Recognition |       | jpeg   | you see                |
+---------------+-------------+-------+--------+------------------------+
| picture_recog | Picture     | image | image/ | Describe               |
|               | Recognition |       | jpeg   | the picture            |
+---------------+-------------+-------+--------+------------------------+
| video_recog   | Video       | video | video/ | Describe               |
|               | Recognition |       | mpeg   | the video              |
+---------------+-------------+-------+--------+------------------------+
| speech_recog  | Speech      | audio | audio/ | Enter the              |
|               | Recognition |       | x-wav  | words you hear         |
+---------------+-------------+-------+--------+------------------------+
| audio_recog   | Audio       | audio | audio/ | Describe the           |
|               | Recognition |       | x-wav  | sound you hear         |
+---------------+-------------+-------+--------+------------------------+
| picture_q     | Picture     | image | image/ | Answer the             |
|               | Question    |       | jpeg   | question you see       |
+---------------+-------------+-------+--------+------------------------+
| video_q       | Video       | video | video/ | Answer the             |
|               | Question    |       | mpeg   | question in video      |
+---------------+-------------+-------+--------+------------------------+
| speech_q      | Speech      | audio | audio/ | Answer the             |
|               | Question    |       | x-wav  | question you hear      |
+---------------+-------------+-------+--------+------------------------+
| qa            | Text Q & A  | text  | text/  | Answer the question    |
|               |             |       | plain  |                        |
+---------------+-------------+-------+--------+------------------------+

* The image portrays random characters that humans can read but OCR
software cannot. 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.


                Figure 1: Information of CAPTCHA challenges


5.  Syntax

   The Captcha header field carries the solution information.  It has
   parameters called 'id' and 'answer'.  The 'id' parameter value is set
   to the same as the 'id' attribute of the CAPTCHA challenge sent to
   the UAC.  The 'answer' parameter value is set to the answer of the
   CAPTCHA challenge.



Tschofenig, et al.       Expires August 28, 2008                [Page 8]

Internet-Draft   CAPTCHA based Robot Challenges for SIP    February 2008


   Example:

   Captcha: id="rjffe32"; answer="2";

   The ABNF for the header is:

   Captcha       = "Captcha" HCOLON captcha-parm *(COMMA captcha-param)
   captcha-param =  captcha-id SEMI captcha-answer *(SEMI generic-param)
   captcha-id   = "id" EQUAL quoted-string
   captcha-answer = "answer" EQUAL quoted-string

   This document updates the Table 2 of [RFC3261] by adding the
   following:


       Header field         where   proxy   ACK  BYE  CAN  INV  OPT  REG
       ------------         -----   -----   ---  ---  ---  ---  ---  ---
       Captcha               R       dr      o    o    -    o    o    o

                                            SUB  NOT  REF  INF  UPD  PRA
                                            ---  ---  ---  ---  ---  ---
                                             o    o    o    o    o    o


6.  Example

   The following XML document shows the content that is provided of a
   CAPTCHA the challenge message sent towards the sending party as shown
   in message (2) of Figure 12.






















Tschofenig, et al.       Expires August 28, 2008                [Page 9]

Internet-Draft   CAPTCHA based Robot Challenges for SIP    February 2008


   <?xml version="1.0" encoding="UTF-8"?>
   <challenge xmlns="urn:ietf:params:xml:ns:captcha"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       id="73DE28A2">

       <media var="urn:ietf:params:xml:ns:captcha:ocr"
                   width="290" height="80">
           <uri type="image/jpeg">
                   http://www.example.com/challenges/ocr.jpeg?F3A6292C
           </uri>
       </media>

       <media var="urn:ietf:params:xml:ns:captcha:audio_recog">
           <uri type="audio/x-wav">
                   http://www.example.com/challenges/audio.wav?F3A6292C
           </uri>
       </media>

       <media var="urn:ietf:params:xml:ns:captcha:qa">
           <data type="text/plain">Type the color of a stop light</data>
       </media>

   </challenge>


7.  XML Schema

   This document defines the XML Schema based on the schema defined in
   Section 12 of [XEP-0158].


   <?xml version='1.0' encoding='UTF-8'?>

   <xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
       targetNamespace="urn:ietf:params:xml:ns:captcha"
       xmlns="urn:ietf:params:xml:ns:captcha"
       elementFormDefault="qualified">

       <xs:element name="challenge" type="challengeType"/>

       <xs:complexType name="challengeType">
           <xs:complexContent>
               <xs:restriction base="xs:anyType">
                   <xs:sequence>
                       <xs:element ref="media"
                           minOccurs="1" maxOccurs="unbounded"/>
                       <xs:any namespace="##other"
                           minOccurs="0" processContents="lax"/>



Tschofenig, et al.       Expires August 28, 2008               [Page 10]

Internet-Draft   CAPTCHA based Robot Challenges for SIP    February 2008


                   </xs:sequence>
                   <xs:attribute name="id"
                       use="required" type="xs:string"/>
                   <xs:attribute name="min_tests" type="xs:unsignedInt"
                       default="1" use="optional" />

               </xs:restriction>
           </xs:complexContent>
       </xs:complexType>

       <xs:element name="media" type="mediaType"/>

       <xs:complexType name="mediaType">
           <xs:complexContent>
               <xs:restriction base="xs:anyType">
                   <xs:choice minOccurs="1" maxOccurs="1">
                       <xs:element ref="uri"
                           minOccurs="0" maxOccurs="unbounded"/>
                       <xs:element ref="data"
                           minOccurs="0" maxOccurs="unbounded"/>
                       <xs:any namespace="##other" minOccurs="0"
                           processContents="lax"/>
                   </xs:choice>
                   <xs:attribute name="var"
                       use="required" type="xs:anyURI"/>
                   <xs:attribute name="required" type="xs:boolean"
                       default="false" use="optional"/>
                   <xs:attribute name="height"
                       type="xs:string" use="optional"/>
                   <xs:attribute name="width"
                       type="xs:string" use="optional"/>
                   <xs:attribute name="instr"
                       type="xs:string" use="optional"/>
                   <xs:anyAttribute namespace="##any"
                       processContents="lax"/>
               </xs:restriction>
           </xs:complexContent>
       </xs:complexType>

       <xs:element name="uri">
           <xs:complexType>
               <xs:simpleContent>
                   <xs:extension base="xs:string">
                       <xs:attribute name="type" use="required"/>
                       <xs:anyAttribute namespace="##any"
                           processContents="lax"/>
                   </xs:extension>
               </xs:simpleContent>



Tschofenig, et al.       Expires August 28, 2008               [Page 11]

Internet-Draft   CAPTCHA based Robot Challenges for SIP    February 2008


           </xs:complexType>
       </xs:element>

       <xs:element name="data">
           <xs:complexType>
               <xs:simpleContent>
                   <xs:extension base="xs:string">
                       <xs:attribute name="type" use="required"/>
                       <xs:anyAttribute namespace="##any"
                           processContents="lax"/>
                   </xs:extension>
               </xs:simpleContent>
           </xs:complexType>
       </xs:element>

   </xs:schema>


8.  Security Considerations

   [Editor's Note: A future version of this document will describe
   security considerations.]


9.  IANA Considerations

   This specification registers a new header and a new response code.
   IANA is requested to make the following updates in the registry at:
   http://www.iana.org/assignments/sip-parameters.  It also registers a
   new namespace and a content type.

9.1.  Captcha Header

   Add the following entry to the header sub-registry.

        Header Name        compact    Reference
        -----------------  -------    ---------
        Captcha                       [RFC-XXXX]

9.2.  4xx Response

   Add the following entry to the response code sub-registry under the
   "Request Failure 4xx" heading.

   4xx CAPTCHA required [RFC-XXXX]






Tschofenig, et al.       Expires August 28, 2008               [Page 12]

Internet-Draft   CAPTCHA based Robot Challenges for SIP    February 2008


9.3.  Namespace

   This section registers a new XML namespace per the procedures in
   [RFC3688].

   URI: urn:ietf:params:xml:ns:captcha

      Registrant Contact: IETF SIPPING Working Group, Hannes Tschofenig
         (hannes.tschofenig@nsn.com).

      XML:

      BEGIN
      <?xml version="1.0"?>
      <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML Basic 1.0//EN"
        "http://www.w3.org/TR/xhtml-basic/xhtml-basic10.dtd">
      <html xmlns="http://www.w3.org/1999/xhtml">
      <head>
        <meta http-equiv="content-type"
              content="text/html;charset=iso-8859-1"/>
        <title>Namespace for CAPTCHA Challenge</title>
      </head>
      <body>
        <h1>Namespace for providing CAPTCHA challenge</h1>
        <h2>urn:ietf:params:xml:ns:captcha</h2>
      <p>See <a href="[URL of published RFC]">RFCXXXX
          [NOTE TO IANA/RFC-EDITOR:
           Please replace XXXX with the RFC number of this
          specification.]</a>.</p>
      </body>
      </html>
      END

9.4.  Content-Type registration for 'application/captcha-challenge+xml'

   This specification requests the registration of a new MIME type
   according to the procedures of RFC 2048 [RFC2048] and guidelines in
   RFC 3023 [RFC3023].













Tschofenig, et al.       Expires August 28, 2008               [Page 13]

Internet-Draft   CAPTCHA based Robot Challenges for SIP    February 2008


   MIME media type name: application

   MIME subtype name: captcha-challenge+xml
   Mandatory parameters: none
   Optional parameters: charset
     Indicates the character encoding of enclosed XML. Default is UTF-8.
   Encoding considerations:

      Uses XML, which can employ 8-bit characters, depending on the
      character encoding used. See RFC 3023 <xref target="RFC3023"/>,
      Section 3.2.

   Security considerations:

      This content type is designed to carry challenges for
      the user agent clients to solve in order to give a proof
      of being a human behind the generated request. This
      action is a part of a spam preventing mechanism.
      Appropriate precautions should be adopted to limit
      disclosure of this information. Please refer to RFCXXXX
      [NOTE TO IANA/RFC-EDITOR: Please replace XXXX
      with the RFC number of this specification.]
      Security Considerations section for more information.

   Interoperability considerations: none

   Published specification: RFCXXXX [NOTE TO IANA/RFC-EDITOR:
   Please replace XXXX with the RFC number of this specification.]
   this document

   Applications which use this media type: SIP applications

   Additional information:

      Magic Number: None
      File Extension: .xml
      Macintosh file type code: 'TEXT'

   Personal and email address for further information: Hannes
      Tschofenig, Hannes.Tschofenig@nsn.com
   Intended usage: LIMITED USE

   Author/Change controller:

      This specification is a work item of the IETF SIPPING working
      group, with mailing list address <xxxxx @ietf.org>.





Tschofenig, et al.       Expires August 28, 2008               [Page 14]

Internet-Draft   CAPTCHA based Robot Challenges for SIP    February 2008


9.5.  CAPTCHA Schema Registration

   URI:  urn:ietf:params:xml:schema:captcha
   Registrant Contact:  IETF SIPPING Working Group, Hannes Tschofenig
      (Hannes.Tschofenig@nsn.com).
   XML:  The XML schema to be registered is contained in Section 7.  Its
      first line is

   <?xml version="1.0" encoding="UTF-8"?>

      and its last line is

   </xs:schema>


10.  Acknowledgments

   Years ago CAPTCHAs have been introduced for XMPP, see 'XEP-0158:
   Robot Challenges' [XEP-0158].  The authors of this document believe
   that there is value in re-using it for SIP for Spam prevention.
   Hence, the authors would like to thank the XMPP community for their
   work on this subject.  In particular, all credits go to Ian Paterson
   (ian.paterson@clientside.co.uk), the author of [XEP-0158].

   We would like to thank Jonathan Rosenberg for his feedback to this
   draft.


11.  Alternative Solution Approaches

   This section shows alternative solution approaches that can be used
   by a proxy to perform CAPTCHA tests.

11.1.  Challenge by Proxy

11.1.1.  Overview

   Figure 11 and Figure 12 present high level messages flows for
   conveying a challenge (e.g., CAPTCHA) to the SIP UAC that initiated a
   dialog forming SIP request.  In Figure 11 the challenge is included
   in the body of the SIP 4xx response while Figure 12 describes a case
   when the challenge is fetched via an URL that was provided with the
   response.  After the user has managed to solve the challenge the UAC
   re-issues the request with the solution.  The proxy removes the
   solution before forwarding the request to the SIP UAS.






Tschofenig, et al.       Expires August 28, 2008               [Page 15]

Internet-Draft   CAPTCHA based Robot Challenges for SIP    February 2008


   SIP                                                SIP
   UAC                       Proxy                     UAS
    |  1) SIP INVITE          |                         |
    |------------------------>|                         |
    |                         |                         |
    |  2) 4xx + CAPTCHA       |                         |
    |<------------------------|                         |
    |                         |                         |
    |                         |                         |
    |                         |                         |
    |                         |                         |
    |                         |                         |
    |                         |                         |
    |  3) Re-INVITE + Solution|                         |
    |------------------------>|  4) SIP INVITE          |
    |                         |------------------------>|
    |                         |  5) 200 OK              |
    |  6) 200 OK              |<------------------------|
    |<------------------------|                         |

      Figure 11: Proxy returns the CAPTCHA directly with the response


       SIP                                                 SIP
       UAC                       Proxy                     UAS
        |  1) SIP INVITE          |                         |
        |------------------------>|                         |
        |                         |                         |
        |  2) 4xx + CAPTCHA-Ref.  |                         |
        |<------------------------|                         |
        |                         |                         |
    +---+----+                    |                         |
    |3) Fetch|                    |                         |
    |CAPTCHA |                    |                         |
    +---+----+                    |                         |
        |                         |                         |
        | 4) Re-INVITE + Solution |                         |
        |------------------------>|  5) SIP INVITE          |
        |                         |------------------------>|
        |                         |  6) 200 OK              |
        |  7) 200 OK              |<------------------------|
        |<------------------------|                         |

                Figure 12: Proxy returns URL to the CAPTCHA







Tschofenig, et al.       Expires August 28, 2008               [Page 16]

Internet-Draft   CAPTCHA based Robot Challenges for SIP    February 2008


11.1.2.  Operation of Proxy when it issues a challenge directly

   The proxy sends a 4xx response with an XML document containing the
   challenge in the body.  The Content-Type used for the XML document is
   'application/captcha-challenge+xml'.

   When the proxy receives a re-issued SIP request from the UAC, it
   validates the answer provided by the UAC in the CAPTCHA header field.
   In case the answer and other possible policies allow the request to
   get proxied further to the UAS, the proxy removes the CAPTCHA header.
   Depending on the policies and functionality of the proxy, the proxy
   may update the authorization policy according to the decision, e.g.,
   insert the AoR of the user of the UAC to a white or black list.  In
   case the answer was not satisfactory, the UAS acts according to a
   defined policy, e.g., rejects the request.

11.1.3.  Operation of UAC on receiving a CAPTCHA challenge from the SIP

   When the UAC receives a 4xx response with a MIME type 'application/
   captcha-challenge+xml' in the body to be solved, the UAC first
   authenticates and authorizes the sender of the challenge.

   The UAC selects the challenges marked as mandatory and possibly some
   additional ones for UAC's execution or to be rendered to the user
   based on, e.g., the device capabilities.  The UAC may also need to
   fetch the challenges from which URL links were provided.  When the
   challenge gets solved, the UAC provides an answer in the CAPTCHA
   header field by re-issuing the SIP request, e.g., by sending a SIP
   re-INVITE.

11.2.  SIP request redirected by the SIP Proxy

11.2.1.  Overview

   In this case, the SIP proxy redirects the INVITE from a SIP UAC to a
   CAPTCHA UAS.  The CAPTCHA UA acknowledges the request for service and
   then, contacts the SIP UAC directly to issue the challenge.  On
   performing the CAPTCHA tests, it initimates the SIP server of the
   result.

   The redirect of the INVITE by the SIP server to a CAPTCHA UA is a
   simple call redirect, negotiation of the parameters for the CAPTCHA
   is done using the standard SDP negotiation.  From the caller point of
   view this is just a call setup, the caller will be presented the
   CAPTCHA test depending on the media it supports (audio, video, text).
   In this way there is no need for additional signaling that would
   reveal the caller that a CAPTCHA needs to be solved.




Tschofenig, et al.       Expires August 28, 2008               [Page 17]

Internet-Draft   CAPTCHA based Robot Challenges for SIP    February 2008


   Figure 13 presents a high level message flow showing a successful
   CAPTCHA test and Figure 14 presents a high level message flow
   conveying a unsuccessful CAPTCHA challenge by a UA.


   SIP                 SIP Proxy                CAPTCHA               SIP
   UAC                 or UA                      UA                  UAS
    |                    |                         |                   |
    |                    |                         |                   |
    |INVITE(Callee)      |                         |                   |
    +------------------->|                         |                   |
    |                    |INVITE(Re-Directed to    |                   |
    |                    |CAPTCHA UA)              |                   |
    |                    +------------------------>|                   |
    |                    |                         |                   |
    |                    |                         |                   |
    |                    |                   200 OK|                   |
    |                    |<------------------------+                   |
    |                    |                         |                   |
    |              200 OK|                         |                   |
    |<-------------------+                         |                   |
    |  /                 |                      \  |                   |
    |//------------------------------------------\\|                   |
    /                                              \                   |
    \    RTP ( AUDIO/ VIDEO Test Performed )       /                   |
    |\\------------------------------------------//|                   |
    |  \                 |                      /  |                   |
    |                    |                         |                   |
    |                    | <IF Test was Successful>|                   |
    |                    |  REFER TO Callee +      |                   |
    |                    |  Crypto cookie          |                   |
    |                    |<------------------------+                   |
    |                    |                         |                   |
    |  REFER TO Callee + |                         |                   |
    |  Crypto cookie     |                         |                   |
    |<-------------------+                         |                   |
    |                    |                         |                   |
    |INVITE(Callee) +    |                         |                   |
    |Crypto cookie       |                         |                   |
    +------------------->|                                             |
    |                    | INVITE (Callee) + Crypto cookie             |
    |                    +---------------------------------------------|
    |                    |                                             |
    |                    |                                       200 OK|
    |                    |<--------------------------------------------+
    |                    |                         |                   |
    |              200 OK|                         |                   |
    |<-------------------+                         |                   |



Tschofenig, et al.       Expires August 28, 2008               [Page 18]

Internet-Draft   CAPTCHA based Robot Challenges for SIP    February 2008


    |                    |                         |                   |
    |  /                 |                         |                \  |
    |//--------------------------------------------------------------\\|
    /                                                                  \
    \                    RTP ( REAL CONVERSATION )                     /
    |\\--------------------------------------------------------------//|
    |  \                 |                         |                /  |
    |                    |                         |                   |


    Figure 13: A case where the Proxy redirects the INVITE to a CAPTCHA
                      UA and gets a SUCCCESS repsonse



   SIP                 SIP Proxy                CAPTCHA               SIP
   UAC                 or UA                      UA                  UAS
    |                    |                         |                   |
    |                    |                         |                   |
    |INVITE(Callee)      |                         |                   |
    +------------------->|                         |                   |
    |                    |INVITE(Re-Directed to    |                   |
    |                    |CAPTCHA UA)              |                   |
    |                    +------------------------>|                   |
    |                    |                         |                   |
    |                    |                         |                   |
    |                    |                   200 OK|                   |
    |                    |<------------------------+                   |
    |                    |                         |                   |
    |              200 OK|                         |                   |
    |<-------------------+                         |                   |
    |  /                 |                      \  |                   |
    |//------------------------------------------\\|                   |
    /                                              \                   |
    \    RTP ( AUDIO/ VIDEO Test Performed )       /                   |
    |\\------------------------------------------//|                   |
    |  \                 |                      /  |                   |
    |                    |                         |                   |
    |                    | <IF Test was NOT        |                   |
    |                    |  SUCCESSFUL >           |                   |
    |                    |  Bye / 4xx              |                   |
    |                    |<------------------------+                   |
    |                    |                         |                   |
    |                    |                         |                   |
    |           Bye / 4xx|                         |                   |
    |<-------------------+                         |                   |
    |                    |                         |                   |




Tschofenig, et al.       Expires August 28, 2008               [Page 19]

Internet-Draft   CAPTCHA based Robot Challenges for SIP    February 2008


    Figure 14: A case where the Proxy redirects the INVITE to a CAPTCHA
                    UA and gets a NOT SUCCCESS repsonse

11.2.2.  Operation of Proxy when it redirects the INVITE to a CAPTCHA UA

   The SIP server redirects the INVITE to a CAPTCHA UA.  The CAPTCHA UA,
   acknowledges the request for service by sending a "200 OK" message.
   The CAPTCHA UA, then proceeds to issue the CAPTCHA challenge to the
   user.

   If the user is successful in solving the CAPTCHA challenge, the
   CAPTCHA UA issues a reference to the Callee along with crypto cookie
   to ensure that a replay attack isn't possible.  The SIP server passes
   this information to the SIP UAC.  The SIP UAC issues a new INVITE
   along with the obtained crypto cookie.  Figure 13 presents the
   message flow.

   If the user is not successful in solving the CAPTCHA challenge, the
   CAPTCHA UA issues a Bye message or a 4xx RESPONSE with an appropriate
   error message.  Figure 14 presents the message flow.

11.2.3.  Operation of UAC when it recieves a challenge from a CAPTCHA UA

   When the UAC receives a challenge from a CAPTCHA UA, the UAC selects
   the challenges marked as mandatory and possibly some additional ones
   for UAC's execution or to be rendered to the user based on e.g. the
   device capabilities.  When the challenge gets solved, the UAC
   provides an answer to the CAPTCHA UA.

11.3.  SIP Application Interaction Framework

   [I-D.ietf-sipping-app-interaction-framework] defines a framework for
   interaction between users and SIP based applications.  The framework
   covers both the "presentation capable" and "presentation free" user
   interfaces (UI) having different solutions to both.  The user
   interaction with the presentation capable UI is handled by using SIP
   REFER and HTTP while the presentation free UI case utilize SIP events
   [RFC3265] (SIP SUBSCRIBE and NOTIFY).  Since there are different
   solutions for different cases, the UAC needs to indicate the
   supported application user interaction mechamisms when issuing a SIP
   request.


12.  References







Tschofenig, et al.       Expires August 28, 2008               [Page 20]

Internet-Draft   CAPTCHA based Robot Challenges for SIP    February 2008


12.1.  Normative references

   [RFC2048]  Freed, N., Klensin, J., and J. Postel, "Multipurpose
              Internet Mail Extensions (MIME) Part Four: Registration
              Procedures", BCP 13, RFC 2048, November 1996.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2141]  Moats, R., "URN Syntax", RFC 2141, May 1997.

   [RFC2648]  Moats, R., "A URN Namespace for IETF Documents", RFC 2648,
              August 1999.

   [RFC3023]  Murata, M., St. Laurent, S., and D. Kohn, "XML Media
              Types", RFC 3023, January 2001.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002.

   [RFC3688]  Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
              January 2004.

   [XML]      Bray, T., "Exensible Markup Language (XML) 1.0 (Second
              Edition)", W3C CR CR-xml11-20011006, October 2000.

12.2.  Informative references

   [I-D.ietf-sipping-app-interaction-framework]
              Rosenberg, J., "A Framework for Application Interaction in
              the Session Initiation Protocol  (SIP)",
              draft-ietf-sipping-app-interaction-framework-05 (work in
              progress), July 2005.

   [I-D.jennings-sip-hashcash]
              Jennings, C., "Computational Puzzles for SPAM Reduction in
              SIP", draft-jennings-sip-hashcash-06 (work in progress),
              July 2007.

   [I-D.tschofenig-sipping-framework-spit-reduction]
              Tschofenig, H., Schulzrinne, H., Wing, D., Rosenberg, J.,
              and D. Schwartz, "A Framework to tackle Spam and Unwanted
              Communication for Internet  Telephony",
              draft-tschofenig-sipping-framework-spit-reduction-02 (work
              in progress), November 2007.




Tschofenig, et al.       Expires August 28, 2008               [Page 21]

Internet-Draft   CAPTCHA based Robot Challenges for SIP    February 2008


   [Inaccessibility-of-CAPTCHA]
              May, M., "Inaccessibility of CAPTCHA; Alternatives to
              Visual Turing Tests on the Web",
              html http://www.w3.org/TR/turingtest/, November 2005.

   [RFC3265]  Roach, A., "SIP-Specific Event Notification", RFC 3265,
              June 2002.

   [RFC5025]  Rosenberg, J., "Presence Authorization Rules", RFC 5025,
              December 2007.

   [RFC5039]  Rosenberg, J. and C. Jennings, "The Session Initiation
              Protocol (SIP) and Spam", RFC 5039, January 2008.

   [XEP-0158]
              Paterson, I., "XEP-0158: Robot Challenges",
              html http://wiki.jabber.org/index.php/Robot Challenges
              (XEP-0158), October 2006.

   [captcha]  von Ahn, L., Blum, M., and J. Langford, "Telling Humans
              and Computers Apart Automatically",
              html http://www.captcha.net, February 2004.

   [hashcash]
              Back, A., "Hashcash - A Denial of Service Counter-
              Measure", html http://hashcash.org, August 2002.


Authors' Addresses

   Hannes Tschofenig
   Nokia Siemens Networks
   Linnoitustie 6
   Espoo  02600
   Finland

   Phone: +358 (50) 4871445
   Email: Hannes.Tschofenig@nsn.com
   URI:   http://www.tschofenig.com


   Eva Leppanen
   Individual
   Finland

   Email: eva.leppanen@hukassa.com





Tschofenig, et al.       Expires August 28, 2008               [Page 22]

Internet-Draft   CAPTCHA based Robot Challenges for SIP    February 2008


   Saverio Niccolini
   NEC Laboratories Europe, NEC Europe Ltd.
   Kurfuersten-Anlage 36
   Heidelberg  69115
   Germany

   Phone: +49 (0) 6221 4342 118
   Email: saverio.niccolini@nw.neclab.eu
   URI:   http://www.nw.neclab.eu


   Mayutan Arumaithurai
   University of Goettingen

   Email: mayutan.arumaithurai@gmail.com
   URI:   http://www.mayutan.org



































Tschofenig, et al.       Expires August 28, 2008               [Page 23]

Internet-Draft   CAPTCHA based Robot Challenges for SIP    February 2008


Full Copyright Statement

   Copyright (C) The IETF Trust (2008).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
   THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
   OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
   THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.


Intellectual Property

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at
   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at
   ietf-ipr@ietf.org.


Acknowledgment

   Funding for the RFC Editor function is provided by the IETF
   Administrative Support Activity (IASA).





Tschofenig, et al.       Expires August 28, 2008               [Page 24]