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GEOPRIV WG                                                    M. Thomson
Internet-Draft                                           J. Winterbottom
Expires: December 30, 2005                                        Nortel
                                                           June 28, 2005

                    Domain Authorization for PIDF-LO

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Copyright Notice

   Copyright (C) The Internet Society (2005).


   This document describes a standard method for digitally signing
   Presence Information Data Format Location Object (PIDF-LO) documents
   using a subset of the XML Digital Signature specification.  A digital
   signature enables the user of a signed PIDF-LO document to attribute
   that information to an authorized source within the domain of the
   target entity.  A schema is defined for including a domain
   authorization element in the PIDF-LO and a set of XML Path Language
   (XPath) filters for selecting the correct elements for signing.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1   Conventions used in this document  . . . . . . . . . . . .  3
   2.  The Domain Authorization Element . . . . . . . . . . . . . . .  5
     2.1   'expires' attribute  . . . . . . . . . . . . . . . . . . .  5
   3.  Signature Elements . . . . . . . . . . . . . . . . . . . . . .  6
     3.1   PIDF-LO Document Transform . . . . . . . . . . . . . . . .  6
     3.2   Algorithms . . . . . . . . . . . . . . . . . . . . . . . .  7
     3.3   Signature Key Data . . . . . . . . . . . . . . . . . . . .  7
   4.  XML Definitions  . . . . . . . . . . . . . . . . . . . . . . .  9
     4.1   XML Schema . . . . . . . . . . . . . . . . . . . . . . . .  9
     4.2   XPath Filter . . . . . . . . . . . . . . . . . . . . . . . 10
   5.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 16
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 17
     7.1   URN Sub-Namespace Registration for
           urn:ietf:params:xml:ns:pidf:geopriv10:domain-auth  . . . . 17
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 18
       Intellectual Property and Copyright Statements . . . . . . . . 19

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1.  Introduction

   Users of location information may desire to be able to attribute this
   information to an authorized entity.  This entity may not have
   provided the information, but they must be able to vouch for its
   accuracy.  In most cases this will mean that the trusted entity
   resides within the same domain, or access network, as the target

   This specification describes a means by which a domain authority may
   assume responsibility for the validity of the location information
   provided in a Presence Information Data Format Location Object
   (PIDF-LO [I-D.ietf-geopriv-pidf-lo]).  A standard form is described
   whereby a domain authority may digitally sign the PIDF-LO document.
   This signature ensures that a user of the PIDF-LO can verify that the
   presentity identified is at the described location within certain
   time bounds.

   Only selected data are signed within a PIDF, allowing a user freedom
   to change other parts of the document without affecting the
   signature.  A signature only applies to a single tuple element,
   allowing separate tuples to be unsigned, or to be signed separately.
   Elements such as notes, contact information and the privacy
   preferences described in [I-D.ietf-geopriv-pidf-lo] are not signed to
   allow for modification.  The signed elements are restricted to:

   the presentity identifier: the entity attribute of the _presence_

   timestamp: the timestamp associated with the location information

   location-info: the actual location information

   expires: the time at which the signature expires, this datum is added
      by this specification

1.1  Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

   The term _presentity_ is used as defined in RFC 2778 [RFC2778].

   The term _location user_ is used to refer to the entity that is
   consuming location information.  In the context of RFC 2778
   [RFC2778], the location user may be a watcher.

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   The term _domain authority_ is used to refer to the entity that
   assumes responsibility for the accuracy of the information provided
   in the PIDF-LO.

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2.  The Domain Authorization Element

   This specification describes a new namespace for a domain
   authorization element.  This element contains the digital signature
   described in RFC 3275 [RFC3275] and an expiry time for the signature.

   The "domain-auth" element is added to the "tuple" element that
   contains the "geopriv" element.  Separate "tuple" elements are signed
   separately.  The XML Schema definition for the "domain-auth" element
   is included in Section 4.1.

   The digital signature signs the PIDF-LO document that includes only
   the "tuple" element that the "domain-auth" element is included
   within.  Section 3.1 describes the specific means by which the
   correct elements are selected.

2.1  'expires' attribute

   The "expires" attribute defines the expiry time for the domain
   authorization provided.  A user of the PIDF-LO document MUST consider
   the document to be unsigned beyond the expiry time.

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3.  Signature Elements

   The XML Signature specification [RFC3275] describes a means to sign
   XML documents.  The "Signature" element consists of three major

   1.  a description of the signed elements, which may be an entire
       document, or selected parts of a document

   2.  a digital signature

   3.  information on the key used to sign the document

3.1  PIDF-LO Document Transform

   Since the content of XML documents is indeterminate based on similar
   data sets, RFC 3275 [RFC3275] describes a set of transforms that may
   be applied to a document before applying a digital signature.

   The input PIDF-LO document MUST be canonicalized using either
   Canonical XML ("http://www.w3.org/TR/2001/REC-xml-c14n-20010315") or
   Exclusive Canonical XML
   ("http://www.w3.org/TR/2002/REC-xml-exc-c14n-20020718/") [W3C.REC-
   xml-exc-c14n-20020718].  Note that both of these canonicalization
   methods remove comments from the source document.

   The signature form selected for this document is an enveloped
   signature.  Therefore the enveloped signature transform
   ("http://www.w3.org/2000/09/xmldsig#enveloped-signature") MUST be
   applied to the document.

   A filter is applied to the input document in order to select the
   correct elements for signing.  It is RECOMMENDED that the transformed
   document is also a valid PIDF-LO.  In addition, the transform should
   exclude "tuple" elements other than the element that is directly
   signed.  This ensures that other content may be included in other
   "tuple" elements, including other digital signatures.

   The following elements MUST be selected:

   o  the "presence" element, which includes the "entity" attribute

   o  the "location-info" element and all of its contents

   o  the "timestamp" element associated with the signed "tuple" element

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   o  the "domain-auth" element

   The minimum set of elements required to ensure that the signed
   document is a valid PIDF-LO SHOULD also be included.

   The XML Path Language (XPath) filter defined in Section 4.2 meets the
   above criteria.  For convenience, and to reduce the size of a signed
   PIDF-LO document, this transform may be identified by the URN

   Note that any elements from other namespaces included within the
   "domain-auth" element are selected by this XPath filter.  This
   ensures that additions to this element are covered by the digital

3.2  Algorithms

   As recommended in RFC 3275 [RFC3275], implementations of this
   specification MUST provide the following algorithms:

   digest algorithm: The SHA1 digest, as identified by the URN

   signature algorithm: DSA with SHA1, as identified by the URN

   canonicalization method: Canonical XML [RFC3076], as identified by
      the URN "http://www.w3.org/TR/2001/REC-xml-c14n-20010315".

   transforms: The enveloped signature transform, as identified by the
      URN "http://www.w3.org/2000/09/xmldsig#enveloped-signature"; and
      the transform defined in Section 4.2, as identified by the URN

   It is also RECOMMENDED that the following are also supported:

   signature algorithm: PKCS1 (RSA-SHA1) signature algorithm, as
      idenfied by "http://www.w3.org/2000/09/xmldsig#rsa-sha1".

   canonicalization method: Exclusive Canonical XML, as identified by
      the URN "http://www.w3.org/TR/2002/REC-xml-exc-c14n-20020718/"

3.3  Signature Key Data

   RFC 3275 [RFC3275] describes a number of methods for describing the
   key used to sign the document.  For this specification, the "KeyInfo"

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   element MUST be provided in the "Signature" element.

   The domain authority MUST also describe a means to retrieve an X.509
   certificate that includes the key used to sign the document.  This
   can be either by including an "X509Certificate" element, or by
   referencing another certificate.

   A reference to a certificate within the same document may be made
   using the "X509SubjectName" element or a fragment identifier URI.  A
   fragment identifier URI might be applicable where multiple signatures
   are applied to different parts of the document.  External certificate
   sources SHOULD be described by URI only in the "RetrievalMethod"
   element.  It is RECOMMENDED that the scheme for the RetrievalMethod
   URI indicates a secure protocol, such as HTTPS.

   The domain authority MAY include additional information in the
   "KeyInfo" element that could assist the location user in validating
   the certificate.  For example a certificate chain and certificate
   revocation list may be added.  However, this specification does not
   specify how the location user validates the certificate.

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4.  XML Definitions

4.1  XML Schema

   The following XML schema describes the "domain-auth" element.  This
   schema defines a new namespace:

   <?xml version="1.0"?>
   <xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema"

     <xsd:import namespace="http://www.w3.org/2000/09/xmldsig#"/>

         This schema defines a means for providing authentication to a
         PIDF-LO.  This schema is also accompanied by a set of
         transforms that should be applied to the signed PIDF-LO.

     <xsd:element name="domain-auth">
           The domain authorization that is applied to the PIDF-LO.
           This element should be included within the scope of a
           &lt;tuple&gt; element.
           <xsd:restriction base="xsd:anyType">
               <xsd:element ref="dsig:Signature"/>
               <xsd:any namespace="##other" processContents="lax"
                        minOccurs="0" maxOccurs="unbounded"/>

             <xsd:attribute name="expires" use="required"
                   The expiry time associated with the authorization.

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4.2  XPath Filter

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   The following XPath transform follows the recommendations in RFC 3275
   [RFC3275] to select the elements for signing.  This specification
   defines a new URN for this transform:

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   <?xml version="1.0"?>
   <dsig:Transform id="PIDF-LO"
   <!-- Select elements -->
   <!-- The enclosing presence element -->
   ((count(self::pidf:presence | here()/ancestor::pidf:presence[1]) = 1)
   <!-- The enclosing pidf:tuple element -->
    or (count(self::pidf:tuple | here()/ancestor::pidf:tuple[1]) = 1)
   <!-- enclosing()/pidf:tuple with the following portions ... -->
    or ((count(ancestor::pidf:tuple[1]
         | here()/ancestor::pidf:tuple[1]) = 1)
   <!-- ... pidf:status, pidf:status/pidf:timestamp[/text()] -->
        and (self::pidf:status or ancestor-or-self::pidf:timestamp
   <!-- ... gp:geopriv, gp:usage-rules -->
             or self::gp:geopriv or self::gp:usage-rules
   <!-- ... gp:location-info and descendants -->
             or ancestor-or-self::gp:location-info))
   <!-- the enclosing da:domain-auth element -->
    or (count(self::da:domain-auth
        | here()/ancestor::da:domain-auth[1]) = 1)
   ) or (
   <!-- Select attributes and xmlns for those elements -->
     (count(self::node() | parent::*/attribute::*
            | parent::*/namespace::*)
        &lt; (count(self::node()) + count(parent::*/attribute::*)
           + count(parent::*/namespace::*)))
   and parent::*[
   <!-- Repeat of element selection -->
   ((self::pidf:presence and (count(ancestor::pidf:presence) = 0))
    or (count(self::pidf:tuple | here()/ancestor::pidf:tuple[1]) = 1)
    or ((count(ancestor::pidf:tuple[1]
         | here()/ancestor::pidf:tuple[1]) = 1)
        and (self::pidf:status
             or self::gp:geopriv or self::gp:usage-rules))
    or (count(self::da:domain-auth
        | here()/ancestor::da:domain-auth[1]) = 1))

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5.  Examples

   A sample, demonstrating a simple form of the signed PIDF-LO document
   is shown below.

   <?xml version="1.0"?>
   <presence entity="pres:user@example.com"
     <tuple id="pidflo1a786c3">
               <gml:Point srsName="urn:EPSG:geographicCRS:4979">
                 <gml:pos>-34.407 150.88001 34</gml:pos>
         <ds:Signature xmlns:ds="http://www.w3.org/2000/09/xmldsig#">
   <ds:Reference URI="">
   <ds:DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>

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         This note may be changed without affecting the signature.

   Note that the digest and signatures are only included to serve as an
   example.  Several elements are included in the above example that
   should not be signed by the domain authority, as described in
   Section 3.1, in particular "usage-rules" and "note-well".

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   The transform described in Section 3.1 removes extraneous elements,
   resulting in the following PIDF-LO, which only includes the signed
   elements (in this example whitespace has been added for readability):

   <presence xmlns="urn:ietf:params:xml:ns:pidf"
     <tuple id="pidflo1a786c3">
               <gml:Point srsName="urn:EPSG:geographicCRS:4979">
                 <gml:pos>-34.407 150.88001 34</gml:pos>

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6.  Security Considerations

   The security limitations of this specification are no more
   significant than those already identified in RFC 3275 [RFC3275].  In
   particular the rules *Only What is Signed is Secure*, *Only What is
   "Seen" Should be Signed*, and *"See" What is Signed* SHOULD be

   It is RECOMMENDED that where certitude of information is important
   that only the signed information is transmitted or stored, that is,
   the PIDF-LO document formed by performing the transform described in
   Section 3.1.  This ensures that no additional information may be
   misconstrued as being verifiable.  This is particular applicable if
   the contents of the PIDF-LO document are displayed on screen.

   A degree of trust must exist between the domain authority and the
   location user.  It is the responsibility of the location user to
   verify the identity of the domain authority and assert the
   appropriate level of trust.  If the location user is unable to
   validate the identity of the domain authority for any reason, then
   the PIDF-LO document MUST be considered unsigned.

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7.  IANA Considerations

7.1  URN Sub-Namespace Registration for

   This document registers a new XML namespace, as per the guidelines in

   URI: The URI for this namespace is
   Registrant Contact: IETF, GEOPRIV working group, (geopriv@ietf.org),
     Martin Thomson (martin.thomson@nortel.com).
       <?xml version="1.0"?>
       <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML Basic 1.0//EN"
       <html xmlns="http://www.w3.org/1999/xhtml">
           <title>GEOPRIV Domain Authorization Extensions</title>
           <h1>Domain Authorization Extensions</h1>
           <p>See <a href="[[[URL of published RFC]]]">RFCXXXX</a>.</p>

8.  References

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

   [RFC2778]  Day, M., Rosenberg, J., and H. Sugano, "A Model for
              Presence and Instant Messaging", RFC 2778, February 2000.

   [RFC3076]  Boyer, J., "Canonical XML Version 1.0", RFC 3076,
              March 2001.

   [RFC3275]  Eastlake, D., Reagle, J., and D. Solo, "(Extensible Markup
              Language) XML-Signature Syntax and Processing", RFC 3275,
              March 2002.

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

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              Peterson, J., "A Presence-based GEOPRIV Location Object
              Format", draft-ietf-geopriv-pidf-lo-03 (work in progress),
              September 2004.

              3rd, D., Boyer, J., and J. Reagle, "Exclusive XML
              Canonicalization Version 1.0", W3C REC REC-xml-exc-c14n-
              20020718, July 2002.

Authors' Addresses

   Martin Thomson
   PO Box U87
   University of Wollongong, NSW  2500

   Phone: +61 2 4254 7515
   Email: martin.thomson@nortel.com
   URI:   http://www.nortel.com/

   James Winterbottom
   PO Box U87
   University of Wollongong, NSW  2500

   Phone: +61 2 4223 3038
   Email: winterb@nortel.com
   URI:   http://www.nortel.com/

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   Funding for the RFC Editor function is currently provided by the
   Internet Society.

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