GEOPRIV M. Thomson
Internet-Draft J. Winterbottom
Intended status: Standards Track Andrew
Expires: August 26, 2007 February 22, 2007
Digital Signature Methods for Location Dependability
draft-thomson-geopriv-location-dependability-00.txt
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Abstract
The dependability of location information is closely related to the
degree of trust placed in the source of that information. This
document describes techniques that can be used to mitigate the impact
of falsifying location information. The application of digital
signatures is described, relating these methods to the attacks that
they address.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Non-Goals . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2. Basic Countermeasures . . . . . . . . . . . . . . . . . . 6
2.3. Signing Location Information . . . . . . . . . . . . . . . 6
3. PIDF-LO Signature . . . . . . . . . . . . . . . . . . . . . . 7
3.1. Signature Design . . . . . . . . . . . . . . . . . . . . . 7
3.2. Signed Elements and Semantics . . . . . . . . . . . . . . 7
3.3. Signature Algorithms . . . . . . . . . . . . . . . . . . . 9
3.4. LIS/Signer Identification . . . . . . . . . . . . . . . . 9
4. Limited Validity . . . . . . . . . . . . . . . . . . . . . . . 10
4.1. Validity Elements . . . . . . . . . . . . . . . . . . . . 10
5. Signing for a User . . . . . . . . . . . . . . . . . . . . . . 11
5.1. The 'entity' Attribute . . . . . . . . . . . . . . . . . . 11
5.2. Target Identity . . . . . . . . . . . . . . . . . . . . . 12
5.3. Protecting User Anonymity . . . . . . . . . . . . . . . . 12
5.4. Authenticated Identity . . . . . . . . . . . . . . . . . . 13
5.5. Multiple Identity Attack . . . . . . . . . . . . . . . . . 13
6. Target Identity Element . . . . . . . . . . . . . . . . . . . 14
6.1. Identity Types . . . . . . . . . . . . . . . . . . . . . . 14
6.2. Identity Hashing . . . . . . . . . . . . . . . . . . . . . 14
6.3. Authentication Indicator . . . . . . . . . . . . . . . . . 15
7. Signature Validation . . . . . . . . . . . . . . . . . . . . . 16
8. Code and Examples . . . . . . . . . . . . . . . . . . . . . . 17
8.1. Dependability Data Schema . . . . . . . . . . . . . . . . 17
8.2. PIDF-LO Transforms . . . . . . . . . . . . . . . . . . . . 18
8.2.1. PIDF-LO Tuple-only Transform . . . . . . . . . . . . . 19
8.2.2. PIDF-LO Selective Transform . . . . . . . . . . . . . 20
8.3. Example . . . . . . . . . . . . . . . . . . . . . . . . . 21
9. Location Reference Attribution . . . . . . . . . . . . . . . . 25
10. Security Considerations . . . . . . . . . . . . . . . . . . . 26
10.1. Signature Rules . . . . . . . . . . . . . . . . . . . . . 26
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27
11.1. URN Sub-Namespace Registration for
urn:ietf:params:xml:ns:pidf:geopriv10:dsig . . . . . . . . 27
11.2. XML Schema Registration . . . . . . . . . . . . . . . . . 27
11.3. URN Sub-Namespace Registration for
urn:ietf:params:xml:ns:pidf:geopriv10:dsig:identity . . . 28
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 29
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 30
13.1. Normative References . . . . . . . . . . . . . . . . . . . 30
13.2. Informative References . . . . . . . . . . . . . . . . . . 30
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 31
Intellectual Property and Copyright Statements . . . . . . . . . . 32
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1. Introduction
Location information about a particular person or device is critical
to a number of applications. The integrity of this information --
whether or not it can be relied upon for correctness -- is also
important to the user of the data. This is especially important if
the recipient of location information expends resources based on the
information.
The quitessential example of an application where the veracity of
location information is critical is emergency calling. Location
information is used both by routing functions to determine the
correct Public Safety Answering Point (PSAP) and by the selected PSAP
to determine where to send personnel. If location information were
faked, the call could be directed to the wrong PSAP, or personnel
could be directed to an incorrect location. In either case, an
attacker wastes PSAP resources and risks delaying their life-critical
interventions for other legitimate emergency callers.
This document details several cryptographic methods that limit the
scope of attacks on location recipients based on faked or stolen
location information. Methods for applying digital signatures are
described so that a location recipient can identify the source of the
location information, either Location Information Server (LIS) or
Target. Identifying the source allows the location recipient to make
a judgement on whether or not to trust the content of the location
information.
Ultimately, these methods are limited in practicality by the
transient nature of the relationships between LIS (the access
network) and the Target. Because these relationships can be
arbitrary and temporary, schemes like authentication are not always
feasible. The basic goal of this draft is to both limit the scope of
attacks and to provide as much information to the location recipient
as possible so that they can make a decision on whether or not to act
on the location information they are provided.
1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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2. Goals
This document describes several measures that can be applied to limit
attacks that rely on using faked or stolen location information.
These attacks leave a user of location information vulnerable to
exploitation by an attacker.
The measures outlined in this document are designed to limit exposure
to the following attacks:
Place Shifting: In place shifting, an attacker selects any location
(presumably somewhere other than where they are currently located)
and constructs a PIDF-LO based on that information.
Time Shifting: In a time shifting, or replay, attack the attacker
uses location information that was valid in the past, but is no
longer valid because the attacker has moved since the location was
generated.
Location Theft: An attacker that is able to observe the Target's
location information can replay this information and thereby
appear to be at the same location.
Location Swapping: Two colluding attackers can conspire to fake
location by exchanging location information. One attacker can
pretend to be at the other's location.
These attacks are a subset of those described in
[I-D.ietf-geopriv-l7-lcp-ps].
2.1. Non-Goals
The measures outlined in this document cannot hope to address all
possible cases of fraud. This section outlines general areas where
this document does not provide guidance; more specific limitations
are included in the relevant sections.
Attacks where the authenticated identity of the Target can be
reliably mimicked are not included. This includes active collusion,
as well as any attacks, network-based or otherwise, on the Target
host that result in complete access to the Target's credentials. In
addition, this includes attacks that require cooperation between the
attacker and Target. If the attacker is able to gain access to the
Target's private key, then to all cryptographic means the attacker
can pretend to be the Target.
Methods for determining trust in either LIS or Target are out of
scope for this document. This document only describes the means by
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which an identity can be verified; the decision over whether or not
to trust the entity is left to the location recipient. Means for
establishing trust will be the topic of a separate work.
Note that where certificate chains are used for authentication, a
domain name-based certificate does not necessarily indicate
trustworthiness in the provision of location information. Therefore,
verification of LIS identity through a certificate alone is not
enough to ensure that the location recipient can trust the LIS, the
recipient needs to use additional criteria to decide on whether to
trust the LIS.
2.2. Basic Countermeasures
A good minimum requirement for the exchange of location information
is that location information is protected from interception and
modification by third parties in all protocol exchanges. Location
protocols that use TLS [RFC4346] are able to meet this requirement.
Confidentiality from third parties and integrity protection are
required for all location using-protocols [RFC3693].
2.3. Signing Location Information
A digital signature provides data integrity and authentication of the
source of information. This document describes how XML-Signature
[RFC3275] can be applied to a Presence Information Data Format -
Location Object (PIDF-LO) [RFC4119]. It also describes the benefits
of signing and how signing can be practically applied.
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3. PIDF-LO Signature
A location recipient can use the signature on a location object to
authenticate the identity of the LIS. This is done through the
certificate that the LIS attaches with the signature. The signature
also ensures that the contents have not been modified since the LIS
signed the location object.
It is important to specify the semantics of a certificate of this
nature. In essence, information that is signed SHOULD be verifiable
by the LIS. However, in some cases it is expedient to include some
unverifiable information (as is shown in later sections). Therefore,
this document assigns a strict semantic to each signed element in the
location object.
3.1. Signature Design
This document uses the XML-Signature [RFC3275] enveloped signature
type; that is, signature elements are included within the normal
structure of the PIDF-LO document. This ensures that the location
object does not appear to be any different from a regular PIDF-LO
document. This permits use of the document in any protocol that
carries PIDF-LO without requiring any changes to the protocol.
Applications that rely on PIDF-LO can simply ignore the signature
elements if they are not supported.
A signature is applied to a single tuple within the PIDF document.
This means that signed location information can be included in a
composite presence document without destroying the signature.
It is also a goal of the signature design to ensure that if unsigned
elements are removed from the PIDF-LO, the document remains a valid
PIDF-LO. This keeps the PIDF-LO usable if the signature and any
unsigned data are stripped out. This is particularly important when
the signature rules (Section 10.1) are applied.
3.2. Signed Elements and Semantics
When location information is signed by a LIS, each unit of data in
the signed document is given certain significance. A location
recipient needs to know what significance the LIS has given to each
field before it can base any decision on the contents of that field.
The following list describes each of the elements that are included
in a signed LO, justifies their inclusion and outlines the intended
semantics of each being signed:
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presence (urn:ietf:params:xml:ns:pidf): The root element of a
presence document, the "presence" element, is signed. The
"entity" attribute is also signed. The "entity" attribute SHOULD
contain an identifier generated by the LIS, see Section 5 and
Section 5.3.
tuple (urn:ietf:params:xml:ns:pidf): Only the tuple that contains
location information is signed. The "id" attribute is signed to
ensure a valid PIDF-LO is produced.
geopriv (urn:ietf:params:xml:ns:pidf:geopriv10): The "geopriv"
element is signed, along with select elements within it.
location-info (urn:ietf:params:xml:ns:pidf:geopriv10): The most
important element of the PIDF-LO, "location-info" contains
location data. This element and all its contents are signed.
usage-rules (urn:ietf:params:xml:ns:pidf:geopriv10): Usage rules are
included to ensure the validity of the PIDF-LO. An empty
"usage-rules" element is valid. The contents of these are not
signed to allow a user to enter their preferences upon receipt of
the signed LO. No decisions can be made on the unsigned content
of usage rules.
method (urn:ietf:params:xml:ns:pidf:geopriv10): The method parameter
is included, and consequently signed, only if known. The LIS
SHOULD verify the accuracy of this field, but MAY opt to include
the element without validation. An unvalidated method is allowed
because of the informational nature of the data it contains.
Method is a metadata element and therefore is not a suitable basis
for decision making, especially where a similar decision can be
based on location information. A recipient SHOULD NOT use the
value of this field as the basis for any decision.
All elements defined in this document: This document defines a
number of elements that are designed for inclusion in the tuple.
These elements limit the effectiveness of certain attacks.
Validity intervals and Target identity are defined in Section 4,
Section 5.
This document defines two transforms that can be applied to a PIDF-LO
in order to limit what is signed Section 8.2. The first is a
selective transform that only selects the elements listed above. The
second simply selects the enveloping tuple. The LIS MAY choose not
to use either transform, but in doing so, all unverified elements
MUST be removed from the signed document.
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3.3. Signature Algorithms
As specified in RFC 3275 [RFC3275], implementations of this
specification MUST provide the following algorithms:
digest algorithm: The SHA1 digest, as identified by the URN
"http://www.w3.org/2000/09/xmldsig#sha1".
signature algorithm: DSA with SHA1, as identified by the URN
"http://www.w3.org/2000/09/xmldsig#dsa-sha1".
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 transforms defined in this document: the tuple-only transform
(Section 8.2.1), as identified by the URN
"urn:ietf:params:xml:ns:pidf:geopriv10:dsig#tuple" and the
selective transform (Section 8.2.2), as identified by the URN
"urn:ietf:params:xml:ns:pidf:geopriv10:dsig#selective".
It is also RECOMMENDED that the PKCS1 (RSA-SHA1) signature algorithm,
as idenfied by "http://www.w3.org/2000/09/xmldsig#rsa-sha1" is also
supported.
3.4. LIS/Signer Identification
RFC 3275 [RFC3275] describes a number of methods for describing the
key used to sign the document. For the purpose of signing a location
object, the "KeyInfo" element MUST be provided in the "Signature"
element.
The LIS MUST include an X.509v3 certificate in the signature. 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 a fragment identifier URI. Internal references could be
applicable where multiple signatures are applied to different parts
of the document.
The LIS SHOULD NOT reference an external source unless there is a
reasonable expectation that the location recipient can successfully
retrieve the certificate. A reference to an external certificate
MUST be described by URI in the "RetrievalMethod" element. The
scheme for the the RetrievalMethod URI MUST be "https:".
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4. Limited Validity
The simplest attack to address is the Time Shifting attack. The LIS
can specify a limited time period where the location information can
be considered valid. Applying a signature ensures that the
information is not tampered with.
A known limitation of this method is that the information could
become invalid at any time after the LIS signs the document. Once
location is generated, the Target can move at any time, thereby
invalidating the location object. Therefore, the LIS SHOULD make the
time period covered by the signature as short as possible to limit
the impact of such movement. The LIS can base any chosen period on
any knowledge it has about the mobility or current speed of the
Target.
Location recipients MAY choose to implement a minimum age policy for
locations, choosing to further restrict the validity interval to
limit the chances of this occurring. It is RECOMMENDED that the
"from" element is used in this case, since the LIS is expected to
validate location before it signs location information.
4.1. Validity Elements
A "validity" element is defined with two sub-elements, "from" and
"until".
The "from" element contains the time that the LIS signs the location
information. Note that this could differ from the time that the
location was generated, which is included in the PIDF "timestamp"
element.
The "until" element contains the last time that the signature can be
considered valid. Choice of an appropriate validity interval is left
to LIS implementations; however, it is RECOMMENDED that this period
not exceed one day. The period chosen SHOULD also consider the type
of network access in use -- location becomes invalid faster in more
mobile networks.
The signature MUST NOT be considered valid if the current time is
outside of the interval specified in these elements.
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5. Signing for a User
Signing location information alone, even with a limited validity
period does not ensure that it is not reused. Signing some sort of
user identifier with the location object provides an additional
degree of protection. Most importantly, the location recipient is
able to detect duplicate location objects for the same Target. In
addition, if some extra data is included from the Target, the
location recipient is also able to link the location object with a
user identity.
5.1. The 'entity' Attribute
The "entity" attribute of a presence document is intended to convey
the identity of the Target. The LIS does not necessarily know this
identity. Nor does the LIS necessarily have the means to
authenticate the Target.
It is RECOMMENDED that this field be generated by the LIS. The LIS
SHOULD construct an "unlinked pseudonym" for the Target that does not
contain any possible identifying information for the target. The
simplest way to meet this requirement is to generate a "pres:" URI
randomly, using a random sequence of characters and the host name of
the LIS (e.g., "pres:f6pc98w1pd49s0p@lis.example.com").
An unlinked pseudonym provides a limited means of ensuring that
location information is not reused or replayed. The presentity
identifier used acts as a serial number for each location object,
allowing each to be uniquely identified. A location recipient is
able to use this identifier to detect multiple uses of the same piece
of location information. This limits the effectiveness of replay
attacks.
Presentity identifiers can be reused for the same Target, provided
that the LIS is able to verify that the Target is the same. This
depends on the means by which location is acquired from the LIS; if
session data that links subsequent requests exists, the LIS MAY reuse
the presentity identifier. Note that the the Target can initiate a
new session to ensure that a new identifier is generated and thereby
ensure that their previous and current positions cannot be correlated
using the presentity identifier.
This does not prevent the use of a "real" presentity in this field.
If the LIS is able to authenticate the Target, and the Target grants
permission to the LIS to use this field, the LIS can include this
information in the "entity" field. These conditions are hard to
meet, which leads to two alternative means of including Target
identity, described in the following sections.
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5.2. Target Identity
The unlinked pseudonym used by the LIS acts as an anonymous
identifier to a location recipient. The only information that this
provides is that two location objects were generated for the same
(anonymous) Target. The location recipient might also wish to link
the location object to the identity of a particular user. For
example, a PSAP might want to link the location object to the
authenticated identity of a emergency caller.
To achieve this linkage between location object and the Target's
identity, the Target sends its identity to the LIS. The LIS includes
this identifier in the signed location object, effectively linking
the identity to the location information.
A location recipient verifies that location information was signed
for a particular Target by authenticating the Target and comparing
the authenticated identity against the one in the signed location
object.
The LIS is not expected to authenticate this identity information,
although it MAY do so. This means that an attacker within the
network could request a signed location object with any identity they
choose. However, the location object could only be used by an entity
that can prove that they have the chosen identity, which limits the
number of potential attackers.
5.3. Protecting User Anonymity
The problem with sending the Target's identity to the LIS is that the
Target might not wish to provide this information to the access
network operator.
This can be addressed by using a cryptographic hash of the user
identity in place of the actual identifier. Since the LIS does not
necessarily authenticate the identity, this information provides the
same attributes as the real identity. Since the hash is not
reversible, the LIS is unable to identify the Target, but the hash
cannot be generated from any identifier other than the one used by
the Target.
The location recipient authenticates the Target's identity, then
compares a hash of the identity to the hash that is included in the
location object to verify that the identity matches.
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5.4. Authenticated Identity
With the above solution, one easy collusion attack exists. One
attacker at the actual location requests a location object with
another attacker's identity. The second, potentially remote,
attacker is able to use this object. If the first attacker is
authenticated by the LIS, this attack is limited, because it requires
that both attackers have access to the same authentication
credentials.
The effectiveness of this approach is limited by the ability of the
LIS to authenticate arbitrary users in the access network. Location
recipients cannot rely on the LIS performing authentication.
5.5. Multiple Identity Attack
The schemes described in this section rely on the Target providing an
identity. A potential attack uses a single attacker in the access
network that requests location information using a number of
different identities. The attacker requests multiple location
objects, using a different identity each time. These objects are
passed to any number of other attackers, who are each able to
authenticate with the identity that is included in the location
object. This potentially allows a large number of distributed
attackers to use the same location information to perform a denial of
service attack.
In some scenarios, multiple identities can be valid. Examples in
Section 3 of [I-D.ietf-geopriv-l7-lcp-ps] show that multiple hosts
can appear from the same network demarcation point. Ideally, the LIS
would still serve these hosts individually because they each have a
valid reason to acquire location information. However, to prevent an
attack where a user requests large numbers of location objects with
different identity information, the LIS SHOULD limit the number of
identities that can be served from any particular network point.
Authenticating the Targets in this scenario could provide some
additional surety that each is legitimate. If multiple Targets
legitimately exist at the same location, then these Targets can
authenticate with the LIS. The LIS MAY use a higher limit for
authenticated Targets.
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6. Target Identity Element
This document defines an XML "identity" element that can be used to
include identity information in a PIDF-LO. This element is used in
addition to a randomized "entity" attribute for several reasons: a
randomized "entity" attribute can be used to detect replays; the
identity is not necessarily authenticated; and the content can be
other than a presentity identifier.
6.1. Identity Types
The content of this element is dependent on the type associated with
the identifier. The "type" attribute is used to define the nature of
the identity that is included. Two values are provided by default:
URI: A value of
"urn:ietf:params:xml:ns:pidf:geopriv10:dsig:identity#uri" for the
type attribute indicates that the contents are a URI. This URI
can include a presentity URI, or other URI that identifies the
target; for example a SIP URI. This type MUST be supported.
An X.509 certificate: A value of
"urn:ietf:params:xml:ns:pidf:geopriv10:dsig:identity#x509"
indicates that the contents are an X.509v3 certificate [X509v3],
in the format described in [RFC3275] for "X509Certificate".
New identity types are identified by URNs, which means that
registration is not required to add new types. A location recipient
that does not support a particular identity type MUST treat the
location object as if no identity information were included.
6.2. Identity Hashing
To allow for anonymity, the content of the "identity" element MAY be
hashed and the hash value included in this element instead. The
"hash" attribute indicates whether the value has been hashed. A
reserved value of "##none" indicates that the actual value is
included. Otherwise, the attribute includes a hash algorithm
identifier, as defined in [RFC3275]. The SHA1 algorithm MUST be
supported; this is identified by the URN
"http://www.w3.org/2000/09/xmldsig#sha1".
Each different identity type requires a procedure for obtaining the
bytes that are hashed.
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URI: For the URI type, the input to the hash algorithm is the UTF-8
bytes of the URI.
X.509: For the X.509 type, the input to the hash algorithm is the
binary value of the certificate; that is, after the base64
encoding [RFC2045] is decoded.
If the "hash" attribute is present and set to a value other than
"##none", the contents of the "identity" element are always the
base64 encoded result from the hash function.
6.3. Authentication Indicator
If the LIS is able to authenticate the Target, the LIS can indicate
this in the "authenticated" attribute.
This indicator can be used irrespective of the value of the "hash"
attribute. This indicates to the location recipient that user
identity included in the "identity" element was authenticated by the
LIS.
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7. Signature Validation
A location recipient performs the following steps to validate a
signed location object:
1. Authenticate the entity that provided the location information
(the sender).
2. Check the integrity of the digital certificate.
3. Extract the identity of the LIS from the digital certificate.
4. Remove all unsigned components from the location object.
5. Ensure the validity interval from the location object covers the
present time.
6. Check that the authenticated identity of the sender matches the
identity in the location object, or that a hash of this identity
matches the hashed identity in the location object.
Once this process is complete, the location recipient has the
following information upon which to base any policy decision:
o Whether the location object was signed.
o Whether the signature on the location object was valid.
o The identity of the sender.
o The identity of the LIS.
o Whether the LIS authenticated the sender in generating the
location object.
o The presentity identifier generated by the LIS that distinguishes
this location object.
Policies are set by individual location recipients and are dictated
by a range of factors. Even a failure in signature validation does
not necessarily require that the location recipient reject the
location information.
For instance, a PSAP might not reject an emergency call with no
signature. The PSAP could instead place a lower priority on such a
call so that in a busy period the call is queued behind calls that
contained valid signatures. Similarly, un-authenticated calls could
be given similar treatment.
Thomson & Winterbottom Expires August 26, 2007 [Page 16]
Internet-Draft Location Dependability February 2007
8. Code and Examples
8.1. Dependability Data Schema
The following XML Schema [W3C.REC-xmlschema-1-20041028] defines the
"dependability" element. This element is intended for use in a
PIDF-LO within a "tuple".
See RFCXXXX.
END Note: Two fragment identifiers ("#tuple" and "#selective") are added to this URN to identify the two transforms defined in RFCXXXX. 11.2. XML Schema Registration This section registers an XML schema as per the guidelines in [RFC3688]. Thomson & Winterbottom Expires August 26, 2007 [Page 27] Internet-Draft Location Dependability February 2007 URI: urn:ietf:params:xml:schema:pidf:geopriv10:dsig Registrant Contact: IETF, GEOPRIV working group, (geopriv@ietf.org), Martin Thomson (martin.thomson@andrew.com). Schema: The XML for this schema can be found in Section 8.1 of this document, between "" and "" (inclusive). 11.3. URN Sub-Namespace Registration for urn:ietf:params:xml:ns:pidf:geopriv10:dsig:identity This section registers a new XML namespace, "urn:ietf:params:xml:ns:pidf:geopriv10:dsig:identity", as per the guidelines in [RFC3688]. URI: urn:ietf:params:xml:ns:pidf:geopriv10:dsig:identity Registrant Contact: IETF, GEOPRIV working group, (geopriv@ietf.org), Martin Thomson (martin.thomson@andrew.com). XML: BEGINSee RFCXXXX.
END Note: Two fragment identifiers ("#uri" and "#x509") are added to this URN to identify the two types of identity defined in RFCXXXX. Thomson & Winterbottom Expires August 26, 2007 [Page 28] Internet-Draft Location Dependability February 2007 12. Acknowledgements The authors would like to acknowledge the contribution of the GEOPRIV WG; the L7 design team; Hannes Tschofenig and Henning Schulzrinne. Thomson & Winterbottom Expires August 26, 2007 [Page 29] Internet-Draft Location Dependability February 2007 13. References 13.1. Normative References [RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies", RFC 2045, November 1996. [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. [RFC4119] Peterson, J., "A Presence-based GEOPRIV Location Object Format", RFC 4119, December 2005. [I-D.ietf-geopriv-l7-lcp-ps] Tschofenig, H. and H. Schulzrinne, "GEOPRIV Layer 7 Location Configuration Protocol; Problem Statement and Requirements", draft-ietf-geopriv-l7-lcp-ps-00 (work in progress), January 2007. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [X509v3] ITU-T Recommendation, "Information Technology - Open Systems Interconnection - The Directory Authentication Framework", ISO/IEC 9594-8:1997, 1997. [W3C.REC-xmlschema-1-20041028] Beech, D., Thompson, H., Maloney, M., and N. Mendelsohn, "XML Schema Part 1: Structures Second Edition", World Wide Web Consortium Recommendation REC-xmlschema-1-20041028, October 2004,