Privacy and Identity Management in a Layered Pervasive Service Platform
ABSTRACT Making pervasive computing reality is a challenging task mainly due to the multitude of functional requirements and technological constraints. In parallel to the honourable research progress in specific technologies, the Daidalos project assessed that in future there will be the need for a pervasive service platform with open interfaces in order to simplify service development and provisioning. The success of such a platform depends on the balance of different aspects, e.g. operational costs with revenue potentials, collection of personal data for context-awareness with privacy protection, manual control and transparency with enhanced user experience and simplicity. In this paper we show the Daidalos approach to privacy protection and identity management for a future pervasive service platform and its architecture. We show how user identities are structured to support dynamic context information while following regulations for privacy protection in Europe. Special focus is put on the trade-off between access control for privacy protection and user experience. This is achieved by automated identity selection, automatic derivation of fine-grained access control policies and their deployment. We also present gathered performance data and implementation details of our ID Broker concept.
Privacy and Identity Management in a
Layered Pervasive Service Platform
Marc BARISCH1, Martin NEUBAUER1, Joao PAGAIME2, Joao GIRAO2, Rui L. AGUIAR3
1University of Stuttgart, Institute of Communication Networks and Computer Engineering,
Pfaffenwaldring 47, 70569 Stuttgart, Germany
Tel: +49 (711)685-60217, Fax: +49(711) 685-50217,
2NEC Europe Ltd., Kurfürsten-Anlage 36, 69115 Heidelberg, Germany
Tel: +49 (6221) 4342-0, Fax: +49 (6221) 4342-155
3 Instituto de Telecomunicaes/Universidade de Aveiro, 3810-193 Aveiro, Portugal
Tel: +351 234 377900, Fax: +351 234 377900
Abstract: Making pervasive computing reality is a challenging task mainly due to
the multitude of functional requirements and technological constraints. In parallel to
the honourable research progress in specific technologies, the Daidalos project as-
sessed that in future there will be the need for a pervasive service platform with open
interfaces in order to simplify service development and provisioning. The success of
such a platform depends on the balance of different aspects, e.g. operational costs
with revenue potentials, collection of personal data for context-awareness with pri-
vacy protection, manual control and transparency with enhanced user experience and
simplicity. In this paper we show the Daidalos approach to privacy protection and
identity management for a future pervasive service platform and its architecture. We
show how user identities are structured to support dynamic context information
while following regulations for privacy protection in Europe. Special focus is put on
the trade-off between access control for privacy protection and user experience. This
is achieved by automated identity selection, automatic derivation of fine-grained ac-
cess control policies and their deployment. We also present gathered performance
data and implementation details of our ID Broker concept.
Keywords: Identity Management, Privacy Protection, Virtual Identity
Academia and industry have been working towards Weiser’s vision  of future comput-
ing. This vision materialises in recent advances in wireless communication and miniaturisa-
tion of technologies. These allow provisioning of computing power, communication, stor-
age and sensing of the surroundings every time and everywhere. Consequently, systems be-
come more dynamic, e.g. interaction partners change more often and interactions them-
selves adapt to changes in context. Context-awareness implies increased collection and
processing of personal data, which amplifies the potential to invade users’ privacy. To miti-
gate this flaw, solutions must be integrated to protect personal data while “they weave
themselves into the fabric of everyday life until they are indistinguishable from it” .
The Daidalos project shares this ubiquitous computing vision and is designing a layered
service platform with inherent privacy protection. The general approach to privacy protec-
tion is to guarantee the right on informational self-determination and allow users to act
pseudonymously in the platform by means of virtual identities (VIDs). Different VIDs can
be used to consume services. The view on user attributes associated to an identity is limited.
Hence, VIDs and identity management (IdM) are key concepts in the enforcement of pri-
vacy protection as well as for success and acceptability of pervasive services. This work is
in accordance with legislative regulations in Europe [2, 3].
Existing approaches do not fulfil all requirements of pervasive service platforms with
respect to privacy. In particular, they mostly regard privacy protection as pure manual con-
trol over which attributes are disclosed to which service on application layer. They neither
provide mechanisms for dynamic access control on attributes nor do they give recommen-
dations whether it is sensible to disclose the attributes in this context. If disclosure of attrib-
utes is not restricted, a malicious service provider could gain a detailed view on an identity.
In addition, if multiple VIDs per entity are supported then it has to be ensured that no
entity can link them by evaluation of information inherent to the system. For example, us-
ing different VIDs for different services that are provided by a single service provider con-
currently are easily linkable in today’s systems by evaluating the IP-address. To overcome
this flaw, cross-layer considerations and solutions such as  should be taken into account.
To the best of our knowledge, existing approaches do not consider linkability of VIDs pre-
viously used in different contexts and across layers.
Eventually, performance of retrieving personal data matters in case of frequent chang-
ing values like location or sensor data. This means, the mobile terminal should not be in-
volved in the retrieval due to the scarce radio resources.
In Section 2, we first give a brief overview of related work in the IdM area. Section 3
gives an example scenario, which can be realised by the Daidalos architecture. The underly-
ing identity model is presented in section 4. Section 5 and 6 take a closer look at the com-
ponents of the proposed IdM architecture. Section 7 summarises and concludes our work.
2. Related Work
The specifications of Shibboleth  and Liberty Alliance  provide federated identity
management with main focus on Single Sign-On and Logout for Web Services. Both are
based on SAML  but address different target groups. Shibboleth concentrates on acade-
mia and provides mechanisms for direct exchange of user attributes between federated insti-
tutions for authorisation purposes. Institutions and users can protect their attributes by so
called attribute release policies, which can be modified by graphical user interfaces . In
contrast, Liberty Alliance concentrates on business environments and has a set of services
based on templates , which allow the management and release of user attributes. The
concrete realisation of attribute release policies is not specified.
Microsoft CardSpace  uses WS-* specifications and requests the user to select an
identity, which is termed identity card. Each identity card comprises a set of attributes, so
called claims. These claims are transferred from the card provider (=identity provider) to
the service provider via the user’s terminal. This means that there is no direct communica-
tion between the identity provider and service provider, which contrasts the approaches of
Shibboleth and Liberty Alliance.
Beyond the web service world, approaches for attribute management are specified by
OMA and 3GPP. OMA has specified the PEEM , which is a generic policy-based ap-
proach for the control of service delivery platforms.
3GPP specified the “Generic User Profile (GUP)” . It allows services to access user
data by contacting a central point and thus has the possibility to hide several data stores.
The GUP concept is closely related to our approach. However, we focus on pervasive com-
puting and privacy protection.
3. Daidalos Architecture and Usage Scenario
Daidalos is a FP6-funded European research project with the goal to create a service plat-
form, which enables pervasive services for mobile users with a major focus on privacy pro-
tection. In addition the platform has to ensure that telecommunication operators as well as
service providers are in the position to offer flexible and innovative services. The following
scenario gives an example for such new services.
A telecommunication operator runs a service platform, which allows integration of 3rd
party services in a flexible manner. This is used by a regional service provider (SP) to offer
a novel tourist guide service (TGS) for the local city. The service can only be discovered by
users who are in this city. The TGS is context-aware and thus provides enhanced user ex-
perience. For this, the TGS has to know personal attributes from its users. For example,
user location and orientation is required in order to provision appropriate information about
nearby famous buildings. If user’s preferred language is provided then the service presents
the information in this language. For some buildings videos with additional historic insights
are available. To view these, the current access network QoS capabilities and user QoS pa-
rameters are considered. Because the user subscribed to the golden QoS-class and the ac-
cess network has sufficient capacity, the video is presented with highest available resolution
including stereo sound.
This scenario demonstrates that (1) there are short-term relations between users and ser-
vice providers, (2) service providers require personal data, (3) telecommunication operators
have to maintain complex business relations, and (4) users require mechanisms for attribute
exchange with service providers while maintaining their privacy. To cope with these chal-
lenges the Daidalos architecture  is divided into two layers as shown in Figure 1.
Pervasive Service Support Layer
Service Provisioning Support Layer
Pervasive Service Support Layer
Service Provisioning Support Layer
Figure 1: The Daidalos Architecture
ployment. This comprises, e.g. support for mobility, quality of service, multimedia services
and basic security functionality, such as access network authentication. These are comple-
mented by context and personalisation in addition to security, privacy and identity man-
agement functions in the Pervasive Service Support (PSS) layer.
The Service Provisioning Support (SPS) layer offers basic functionality for service de-
4. Daidalos IdM Concepts
In Daidalos, we generalise the understanding of identity concept. It covers users, operators
as well as service providers. To avoid confusion we introduced the term entity and limited it
to natural and legal persons whose privacy has to be protected. Furthermore, a partial or
virtual identity is “a subset of attributes of a complete identity” which “(each) represents the
person in a specific context or role” . In Daidalos the term VID is adopted. The previ-
ously referred “subset of attributes of a complete identity” thus reflects the set of personal
data related to a VID and may comprise later related data items as well. Consequently, the
management of VIDs becomes a key functionality to privacy protection. In the remainder,
the major focus is on virtual identities with respect to users.
Our virtual identity model reflects the inherent distribution of personal data across vari-
ous systems and several administrative domains as well as the association of data to virtual
identities. Figure 2 shows that each entity can own several entity profile parts (EPPs). An
EPP is a set of attributes that comprise a consistent whole and thus can be regarded useless
if decomposed. EPPs of one entity are virtually aggregated to the entity profile (EP). An
entity profile comprises all existing EPPs of an entity. Based on the entity profile multiple
views can be defined. Each entity profile view (EPV) corresponds to a VID. This allows
creation of several views (VID) on existing data (EPP). For more information on the Dai-
dalos VID concept, please consult .
EPV 1 = VID
EPV 1 = VID
EPV 1 = VID
Figure 2: The Daidalos Virtual Identity Model
5. Daidalos Identity Management Architecture
In the following, the Daidalos Architecture introduced in section 3 will be detailed with re-
spect to IdM and application of the VID model. Figure 3 shows the components of the IdM
implementation, which is the basis for subsequent explanations. The components perform a
two-stage process to protect user privacy and improve user convenience.
In the first phase (service preparation phase), a VID matching the characteristics and
requirements of the service is selected. In the second phase (service consumption and at-
tribute retrieval phase), the service provider can retrieve user attributes. In the following
both phases are explained in more detail, including the involved components.
Figure 3: Daidalos IdM Components
5.1 Service Preparation Phase
The goal of this phase is authentication of the user towards the SP and to grant access to
user’s attributes, which are required for service provisioning.
Conventional systems describe the required user attributes in static privacy policies.
This does not consider the trustworthiness of the service providers and that additional in-
formation is disclosed during retrieval of privacy policies. For example the IP address could
be used to infer more details about the requesting user.
attributes, disclosure to other entities as well as the purpose of processing and storage.
It evaluates the statements about required data. Then it decides which VID of the entity in
question should be used in order to fulfil the service requirements while protecting privacy.
This functionality enhances the user experience in pervasive environments  in which a
much higher dynamic with respect to changing business partners is expected.
Up to now, no personal information is revealed to the service provider. All interactions
are anonymous and do not result in invasion of privacy. If the user or the VID-Selector act-
ing on his behalf agrees to the requirements of the SP, the SP gets informed about the user’s
VID. This is achieved by the VID-Exchange protocol defined by the Daidalos project. This
Eventually, the selected VID is activated. First, the VID is authenticated against the
network. Second, the service provider must be granted access to required EPPs. This is the
task of the Access Control Manager. It performs a two stage policy deployment process. In
the first stage, access control rules are only deployed on the ID-Broker, which is the central
hub for EPP retrieval. In the second stage, access control rules are deployed on the actual
storage (EPPHolder). After successful completion of these steps the service consumption
can take place.
5.2 Service Consumption and Attribute Retrieval Phase
The SP requires user attributes for the actual service provisioning. In the following, the user
attribute retrieval is explained in detail.
Every VID has one ID-Broker assigned. Each ID-Broker acts as the central hub for at-
tribute retrieval and is supposed to be operated by a telecommunication operator. Therefore,
the SP queries the VID’s ID-Broker to retrieve attributes (EPPs). The ID-Broker itself does
not store EPPs but manages references to EPPs stored in EPPHolders (EPPH).
The ID-Broker provides two modes for attribute retrieval to support administrative do-
mains and highly dynamic EPPs. Figure 4 and Figure 5 illustrate both modes, Proxy Mode
and Refer Mode.
Telecommunication Operator Domain
Service Provider Domain
(1) Get ‘Data Item Z’
(3) Get Data XYZ
Figure 4: Proxy Mode
Telecommunication Operator Domain
Service Provider Domain
(1) Get ‘Data Item Z’
(4) Get Data
Figure 5: Refer Mode
which performs the first permission check (2).
In case of Proxy Mode, the ID-Broker contacts the EPPH (3) which optionally checks
the permissions depending on the trust relationship between the ID-Broker and the EPPH.
Finally, the ID-Broker receives the EPP and forwards it to the SP (5). The EPP can be pro-
vided in encrypted form to avoid that the ID-Broker can learn information about the user it
is not privy to.
In case of Refer Mode, the ID-Broker only returns a reference to the actual EPPH (3).
The reference allows inferring the required protocol to contact the EPPH (4). The EPPH
performs an access control check (5), which is required because of possible reference cach-
ing, and returns the queried information (6).
The Refer Mode is faster if the same EPP is accessed several times, because the ID-
Broker is only involved in the first access (reference caching). It also avoids the ID-Broker
as a bottleneck. A drawback is that the actual storage location of the EPP is revealed. This
information might be used to infer additional information about a VID and potentially link
two VIDs, e.g. in case the revealed reference contains entity-specific information.
Both modes have in common that the initial EPP query (1) is directed to the ID-Broker,
6. ID-Broker Implementation and Performance
The presented concept is prototypically implemented in the Daidalos project. In the follow-
ing, details about the ID-Broker implementation including performance measurements are
6.1 – ID-Broker Software Architecture
We built our ID-Broker on top of existing standards, like SAML  and XACML , to
ensure a maximum compatibility with other identity management solutions.
In our solution, the ID-Broker and the EPPH are combined with required IdP functions.
Our ID-Broker can thus be deployed as an EPPH enforcing access control, as an IdP, or as a
“proxy” that redirects requests to the appropriate EPPH or to other ID-Brokers.
Figure 6 shows the ID-Broker internal architecture including the SAML engine and
VID data model implementation (instantiated in a database).
Figure 6: ID-Broker Software Architecture
rently provide a SAML 2.0 connector that implements both SAML’s HTTP and SOAP
bindings, which are executed as web applications within Apache Tomcat. All requests are
forwarded in a common format to the Core component, which keeps authentication and ses-
Connectors allow for different protocols to communicate with the ID-Broker. We cur-
the request. We support different Policy Engines to be plugged in, and we are currently us-
ing an XACML engine. EPP resolution is delegated to the EPP Storage module (realises
the EPPH), which contains plug-ins that translate identity information from existing com-
ponents to a common format. We currently have plug-ins that provide local storage in a
MySQL database and that translate data from an existing 3GPP Home Subscriber Server
(HSS). Expansion of references to remote EPPs is handled by the Core, which uses a suit-
able Connector to expand the reference.
The Core also triggers relevant policies for each request that determine how to handle
6.2 – Identity Broker Performance
These values were measured on a Linux-based system with kernel 2.6.22. The host machine
was an AMD Opteron clocked at 2410 MHz, 512 MB RAM and 256 MB swap. We used
Sun’s Java SDK 1.6.0_03-b05, Apache Tomcat 5.5.23 and MySQL 5.0.45.
Table 1: Average Response Time for 100 Requests (ms)
Run #1 #2 #3 #4 #5 #6 #7 #8 #9 #10
Each measured time includes the sending and receiving of the request over the network,
and each response produced includes an XML Digital Signature, which is where most of
the time is spent. From these figures, the current implementation can process about 6 to 7
requests per second; this can be improved by distributing the load among more ID-Brokers
making full use of the distributed nature of the protocol. There is also ample room for im-
provement in the current implementation; these metrics should be interpreted as meaning
our approach is feasible.
We presented a design for a layered privacy and identity management system. It integrates
pervasive services into telecommunication operator platforms. The system puts the tele-
communication operator in the middle of all IdM processes. He acts as the central hub for
information exchange about users without having global views on them. The user can take
full advantage of all identity related information across administrative domains without en-
tering information several times. Privacy invasion is prevented by an automated process,
which selects virtual identities on behalf of users and deploys access control rules. In con-
sequence SPs only have a restricted but sufficiently detailed view on their customers.
During system design we faced several challenges. First, we had to solve the problem of
data ownership. Second, we had to deal with different representations of personal informa-
tion in different layers. This is related to different understanding of terminology and differ-
ent semantics of available information.
Our first implementation shows that our architecture can improve privacy and identity
management in open pervasive telecommunication platforms for the operator of the plat-
form, the user of the platforms as well as for connected service providers. Although the ar-
chitecture is based on well-known IdM standards, interoperability with existing IdM solu-
tion is not automatically ensured. In future, support for groups of identities as well as im-
provement of the existing processes should be addressed.
We thank our colleagues in the Daidalos project developing the pervasive system. Special
thanks to J. Kögel, C. Hauser, A. Matos, A. G. Skarmeta, A. Sarma, T. Mota and P. Bran-
dão for their contributions to the VID IdM design.
This work was supported in part by the European Union under the FP6 programme
(Daidalos project). Apart from partial funding the Daidalos project, the European Commis-
sion has no responsibility for the content of this paper. The information in this document is
provided as is and no guarantee or warranty is given that it is fit for any particular purpose.
The user thereof uses the information at his sole risk and liability.
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