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70<br />
• Instant postcard: A Bluetooth enabled digital<br />
camera can be used to immediately send a picture<br />
as an e-mail attachment to any recipient<br />
via a cellular phone. A text message can be<br />
supplied through the cellular phone keyboard<br />
or through a PDA.<br />
• Data access point: Bluetooth equipped devices<br />
may serve as access points to an arbitrary<br />
backbone network. For instance, at an<br />
airport lounge such devices can be installed in<br />
a grid so that all seats are covered by at least<br />
one Bluetooth cell. These will give the users<br />
access to the airport local area network<br />
(LAN) that is connected to the Internet. Similarly,<br />
small office buildings and hotels can<br />
offer wireless access to their backbone LAN.<br />
• Hidden computing: Ubiquitous computing<br />
means that small computers, dedicated to<br />
some specific task, are present almost everywhere<br />
without us even noticing them. Scattered<br />
all over a room, one of the largest problem<br />
is how to achieve the communication that<br />
is necessary for this scenario to work. Bluetooth<br />
is a very attractive candidate for this. A<br />
simpler variant of this theme is the so-called<br />
“briefcase trick”. As a user disembarks an<br />
aeroplane and switches on his cellular phone,<br />
the laptop stored in his briefcase automatically<br />
synchronises its e-mail inbox through a Bluetooth<br />
connection to the cellular phone, which<br />
in turn connects to the corporate mail server<br />
over air. All this takes place without any specific<br />
user interaction.<br />
3.2 Ad-hoc Connectivity<br />
We define ad-hoc networking as forming and<br />
maintaining networks without any central<br />
administration. For many interesting user scenarios,<br />
this is a necessary functionality, and Bluetooth<br />
may serve as an enabling technology for<br />
these scenarios. Clearly, the piconet is by definition<br />
a sort of ad-hoc network. As already described,<br />
the Bluetooth specification provides<br />
means for creating sessions between devices<br />
without any a priori information about the other<br />
devices regarding addresses, clocks, and which<br />
services are run on devices in the vicinity. The<br />
specification also describes how units can enter<br />
and leave existing piconets, which facilitates<br />
dynamic network topologies.<br />
The scatternets and interpiconet communication<br />
makes things even more interesting. By utilising<br />
relaying nodes, it is possible to perform multihop<br />
communication. A device can communicate<br />
with a node out of range using one or more intermediate<br />
nodes to reach the destination. How-<br />
ever, for this to work it is necessary to develop<br />
routing in ad-hoc networks. This is a challenge,<br />
since the networks are not static. Users may<br />
enter and leave piconets at random time instants,<br />
and consequently the routes between source and<br />
destination may change frequently. Moreover,<br />
the quality of each link may change over time<br />
(people passing by, shadowing, fading), so for<br />
the entire route it is possible that certain services<br />
will encounter problems that are not seen on single-hop<br />
connections.<br />
For a more thorough discussion on wireless adhoc<br />
networking, see [4].<br />
3.3 Security Issues<br />
The need for some security support in Bluetooth<br />
was recognised at an early stage of the system<br />
development. In general, radio communication is<br />
quite easy to listen in to without revealing this to<br />
the victim. Even though the pseudo-randomness<br />
of the FH channel gives some protection towards<br />
a casual eavesdropper (e.g. a person sitting close<br />
to you at an airport, following what websites you<br />
are reading while waiting in the departure hall),<br />
it provides no privacy in a cryptographic sense.<br />
For this purpose, a ciphering mechanism has<br />
been included in the specification. For some services,<br />
it is clear that means for access control is<br />
desirable. A Bluetooth headset connected to a<br />
mobile phone may serve as an example of this.<br />
Clearly, the phone owner would not like an arbitrary<br />
headset being able to connect to the<br />
mobile, since then someone could make calls<br />
using a personal headset while charging the<br />
phone owner without their knowledge. Only<br />
authorised headsets should be allowed access<br />
to the mobile.<br />
To accommodate for security, all units must be<br />
able to prove their identity, the BD_ADDR. For<br />
this purpose, the pairing protocol is a prerequisite.<br />
Pairing is done when two units set up a connection<br />
for the first time. In this procedure, the<br />
units exchange a 128 bit secret key, denoted link<br />
key. This key is unique 1) for each link. Thus, a<br />
device needs to remember one link key for each<br />
Bluetooth device it is paired to. The pairing<br />
involves some user interaction since a common<br />
pass key is necessary in both devices in order to<br />
generate the link key. The length of the pass key<br />
is defined to be from 1 to 16 bytes. In its simplest<br />
practical form, it can be an arbitrary 4-digit<br />
number entered on a keypad. Of course, the<br />
longer pass key used, the better security. For<br />
sensitive applications that need particular long<br />
pass keys, an alternative method more suitable<br />
for humans is to exchange it using some key<br />
agreement scheme (e.g. Diffie-Hellman).<br />
1) Exceptions to this do exist, unit keys and master keys are not unique. For details, see [1], chapter 14.<br />
Telektronikk 1.2001
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