Clare McGrath, Ghazanfar Ali Safdar and Máire McLoone
Institute of Electronics, Communications and Information Technology (ECIT)
Queens University Belfast, Belfast, Northern Ireland
Keywords: Ad Hoc Network, Key Management, Identity Based, Security.
Abstract: In this paper a novel identity based public key exchange (IDPKE) protocol is proposed for wireless ad hoc
networks, where the network node IDs are used as public keys. Previous research into ID based key
management schemes assumes that node IDs are well known and have been distributed amongst the nodes
at the time of network formation. However, this assumption limits the application of these schemes to many
ad hoc networking scenarios. Our proposed IDPKE protocol addresses this disadvantage. It assumes that
node IDs are not known prior to network formation and provides secure and authentic ID exchange between
nodes, thus allowing employment in a wider range of applications. The IDPKE protocol is an extension to
an existing certificate based scheme and it provides an increase in security and a reduction in computation
and bandwidth by comparison.
Ad hoc networks are used by the government and
military and in every day applications such as
security surveillance and traffic monitoring. As
such, they need to be secured, and efficient and
secure key management/authentication schemes
designed specifically for ad hoc networks are
required. A significant number of such schemes
have been proposed in the literature and are
reviewed and categorised in (McGrath et al., 2006,
Hoeper and Gong, 2004).
In this research we investigate identity (ID)
based schemes due to the fact that they can cut down
on complexity and the computational and memory
requirements of nodes compared to certificate-based
techniques (Khalili et al., 2003). However, one
outstanding issue with these schemes, which we
address in this paper, is the assumption that node
IDs are known prior to communication with each
other in a network. This assumption is made because
the node IDs are considered to be well known pieces
of information. However, IDs are strings of
information bound to particular entities and this type
of information will depend on the entity and
application (Hoeper and Gong, 2006). It cannot
always be assumed that IDs will be known by other
users prior to communication. Also, this limits the
type of ID that can be used and therefore limits the
application. If IDs are assumed to be unknown
when the network forms and a secure and authentic
ID distribution scheme is provided, then a wider
range of IDs can be used and the scheme can be
applied to a wider number of applications.
This paper presents a novel ID-based Public Key
Exchange (IDPKE) protocol where nodes can
distribute and authenticate their IDs/public keys for
use in an ID-based cryptographic scheme. To the
authors knowledge this is a new concept compared
to the ID-based key exchange protocols that have
been previously proposed. The IDPKE protocol
meets the specifications of the key exchange
framework proposed by McGrath et al. (2006) and
builds upon a certificate-based scheme known as
OPKM (Li, X. et al., 2004). We show how savings
can be made on bandwidth and computational
requirements and how an improvement in security is
achieved in comparison to the OPKM scheme.
The paper is organised as follows: Section 2
discusses previous research in this area. Section 3
outlines the proposed IDPKE scheme while section
4 reviews its computational and communicational
aspects. Section 5 discusses the protocol’s security
strengths and conclusions are provided in section 6.
McGrath C., Ali Safdar G. and McLoone M. (2007).
In Proceedings of the Second International Conference on Security and Cryptography, pages 167-170
DOI: 10.5220/0002123101670170
In 2004, Li, X. et al. (2004) proposed a fully self-
organised certificate based key management and
authentication scheme which tackled the problems
of earlier similar schemes by Zhou and Haas (1999)
and Capkun et al. (2003). These problems included
the lack of node availability to form a key
management service, the use of heavy computation
in the form of threshold cryptography and
maintaining certificate graphs. However Li et al.’s
scheme introduced some new security issues that are
described in section 5.
Also, it can be argued that the use of certificates
at all in these schemes requires too many
computational/communicational resources than can
be provided by the nodes of an ad hoc network
(Hoeper and Gong, 2004). The aim of ID-based
schemes (Deng et al., 2004, Khalili et al., 2003) is to
remove certificates completely and therefore reduce
computation and bandwidth whilst still providing the
authentication that certificates provide. However,
these schemes also suffer from the same availability
problems and the heavy threshold cryptography
computation mentioned previously. Hoeper and
Gong (2006) proposed a scheme which investigated
the disadvantage of unavailable online PKGs but
their scheme assumed that the IDs of the nodes are
known prior to joining the network. In this paper, we
introduce a secure key management scheme, which
does not assume knowledge of IDs prior to
3.1 Network Model and Assumptions
The network consists of an offline trusted
central entity (Private Key Generator (PKG))
with enough computing power to handle
identity-based private-key generation and
sufficient memory to store the identities of all
the nodes on the network.
The PKG generates the public and private
system parameters (can be based on those of the
Boneh-Franklin identity-based encryption
scheme (Boneh and Franklin, 2001) or others
that have improved on this).
The network contains a variable number of
nodes that can join and leave at any time. When
joining the network, the nodes must first make
contact with the PKG (using a physical wired
connection or secure side channel).
The PKG assigns each node a unique ID which
is also the node’s public key and it is unknown
to other nodes until the IDPKE sequence has
been carried out. For example, this could be the
network IP address of the node and no two
nodes will be assigned the same address. An
assigned ID will also never be reassigned with
the same master parameters, even after the
original node has left the network. The PKG
also calculates a private key using this ID and
the master private key associated with the ID-
based encryption schemes (parameter known
only to the PKG) and loads the node with all the
public parameters of the scheme and the node’s
private key.
Online nodes act as transceivers and can be
stationary or mobile. They may be identical in
terms of computing power and memory but they
must have enough computational ability to
handle ID-based encryption and enough storage
to hold a sufficient number of IDs depending on
the application. They will also have omni-
directional broadcasting capabilities which will
most likely be within a short range. They can
store two tables of IDs, the one-hop and two-
hop table, for nodes that are within a one-hop or
two-hop range respectively.
3.2 IDPKE Scheme
IDPKE describes the events that take place when a
new node joins the network or moves to a new
position. It proceeds as follows:
After contacting the PKG, a node N
joins the
network and broadcasts a “hello” message to its n
one-hop neighbours (N
). The contents of the
message are that of the hello field and a timestamp
) to ensure data freshness and to prevent replay
attacks. These contents are then encrypted with the
private key of the sender.
: (ID
), [Epk
( “hello”, TS
Message 1 shows that the ID of the sender node
) is also present since the ID will automatically
be sent within the address field of the packet.
The one-hop neighbours use the ID/public key to
decrypt the contents of the message and verify that
the message was sent by node N
, since only node N
holds the corresponding private key. The
verification also provides an assurance that the
sending nodes contacted the PKG before joining the
network to receive a legitimate ID and
SECRYPT 2007 - International Conference on Security and Cryptography
corresponding private key. The one-hop receiver
nodes update their one-hop ID table using this
unique address.
Next, the receiver nodes reply to N
broadcastingwelcome” messages, which also
include a timestamp and the IDs of their m one-hop
neighbours (N
), [ Epk
(“welcome”, TS
, ID
Again the contents of the messages are encrypted
with each of the sender nodes’ private keys. N
verify that the public key in the address of each
packet corresponds to the private key used to
encrypt it by decrypting each message successfully.
The replies of the sending nodes are sent after
chosen random times T
to avoid bombarding N
with incoming messages. Since nodes can hear the
replies their one-hop neighbours broadcast to N
, all
nodes within two hops of N
can verify that their
public information (ID) has been distributed. If
nodes can record incorrect/omitted publications from
others, malicious behaviour can be caught out. This
is Li et al.s (2004) process known as
neighbourhood monitoring.
Once each node, including N
, is satisfied that the
information is correct, they update their one and
two-hop ID tables according to the IDs in the reply
messages and the ID of the sender itself, if these IDs
are new. If a node finds the information from a
particular node is not satisfactory, i.e. its ID was
omitted from the broadcast of a certain node, it can
broadcast a correction message, which will include
the correct information.
: (ID
), [Epk
(“correction”, ID
, TS
The correct ID of the omitted node (ID
) is the
address from which the correction message
originated. The contents of the message include a
field indicating it is a correction message along with
the ID of the node that sent the incorrect message
) and a timestamp. Nodes receiving the
correction message can forward the message,
including the omitted ID within the message
contents, to ensure the correction message reaches
all the two-hop neighbours of the omitted node.
Nodes may then set a “Don’t trust” flag against
the sender of the incorrect message such that they
know this node may not be trusted in the future.
After time T
, N
broadcasts an update message to its
one-hop neighbours. T
is defined to ensure N
able to receive all the reply messages from its one-
hop neighbours. The update message contains all its
one-hop neighbours’ IDs. Nodes receiving this
message verify the information against their own
and update their tables if any new information is
(“update”, TS
, ID
…., ID
If after time T
, N
has not received any
correction messages, it may assume that the IDs it
has received are correct. IDPKE thus enables each
node to obtain all the IDs (IP addresses) and
therefore all the public keys of its neighbours within
two hops in a trustworthy manner. When a node
moves to a new position in the network, it initiates
the process again. This allows nodes to ensure that
the information they have stored about other nodes
in the network remains current even while nodes are
moving about and are joining or leaving the
network. Once nodes have received the IDs of
nodes in the local neighbourhood, they can proceed
with secure encrypted messaging.
The ID-based encryption used in the IDPKE
messages is based on elliptic curve cryptography
giving savings in computation compared to the
RSA-based digital signature schemes (Khalili et al.,
2003) used in OPKM. There is no hashing involved
in IDPKE, unlike OPKM, which involves both
hashing and encryption. Also, RSA-based schemes
use public keys that are 1024 bits in size as opposed
to the much shorter keys used in ID-based
cryptosystems (e.g. if using an IP address, this is
only 32 bits in size). The use of smaller keys reduces
computation, communication overhead and storage
(Deng et al., 2004). Communication overhead is also
reduced because IDPKE distributes less data than
OPKM which distributes certificates. In addition,
storage overhead is reduced in IDPKE since there
are no certificates. Finally the use of the offline
entity in IDPKE shifts key generation computation
from the individual nodes, thus reducing node
computation in comparison to OPKM.
The main attacks that key management schemes for
ad hoc networks have to face are denial of service
(DoS) attacks and man-in-the-middle (MiM) attacks.
These can be carried out in different ways:
illegitimately, legitimately or by impersonation.
The illegitimate node attack is when a malicious
node joins the network with a fake ID and without
contacting the PKG. It will therefore not receive a
matching private key and will behave maliciously
when it joins the network. Any messages they send
however will be ignored as they will have no private
key to sign them. This attack can therefore be
prevented in IDPKE.
The legitimate node attack is more serious and it
happens when a node joins the network legitimately
but acts maliciously. It therefore contacts the PKG
and receives a legitimate ID and matching private
key but it behaves maliciously when it joins the
network by trying to flood the network with false
IDs (DoS) or by ignoring or forwarding on
false/modified IDs (MiM). The attack worsens if a
number of legitimate nodes decide to act together
and behave maliciously. If false information has
been transmitted by a legitimate node, it can be
defended against using neighbourhood monitoring in
both IDPKE and OPKM. An attack involving a
number of legitimate nodes acting together and
behaving maliciously cannot be defended against (in
both IDPKE and OPKM) as it would be impossible
to determine which information is correct or
The impersonation attack is when a malicious
node joins the network and masquerades as another
legitimate node in the network in order to modify
public information or flood the network with false
information. The OPKM approach does not prevent
such attacks outright. However, IDPKE can defend
against impersonation attacks. In IDPKE the
legitimate node has a matching private key and will
sign all outgoing messages with this key. A
malicious node will not be able to derive this key
and will not be able to contact the PKG for access to
the key as the PKG will only issue one private key
per node ID, therefore any unsigned outgoing
messages from this node will be ignored. It will also
not be able to decrypt any secure messages that have
been encrypted with that ID/public key. Hence
signing with the private key ensures the receiver that
the message definitely originated from the node with
the ID contained in the message and that the ID was
not modified.
By modifying a fully self organised certificate based
key management scheme to a partially self-
organised identity-based scheme, we have provided
a more complete solution for existing ID-based
schemes, allowing the use of different types of IDs
and more accessibility to different ad hoc scenarios
and applications. The proposed IDPKE protocol
improves on the certificate based schemes in terms
of security against impersonation attacks and in
terms of computation and communication overhead
making it more accessible to constrained devices.
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