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G → N 8 : E k8 (E k9 (E k3 (E k10 (E k6 (E k11 (G, pad, E G (nonce)), N 11 , down 6 ), N 6 , pad),
N 10 , down 3 ), N 3 , pad), N 9 , pad)
N 8 → N 9 : E k9 (E k3 (E k10 (E k6 (E k11 (G, pad, E G (nonce)), N 11 , down 6 ), N 6 , pad),
N 10 , down 3 ), N 3 , pad)
N 9 → N 3 : E k3 (E k10 (E k6 (E k11 (G, pad, E G (nonce)), N 11 , down 6 ), N 6 , pad),
N 10 , down 3 )
N 3 → N 10 : E k10 (E k6 (E k11 (G, pad, E G (nonce)), N 11 , down 6 ), N 6 , pad)
N 10 → N 6 : E k6 (E k11 (G, pad, E G (nonce)), N 11 , down 6 )
N 6 → N 11 : E k11 (G, pad, E G (nonce))
N 11 → G : E G (nonce)
4. Uplink Phase
In the uplink phase, mesh nodes send uplink data to gateway anonymously. If a
node has any uplink data to send, it employs the same approach used to make access requests.
They wait for an uplink carrier arrives, insert the desired data, and then chooses a
random node to forward the carrier. After visiting r hops, the carrier is sent back to gateway
which performs the protocol check, in the same way as described in the access phase.
After visiting r nodes of a random route, N 1 , N 2 , . . . , N r , an uplink carrier has the following
format: E kr (...E k2 (E k1 (uplink 1 ), uplink 2 )..., uplink r ), E G (k 1 )‖E G (k 2 )‖...‖E G (k r ).
Note that in all protocol phases we have included mechanisms to enable various
nodes to anonymously communicate using a single packet. This is a feature WuLi’s protocol
fails to provide [Wu and Li 2006]. In their both schemes, when two or more nodes
make a request, always the node closer to the gateway gets granted, since it replaces the
onion. In a networks with a large number of nodes, such as metropolitan-scale WMNs,
this turns out to be a real concern, because simultaneously communication is very likely
to occur.
3.5. Sketch of Security Analysis
The security of our protocol is based on the uniform behaviour of mesh nodes while
following the protocol. That is, the activities for an active node is indistinguishable from
the activities of an inactive one. This is achieved by employing modified onion routing
schemes associated with redundancy and padding techniques. Nodes may either encrypt
or decrypt onions according to the protocol phase. Padding bits are included into the
onion to defend against message’s size attacks. Redundancy, in turn, is the key technique
for achieving anonymity. That is, packets visit several mesh nodes, so that an active one
is hidden among them. Hence, the anonymity level achieved can be measured by the
number of redundant nodes involved in a given protocol phase. In other words, the more
nodes the network has, the better the anonymity level will be. However, a large number
of nodes would introduce relevant overhead over the network performance.
In addition, onion routing techniques also provides privacy to mesh nodes’ data,
since each layer is encrypted with a shared symmetric key. The security of the data is
then guaranteed by the underline symmetric cryptosystem. In setup phase, more sophisticated
key agreement protocol, such as [Wan et al. 2009], may be employed to establish
more secure symmetric keys. Asymmetric cryptography is also employed to secure other
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