Anais - Engenharia de Redes de Comunicação - UnB

carrier visited in a random route. We implement nodes’ encrypted secret parameter by
encrypting the previously mentioned shared symmetric key k i (a unique parameter) with
the public key G of the gateway, to obtain E G (k i ) – though, there are other ways to
implement this. Each ciphertext is inserted into the route part of carrier and concatenated
to the previous ones. At the end of a route of length r, the route information would be as
follows: E G (k 1 )‖E G (k 2 )‖...‖E G (k r−1 )‖E G (k r )
Due to this concatenation structure, any mesh node can count how many secret parameters
have been included in the carrier. However, they cannot determine the originator
node, since these parameters are encrypted. Hence, insiders and outsiders are unable to
know the entire route the carrier took. That count is the criterion that nodes use to stop
forwarding the carrier. When a carrier reaches the gateway, it decrypts each nodes’ secret
parameter to discover which hops the carrier has visited. This information is important to
check the correctness of the protocol, as detailed in the next section.
3.4. The Protocol Description
The protocol is composed of three phases: access, downlink and uplink phases. These
phases are preceded by a setup phase.
1. Setup Phase
Before the nodes begin sending or receiving data to/from the gateway, they first
need to generate and distribute key material. In particular, the gateway generates a pair
of asymmetric keys and sends its public key to every mesh node. After that, each node
generates a symmetric key and shares it with the gateway. To perform this, each node
encrypts its key with the gateway’s public key and sends it to the gateway. Additionally,
a key sharing protocol must be performed between each mesh node and their neighbours
(i.e. the nodes within the communication range). These keys will be used for the nodes
single-hop communication.
In addition, the gateway defines the number of nodes r that a carrier should visit
before being forwarded back to the gateway. This value may consider the number of
mesh nodes (and thus estimating network traffic) and the level of anonymity that should
be achieved. The anonymity level is further commented in the Section 3.5.
2. Access Phase
The protocol starts with the access carriers generation by the gateway. Initially,
this carrier contains only an encrypted nonce value, used to protect the network against
replay attacks. The gateway periodically generates access carriers and casts them to the
mesh network. When a node receives a carrier and wants to make a request, it inserts a
valid access request into the onioned data section, encrypts the result with its shared key,
and then forwards the carrier to any hop within its communication range. However, if a
node has no request to make, it proceeds as before, except that it inserts padding bits into
onioned data part, instead of an actual request.
Besides operating on the first part of the carrier, each node adds its encrypted
secret parameter into the route information part. Later on, the gateway will use this information
to verify which hops the carrier visited. Since the route is randomly taken, having
this knowledge ensures that the carrier visited different and valid nodes in the network.
343