Anais - Engenharia de Redes de Comunicação - UnB

The access carriers travels through r random nodes before being forwarded back to
the gateway. Each node checks how many times a carrier has been forwarded by counting
the number of encryptions on the route information part. Based on the count, the node
determines the packet’s destination. That is, if the count is less then r (r was defined in the
setup phase) the node continues forwarding the packet. Otherwise, if the count is equal
to r, that means the current node is the last one in the random route and should send the
carrier back to the gateway. The gateway receives the carrier sent back from a node and
verifies its correctness. In order to perform this, the gateway first decrypts each encrypted
secret parameter. After that, it obtains the plaintext of each shared symmetric key which
works as node’s identification. Based on that, it discovers the carrier’s route by checking
the order of these keys. From this knowledge, the onioned data can be decrypted. At the
end, the gateway may have a set of plain access requests, and thus, will be able to provide
access to the requesters. The protocol is verified based on the success of decryption
operation over the onioned data. If it is not successful, the protocol has not been properly
followed and the gateway drops the carrier.
As an example, suppose a gateway G, a parameter r = 6, and a sequence of
nodes N 1 , . . . , N 7 , hops of a random route. Suppose N 1 , N 2 , N 4 , N 5 , and N 7 are just
forwarders, whereas N 3 and N 6 are requesters. At first, G generates an access carrier
E k1 (nonce) containing a nonce value encrypted with the shared symmetric key k 1 . This
encryption is performed as a mean to authenticate the packet to N 1 . After receiving the
carrier, N 1 processes it by padding onioned data with dummy bits and then encrypting
the result using k 1 to obtain E k1 (nonce, pad). Then N 1 generates its encrypted secret
parameter E G (k 1 ) and inserts it into to the route part of the carrier. Finally, N 1 sends the
new carrier, E k1 (nonce, pad), E G (k 1 ), to the node N 2 .
N 2 , N 4 , N 5 operates exactly as N 1 .On the other hand, N 3 and N 6 proceeds slightly
different. As requesters, rather than adding padding bits, they add an actual request into
the onioned data. After finishing, N 6 sends the carrier to N 7 . As any other node in the
route, N 7 verifies if the number of parameters on the route part are equal to r. Up to that
point, as the route part has 6 encryptions and r = 6, N 7 just forwards the carrier back to
G. The following message flow summarizes this example.
G → N 1 : E k1 (nonce)
N 1 → N 2 : E k1 (nonce, pad), E G (k 1 )
N 2 → N 3 : E k2 (E k1 (nonce, pad), pad), E G (k 1 )‖E G (k 2 )
N 3 → N 4 : E k3 (E k2 (E k1 (nonce, pad), pad)req 3 ), E G (k 1 )‖E G (k 2 )‖E G (k 3 )
N 4 → N 5 : E k4 (E k3 (E k2 (E k1 (nonce, pad), pad), req 3 ), pad),
E G (k 1 )‖E G (k 2 )‖E G (k 3 )‖E G (k 4 )
N 5 → N 6 : E k5 (E k4 (E k3 (E k2 (E k1 (nonce, pad), pad), req 3 ), pad), pad),
E G (k 1 )‖E G (k 2 )‖E G (k 3 )‖E G (k 4 )‖E G (k 5 )
N 6 → N 7 : E k6 (E k5 (E k4 (E k3 (E k2 (E k1 (nonce, pad), pad), req 3 ), pad), pad), req 6 ),
E G (k 1 )‖E G (k 2 )‖E G (k 3 )‖E G (k 4 )‖E G (k 5 )‖E G (k 6 )
N 7 → G : E k6 (E k5 (E k4 (E k3 (E k2 (E k1 (nonce, pad), pad), req 3 ), pad), pad), req 6 ),
E G (k 1 )‖E G (k 2 )‖E G (k 3 )‖E G (k 4 )‖E G (k 5 )‖E G (k 6 )
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