[Studies in Computational Intelligence 481] Artur Babiarz, Robert Bieda, Karol Jędrasiak, Aleksander Nawrat (auth.), Aleksander Nawrat, Zygmunt Kuś (eds.) - Vision Based Systemsfor UAV Applications (2013, Sprin

The Dedicated Wireless Communication Device for Group of Unmanned Vehicles 253
removes the security gap resulting from the use of an iterative hash functions. The
HMAC value is calculated for each package separately and sent along with it.
In order to assure safety of keys exchange between the devices it was decide to
use the Diffie-Hellman key-exchange protocol. It allows to establish a common
secret between two devices with the use of an unprotected channel. In order to run
the DH protocol it is required to select group used for executing operations of
exponentiation of large numbers. During experiments nine numbers were chosen.
Each of them is a safe prime number p. It means that both p and number p 2 computed
by the equation:
1
2 , (1)
are prime numbers.
6 Data Encryption
The communication protocol consists of two main parts: establishing and maintaining
the connection. It is assumed that the addresses of the devices are in the
range from 0 to 16 383 (within 14 bits).
6.1 Establishing Connection
The process of establishing connection was divided into two phases. During first
phase the device initiating the communication sends the PING packet to the receiver
which replies with the PONG packet. Both packets are time and quantity
secured by using counters of number of starts and time of execution. After receiving
the PONG packet, the sender is assured about the existence of the receiver.
Then the synchronizing SYN packet is send. The last 16 bytes of the SYN packet
contains random value used as an aid during generating random temp keys. The
response for the synchronizing packet is SYNACK with the same structure as the
SYN packet. After exchanging those two packets both devices generate common
temporary key using the function:
tk := SHA256(AddrA || RunCntA || RndA || AddB ||
RunCntB || RndB || AuthKey)
6.2 Maintaining the Connection
During proper communication between the devices a smaller packet is used. The
header’s length is 4 bytes and authorization value’s length is 8 bytes. Transferred
data is encoded by estimated channel. Its number is in the range from 0 to 32767
and is written in first two bytes of the data. Third and fourth bytes contain the
sequential number of the packet. Due to the low throughput of the transmission
there are no other protocol limitations.