[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

252 K. Daniec et al.
by the ability to generate arbitrarily long string of blocks (keystream). In order to
generate it, it is required a key and a nonce value (the value for a single call). The
resultant sequence of blocks is used to convert non-confidential data to an encrypted
form by a simple transformation of the XOR between the plaintext block and the
block of keystream. In the case of a length not a multiple of the block, one can leave
the appropriate number of bytes in the stream key, because it does not interfere with
the possibility to decrypt the data. Due to the advantages of CTR: No need for a decoding
function, no need for padding, and a large freedom in the implementation, it
was decided that other modes are no needed.
Due to the symmetry of the encoding and decoding operations in the CTR
mode a single crypto_encproc function was implemented in the hardware. The
function parameters are block of data of any length, the encryption key, the size of
the header and the unique value NOONCE. Inside the function crypto_encproc a
key stream is generated. The input to the cipher are 16-byte blocks in which the
first 4 bytes of a given nonce value stored in the byte order from the youngest,
then it is from 0 to 8 bytes of the packet header, completed to 8 bytes of zeros
followed by four bytes describing the shift of the first byte in the packet, which
will be covered by the block. By design, the header is no longer than 8 bytes. All
cryptographic keys used by the hardware are 256 bits (32 bytes).
The second most important function in the whole system is a hash function. It is
necessary because of the need to generate and verify the HMAC authorizing the
values for all packages, and for the separation of certain parts of the system.
The hash function used was SHA-256. It maps a block of data of any length in
a short fixed length of 256 bits (32 bytes). Function selection was dictated by the
current state of knowledge about the safety of different hash function, the size of
the hash has been chosen to equal the length of the key in the encryption algorithm,
making it possible to easily implement certain procedures for the compilation
of the connection between the devices.
The primary use of the hash function is to determine the value of authorizing
for packages. The authorization allows to detect falsification of data and prevents
an attacker to set own packages without knowing the correct keys.
Computation of the value of an authorizing mechanism was implemented within
the function crypto_authproc:
HMAC (key, nonce, msg): = H (key | | H (key | | nonce | | msg) | | NOONCE)
Where H () represents a single calculation using the hash function, | | means concatenation
of data blocks. Using the key in the HMAC function ensures that the
attacker is not able to generate the correct value of an authorizing, because they do
not have complete information required to calculate the correct hash function
value.
Dual use of hash function is to eliminate certain defects, which derives from the
hash function SHA-256. In case of single call, the attacker would have not access
to the authorizing value, but he would be able to generate the correct value for the
extended package on the basis of acquired basic package. In order to prevent this,
the result of the internal hash function is additionally mixed with a key. This