The value of the information came to be reviewed by an explosive spread
of the Internet in recent years.
Therefore, the problem concerning information security like tapping, the
falsification, and the unlawful computer access etc. of information has
become important in the network society of today.
The cipher is known as a technology that does the basis in
information security.
The cipher is a technology that makes information to a scramble, and can
prevent the leak to others.
There is a symmetric key
block cipher in one of such the encryption technologies.
The symmetric key block cipher is a cipher to which it is composed
of simple operations of exclusiveOR, the additive, the subtraction,
multiplication, and the cyclic shift, etc , and data is stirred with
the common key by repeating a simple encryption function.
The symmetric key block cipher computes at very compact and high speed
compared with the public key cryptosystem based on the number theory .
Therefore, the symmetric key block cipher is used with the equipment
with lowspeed CPU such as IC cards and mobile phones
and to encrypt a large amount of data.
The security of the symmetric key block cipher is decided by the number
of rounds that can be attacked more efficiently than the exhaustive
search.
``Attack to n rounds possible'' means that it is possible to cracked up
to n rounds earlier than the exhaustive search.
In this paper, we evaluate the security of the algorithm that is
called RC6
in the symmetric key block cipher from which various algorithms are
proposed.
RC6 was a symmetric key block cipher proposed by Rivest, and
it was one
of the final candidates of AES(Advanced Encryption Standard) selected by
NIST（National Institute of Standards and Technology).
Since RC6 was a suitable algorithm for the software implementation that
was composed by using the arithmetic operation and the bit shift , it
was fastest in the software implementation in the candidate of AES.
In general, RC6 that is composed of w words, r rounds, and b bits is
described
RC6w/r/b. Moreover, it is a parameter from which r=20, w=32, and
b=128, 192, 256 are recommended in RC6. RC632 is only described RC6.
Knudsen and Meier showed that the chisquare attack using a statistical
weakness was especially effective in various researches about the
safety of RC6. Afterwards, a lot of papers that used the chisquare
attack were suggested in RC6.
Knudsen and Meier guessed the round key by fixing the data rotation
shift
of F function of the first round to 0. Shimoyama at
el showed a theoretical analysis the Distinguish attack by the
chisquare attack. Matunaka showed the method of theoretically deriving
the success probability of the Key Recovery attack from the consequence
of the Disinguish attack. The success probability of the chisquare
attack can be theoretically derived from these consequences. Moreover,
they guessed the key from the final round by the chisquare value when
decrypting by one round in RC6P. In addition, Takano improved the Key
Recovery algorithm in RC6P by fixing the data rotation shift of the
final stage to 0, and applied to RC6. As a result, He showed that RC6
was able to be cracked from the exhaustive search efficiently up to 16
rounds.
A basic idea in a current research is to make the output biased fixing
the data rotation shift to 0. However, there is a problem that a
computational complexity necessary for the attack in this idea
increases. Because both of round keys (S[2r+2] and S[2r+3])of the final
round are guessed at
the same time, the computational complexity and the memory are hugely
needed in a current research. Therefore, to reduce a computational
complexity and a memory necessary for the attack, we have improved
the algorithm of the chisquare attack.
The computational complexity and the memory needed for the attack depend
on the key bit length. Then, we have reduced the computational
complexity
and the memory by guessing the round key guessed to the asymmetrical.
Moreover, the suggesting method greatly takes sets of keys
such that the data rotation shift becomes 0, that is,
the computational complexity has been reduced with the coarsesieve with
a key.
In addition, it was confirmed to be able to do the key recovery attack
even by the asymmetrical though the test bit had to be symmetry in an
existing algorithm.
