ABSTRACT
Symmetrical cryptosystem is an important branch of modern cryptography, with
its appearance and development there are wide applicant background and theorial
value. There are lot theorial and applicant problems need to be studied and optimized,
such as: how to design a provable safe cryptosystem, how to strengthen the safty of
algorithms and working modules which are already available, how to test the safty of
a cipher algorithm, how to design safe components of a cryptosystem, as S-boxes,
diffusing layers, and key-expanding processes, etc.
The general architecture of symmetrical cryptosystem at present can be sorted as
Feistel (CAST-256, DEAL, DFC E2, etc.), SP network (Safer+, Serpent, etc.) and
other architectures (Frog, HPC). Symmetry is the most distinct character of Feistel,
while SP network has a good deffuse capability. AES inherited SQUARE in
designation, and added in a lot of matrix operations. This causes a bit different
between encryption and decryption, but it optimizes the efficiency of the algorism.
AES is a rapid progress in cryptosystem development, however, it needs to be
ameliorated yet. The efficiency of AES may be boosted, and new working module is
also necessary to be developed.
This paper introuduces the theory of semmetrical cryptography and the working
process of AES algorithm, improves a conventional means of increasing the
encrypting speed based on table-looking-up method, proposes its optimized algorism,
which can greately increase the encrypting/decrypting speed, or save on memory
space. Besides, a high-efficient method of generating S-box is proposed.
And, the working module of AES is also a hot point of researching at present. A
working module based on random block length is designed taking the advantage of
Rijndael that it has a obvious agility in the length of block and cipher key, and this
module can strengthen the safty of Rijndael algorithm. that completely crack this
module is a imporsable case at the applicant level of present is proved. This module is
also able to be composed with CFB, CBC, and other available modules, strengthening
safty and remaining aviable merits. And, other characters of this module are analysed
in this paper, based this, some advices of applying it are proprsed.
Keywords: symmetrical cryptography, Rijndael, finite field, working module