대칭단을 이용한 암호와 복호가 다른 블록 암호의 재설계

Abstract

The hardware implementation of block cipher algorithm is closely related to the structure of a cipher algorithm. Because Feistel structure has same encryption and decryption algorithm, only one of them is required to process both encryption and decryption, when implemented by hardware. However, since SPN structure has different encryption and decryption algorithm, both algorithms have to be implemented by hardware when both are needed. Therefore the block cipher algorithm, which has different encryption and decryption, has the significant disadvantage that hardware area increases, compared to the one, which has same encryption and decryption algorithm. In this paper, in order to improve the block cipher which has different encryption and decryption algorithm, it has been composed of same encryption and decryption, using symmetric layer consisting of simple logical operations. The whole algorithm is organized with R round. A function (encryption algorithm) is used from first round to ((R/2) - 1)round and the inversed function (decryption algorithm) is used from (R/2) + 1 round to R round. Moreover, an irregular symmetric block, an independent round symmetric formed-layer, is inserted into the middle of the function and the inversed function to avoid the repetition of regular rounds. The block ciphers which have different encryption and decryption algorithm such as AES, RC6, SHACAL-1, and SHACAL-2, have been recomposed by software, adapting symmetric layer. The symmetric adapted-block ciphers are about 10% slower than original ones, and it is estimated that the adapted symmetric layers don’t have a significant effect on encryption execution time. The symmetric layer can be easily applied to not only 128 bits block cipher but also 160 and 256 bits block cipher algorithms. It means that the symmetric layer can be applied to all block cipher algorithms which have different encryption and decryption. In the aspect of security of symmetric layer adapted block algorithms, the symmetric layer prevents from the attacks such as Differential, Linear, Square, Impossible Differential, Boomerang, Related-key Slide, Related-key Differential-Nonlinear, Related-key Rectangle. It shows that symmetric layer is very efficient in the improvement of block cipher security. Like this, the reimplementation which applies characteristic about execution time and security of symmetric layer can be an efficient idea that is able to be easily adapted to a new block cipher algorithm design.