The present invention relates to data encryption, and more particularly to the improvements in processing efficiency of encryption and cipher strength to any cryptanalysis. Furthermore, the present invention relates to the encryption involving data compression and more particularly to the improvements in processing efficiency of data compression and encryption and strength to cryptanalysis.
With increase of the computerized central information of a system and the data communication through network, importance is now being placed on a technique of encrypting data for keeping the computerized data from being tapped and tampered. As describes in pages 27 to 32 of "Introduction to Cryptography Theory" Kyoritu edit., 1993, the encryption is roughly divided into a symmetric key cryptosystem and an asymmetric key cryptosystem. The present invention is intended for the improvement in symmetric cryptosystem which is suitable for encrypting a large amount of data. Later, a secret key cryptosystem is simply called cryptosystem.
At first, the description will be oriented to the basic terms about the cryptosystem. As is described in pages 33 to 59 of the foregoing writing, the cryptosystem is executed to convert plaintext into ciphertext through secret parameters. The decryptosystem is executed to transform the ciphertext into the original plaintext through the effect of reverse transform with the same secret parameters as those used in the cryptosystem. The secret parameters are generally called a crypt-key (or just a key). The encrypting procedure is composed of repetition of one or more kinds of fundamental functions. The repetitive times are called rounds. In applying the encrypting procedure, the input data is divided into parts each of which has the same size and the encrypting procedure is applied to each data part. Each data part is called a crypt-block (or just a block).
In designing and promoting the encryption, an important factor is a defense for various kinds of decrypting methods. The most frequently used decrypting method is an extensive search for keys. In recent days, however, remarks are placed on more efficient differential cryptanalysis and linear cryptanalysis than the extensive search.
In the pages 163 to 166 of the aforementioned writing and the linear cryptanalysis of the DES (Data Encryption Standard) published in "The 1993 Symposium on Cryptography and Information Security", the differential and the linear cryptanalyses utilize the correlation among the plaintext, the ciphertext, and the keys, which are proper to the encrypting system, and is executed to collect lots of inputs and outputs (plaintext and ciphertext) to be encrypted or decrypted by the same key and perform the statistical operation about these inputs and outputs for estimating the key.
The conventional method for defending the differential or linear cryptanalysis in the conventional encrypting system is executed to reduce the correlation among the plaintext, the ciphertext, and the key by increasing the rounds.