The symmetric block cipher, which encrypts plain data into an unreadable ciphertext at high speed, is a key component of the information technology society. The Data Encryption Standard (DES), the U.S. federal standard cipher, has been the de facto standard so far. However, the security of DES is no longer adequate because of the rapid improvement of CPU power. The need for 128-bit block ciphers for the next generation has become widely accepted, and the trend of standardization is now active. To meet this requirement, we have designed a new symmetric block cipher family called Hierocrypt, which is characterized by the nested substitution permutation network (SPN) structure. The nested SPN structure is very simple, and guarantees high security against differential attacks and linear attacks, which are efficient cryptanalytical methods. Furthermore, the Hierocrypt cipher is compact yet it very rapidly encrypts almost all implementations on both software and hardware. We have proposed the Hierocrypt cipher in some standardization projects, and plan to make it widely available for use in many fields such as middleware and smart cards.
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