"A study on Cryptanalysis Regarding Block Chipers"

Information security is one of inevitable features to be established in our network society. Cryptography and other security technologies are essential to realize the reliable global network society. The technologies of a secrecy, a guarantee, and so on of the contents have been brought especially to public attention.

Symmetric block ciphers are mentioned as one of the important technology which realize this.However, the computer throughput becomes remarkably high recently. And some analytic methods were proposed against known used algorithms. Therefore many known cryptosystems could not keep the high security.

In 1997, the National Institute of Standards and Technology (NIST) advertised a competition. Out of the 15 original candidates that were discussed in 1998, ten were picked out till 1999. The remaining algorithms were: MARS by IBM, RC6 by RSA Laboratories, Rijndael by Joan Daemen and Vincent Rijmen, Serpent by Ross Anderson, Eli Biham and Lars Knudsen as well as Twofish by Bruce Schneier. In October 2000, the winner was definitive: The algorithm Rijndael by Joan Daemen and Vincent Rijmen from Belgium was established as the future Advanced Encryption Standard. Presently, the NESSIE project selection and the CRYPTREC project selection are held in European Union and Japan, respectively.

In our researches, we evaluate the security of RC5 and RC6 block ciphers by using chi-square attacks.RC5 is a symmetric-key block cipher proposed by R. Rivest in 1994. RC6 which is a block cipher designed by Rivest et al.$ in 1998 is the next version of RC5. This block cipher has a wonderful capability for performing high-speed software implementation especially on Intel processors (e.g., Penteium III). Furthermore, we investigate the theoretical relation between the distinguishing attack and the key recovery attack, and prove one theorem to evaluate the exact security against the key recovery attack by using the results of the distinguishing attack.

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