Study on Highly Secure and Efficient Symmetric Key Cryptosyste

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We obtain the following main results.・We propose a construction of a tweakable block cipher whose block length is shorter or longer than 2n bits, from an n-bit tweakable block cipher.・ We propose constructions of a permutation layer for Type-1 generalized Feistel structures so that they have a good diffusion property.・We show that EAX-prime allows various efficient attacks. We also prove its security if the input data called a cleartext is always longer than the block length of theunderlying block cipher.・We show that the original security proof of GCM contains a flaw. We also show that the flaw can be fixed and therefore GCM maintains its provable security.