Patent ID: 8577032

Claim:
A common key block encryption device, comprising: a first hash unit that applies locked key permutation to a plaintext of a variable-length of s bit, and outputs a first block of a fixed-length of n bit and a second block of (s-n) bit; a first encryption processing unit that outputs a third block encrypted by means of n-bit block tweakable block cipher using tweak, receiving said first block as input; a second encryption processing unit that generates a random number block of (s-n) bit with a result of group computation of said third block and said first block as input by using an arbitrary cipher having theoretical security at least against a known-plaintext attack; a second hash unit that applies the locked key permutation to the result of the group computation of said random number block and said second block, and to said third block to output a fifth block of n bit and a sixth block of (s-n) bit; and an output unit outputs an s-bit encryption by concatenating said fifth and sixth blocks; wherein the length of n bit is selected such that the number of calls of the ciphers is m and a security indicator is calculated to be (mq) 2 /2 n where q is a number of times an attacker performs a chosen-ciphertext attack.