Common key block encryption device, common key block encryption method, and program

A common key block encryption method, comprising:

• when data is encrypted and written to storage, applying locked key permutation to a plaintext of a variable-length of s bit, and outputting a first block of a fixed-length of n bit and a second block of an (s-n) bit;
• outputting a third block encrypted by means of n-bit block tweakable block cipher using tweak, receiving said first block as input;
• generating a random number block of (s-n) bit, receiving 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;
• applying locked key permutation to the result of the group computation of said random number block and said second block, and to said third block, and outputting a fifth block of n bit and a sixth block of (s-n) bit;
• and
• concatenating said fifth and sixth blocks to form an s-bit ciphertext, and writing the s-bit ciphertext to said storage,
• wherein the length of n bit is selected such that a security indicator for a chosen-cipher attack, where the number of calls of the n-bit block tweakable block cipher and the arbitrary cipher is m and a number of times an attacker performs the chosen-ciphertext attack is q, is (mq)2/2n.

A non-transitory computer-readable storage medium storing a program for encrypting data and writing the result to storage, the program configured to cause a processing unit of a computer execute the steps of:

• encrypting by means of n-bit block tweakable block cipher using tweak, receiving a first block of an n bit obtained from a bit sequence obtained by applying locked key s-bit permutation to a plaintext of an s bit as input;
• applying locked key permutation to a plaintext of a variable-length of s bit, and outputting a first block of a fixed-length of n bit and a second block of (s-n) bit;
• outputting a third block encrypted by means of n-bit block tweakable block cipher using tweak, receiving said first block as input;
• generating a random number block of (s-n) bit with the 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;
• applying 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, and outputting a fifth block of n bit and a sixth block of (s-n) bit;
• and
• outputting an s-bit encryption by concatenating said fifth and sixth blocks, and writing the s-bit encryption to said storage,
• wherein the length of n bit is selected such that a security indicator for a chosen-cipher attack, where the number of calls of the n-bit block tweakable block cipher and the arbitrary cipher is m and a number of times an attacker performs the chosen-ciphertext attack is q, is (mq)2/2n.