ENCRYPTION APPARATUS, DECRYPTION APPARATUS, ENCRYPTION METHOD, DECRYPTION METHOD, SECURITY METHOD, PROGRAM, AND RECORDING MEDIUM

摘要

Security against CCA is improved without providing space for just improving the security against CCA in a ciphertext space. In encryption processing, a first ciphertext C 1 is calculated from a plaintext M and a symmetric key K; a value containing a value corresponding to the symmetric key K and a value corresponding to the first ciphertext C 1 is put into a collision-resistant function H to calculate a function value of the function H; and r corresponding to the function value is used to calculate second ciphertext C 2 = r·(£ µ=1 n v µ ·b µ ) + Á·b n+1 ˆˆ G 1 n+1 . In decryption processing, the second ciphertext C 2 ˆˆ G 1 n+1 and key information D 1 * = ±·(£ µ=1 n w µ ·b µ * ) + b n+1 * ˆˆ G 2 n+1 are put into a bilinear function e to calculate a function value S ˆ¼ = e(C 2 , D 1 * ) ˆˆ G T ; a value corresponding to the function value S ˆ¼ and the first ciphertext C 1 are put into the collision-resistant function H to calculate a function value of the function H; and it is judged whether r ˆ¼ corresponding to the function value satisfies C 2 = r ˆ¼ ·(£ µ=1 n v µ ·b µ ) + Á ˆ¼ ·b n+1 ˆˆ G 1 n+1 .