Patent ID: 8515060

Claim:
An encryption apparatus comprising: a symmetric key encryption unit adapted to calculate a ciphertext C 1 by putting a plaintext M and a symmetric key K into a symmetric key encryption function and to output the ciphertext C 1 ; a function operation unit adapted to calculate a function value of a collision-resistant function H by putting a value containing a value corresponding to the symmetric key K and a value corresponding to the ciphertext C 1 into the function H; and a vector operation unit adapted to calculate a second ciphertext C 2 =r·(Σ μ=1 n v μ ·b μ )+ρ·b n+1 ∈G 1 n+1 by using a value r corresponding to the function value of the function H, a value ρ corresponding to the symmetric key K, elements v μ (μ=1, . . . , n) (n≧1) of an n-dimensional vector v → =(v 1 , . . . , v n ), and n+1 dimensional basis vectors b i ∈G 1 n+1 (i=1, . . . , n+1) composed of n+1 elements of a cyclic group G 1 and to output the second ciphertext C 2 .