Key agreement system, shared-key generation apparatus, and shared-key recovery apparatus

摘要

Provided is a content distribution system that prevents different keys to be derived between an encryption apparatus and a decryption apparatus. A random-number generating unit 112, in an encryption apparatus 110, generates a random number s. A first function unit 113 generates a functional value G(s) of the random number s, and generates a random-number value u and a shared key K from the functional value G(s). An encryption unit 114 generates a first cipher text c1 of the random number s, using a public-key polynomial h and the random-number value u. A decryption unit 123, in a decryption apparatus 120, decrypts the first cipher text c1 using a secret-key polynomial f, to generate a decryption random number s′ A second function unit 126 generates a functional value G(s′) of the decryption random number s′, and generates a random-number value u′ and a shared key K′from the functional value G(s′ ). A comparison unit 127 generates a first re-cipher text c1′, using the random-number value u′ and the shared key K′, and outputs the shared key K′ if the first cipher text c1 is equal to the first re-cipher text c1′.