Patent attributes
A communication terminal shares a session key with and sends cipher text to another communication terminal via a server device, including: a common key cipher text obtaining unit that encrypts a message based on a common key to obtain common key cipher text; a function computation result obtaining unit that computes the common key and the session key based on a predetermined first function to obtain a function computation result; a public key cipher text obtaining unit that encrypts the function computation result based on a public key to obtain public key cipher text; and a cipher text sending unit that sends the common key cipher text and the public key cipher text to the server device. The communication terminal can update data previously saved in a server to data that can be decrypted on the communication terminal side using an updated session key, without the server decrypting the data.
