Patent ID: 11765137

Abstract:
The present invention makes it possible to improve confidentiality. A communication terminal stores a session key shared with other communication terminals (S3). The communication terminal makes a pair of an index generated by using the session key for a character string relating to a message and a message identifier, and transmits it to a server apparatus (S4). The server apparatus stores the pair of the index and the message identifier (S5). The communication terminal generates, when the session key is updated, a re-encryption key with a session key before update and a session key after update (S8). The server apparatus updates, by using the re-encryption key, the stored index to an index generated by using the session key after update (S9). The communication terminal encrypts a search keyword with the session key to generate a search query (S10). The server apparatus extracts a message identifier of which the index matches the search query (S11).