Patent ID: 11902421
Assignee: NIPPON TELEGRAPH AND TELEPHONE CORPORATION
Field: Digital communication (Electrical engineering)
Classification: CPC H | IPC H

Claim 3:
4. A communication terminal used in a communication system that executes transmission and reception of messages via a server device among a plurality of communication terminals forming a group to which a group identifier is assigned, while concealing the messages to the server device, the communication terminal comprising:
a memory, and
a processor configured to execute operations comprising:
generating a message key from a shared key shared among the plurality of communication terminals belonging to the group while excluding the server device and the group identifier, and the message key is to be re-encryptable by using the shared key;
generating an encrypted text of the message key by encrypting the message key to be re-encryptable, by using the shared key;
generating a message key identifier as an identifier of the message key;
generating an encrypted text of a message by encrypting the message with the message key; and
transmitting the encrypted text of the message key, the message key identifier, and the encrypted text of the message to the server device, wherein the encrypted text is to be re-encryptable by overwriting the encrypted text by the server device using a re-encryption key.