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

Claim 0:
1. A server device used in a communication system that executes transmission and reception of messages via the 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 server device comprising:
a memory, and
a processor configured to execute operations comprising:
saving, in the memory, an encrypted text of a message generated by encrypting the message with a message key generated from a shared key shared among the plurality of communication terminals belonging to the group while excluding the server device and the group identifier, by using a message key identifier as an identifier of the message key as a key;
saving, in the memory, an encrypted text of the message key using the message key identifier as an index key, wherein the encrypted text of the message key is generated by a first communication terminal of the plurality of communication terminals, and the first communication terminal encrypting the message key to be re-encryptable by using the shared key; and
receiving, after the shared key has been updated by a second communication terminal of the plurality of communication terminals, a re-encryption key from a third communication terminal of the plurality of communication terminals, wherein the second communication terminal has generated the re-encryption key by using the shared key and the updated shared key;
generating, by the processor, a re-encrypted encrypted text of the message key using the re-encryption key, to overwrite the encrypted text of the message key with the re-encrypted encrypted text of the message key to be saved, in the memory.