A server device includes a memory and a processor to execute saving an encrypted text of a message generated by encrypting the message with a message key generated from a shared key and a group identifier, by using a message key identifier as an identifier of the message key as a key; saving an encrypted text of the message key generated by encrypting the message key to be re-encryptable by using the shared key, by using the message key identifier as a key; and receiving, after update of the shared key, a re-encryption key from a communication terminal that has generated the re-encryption key by using the shared key and the updated shared key, using the re-encryption key to generate a re-encrypted encrypted text of the message key, and overwriting the encrypted text of the message key before re-encryption with the re-encrypted encrypted text of the message key.