Server device, communication terminal, communication system, and program

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.

2. The server device as claimed in claim 1 , wherein the message key is a key generated by inputting the shared key and the group identifier into a twisted pseudorandom function or a pseudorandom function.

3. The server device according to claim 1 , wherein the server device includes a non-transitory computer-readable recording medium having computer-readable instructions stored thereon, which when executed, causes a computer to function as the server device as claimed in claim 1 .

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.

5. The communication terminal as claimed in claim 4 , further comprising:

generating, after the shared key has been updated, the re-encryption key by using the shared key and the updated shared key,

wherein the transmitting transmits the re-encryption key to the server device.

6. The communication terminal according to claim 4 , wherein the communication terminal includes a non-transitory computer-readable recording medium having computer-readable instructions stored thereon, which when executed, causes a computer to function as the communication terminal as claimed in claim 4 .

7. 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,

wherein each of the plurality of communication terminals belonging to the group includes

a memory, and

a processor configured to execute

generating a message key from a shared key shared among the plurality of communication terminals belonging to the group, and the group identifier,

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 server device includes

a memory, and

a processor configured to execute

saving, in the memory, the encrypted text of the message by using the message key identifier as a key,

saving, in the memory, the encrypted text of the message key by using the message key identifier as a 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 encrypts the message kay to be re-encryptable by using the shared key,

transmitting the encrypted text of the message key and the encrypted text of the message to a communication terminal belonging to the group, 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 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 and to store in the memory.