Method for hiding the identity of communication devices in a continuous group key agreement

A communication device for use in a method for hiding the identity of two or more communication devices in a group communicating using a continuous group key agreement, wherein the two or more communication devices may communicate with each other within the group via a server, wherein the communication device is configured to perform a continuous group key agreement protocol comprising:

• generating one or more proposals for updating a group state and causing the one or more proposals to be stored on the server;
• committing a function of zero or more of the one or more proposals stored on the server, wherein committing the function of zero or more proposals includes updating a group state stored at the communication device based on the committed function of zero or more proposals and, as a result of committing the function of zero or more proposals, sending a commit related to the function of zero or more proposals for storage on the server, wherein committing the function of zero or more proposals starts a new epoch for the group;
• downloading a commit and updating the group state based on the commit;
• and
• determining a group secret for the new epoch based on a downloaded commit to allow the communication device to generate a common secret key for the epoch;
• the communication device being further configured to perform a method for hiding the identity of the two or more communications devices comprising:
• sending a signed challenge value to the server in connection with a request for an operation, the signed challenge value having been generated by signing a challenge value with a group signature key, wherein the requested operation comprises one of:
• storing one or more proposals at the server, storing a commit relating to the function of zero or more proposals at the server, and downloading the commit, whereby the server can check the signed challenge value using a group verification key stored at the server and the challenge value, and the server can perform the requested operation if the signed challenge value shows that the communication device is in possession of the group signature key and not perform the requested operation if the signed challenge value does not confirm that the communication device is in possession of the group signature key.

A non-transitory computer-readable storage medium storing one or more computer programs that, when executed by two or more communication devices and a server configured so that the two or more communication devices communicate with each other within a group via the server, causes the communication devices and the server to perform a method for hiding the identity of two or more communication devices in a group communicating using a continuous group key agreement, wherein the continuous group key agreement protocol comprises:

• generating, by one or more of the communication devices of the group, one or more proposals for updating a group state and causing the one or more proposals to be stored on the server;
• committing, by a committing communication device of the group, a function of zero or more of the one or more proposals stored on the server, wherein the committing communication device updates a group state stored at the committing communication device based on the committed function of zero or more proposals and, as a result of committing the function of zero or more proposals, a commit relating to the function of zero or more proposals is stored on the server, and wherein committing the function of zero or more proposals starts a new epoch for the group;
• downloading, by one or more downloading communication devices that are members of the group other than the committing communication device, the commit and updating a group state at each of the one or more downloading communications device based on the commit;
• and
• determining, by each of the one or more downloading communication devices, a group secret for the new epoch based on the downloaded commit to allow the one or more downloading communication devices and the committing communication device within the group to generate a common secret key for the epoch;
• the method for hiding the identity of the two or more communications devices comprising:
• a requesting communication device sending a signed challenge value to the server in connection with a request for an operation, the signed challenge value having been generated by signing a challenge value with a group signature key, wherein the requested operation comprises one of:
• storing one or more proposals at the server, storing a commit relating to the function of zero or more proposals at the server, and downloading the commit;
• the server checking the signed challenge value using a group verification key stored at the server and the challenge value;
• and
• the server performing the requested operation if the signed challenge value shows that the requesting communication device is in possession of the group signature key and does not perform the requested operation if the signed challenge value does not confirm that the requesting communication device is in possession of the group signature key.