Communication apparatus, communication system, and computer program product

A communication system including a plurality of communication apparatuses, each of the communication apparatuses comprising:

• a hardware processor that
• shares a first cryptographic key with another communication apparatus connected via a link, and further generates one or more second cryptographic keys and shares the second cryptographic keys with the other communication apparatus through secret communication using the first cryptographic key;
• generates an extended key by extending a length of one of the second cryptographic keys;
• transmits the extended key to a first application which cryptographically communicates with a second application using the extended key as a cryptographic key, the other communication apparatus providing the extended key to the second application;
• determines an extension method, which is executable in common with the other communication apparatus, from among a plurality of extension methods of the second cryptographic keys;
• transmits to the other communication apparatus, identification information identifying one of the second cryptographic keys that starts execution of the extension method;
• and
• generates the extended key by extending the length of the one of the second cryptographic keys according to the determined extension method, by starting the execution of the determined extension method to the one of the second cryptographic keys identified by the identification information,
• wherein the hardware processor transmits to the other communication apparatus extension method data including a priority order of the extension methods, receives from the other communication apparatus the extension method determined by the other communication apparatus based on the priority order from among the extension methods indicated by the extension method data, and determines the received extension method as the extension method which is executable in common with the other communication apparatus.

A computer program product comprising a non-transitory computer-readable medium containing a computer program that causes a computer execute:

• sharing a first cryptographic key with an external apparatus connected via a link, and further generating one or more a second cryptographic keys and sharing the second cryptographic keys with the external apparatus through secret communication using the first cryptographic key;
• generating an extended key by extending a length of one of the second cryptographic keys;
• transmitting the extended key to a first application which cryptographically communicates with a second application using the extended key as a cryptographic key the external apparatus providing the extended key to the second application;
• determining an extension method, which is executable in common with the external apparatus, from among a plurality of extension methods of the second cryptographic keys;
• transmitting to the external apparatus, identification information identifying one of the second cryptographic keys that starts execution of the extension method;
• and
• generating the extended key by extending the length of the one of the second cryptographic keys according to the determined extension method, by starting the execution of the determined extension method to the one of the second cryptographic keys identified by the identification information,
• wherein said transmitting to the external apparatus transmits extension method data including a priority order of the extension methods, receives from the external apparatus the extension method determined by the external apparatus based on the priority order from among the extension methods indicated by the extension method data, and determines the received extension method as the extension method which is executable in common with the external apparatus.