Managing device keys in cryptographic communication

A method for managing a device key for cryptographic communication, and for performing cryptographic communication between a first communication apparatus and a second communication apparatus, comprising the steps of:

• storing first device keys shared with the second communication apparatus in a memory;
• generating a computed result as second device keys by performing a computation by passing the first device keys read from the memory to a keyed one-way function;
• and
• outputting the second device keys to the first communication apparatus in order to allow the first communication apparatus and the second communication apparatus to make the cryptographic communication with each other by using second device keys, and outputting the key of the one-way function to the first communication apparatus in order for the second communication apparatus to receive the key, wherein the first device keys are stored in a managing apparatus;
• the managing apparatus comprising:
• a key storing section that stores a first device key shared with the second communication apparatus;
• a key generating section that performs a computation by passing the first device key read from the key storing section to a keyed one-way function to generate a computed result as a second device key;
• a key outputting section that outputs the second device key and a key of the one-way function to the first communication apparatus;
• the first communication apparatus comprising:
• a key generation controlling section that transmits to the second communication apparatus, the key of the one-way function received from the managing apparatus, and which thereby causes the second communication apparatus to perform a computation by passing the stored first device key to the keyed one-way function, and thus to generate a second device key;
• and
• a cryptographic communication section that performs the cryptographic communication with the second communication apparatus by using the second device key received from the managing apparatus;
• in order to selectively enable at least one of a plurality of second communication apparatus to decrypt encrypted data broadcasted to the plurality of second communication apparatus, the key storing section stores a plurality of first device keys, each shared by the managing apparatus and the at least one of the second communication apparatus;
• the key generating section receives an input specifying at least one of the plurality of second communication apparatus to be enabled to decrypt encrypted data broadcasted by the first communication apparatus, reads the first device keys, each shared with the specified second communication apparatus from the key storing section, and generates, as second device keys, a computed result by performing a computation by passing each of the first device keys to the keyed one-way function;
• the key generation controlling section broadcasts to the plurality of second communication apparatus keys of the one-way function received from the managing apparatus;
• the cryptographic communication section performs cryptographic communication with the specified second communication apparatus, by encrypting and broadcasting data by using the second device keys received from the managing apparatus;
• the key storing section stores the plurality of first device keys in association with different nodes of a multiway tree structure data one to one;
• leaf nodes of the multiway tree structure data are associated with the second communication apparatus one to one;
• first device keys delivered in advance to each of the second communication apparatus, said first device keys corresponding to nodes in a path from the leaf nodes corresponding to the second communication apparatus to the root node;
• the key generating section receives an input specifying a group of second communication apparatus to be enabled to decrypt encrypted data broadcasted by the first communication apparatus, reads, from the key storing section, each of the first device keys in subtrees including the specified group in the multiway tree, performs a computation by passing each of the first device keys to the keyed-one way function to generate a computed result as a corresponding second device key;
• the key outputting section outputs each of the second device keys and the key of the one-way function to the first communication apparatus;
• the key generation controlling section transmits the key of the one-way function received from the managing apparatus to the specified group, and causes each of the second communication apparatus in the group to perform a computation by passing the stored first device key to the keyed one-way function to generate the second device key;
• the cryptographic communication section encrypts data by using at least one of the plurality of second device keys received from the managing apparatus, and thereby performs cryptographic communication with at least one of the second communication apparatus in the specified group;
• the cryptographic communication section generates post-encrypted device keys by encrypting device keys used for encrypting data with second device keys generated from first device keys, and then broadcasts the post encrypted device attached to the encrypted data, the first device keys having already been delivered to at least one of the second communication apparatus to be enabled to decrypt the data, but having not been delivered to the other second communication apparatus;
• the key generation controlling section further attaches the key of the one-way function received from the managing apparatus to the data to be broadcasted by the cryptographic communication section, and then transmits the key and the data;
• the managing apparatus further comprising a leakage managing section which receives an input indicating one of the plurality of first device keys that has become unusable as a result of leakage thereof to a third party, and which notifies the first communication apparatus that the second device key generated from the indicated first device key by the key generating section is invalidated;
• a plurality of first communication apparatus, wherein
• the first communication apparatus receiving the second device key further comprises:
• a key generating section for performing a computation by passing the second device key received from the managing apparatus to a keyed one-way function to generate a computed result as a third device key;
• a key outputting section for outputting, to a different first communication apparatus, the third device key, the key of the one-way function received from the managing apparatus, and the key of the one-way function that the first communication apparatus has used for the computation performed by the key generating section;
• in the different first communication apparatus receiving the third device key;
• the key generation controlling section transmits the received two keys of the respective one-way functions to the second communication apparatus, and causes the second communication apparatus to generate a computed result as a third device key by performing a computation in which the stored first device key is passed to the two one-way functions with the respective two keys sequentially;
• the cryptographic communication section performs cryptographic communication with the second communication apparatus by using the received third device key, and
• the first device keys are the device key managed in the second communication apparatus so as not to be readable and rewritable from outside.