Key distribution system

A key distribution system comprising:

• a terminal;
• and
• a server connected to said terminal by way of a communication network,
• wherein
• said terminal comprises:
• a first key storage unit configured to store a communication secret key in a rewritable manner;
• a first primitive root storage unit configured to store a predetermined primitive root;
• a first communication unit configured to establish encrypted communication with said server by use of a common key cryptography using the communication secret key stored in said first key storage unit;
• and
• a key request unit configured to, when requesting said server to issue new communication keys,
• generate newly a secret value defined as a numerical value, and
• generate a first public key based on the Diffie-Hellman key agreement protocol by use of the first secret value and the primitive root stored in said first primitive root storage unit, and
• send the first public key to said server,
• wherein
• said server comprises:
• a second key storage unit configured to store the communication secret key;
• a second primitive root storage unit configured to store the predetermined primitive root;
• a second communication unit configured to establish the encrypted communication with said terminal by use of the common key cryptography using the communication secret key stored in said second key storage unit;
• and
• a key issuance unit configured to
• generate newly a second secret value defined as a numerical value in response to a request of issuing new communication keys from said terminal, and
• generate a common key based on the Diffie-Hellman key agreement protocol by use of the first public key received from said terminal and the second secret value, and
• generate a second public key based on the Diffie-Hellman key agreement protocol by use of the second secret value and the primitive root stored in said second primitive root storage unit, and
• encrypt the new communication secret key with the common key based on the Diffie-Hellman key agreement protocol, and
• send the encrypted new communication secret key and the second public key to said terminal requesting issue of a new communication secret key, and
• wherein
• said key request unit is configured to, upon receiving the encrypted new communication secret key and the second public key from said server,
• generate the common key based on the Diffie-Hellman key agreement protocol by use of the newly generated first secret value and the second public key received from said server, and
• decrypt the encrypted new communication secret key with the generated common key based on the Diffie-Hellman key agreement protocol to obtain the new communication secret key, and
• store the new communication secret key in said first key storage unit,
• said first communication unit being configured to, after the new communication secret key is stored in said first key storage unit, use the new communication secret key for establishing the encrypted communication with said server by use of the common key cryptography, and
• said second communication unit being configured to, after the new communication secret key is stored in said second key storage unit, use the new communication secret key for establishing the encrypted communication with said terminal by use of the common key cryptography.