Cryptographic communication system, communication device, key distribution device, and cryptographic communication method

A communication device comprising:

• a common key storing unit configured to store a common key used to communicate with another communication device, and an encrypted common key obtained by encrypting said common key by using an encryption key held by said other communication device;
• and
• a common key transmitting unit configured to transmit said encrypted common key to another communication device serving as a communication destination.

A communication device that stores a master key serving as an encryption key, comprising a common key obtaining unit configured to receive, from another communication device serving as a communication destination, an encrypted common key generated by encrypting a common key used during communication by using said master key, and obtaining said common key by decrypting said received encrypted common key by using said master key.

A key distribution device comprising:

• a key pair distributing unit configured to transmit, to a first communication device, a key pair constituted by a first key, which is a common key used by said first communication device to communicate with a second communication device, and a second key obtained by encrypting said first key by using a master key held by said second communication device,
• said key pair distributing unit transmits said key pair corresponding to a second communication devices existing within a predetermined distance from a location in which said first communication device receives said key pair.