In order to perform cipher communication in a secret key cipher system, both parties which make communication are required to hold in common a key in advance. Hitherto, sharing of the key has been performed by a system as shown in FIG. 11 for example. Referring to FIG. 11, numeral 41 designates a coder which is used by a sending party of a message (hereinafter is simply referred to as a sending party), and 42 designates a decoder which is used by a receiving party of the message (hereinafter is simply referred to as a receiving party), and the coder 41 comprises a random number generating means 43, a first enciphering means 44 and a second enciphering means 45, and the decoder 42 comprises a first decoding means 46 and a second decoding means 47.
Operation of the conventional common ownership system of the key composed like this is described hereafter. Since the key for enciphering a plain text of a message (hereinafter is simply referred to as a plain text) m is required to be changed frequently from the aspect of safety, a random number which is generated by the random number generating means 43 is used. Hereinafter, this is described as a session key. The sending party sends an output r1 (it is called a session key as another name) of the random number generating means 43 which is held in the coder 41 to the receiving party in order to hold in common with the receiving party; but if r1 is sent as the sate of raw data it is liable to be tapped on the communication line between the coder 41 and the decoder 42, and hence 41 is enciphered by the enciphering means 44 and is sent. The key which is used to encipher r1 is called as a master key km, and it is the key which is held in common by the sensing party and receiving party in advance. The master key is used only when the session key is sent by enciphering, and is generally fixed during a long time period. The decoder 42 of the receiving party decodes the enciphered random number by the master key km and restores r1 by using the first decoding means 46. Thereby, since the sending party and receiving party have held in common the session key r1, thereafter, cipher communication of the plain text m can be accomplished by the session key r1 by using the second enciphering means 45 and the second decoding means 47. In the event that the session key is changed, a new random number r2 (not shown) is generated by the random number generating means 43; and in the same manner as described above, r2 is held in common by the cipher communication by means of the master key km and is made to the session key.
Now, the case of cipher communication between two communication parties A and B is assumed. The communication party A and the communication party B encipher the session key r1 by using the master key km and send. When another communication party C holds the master key km, all cipher text which is exchanged between the communication parties A and B can be deciphered since the communication party C can decipher the session key r1. Therefore, it is required that the master key km is known by only the communication party A and the communication party B, and in the event that, for example, the communication party A carries out cipher communication with the communication party C, a master key which is other than km is used. Namely, the communication party A must hold the master keys which are identical with the number of the parties which carry out the cipher communication.
In such conventional system, there is no problem in the event of a small number of parties are to be communicated; but in a network having unspecified many subscribers, communication parties to be communicated become large number, and management of the key becomes a big problem. As a means for solving it, the method in which a center for performing management of the key is provided, and prior to prosecution of the cipher communication, the center delivers (or transmit) the common session key to both the parties, is generalized, but there is a defect that the center must intervene in every common holding operation of the key. On the other hand, as other solution, there is a method using the public key cipher system which is superior in management of the key, but the public key cipher system entails a much longer processing time in comparison with the secret key cipher system. As mentioned above, in the conventional cipher communication method, a big problem has existed with respect to the management of the key.
In view of such a problem, the present invention is directed to provide a data carrier which is safely, easily and speedy realizable common ownership of the key, on the basis of the futures in which an internal data is physically safety and calculation ability exist, and a data communication apparatus using it.