PURPOSE: To devise the system such that a transmission station sends information being combination of identification information such as a name of a user of an opposite station, secret information of own station user, public information resulting from coding a communication opposite party and a random number to the opposite station and a reception station obtains the identification information to verify the transmission station and generates a cryptographic key in common between both the stations by using the secret information of the own station user.

CONSTITUTION: A user A receives public information of a user B from an opposite public information input terminal 5 and uses it and a random number generated by a random number generator 6 to apply arithmetic processing at an arithmetic circuit 7. Moreover, secret information stored in a secret information memory 1 in the arithmetic circuit 8 is generated by a time stamp publicating device 9 and a stamp through a unidirectional function device 10 is used to apply arithmetic processing, and an arithmetic circuit 11 uses a value stored in a memory 4 to apply arithmetic processing. Then the value obtained in this way is coupled by using a coupler 12 and the result is sent to the user B from a communication text output terminal 18. Moreover, when the user A makes communication to the user C, a cryptographic key in common between the stations is similarly obtained.