Mutual authentication/cipher key delivery system

Claim 1

A mutual authentication system in which a communication network and all its users have devices for implementing a common key cryptosystem; identifier ID i of a user named i is made public in said network; an authentication key K i of the user named i is known only to the network and the user named i; wherein said network obtains a value by processing, with a specific function, a random number generated by said each user for the authentication of said network and sent to said network and a random number generated by said network, encrypts said value, using said authentication key of said user concerned as a cipher key and sends the encrypted value to said user concerned; and wherein said network generates a data sequence for the authentication of each user and sends it to the user concerned, said user concerned decrypts it using his authentication key as a cipher key and processes the decrypted data with a specific function, processes a part of said processed data and a newly generated random number with another specific function, encrypts the processed value, using said authentication key of said user concerned as a cipher key and sends the encrypted data to said network, thereby making it impossible for a third party to make a chosen plaintext attack and a known plaintext attack on the encryption algorithm used.