Cryptographic communication method and cryptographic communication device

Claim 2

A cryptographic communication method for carrying out cryptographic communication to transmit a message M from a sending station to a receiving station through a communication line by utilizing public information P2 of the receiving station, secret information S1 of the sending station, an identification number 101 of the sending station, and a secret key d2 of the receiving station, comprising the steps of; obtaining the public information P2 in the sending station; generating a first random number r in the sending station; generating a second random number R according to a prescribed procedure by utilizing both the public information P2 and the first random number r in the sending station; generating a third random number X according to a prescribed procedure by utilizing the second random number R in the sending station: generating a cryptographic key K according to a prescribed procedure by utilizing the first random number r in the sending station: producing a ciphertext C from the message M by utilizing the cryptographic key K in the sending station; generating a first hashing result E by implementing a one-way hashing to hash both the ciphertext C and the third random number X in the sending station; generating key-distribution-information Y according to a prescribed procedure by utilizing the secret information S1, the second random number R, and the hashing result E in the sending station; sending the third random number X, the identification number ID1, the ciphertext C, and the key-distribution-information Y to the receiving station through the communication line: generating a second hashing result EO by implementing the one-way hashing to hash both the ciphertext C and the third random number X in the receiving station; restoring the cryptographic key K according to a prescribed procedure by utilizing both the secret key d2 and the third random number X in the receiving station; and obtaining the message M by decrypting the ciphertext C by utilizing the cryptographic key K in the receiving station.

Claim 3

A cryptographic communication method for carrying out cryptographic communication to transmit a message M from a sending station to a receiving station through a communication line by utilizing public information P2 of the receiving station, secret information S1 of the sending station, an identification number ID1 of the sending station, and a secret key d2 of the receiving station, comprising the steps of: obtaining the public information P2 in the sending station; generating a first random number r in the sending station; generating a second random number R according to a prescribed procedure by utilizing both the public information P2 and the first random number r in the sending station; generating a third random number X according to a prescribed procedure by utilizing the second random number R in the sending station: generating a cryptographic key K according to a prescribed procedure by utilizing the first random number r in the sending station; producing a ciphertext C from the message M by utilizing the cryptographic key K in the sending station; generating a first hashing result E by implementing a one-way hashing to hash both the ciphertext C and the third random number X in the sending station; generating key-distribution-information Y according to a prescribed procedure by utilizing the secret information S1, the second random number R, and the hashing result E in the sending station: sending the hashing result E, the identification number ID1, the ciphertext C, and the key-distribution-information Y to the receiving station through the communication line; generating a fourth random number XO according to a prescribed procedure by utilizing the hashing result E, the key-distribution-information Y, and the identification number ID1 in the receiving station: generating a second hashing result EO by implementing the one-way hashing to hash both the ciphertext C and fourth random number XO in the receiving station; restoring the cryptographic key K according to a prescribed procedure by utilizing both the secret key d2 and the fourth random number XO in the receiving station; and obtaining the message M by decrypting the ciphertext C by utilizing the cryptographic key K in the receiving station.

Claim 4

A cryptographic communication method for carrying out cryptographic communication to transmit a message M from a sending station to a receiving station through a communication line by utilizing public information P2 of the receiving station, secret information S1 of the sending station, an identification number ID1 of the sending station, and a secret key d2 of the receiving station, comprising the steps of: obtaining the public information P2 in the sending station; generating a first random number r in the sending station; generating a second random number R according to a prescribed procedure by utilizing both the public information P2 and the first random number r in the sending station; generating a third random number X according to a prescribed procedure by utilizing the second random number R in the sending station: generating a cryptographic key K according to a prescribed procedure by utilizing the first random number r in the sending station; producing a ciphertext C from the message M by utilizing the cryptographic key K in the sending station; generating a first hashing result E by implementing a one-way hashing to hash both the ciphertext C and the third random number X in the sending station; generating key-distribution-information Y according to a prescribed procedure by utilizing the secret information S1, the second random number R, and the hashing result E in the sending station: sending the hashing result E, the identification number ID1, the ciphertext C, and the key-distribution-information Y to the receiving station through the communication line; generating a fourth random number XO according to a prescribed procedure by utilizing the hashing result E, the key-distribution-information Y, and the identification number ID1 in the receiving station: generating a second hashing result EO by implementing the one-way hashing to hash both the ciphertext C and fourth random number XO in the receiving station; restoring the cryptographic key K according to a prescribed procedure by utilizing both the secret key d2 and the fourth random number XO in the receiving station; and obtaining the message M by decrypting the ciphertext C by utilizing the cryptographic key K in the receiving station.

Claim 5

A cryptographic communication method for carrying out cryptographic communication to transmit a message M from a sending station to a receiving station through a communication line by utilizing public information P2 of the receiving station, secret information S1 of the sending station, an identification number ID1 of the sending station, and a secret key d2 of the receiving station, comprising the steps of: obtaining the public information P2 in the sending station; generating a first random number r in the sending station; generating a second random number R according to a prescribed procedure by utilizing both the public information P2 and the first random number r in the sending station; generating a cryptographic key K according to a prescribed procedure by utilizing the first random number r in the sending station; producing a ciphertext C from the message M by utilizing the cryptographic key K in the sending station; generating a first hashing result E by implementing a one-way hashing to hash both the cryptographic key K and the ciphertext C in the sending station; generating key-distribution-information Y according to a prescribed procedure by utilizing the secret information S1, the second random number R, and the hashing result E in the sending station; sending the hashing result E, the identification number ID1, the ciphertext C, and the key-distribution-information Y to the receiving station through the communication line; generating a fourth random number XO according to a prescribed procedure by utilizing the hashing result E, the identification number ID1, and the key-distribution-information Y in the receiving station; restoring the cryptographic key K according to a prescribed procedure by utilizing the secret key d2 and the fourth random number XO in the receiving station; generating a second hashing result EO by implementing the one-way hashing to hash both the ciphertext C and the cryptographic key K in the receiving station; and obtaining the message M by decrypting the ciphertext C by utilizing the cryptographic key K in the receiving station.

Claim 6

A cryptographic communication method for carrying out cryptographic communication to transmit a message M from a sending station to a receiving station through a communication line by utilizing public information P2 of the receiving station, secret information S1 of the sending station, an identification number ID1 of the sending station, and a secret key d2 of the receiving station, comprising the steps of: obtaining the public information P2 in the sending station; generating a first random number r in the sending station; generating a second random number R according to a prescribed procedure by utilizing both the public information P2 and the first random number r in the sending station; generating a cryptographic key K according to a prescribed procedure by utilizing the first random number r in the sending station; producing a ciphertext C from the message M by utilizing the cryptographic key K in the sending station; generating a first hashing result E by implementing a one-way hashing to hash both the cryptographic key K and the ciphertext C in the sending station; generating key-distribution-information Y according to a prescribed procedure by utilizing the secret information S1, the second random number R, and the hashing result E in the sending station; sending the hashing result E, the identification number ID1, the ciphertext C, and the key-distribution-information Y to the receiving station through the communication line; generating a fourth random number XO according to a prescribed procedure by utilizing the hashing result E, the identification number ID1, and the key-distribution-information Y in the receiving station; restoring the cryptographic key K according to a prescribed procedure by utilizing the secret key d2 and the fourth random number XO in the receiving station; generating a second hashing result EO by implementing the one-way hashing to hash both the ciphertext C and the cryptographic key K in the receiving station; and obtaining the message M by decrypting the ciphertext C by utilizing the cryptographic key K in the receiving station.

Claim 7

A cryptographic communication method for carrying out cryptographic communication to transmit a message M from a sending station to a receiving station through a communication line by utilizing public information P2 of the receiving station, secret information S1 of the sending station, an identification number ID1 of the sending station, and a secret key d2 of the receiving station, comprising the steps of; obtaining the public information P2 in the sending station; generating a first random number r in the sending station; generating a second random number R according to a prescribed procedure by utilizing both the public information P2 and the first random number r in the sending station; generating a cryptographic key K according to a prescribed procedure by utilizing the first random number r in the sending station; producing a ciphertext C from the message M by utilizing the cryptographic key K in the sending station: generating a first hashing result E by implementing a one-way hashing to hash the ciphertext C in the sending station; generating key-distribution-information Y according to a prescribed procedure by utilizing the secret information S1, the second random number R, and the hashing result E in the sending station; sending the identification number ID1, the ciphertext C and the key-distribution-information Y to the receiving station through the communication line: generating a second hashing result EO by implementing the one-way hashing to hash the ciphertext C in the receiving station; generating a fourth random number XO according to a prescribed procedure by utilizing the second hashing result EO, the identification number ID1, and the key-distribution-information Y in the receiving station; restoring the cryptographic key K according to a prescribed procedure by utilizing the secret key d2 and the fourth random number XO in the receiving station; and obtaining the message M by decrypting the ciphertext C by utilizing the cryptographic key K in the receiving station.

Claim 8

A cryptographic communication method for carrying out cryptographic communication to transmit a message M from a sending station to a receiving station through a communication line by utilizing public information P2 of the receiving station, secret information S1 of the sending station, an identification number ID1 of the sending station, and a secret key d2 of the receiving station, comprising the steps of; obtaining the public information P2 in the sending station; generating a first random number r in the sending station; generating a second random number R according to a prescribed procedure by utilizing both the public information P2 and the first random number r in the sending station; generating a cryptographic key K according to a prescribed procedure by utilizing the first random number r in the sending station; producing a ciphertext C from the message M by utilizing the cryptographic key K in the sending station: generating a first hashing result E by implementing a one-way hashing to hash the ciphertext C in the sending station; generating key-distribution-information Y according to a prescribed procedure by utilizing the secret information S1, the second random number R, and the hashing result E in the sending station; sending the identification number ID1, the ciphertext C and the key-distribution-information Y to the receiving station through the communication line: generating a second hashing result EO by implementing the one-way hashing to hash the ciphertext C in the receiving station; generating a fourth random number XO according to a prescribed procedure by utilizing the second hashing result EO, the identification number ID1, and the key-distribution-information Y in the receiving station; restoring the cryptographic key K according to a prescribed procedure by utilizing the secret key d2 and the fourth random number XO in the receiving station; and obtaining the message M by decrypting the ciphertext C by utilizing the cryptographic key K in the receiving station.

Claim 9

A cryptographic communication device for carrying out cryptographic communication to transmit a message M from a sending station to a receiving station through a communication line by utilizing public information P2 of the receiving station, secret information S1 of the sending station, an identification number ID1 of the sending station, and a secret key d2 of the receiving station, comprising: obtaining means in the sending station for obtaining the public information P2; first generating means in the sending station for generating a first random number r; second generating means in the sending station for generating a second random number R according to a prescribed procedure by utilizing both the public information P2 obtained by the obtaining means and the first random number r generated by the first generating means; third generating means in the sending station for generating a third random number X according to a prescribed procedure by utilizing the second random number R generated by the second generating means; fourth generating means in the sending station for generating a cryptographic key K according to a prescribed procedure by utilizing the first random number r generated by the first generating means; producing means in the sending station for producing a ciphertext C from the message M by utilizing the cryptographic key K generated by the fourth generating means; fifth generating means in the sending station for generating a first hashing result E by implementing a one-way hashing to hash both the ciphertext C produced by the producing means and the third random number X generated by the third generating means; sixth generating means in the sending station for generating key-distribution-information Y according to a prescribed procedure by utilizing the secret information S1, the second random number R generated by the second generating means, and the hashing result E generated by the fifth generating means; sending means for sending the third random number X generated by the third generating means, the identification number ID1, the ciphertext C produced by the producing means, and the key-distribution-information Y generated by the sixth generating means to the receiving station through the communication line; seventh generating means in the receiving station for generating a second hashing result EO by implementing the one-way hashing to hash the ciphertext C and the third random number X sent by the sending means; restoring means in the receiving station for restoring the cryptographic key K according to a prescribed procedure by utilizing the secret key d2 and the third random number X sent by the sending means; and obtaining means in the receiving station for obtaining the message M by decrypting the ciphertext C sent by the sending means by utilizing the cryptographic key K restored by the restoring means.

Claim 10

A cryptographic communication device for carrying out cryptographic communication to transmit a message M from a sending station to a receiving station through a communication line by utilizing public information P2 of the receiving station, secret information S1 of the sending station, an identification number ID1 of the sending station, and a secret key d2 of the receiving station, comprising: obtaining means in the sending station for obtaining the public information P2; first generating means in the sending station for generating a first random number r; second generating means in the sending station for generating a second random number R according to a prescribed procedure by utilizing both the public information P2 obtained by the obtaining means and the first random number r generated by the first generating means; third generating means in the sending station for generating a third random number X according to a prescribed procedure by utilizing the second random number R generated by the second generating means; fourth generating means in the sending station for generating a cryptographic key K according to a prescribed procedure by utilizing the first random number r generated by the first generating means; producing means in the sending station for producing a ciphertext C from the message M by utilizing the cryptographic key K generated by the fourth generating means; fifth generating means in the sending station for generating a first hashing result E by implementing a one-way hashing to hash both the ciphertext C produced by the producing means and the third random number X generated by the third generating means; sixth generating means in the sending station for generating key-distribution-information Y according to a prescribed procedure by utilizing the secret information S1, the second random number R generated by the second generating means, and the hashing result E generated by the fifth generating means; sending means for sending the third random number X generated by the third generating means, the identification number ID1, the ciphertext C produced by the producing means, and the key-distribution-information Y generated by the sixth generating means to the receiving station through the communication line; seventh generating means in the receiving station for generating a second hashing result EO by implementing the one-way hashing to hash the ciphertext C and the third random number X sent by the sending means; restoring means in the receiving station for restoring the cryptographic key K according to a prescribed procedure by utilizing the secret key d2 and the third random number X sent by the sending means; and obtaining means in the receiving station for obtaining the message M by decrypting the ciphertext C sent by the sending means by utilizing the cryptographic key K restored by the restoring means.

Claim 11

A cryptographic communication device for carrying out cryptographic communication to transmit a message M from a sending station to a receiving station through a communication line by utilizing public information P2 of the receiving station, secret information S1 of the sending station, an identification number ID1 of the sending station, and a secret key d2 of the receiving station, comprising: obtaining means in the sending station for obtaining the public information P2; first generating means in the sending station for generating a first random number r; second generating means in the sending station for generating a second random number R according to a prescribed procedure by utilizing both the public information P2 obtained by the obtaining means and the first random number r generated by the first generating means; third generating means in the sending station for generating a third random number X according to a prescribed procedure by utilizing the second random number R generated by the second generating means; fourth generating means in the sending station for generating a cryptographic key K according to a prescribed procedure by utilizing the first random number r generated by the first generating means; producing means in the sending station for producing a ciphertext C from the message M by utilizing the cryptographic key K generated by the fourth generating means; fifth generating means in the sending station for generating a first hashing result E by implementing a one-way hashing to hash both the ciphertext C produced by the producing means and the third random number X generated by the third generating means; sixth generating means in the sending station for generating key-distribution-information Y according to a prescribed procedure by utilizing the secret information S1, the second random number R generated by the second generating means, and the hashing result E generated by the fifth generating means; sending means for sending the first hashing result E generated by the fifth generating means, the identification number ID1, the ciphertext C produced by the producing means, and the key-distribution-information Y generated by the sixth generating means to the receiving station through the communcation line; seventh generating means in the receiving station for generating a fourth random number XO according to a prescribed procedure by utilizing the hashing result E, the key-distribution-information Y, and the identification number ID1 sent by the sending means; eighth generating means in the receiving station for generating a second hashing result EO by implementing the one-way hashing to hash the ciphertext C and the fourth random number XO sent by the sending means; restoring means in the receiving station for restoring the cryptographic key K according to a prescribed procedure by utilizing the secret key d2 and the fourth random number XO sent by the sending means; and obtaining means in the receiving station for obtaining the message M by decrypting the ciphertext C sent by the sending means by utilizing the cryptographic key K restored by the restoring means.

Claim 12

A cryptographic communication device for carrying out cryptographic communication to transmit a message M from a sending station to a receiving station through a communication line by utilizing public information P2 of the receiving station, secret information S1 of the sending station, an identification number ID1 of the sending station, and a secret key d2 of the receiving station, comprising: obtaining means in the sending station for obtaining the public information P2; first generating means in the sending station for generating a first random number r; second generating means in the sending station for generating a second random number R according to a prescribed procedure by utilizing both the public information P2 obtained by the obtaining means and the first random number r generated by the first generating means; third generating means in the sending station for generating a third random number X according to a prescribed procedure by utilizing the second random number R generated by the second generating means; fourth generating means in the sending station for generating a cryptographic key K according to a prescribed procedure by utilizing the first random number r generated by the first generating means; producing means in the sending station for producing a ciphertext C from the message M by utilizing the cryptographic key K generated by the fourth generating means; fifth generating means in the sending station for generating a first hashing result E by implementing a one-way hashing to hash both the ciphertext C produced by the producing means and the third random number X generated by the third generating means; sixth generating means in the sending station for generating key-distribution-information Y according to a prescribed procedure by utilizing the secret information S1, the second random number R generated by the second generating means, and the hashing result E generated by the fifth generating means; sending means for sending the first hashing result E generated by the fifth generating means, the identification number ID1, the ciphertext C produced by the producing means, and the key-distribution-information Y generated by the sixth generating means to the receiving station through the communcation line; seventh generating means in the receiving station for generating a fourth random number XO according to a prescribed procedure by utilizing the hashing result E, the key-distribution-information Y, and the identification number ID1 sent by the sending means; eighth generating means in the receiving station for generating a second hashing result EO by implementing the one-way hashing to hash the ciphertext C and the fourth random number XO sent by the sending means; restoring means in the receiving station for restoring the cryptographic key K according to a prescribed procedure by utilizing the secret key d2 and the fourth random number XO sent by the sending means; and obtaining means in the receiving station for obtaining the message M by decrypting the ciphertext C sent by the sending means by utilizing the cryptographic key K restored by the restoring means.

Claim 13

A cryptographic communication device for carrying out cryptographic communication to transmit a message M from a sending station to a receiving station through a communication line by utilizing public information P2 of the receiving station, secret information S1 of the sending station, an identification number ID1 of the sending station, and a secret key d2 of the receiving station, comprising: obtaining means in the sending station for obtaining the public information P2; first generating means in the sending station for generating a first random number r; second generating means in the sending station for generating a second random number R according to a prescribed procedure by utilizing both the public information P2 obtained by the obtaining means and the first random number r generated by the first generating means: third generating means in the sanding station for generating a cryptographic key K according to a prescribed procedure by utilizing the first random number r generated by the first generating means; producing means in the sending station for producing a ciphertext C from the message M by utilizing the cryptographic key K generated by the third generating means; fourth generating means in the sending station for generating a first hashing result E by implementing a one-way hashing to hash both the ciphertext C produced by the producing means and the cryptographic key K generated by the third generating means; fifth generating means in the sending station for generating key-distribution-information Y according to a prescribed procedure by utilizing the secret information S1, the second random number R generated by the second generating means, and the hashing result E generated by the fourth generating means; sending means for sending the first hashing result E generated by the fourth generating means, the identification number ID1, the ciphertext C produced by the producing means, and the key-distribution-information Y generated by the fifth generating means to the receiving station through the communication line; sixth generating means in the receiving station for generating a fourth random number XO according to a prescribed procedure by utilizing the hashing result E, the key-distribution-information Y, and the identification number ID1 sent by the sending means; restoring means in the receiving station for restoring the cryptographic key K according to a prescribed procedure by utilizing the secret key d2 and the fourth random number XO sent by the sending means: and seventh generating means in the receiving station for generating a second hashing result EO by implementing the one-way hashing to hash the ciphertext C sent by the sending means and the cryptographic key K restored in the restoring means; obtaining means in the receiving station for obtaining the message M by decrypting the ciphertext C sent by the sending means by utilizing the cryptographic key K restored by the restoring means.

Claim 14

A cryptographic communication device for carrying out cryptographic communication to transmit a message M from a sending station to a receiving station through a communication line by utilizing public information P2 of the receiving station, secret information S1 of the sending station, an identification number ID1 of the sending station, and a secret key d2 of the receiving station, comprising: obtaining means in the sending station for obtaining the public information P2; first generating means in the sending station for generating a first random number r; second generating means in the sending station for generating a second random number R according to a prescribed procedure by utilizing both the public information P2 obtained by the obtaining means and the first random number r generated by the first generating means: third generating means in the sanding station for generating a cryptographic key K according to a prescribed procedure by utilizing the first random number r generated by the first generating means; producing means in the sending station for producing a ciphertext C from the message M by utilizing the cryptographic key K generated by the third generating means; fourth generating means in the sending station for generating a first hashing result E by implementing a one-way hashing to hash both the ciphertext C produced by the producing means and the cryptographic key K generated by the third generating means; fifth generating means in the sending station for generating key-distribution-information Y according to a prescribed procedure by utilizing the secret information S1, the second random number R generated by the second generating means, and the hashing result E generated by the fourth generating means; sending means for sending the first hashing result E generated by the fourth generating means, the identification number ID1, the ciphertext C produced by the producing means, and the key-distribution-information Y generated by the fifth generating means to the receiving station through the communication line; sixth generating means in the receiving station for generating a fourth random number XO according to a prescribed procedure by utilizing the hashing result E, the key-distribution-information Y, and the identification number ID1 sent by the sending means; restoring means in the receiving station for restoring the cryptographic key K according to a prescribed procedure by utilizing the secret key d2 and the fourth random number XO sent by the sending means: and seventh generating means in the receiving station for generating a second hashing result EO by implementing the one-way hashing to hash the ciphertext C sent by the sending means and the cryptographic key K restored in the restoring means; obtaining means in the receiving station for obtaining the message M by decrypting the ciphertext C sent by the sending means by utilizing the cryptographic key K restored by the restoring means.

Claim 15

A cryptographic communication device for carrying out cryptographic communication to transmit a message M from a sending station to a receiving station through a communication line by utilizing public information P2 of the receiving station, secret information S1 of the sending station, an identification number ID1 of the sending station, and a secret key d2 of the receiving station, comprising: obtaining means in the sending station for obtaining the public information P2; first generating means in the sending station for generating a first random number r; second generating means in the sending station for generating a second random number R according to a prescribed procedure by utilizing both the public information P2 obtained by the obtaining means and the first random number r generated by the first generating means; third generating means in the sending station for generating a cryptographic key K according to a prescribed procedure by utilizing the first random number r generated by the first generating means; producing means in the sending station for producing a ciphertext C from the message M by utilizing the cryptographic key K generated by the third generating means; fourth generating means in the sending station for generating a first hashing result E by implementing a one-way hashing to hash the ciphertext C produced by the producing means: fifth generating means in the sending station for generating key-distribution-information Y according to a prescribed procedure by utilizing the secret information S1, the second random number R generated by the second generating means, and the hashing result E generated by the fourth generating means; sending means for sending the first hashing result E generated by the fourth generating means, the identification number ID1, the ciphertext C produced by the producing means, and the key-distribution-information Y generated by the fifth generating means to the receiving station through the communication line; sixth generating means in the receiving station for generating a second hashing result EO by implementing the one-way hashing to hash the ciphertext C sent by the sending means; seventh generating means in the receiving station for generating a fourth random number XO according to a prescribed procedure by utilizing the second hashing result EO generated by the sixth generating means, the key-distribution-information Y, and the identification number ID1 sent by the sending means; restoring means in the receiving station for restoring the cryptographic key K according to a prescribed procedure by utilizing the secret key d2 sent by the sending means and the fourth random number XO generated by the seventh generating means; and obtaining means in the receiving station for obtaining the message M by decrypting the ciphertext C sent by the sending means by utilizing the cryptographic key X restored by the restoring means.

Claim 16

A cryptographic communication device for carrying out cryptographic communication to transmit a message M from a sending station to a receiving station through a communication line by utilizing public information P2 of the receiving station, secret information S1 of the sending station, an identification number ID1 of the sending station, and a secret key d2 of the receiving station, comprising: obtaining means in the sending station for obtaining the public information P2; first generating means in the sending station for generating a first random number r; second generating means in the sending station for generating a second random number R according to a prescribed procedure by utilizing both the public information P2 obtained by the obtaining means and the first random number r generated by the first generating means; third generating means in the sending station for generating a cryptographic key K according to a prescribed procedure by utilizing the first random number r generated by the first generating means; producing means in the sending station for producing a ciphertext C from the message M by utilizing the cryptographic key K generated by the third generating means; fourth generating means in the sending station for generating a first hashing result E by implementing a one-way hashing to hash the ciphertext C produced by the producing means: fifth generating means in the sending station for generating key-distribution-information Y according to a prescribed procedure by utilizing the secret information S1, the second random number R generated by the second generating means, and the hashing result E generated by the fourth generating means; sending means for sending the first hashing result E generated by the fourth generating means, the identification number ID1, the ciphertext C produced by the producing means, and the key-distribution-information Y generated by the fifth generating means to the receiving station through the communication line; sixth generating means in the receiving station for generating a second hashing result EO by implementing the one-way hashing to hash the ciphertext C sent by the sending means; seventh generating means in the receiving station for generating a fourth random number XO according to a prescribed procedure by utilizing the second hashing result EO generated by the sixth generating means, the key-distribution-information Y, and the identification number ID1 sent by the sending means; restoring means in the receiving station for restoring the cryptographic key K according to a prescribed procedure by utilizing the secret key d2 sent by the sending means and the fourth random number XO generated by the seventh generating means; and obtaining means in the receiving station for obtaining the message M by decrypting the ciphertext C sent by the sending means by utilizing the cryptographic key X restored by the restoring means.