US 20040174997 A1 Abstract Provided is a content distribution system that prevents different keys to be derived between an encryption apparatus and a decryption apparatus. A random-number generating unit
112, in an encryption apparatus 110, generates a random number s. A first function unit 113 generates a functional value G(s) of the random number s, and generates a random-number value u and a shared key K from the functional value G(s). An encryption unit 114 generates a first cipher text c1 of the random number s, using a public-key polynomial h and the random-number value u. A decryption unit 123, in a decryption apparatus 120, decrypts the first cipher text c1 using a secret-key polynomial f, to generate a decryption random number s′ A second function unit 126 generates a functional value G(s′) of the decryption random number s′, and generates a random-number value u′ and a shared key K′from the functional value G(s′ ). A comparison unit 127 generates a first re-cipher text c1′, using the random-number value u′ and the shared key K′, and outputs the shared key K′ if the first cipher text c1 is equal to the first re-cipher text c1′. Claims(43) 1. A key agreement system comprising a shared-key generation apparatus and a shared-key recovery apparatus, each apparatus establishing therein a same shared key in secrecy, wherein
the shared-key generation apparatus includes:
a seed-value generating unit operable to generate a seed value;
a first shared-key generating unit operable to generate a blind value and a shared key, from the seed value;
an encryption unit operable to encrypt the seed value based on the blind value, to generate encryption information; and
a transmitting unit operable to transmit the encryption information, and
the shared-key recovery apparatus includes:
a receiving unit operable to receive the encryption information;
a decryption unit operable to decrypt the encryption information, to generate a decryption seed value;
a second shared-key generating unit operable to generate a decryption blind value and a decryption shared key, using the decryption seed value and according to a same method as used in the first shared-key generating unit;
a re-encryption unit operable to encrypt the decryption seed value based on the decryption blind value, to generate re-encryption information;
a judging unit operable to judge, based on the encryption information and the re-encryption information, whether the decryption shared key should be outputted; and
an outputting unit operable, when the judging unit has judged affirmatively, to output the decryption shared key.
2. The key agreement system of the shared-key generation apparatus further includes:
an obtaining unit operable to obtain a content; and
an encryption unit operable to encrypt the obtained content using the shared key, to generate an encrypted content,
the transmitting unit further transmits the encrypted content, the receiving unit further receives the encrypted content, and the shared-key recovery apparatus further includes:
a decryption unit operable to decrypt the received encrypted content using the decryption shared key, to generate a decrypted content; and
an outputting unit operable to output the decrypted content.
3. A shared-key generation apparatus that notifies a destination apparatus about a shared key in secrecy, the shared-key generation apparatus comprising:
a seed-value generating unit operable to generate a seed value; a shared-key generating unit operable to generate a blind value and a shared key, from the seed value; an encryption unit operable to encrypt the seed value based on the blind value, to generate encryption information; and a transmitting unit operable to transmit the encryption information. 4. The shared-key generation apparatus of the shared-key generating unit performs a one-way function on the seed value, to generate a functional value, and generates the blind value and the shared key from the functional value, the encryption unit includes:
a public-key obtaining subunit operable to obtain a public key; and
a public-key encryption subunit operable to perform a public-key encryption algorithm on the seed value, using the public key and the blind value, to generate an encryption seed value as the encryption information.
5. The shared-key generation apparatus of the public-key encryption algorithm conforms to an NTRU cryptosystem, the public-key obtaining subunit obtains a public-key polynomial generated according to a key-generation algorithm of the NTRU cryptosystem, as the public key, the public-key encryption subunit generates a seed-value polynomial from the seed value, generates a blind-value polynomial from the blind value, and encrypts the seed-value polynomial according to an encryption algorithm of the NTRU cryptosystem, using the public-key polynomial as a key, and using the blind-value polynomial to randomize the seed-value polynomial, to generate an encryption seed-value polynomial as the encryption seed value, and the transmitting unit transmits the encryption seed-value polynomial as the encryption seed value. 6. The shared-key generation apparatus of the encryption unit includes:
a public-key obtaining subunit operable to obtain a public key;
a public-key encryption subunit operable to generate a blind value, perform the public-key encryption algorithm on the seed value using the public key and the blind value, to generate a public-key cipher text; and
a function subunit operable to perform a second one-way function on at least one of the seed value, the blind value, and the shared key, to generate a second functional value, and
the encryption unit generates the encryption information that includes the public-key cipher text and the second functional value. 7. The shared-key generation apparatus of the shared-key generating unit performs a one-way function on the seed value, to generate a functional value, and generates the blind value and the shared key from the functional value. 8. The shared-key generation apparatus of the shared-key generating unit performs a first one-way function on the seed value, to generate a first functional value, and generates the shared key from the first functional value, instead of generating the blind value and the shared key. 9. The shared-key generation apparatus of the public-key encryption algorithm conforms to an NTRU cryptosystem, the public-key obtaining subunit obtains a public-key polynomial generated according to a key-generation algorithm of the NTRU cryptosystem, as the public key, the public-key encryption subunit generates a seed-value polynomial from the seed value, generates a blind-value polynomial from the blind value, encrypts the seed-value polynomial according to an encryption algorithm of the NTRU cryptosystem, using the public-key polynomial as a key, and using the blind-value polynomial to randomize the seed-value polynomial, to generate an encryption seed-value polynomial as the public-key cipher text, and the encryption unit generates the encryption information that includes the encryption seed-value polynomial as the public-key cipher text and the second functional value. 10. The shared-key generation apparatus of the shared-key generating unit performs a one-way function on the seed value, to generate a functional value, and generates a verification value, the blind value, and the shared key, from the functional value, the encryption unit includes:
a public-key obtaining subunit operable to obtain a public key;
a first encryption subunit operable to perform a public-key encryption algorithm on the verification value, using the public key and the blind value, to generate a first cipher text; and
a second encryption subunit operable to perform, on the seed value, a computation algorithm different from the public-key encryption algorithm, to generate a second cipher text, and
the encryption unit generates the encryption information that includes the first cipher text and the second cipher text. 11. The shared-key generation apparatus of the public-key encryption algorithm conforms to an NTRU cryptosystem, the public-key obtaining subunit obtains a public-key polynomial generated according to a key-generation algorithm of the NTRU cryptosystem, as the public key, the first encryption subunit generates a verification-value polynomial from the verification value, generates a blind-value polynomial from the blind value, and encrypts the verification-value polynomial according to an encryption algorithm of the NTRU cryptosystem, using the public-key polynomial as a key, and using the blind-value polynomial to randomize the verification-value polynomial, to generate an encryption verification-value polynomial as the first cipher text, and the encryption unit generates the encryption information that includes the encryption verification-value polynomial as the first cipher text and the second cipher text. 12. The shared-key generation apparatus of the different computation algorithm is a symmetric key encryption algorithm, and the second encryption subunit performs the symmetric key encryption algorithm on the seed value using the verification value as a key, to generate the second cipher text. 13. The shared-key generation apparatus of the different computation algorithm is bitwise exclusive-or, and the second encryption subunit performs the bitwise exclusive-or on the verification value and the seed value, to generate the second cipher text. 14. The shared-key generation apparatus of the different computation algorithm is addition, and the second encryption subunit performs the addition on the verification value and the seed value, to generate the second cipher text. 15. The shared-key generation apparatus of the different computation algorithm is multiplication, and the second encryption subunit performs the multiplication on the verification value and the seed value, to generate the second cipher text. 16. The shared-key generation apparatus of the seed-value generating unit generates a random number, as the seed value. 17. The shared-key generation apparatus of the shared-key generating unit performs a one-way function on the seed value, to generate a functional value, and generates the blind value and the shared key from the functional value. 18. The shared-key generation apparatus of the one-way function is a hash function, and the shared-key generating unit performs the hash function on the seed value. 19. The shared-key generation apparatus of the shared-key generating unit generates the blind value by setting a part of the functional value as the blind value, and generates the shared key by setting another part of the functional value as the shared key. 20. The shared-key generation apparatus of obtaining unit operable to obtain a content; and an encryption unit operable to encrypt the obtained content using the shared key, to generate an encrypted content, wherein the transmitting unit further transmits the encrypted content. 21. A shared-key recovery apparatus that receives a shared key from a shared-key generation apparatus in secrecy, the shared-key generation apparatus generating a seed value, generating a blind value and a shared key from the seed value, encrypting the seed value based on the blind value to generate encryption information, and transmitting the encryption information, the shared-key recovery apparatus comprising:
a receiving unit operable to receive the encryption information; a decryption unit operable to decrypt the encryption information, to generate a decryption seed value; a shared-key generating unit operable to generate a decryption blind value and a decryption shared key, using the decryption seed value and according to a same shared-key generating method used in the shared-key generation apparatus; a re-encryption unit operable to encrypt the decryption seed value based on the decryption blind value, to generate re-encryption information; a judging unit operable to judge, based on the encryption information and the re-encryption information, whether the decryption shared key should be outputted; and an outputting unit operable, when the judging unit has judged affirmatively, to output the decryption shared key. 22. The shared-key recovery apparatus of the shared-key generation apparatus performs a one-way function on the seed value to generate a functional value, generates the blind value and the shared key from the functional value, obtains a public key, performs a public-key encryption algorithm on the seed value using the public key and the blind value, to generate an encryption seed value as the encryption information, and transmits the encryption seed value, the receiving unit receives the encryption seed value as the encryption information, the decryption unit includes:
a secret-key obtaining subunit operable to obtain a secret key that corresponds to the public key; and
a public-key decryption subunit operable to perform, on the received encryption seed value, a public-key decryption algorithm that corresponds to the public-key encryption algorithm, using the obtained secret key, to generate the decryption seed value,
the shared-key generating unit performs the one-way function on the decryption seed value to generate a decryption functional value, and generates the decryption blind value and the decryption shared key from the decryption functional value, the re-encryption unit includes:
a public-key obtaining subunit operable to obtain the public key; and
a re-encryption subunit operable to perform the public-key encryption algorithm on the decryption seed value using the public key and the decryption blind value, to generate a re-encryption seed value as the re-encryption information, and
the judging unit judges whether the encryption seed value is identical to the re-encryption seed value, and when judging affirmatively, determines that the decryption shared key should be outputted. 23. The shared-key recovery apparatus of the public-key encryption algorithm and the public-key decryption algorithm conform to an NTRU cryptosystem, the shared-key generation apparatus obtains a public-key polynomial generated according to a key-generation algorithm of the NTRU cryptosystem, as the public key, generates a seed-value polynomial from the seed value, generates a blind-value polynomial from the blind value, encrypts the seed-value polynomial according to an encryption algorithm of the NTRU cryptosystem, using the public-key polynomial as a key, and using the blind-value polynomial to randomize the seed-value polynomial, to generate an encryption seed-value polynomial as the encryption seed value, and transmits the encryption seed-value polynomial as the encryption seed value, the receiving unit receives the encryption seed-value polynomial as the encryption seed value, the secret-key obtaining subunit obtains a secret-key polynomial generated according to the key-generation algorithm of the NTRU cryptosystem, as the secret key, the public-key decryption subunit decrypts the received encryption seed-value polynomial according to a decryption algorithm of the NTRU cryptosystem and using the obtained secret-key polynomial as a key, to generate a decryption seed-value polynomial, and generates the decryption seed value from the decryption seed-value polynomial, the public-key obtaining subunit obtains the public-key polynomial as the public key, the re-encryption subunit generates a seed-value polynomial from the decryption seed value, generates a blind-value polynomial from the decryption blind value, and encrypts the seed-value polynomial according to the encryption algorithm of the NTRU cryptosystem, using the public-key polynomial as a key, and using the blind-value polynomial to randomize the seed-value polynomial, to generate a re-encryption seed-value polynomial, and the judging unit judges whether the encryption seed-value polynomial is identical to the re-encryption seed-value polynomial. 24. The shared-key recovery apparatus of the shared-key generation apparatus obtains a public key, generates a blind value, performs a public-key encryption algorithm on the seed value using the public key and the blind value to generate a public-key cipher text, performs a second one-way function on at least one of the seed value, the blind value, and the shared key to generate a second functional value, generates the encryption information that includes the public-key cipher text and the second functional value, and transmits the encryption information, the receiving unit receives the encryption information that includes the public-key cipher text and the second functional value, the decryption unit includes:
a secret-key obtaining subunit operable to obtain a secret key that corresponds to the public key;
a public-key decryption subunit operable to perform, on the public-key cipher text included in the received encryption information, a public-key decryption algorithm that corresponds to the public-key encryption algorithm, to generate a decryption seed value; and
a function subunit operable to perform the second one-way function on at least one of the decryption seed value, the decryption blind value, and the decryption shared key, to generate a decryption second functional value, and
the judging unit judges whether the second functional value included in the received encryption information is identical to the decryption second functional value instead of performing judging based on the encryption information and the re-encryption information and when judging affirmatively, determines that the decryption shared key should be outputted. 25. The shared-key recovery apparatus of the shared-key generation apparatus performs a one-way function on the seed value to generate a functional value, and generates the blind value and the shared key from the functional value, and the shared-key generating unit performs the first one-way function on the decryption seed value to generate a decryption functional value, and generates the decryption blind value and the decryption shared key from the decryption functional value. 26. The shared-key recovery apparatus of the shared-key generation apparatus performs a first one-way function on the seed value to generate a first functional value, and generates the shared key from the first functional value, instead of generating the blind value and the shared key, and the shared-key generating unit performs the first one-way function on the decryption seed value to generate a decryption functional value, and generates the decryption shared key from the decryption functional value, instead of generating the decryption blind value and the decryption shared key. 27. The shared-key recovery apparatus of the shared-key generation apparatus obtains a public-key polynomial generated according to a key-generation algorithm of the NTRU cryptosystem, as the public key, generates a seed-value polynomial from the seed value, generates a blind-value polynomial from the blind value, encrypts the seed-value polynomial according to an encryption algorithm of the NTRU cryptosystem using the public-key polynomial as a key and using the blind-value polynomial to randomize the seed-value polynomial, to generate an encryption seed-value polynomial as the public-key cipher text, and generates the encryption information that includes the encryption seed-value polynomial as the public-key cipher text and the second functional value, the secret-key obtaining subunit obtains a secret-key polynomial generated according to the key-generation algorithm of the NTRU cryptosystem, as the secret key, and the public-key decryption subunit generates a public-key cipher-text polynomial from the public-key cipher text, decrypts the public-key cipher-text polynomial according to a decryption algorithm of the NTRU cryptosystem using the secret-key polynomial as a key to generate a decryption seed-value polynomial, and generates the decryption seed value from the decryption seed-value polynomial. 28. The shared-key recovery apparatus of the shared-key generation apparatus performs a one-way function on the seed value to generate a functional value, generates a verification value, the blind value, and the shared key from the functional value, obtains a public key, performs a public-key encryption algorithm on the verification value using the public key and the blind value to generate a first cipher text, performs, based on the verification value, a computation algorithm different from the public-key encryption algorithm on the seed value, to generate a second cipher text, generates the encryption information that includes the first cipher text and the second cipher text, and transmits the encryption information, the receiving unit receives the encryption information that includes the first cipher text and the second cipher text, the decryption unit includes:
a secret-key obtaining subunit operable to obtain a secret key that corresponds to the public key;
a public-key decryption subunit operable to perform, on the first cipher text included in the received encryption information, a public-key decryption algorithm that corresponds to the public-key encryption algorithm, to generate a decryption verification value; and
a computation decryption subunit operable to perform, on the second cipher text included in the received encryption information, a computation algorithm for performing an inverse computation of the different computation algorithm, to generate a decryption seed value,
the shared-key generating unit performs the one-way function on the decryption seed value to generate a decryption functional value, and generates a decryption verification value, the decryption blind value, and the decryption shared key, from the decryption functional value, the re-encryption unit includes:
a public-key obtaining subunit operable to obtain the public key; and
a re-encryption subunit operable to perform, on the decryption verification value, the public-key encryption algorithm using the public key and the decryption blind value, to generate the re-encryption information, and
the judging unit judges whether the first cipher text included in the encryption information is identical to the re-encryption information, and when judging affirmatively, determines that the decryption shared key should be outputted.
29. The shared-key recovery apparatus of the public-key encryption algorithm and the public-key decryption algorithm conform to an NTRU cryptosystem, the shared-key generation apparatus obtains a public-key polynomial generated according to a key-generation algorithm of the NTRU cryptosystem, as the public key, generates a verification-value polynomial from the verification value, generates a blind-value polynomial from the blind value, encrypts the verification-value polynomial according to an encryption algorithm of the NTRU cryptosystem, using the public-key polynomial as a key, and using the blind-value polynomial to randomize the verification-value polynomial, to generate an encryption verification-value polynomial as the first cipher text, generates the encryption information that includes the encryption verification-value polynomial as the first cipher text and the second cipher text, and transmits the encryption information, the receiving unit receives the encryption information that includes the encryption verification-value polynomial and the second cipher text, the secret-key obtaining subunit obtains a secret-key polynomial generated according to the key-generation algorithm of the NTRU cryptosystem, as the secret key, the public-key decryption subunit generates a first cipher-text polynomial from the first cipher text, decrypts the first cipher-text polynomial according to a decryption algorithm of the NTRU cryptosystem using the secret-key polynomial as a key, to generate a decryption verification polynomial, and generates the decryption verification value from the decryption verification-value polynomial, the public-key obtaining subunit obtains the public-key polynomial, the re-encryption subunit generates a decryption verification-value polynomial from the decryption verification value, generates a blind-value polynomial from the decryption blind value, and encrypts the decryption verification-value polynomial according to the encryption algorithm of the NTRU cryptosystem, using the public-key polynomial as a key, and using the blind-value polynomial to randomize the decryption verification-value polynomial, to generate a re-encryption verification-value polynomial as the re-encryption information, and the judging unit judges whether the encryption verification-value polynomial as the first cipher text is identical to the re-encryption verification-value polynomial as the re-encryption information. 30. The shared-key recovery apparatus of the different computation algorithm is a symmetric key encryption algorithm, and the computation algorithm for performing the inverse computation is a corresponding symmetric key decryption algorithm, and the computation decryption subunit performs the symmetric key decryption algorithm on the second cipher text, using the decryption verification value as a key, to generate the decryption seed value. 31. The shared-key recovery apparatus of the different computation algorithm and the computation algorithm for performing the inverse computation are bitwise exclusive-or, and the computation decryption subunit performs the bitwise exclusive-or on the decryption verification value and the second cipher text, to generate the decryption seed value. 32. The shared-key recovery apparatus of the different computation algorithm is addition and the computation algorithm for performing the inverse computation is subtraction, and the computation decryption subunit performs the subtraction on the decryption verification value and the second cipher text, to generate the decryption seed value. 33. The shared-key recovery apparatus of the different calculation algorithm is multiplication and the computation algorithm for performing the inverse computation is division, and the computation decryption subunit performs the division on the decryption verification value and the second cipher text, to generate the decryption seed value. 34. The shared-key recovery apparatus of the shared-key generating unit performs a one-way function on the decryption seed value to generate a functional value, and generates the decryption blind value and the decryption shared key from the functional value. 35. The shared-key recovery apparatus of the one-way function is a hash function, and the shared-key generating unit performs the hash function on the decryption seed value. 36. The shared-key recovery apparatus of the shared-key generating unit generates the decryption blind value by setting a part of the functional value as the decryption blind value, and generates the decryption shared key by setting another part of the functional value as the decryption shared key. 37. The shared-key recovery apparatus of the shared-key generation apparatus further obtains a content, encrypts the obtained content using the shared key to generate an encrypted content, and transmits the encrypted content, and the shared-key recovery apparatus further includes:
a content receiving unit operable to receive the encrypted content;
a decryption unit operable to decrypt the received encrypted content using the outputted decryption shared key, to generate a decrypted content; and
a playback unit operable to playback the decrypted content.
38. A shared-key generating method used in a shared-key generation apparatus that notifies a destination apparatus about a shared key in secrecy, the shared-key generating method comprising:
a seed-value generating step of generating a seed value; a shared-key generating step of generating a blind value and a shared key, from the seed value; an encryption step of encrypting the seed value based on the blind value, to generate encryption information; and a transmitting step of transmitting the encryption information. 39. A shared-key generating program used in a shared-key generation apparatus that notifies a destination apparatus about a shared key in secrecy, the shared-key generating program comprising:
a seed-value generating step of generating a seed value; a shared-key generating step of generating a blind value and a shared key, from the seed value; an encryption step of encrypting the seed value based on the blind value, to generate encryption information; and a transmitting step of transmitting the encryption information. 40. The shared-key generating program of the shared-key generating program is recorded in a computer-readable recording medium. 41. A shared-key recovery method used in a shared-key recovery apparatus that receives a shared key from a shared-key generation apparatus in secrecy, the shared-key generation apparatus generating a seed value, generating a blind value and a shared key from the seed value, encrypting the seed value based on the blind value to generate encryption information, and transmitting the encryption information, the shared-key recovery method comprising:
a receiving step of receiving the encryption information; a decryption step of decrypting the encryption information, to generate a decryption seed value; a shared-key generating step of generating a decryption blind value and a decryption shared key, using the decryption seed value and according to a same shared-key generating method used in the shared-key generation apparatus; a re-encryption step of encrypting the decryption seed value based on the decryption blind value, to generate re-encryption information; a judging step of judging, based on the encryption information and there-encryption information, whether the decryption shared key should be outputted; and an outputting step, when the judging unit has judged affirmatively, of outputting the decryption shared key. 42. A shared-key recovery program used in a shared-key recovery apparatus that receives a shared key from a shared-key generation apparatus in secrecy, the shared-key generation apparatus generating a seed value, generating a blind value and a shared key from the seed value, encrypting the seed value based on the blind value to generate encryption information, and transmitting the encryption information, the shared-key recovery program comprising:
a receiving step of receiving the encryption information; a decryption step of decrypting the encryption information, to generate a decryption seed value; a shared-key generating step of generating a decryption blind value and a decryption shared key, using the decryption seed value and according to a same shared-key generating method used in the shared-key generation apparatus; a re-encryption step of encrypting the decryption seed value based on the decryption blind value, to generate re-encryption information; a judging step of judging, based on the encryption information and the re-encryption information, whether the decryption shared key should be outputted; and an outputting step, when the judging unit has judged affirmatively, of outputting the decryption shared key. 43. The shared-key recovery program of The shared-key recovery program is recorded in a computer-readable recording medium. Description [0001] 1. Field of the Invention [0002] The present invention relates to a cryptographic technology used as an information security technology. The present invention particularly relates to a technology of distributing a key under a condition that any third party cannot know the content of the key. [0003] 2. Description of Related Art [0004] Conventionally, the public-key cryptosystem has been used for transmitting information from a transmission apparatus to a reception apparatus in secrecy. [0005] In the public-key cryptosystem, a transmission apparatus encrypts a communication content using the public key of a reception apparatus, and sends the encrypted communication content to the reception apparatus. The reception apparatus receives the encrypted communication content, and decrypts the encrypted communication content using a secret key, thereby obtaining the original communication content (e.g. refer to the non-patent reference 1). [0006] In the year of 1996, the NTRU cryptosystem was proposed, as a public-key cryptosystem for high-speed processing (e.g. refer to the non-patent reference 2). The NTRU cryptosystem performs encryption/decryption using a polynomial operation that enables high-speed computation. The NTRU cryptosystem enables higher-speed processing using software, compared to the conventional public-key cryptosystems such as the RSA cryptosystem and the elliptic curve cryptosystem, the RSA cryptosystem performing exponentiation, and the elliptic curve cryptosystem performing scalar multiplication on a point of an elliptic curve. [0007] In this NTRU cryptosystem, a decrypted text is generated by the processes in which the plaintext is encrypted using the public key to generate a cipher text, and then this cipher text is decrypted using the secret key. However, the mentioned processes have a possibility of yielding decrypted text that is different from the original plaintext. This phenomenon is called “decryption error”. Here, the patent reference 1, for example, discloses a method of avoiding such decryption errors. In this method, a plaintext is added additional information before being encrypted, and the cipher text is transmitted together with the hash value of the plaintext. [0008] Meanwhile, a mechanism called “key encapsulation mechanism” has recently been proposed as a new notion of the public-key cryptosystem (e.g. refer to the non-patent reference 3). This key encapsulation mechanism is an algorithm that enables distribution of a shared key between a transmission apparatus and a reception apparatus, using the public-key cryptosystem. In this mechanism, the transmission apparatus inputs a public key pk of a receiver into an encryption algorithm E, to generate a cipher text C and a shared key K, and transmits this cipher text C to the reception apparatus. Next, the reception apparatus inputs a secret key sk and the cipher text C into a decryption algorithm D, thereby obtaining the same shared key K as that the transmission apparatus owns. [0009] After both of the transmission apparatus and the reception apparatus have established therein the shared key K using the key encapsulation mechanism, as described above, the transmission apparatus encrypts the plaintext to be transmitted to the reception apparatus, according to the symmetric key cryptography and using the shared key K, to generate a cipher text, and transmits the generated cipher text to the reception apparatus. The reception apparatus, in turn, receives the cipher text, and decrypts the received cipher text according to the same symmetric key cryptography and using the shared key K, to generate decrypted text. [0010] With the key encapsulation mechanism, a transmitter cannot take a whole liberty with creation of a shared key, therefore is prevented from committing fraud even though information is only allowed to be distributed from the transmitter to the receiver. This is the distinctive feature that the conventional arts do not have. [0011] As one example of the mentioned key encapsulation mechanism, an algorithm called RSA-KEM is disclosed (e.g. the non-patent references 3). The following describes the RSA-KEM algorithm disclosed in the non-patent reference 3. [0012] (1) System Parameter of RSA-KEM [0013] The RSA-KEM has the following system parameter: [0014] hash function: G [0015] Note here that the hash function is detailed in the non-patent reference 1, and so will not be described here. [0016] (2) Public Key and Secret Key of RSA-KEM [0017] Prime numbers p and q are selected, to generate n=p*q. [0018] The least common multiple between (p−1) and (q−1) is calculated, and the result thereof is set as L. [0019] e that is coprime to L is randomly selected. Note that e is an element of ZL. d=1/e mod L is calculated. Here, ZL is a set comprised of {0, 1, 2 . . . , L−1}. [0020] A public key pk is set as (e, n), and a secret key sk as (d, n). [0021] (3) Encryption of RSA-KEM [0022] In encryption, the public key pk is inputted into an encryption algorithm KemE detailed below, to output a shared key K and a cipher text C. The encryption algorithm KemE is specifically as follows. [0023] Randomly generates, which is an element of Zn, where Zn is a set comprised of {0, 1, 2, . . . n−1}. [0024] Generate K=G(s). [0025] Generate C=s [0026] Output the shared key K and the cipher text C. [0027] (4) Decryption of RSA-KEM [0028] In decryption, the cipher text C and the secret key sk are inputted into a decryption algorithm KemD detailed below, to output a shared key K. The decryption algorithm KemD is specifically as follows. [0029] Generate s=C [0030] Generate G(s), and set K=G(s). [0031] Output the shared key K. [0032] When this RSA-KEM algorithm is applied to the cryptosystem where cryptographic communication is performed between its transmission apparatus and reception apparatus, first of all, the transmission apparatus acquires a public key pk of the reception apparatus that is the communication destination, derives a shared key K and a cipher text C by inputting the acquired public key pk into the aforementioned encryption algorithm KemE, and transmits the cipher text C to the reception apparatus. [0033] Next, the reception apparatus receives the cipher text C from the transmission apparatus, and derives a shared key K by inputting, into the aforementioned decryption algorithm KemD, the cipher text C that is received and a secret key sk that is owned by the reception apparatus. Here, the shared key K that the reception apparatus has derived is the same as that obtained by the transmission apparatus. [0034] The above-described RSA-KEM algorithm is summarized as follows. In the encryption algorithm KemE, a randomly generated element s is encrypted using a public key pk, to generate a cipher text C. Next, in the decryption algorithm KemD, the cipher text C is decrypted using a secret key sk, to obtain the random element s which is the same as that generated by the encryption algorithm KemE. In both of the encryption algorithm KemE and the decryption algorithm KemD, the same value for s can be inputted in the hash function G. Therefore, each algorithm can derive the same shared key K. [0035] As a result, the reception apparatus owning the secret key sk can derive a shared key K which is the same as that derived by the transmission apparatus. [0036] On the contrary, other reception apparatuses that do not know about the secret key sk cannot obtain the element s from the cipher text C, even if they have acquired the public key pk and received the cipher text C. This means that these reception apparatuses cannot derive the same shared key K as that derived by the transmission apparatus. [0037] As described above, the transmission apparatus and the reception apparatus are enabled to secretly share a shared key K. On this premise, the transmission apparatus encrypts communication content data to be transmitted to the reception apparatus, according to the symmetric key cryptography and using the shared key K, thereby transmitting the generated cipher text. After this, the reception apparatus receives the cipher text, and decrypts this cipher text according to the same symmetric key cryptography and using the same shared key K, to obtain the original communication content data. [0038] (Patent Reference 1) [0039] Japanese Laid-Open Patent application 2002-252611 [0040] (Non-Patent Reference 1) [0041] Tatsuaki Okamoto, Hirosuke Yamamoto “Modern cryptography”, Series/Mathematics in Information Science, Sangyotosho, 1997 (ISBN4-7828-5353-X C3355) [0042] (Non-Patent Reference 2) [0043] Jeffery Hoffstein, Jill Pipher, and Joseph H. Silverman, “NTRU: A ring based public key cryptosystem,” Lecture Notes in Computer Science, 1423, pp. 267-288, Springer-Verlag, 1998. [0044] (Non-Patent Reference 3) [0045] Victor Shoup, “A proposal for an ISO standard for public key encryption (version 2.1)”, online, Dec. 20, 2001 (retrieved on Sep. 29, 2002 on the Internet<URL: http://shoup.net/papers/iso-2 [0046] As described above, in the RSA-KEM algorithm, an element s is inputted in the hash function G to derive the shared key K, where the element s being hard to be derived from the cipher text C unless the secret key is known. Accordingly, the shared key K will not be derived unless the secret key is known. [0047] However, suppose that a decryption error occurs in the NTRU cryptography, in an attempt to destribute the shared key with use of the NTRU cryptography to which the RSA-KEM algorithm (i.e. key encapsulation mechanism) is applied. If such a decryption error occurs, then the correct element s will not be derived even with use of the secret key, and so the correct shared key K will not be derived. This is a case where different shared keys are derived between the transmission apparatus and the reception apparatus, which leads to a problem that cryptographic communication, from the transmission apparatus to the reception apparatus, is not performed with reliability. [0048] The object of the present invention, in view of the above-described problems, is to provide a key agreement system, a shared-key generation apparatus, a shared-key recovery apparatus, a shared-key generating method, a shared-key recovery method, a shared-key generating program, and a shared-key recovery program, which prevents derivation of different keys between the shared-key generation apparatus and the shared-key recovery apparatus. [0049] So as to achieve the above object, the present invention provides a key agreement system having a shared-key generation apparatus and a shared-key recovery apparatus, each apparatus establishing therein a same shared key in secrecy, where the shared-key generation apparatus includes: a seed-value generating unit operable to generate a seed value; a first shared-key generating unit operable to generate a blind value and a shared key, from the seed value; an encryption unit operable to encrypt the seed value based on the blind value, to generate encryption information; and a transmitting unit operable to transmit the encryption information, and the shared-key recovery apparatus includes: a receiving unit operable to receive the encryption information; a decryption unit operable to decrypt the encryption information, to generate a decryption seed value; a second shared-key generating unit operable to generate a decryption blind value and a decryption shared key, using the decryption seed value and according to a same method as used in the first shared-key generating unit; a re-encryption unit operable to encrypt the decryption seed value based on the decryption blind value, to generate re-encryption information; a judging unit operable to judge, based on the encryption information and the re-encryption information, whether the decryption shared key should be outputted; and an outputting unit operable, when the judging unit has judged affirmatively, to output the decryption shared key. [0050] According to this construction, the shared-key generation apparatus encrypts the generated seed value to generate encryption information, and transmits the generated encryption information, and the shared-key recovery apparatus generates a decryption seed value from the received encryption information, re-encrypts the decryption seed value to generate re-encryption information, and judges, based on the received encryption information and the re-encryption information newly generated, whether to output the decryption shared key. Therefore, a decryption shared key will be outputted if a shared key generated in the shared-key generation apparatus is identical to a decryption shared key generated in the shared-key recovery apparatus. In other words, the construction has an effect of preventing a decryption shared key from being outputted, if a shared key generated in the shared-key generation apparatus is not identical to a decryption shared key generated in the shared-key recovery apparatus. [0051] This is realized as follows. In this construction, the shared-key recovery apparatus generates a decryption blind value from the generated decryption seed value, in the same method used in the shared-key generation apparatus. Accordingly, if a correct decryption seed value is generated by the decryption unit of the shared-key recovery apparatus, then re-encryption information generated by the shared-key recovery apparatus is expected to be identical to encryption information generated by the shared-key generation apparatus. [0052] In addition, the shared-key generation apparatus generates a shared key and a blind value from the seed value, and encrypts the generated seed value based on the generated blind value. Therefore this construction has an effect of scrambling the seed value. [0053] Here, the shared-key generation apparatus may further include: an obtaining unit operable to obtain a content; and an encryption unit operable to encrypt the obtained content using the shared key, to generate an encrypted content, the transmitting unit may further transmit the encrypted content, the receiving unit may further receive the encrypted content, and the shared-key recovery apparatus may further include: a decryption unit operable to decrypt the received encrypted content using the decryption shared key, to generate a decrypted content; and an outputting unit operable to output the decrypted content. [0054] According to this construction, the shared-key generation apparatus encrypts the obtained content using the generated shared key, to generate an encrypted content, and the shared-key recovery apparatus decrypts the received encrypted content, using the outputted decryption shared key, to generate a decrypted content. Therefore, the construction has an effect of transmitting a content from the shared-key generation apparatus to the shared-key recovery apparatus, in secrecy. [0055] In addition, the present invention is a shared-key generation apparatus that notifies a destination apparatus about a shared key in secrecy, the shared-key generation apparatus including: a seed-value generating unit operable to generate a seed value; a shared-key generating unit operable to generate a blind value and a shared key, from the seed value; an encryption unit operable to encrypt the seed value based on the blind value, to generate encryption information; and a transmitting unit operable to transmit the encryption information. [0056] According to this construction, the shared-key generation apparatus generates a blind value from the seed value, and encrypts the generated seed value, based on the generated blind value. Therefore there is an effect of scrambling the seed value. [0057] Here, the shared-key generating unit may perform a one-way function on the seed value, to generate a functional value, and generate the blind value and the shared key from the functional value, the encryption unit may include: a public-key obtaining subunit operable to obtain a public key; and a public-key encryption subunit operable to perform a public-key encryption algorithm on the seed value, using the public key and the blind value, to generate an encryption seed value as the encryption information. [0058] According to this construction, the shared-key generation apparatus performs a one-way function on the seed value to generate a functional value, and generates the blind value and the shared key from the generated functional value. Therefore, it can be expected that the destination apparatus that receives the encryption information generates a blind value and a shared key respectively being the same as the blind value and the shared key, from the seed value that the destination apparatus has decrypted using the same method. [0059] In addition, the shared-key generation apparatus obtains a public key, and performs a public-key encryption algorithm on the seed value, using the obtained public key, to generate the encryption information. Therefore, the construction enables adoption of more secured public-key cryptosystem. [0060] Here, the public-key encryption algorithm may conform to an NTRU cryptosystem, the public-key obtaining subunit may obtain a public-key polynomial generated according to a key-generation algorithm of the NTRU cryptosystem, as the public key, the public-key encryption subunit may generate a seed-value polynomial from the seed value, generate a blind-value polynomial from the blind value, and encrypt the seed-value polynomial according to an encryption algorithm of the NTRU cryptosystem, using the public-key polynomial as a key, and using the blind-value polynomial to randomize the seed-value polynomial, to generate an encryption seed-value polynomial as the encryption seed value, and the transmitting unit may transmit the encryption seed-value polynomial as the encryption seed value. [0061] According to this construction, an NTRU encryption algorithm may be adopted as the public-key encryption algorithm. [0062] Here, the encryption unit may include: a public-key obtaining subunit operable to obtain a public key; a public-key encryption subunit operable to generate a blind value, perform the public-key encryption algorithm on the seed value using the public key and the blind value, to generate a public-key cipher text; and a function subunit operable to perform a second one-way function on at least one of the seed value, the blind value, and the shared key, to generate a second functional value, and the encryption unit may generate the encryption information that includes the public-key cipher text and the second functional value. [0063] According to this construction, the shared-key generation apparatus performs a second one-way function on the generated seed value, to generate a second functional value, and transmits the encryption information that includes the second functional value. Therefore, the destination apparatus can perform judgment, using the second functional value, on whether to output a decryption shared key, without performing re-encryption. [0064] Here, the shared-key generating unit may perform a one-way function on the seed value, to generate a functional value, and generate the blind value and the shared key from the functional value. [0065] According to this construction, the shared key is generated from the first functional value obtained by performing a first one-way function on the seed value. Therefore even when the seed value is revealed, it is still difficult to estimate the shared key. [0066] Here, the shared-key generating unit may perform a first one-way function on the seed value, to generate a first functional value, and generate the shared key from the first functional value, instead of generating the blind value and the shared key. [0067] According to this construction, the shared key is generated from the first functional value obtained by performing a first one-way function on the seed value. Therefore even when the seed value is revealed, it is still difficult to estimate the shared key. [0068] Here, the public-key encryption algorithm may conform to an NTRU cryptosystem, the public-key obtaining subunit may obtain a public-key polynomial generated according to a key-generation algorithm of the NTRU cryptosystem, as the public key, the public-key encryption subunit may generate a seed-value polynomial from the seed value, generate a blind-value polynomial from the blind value, encrypt the seed-value polynomial according to an encryption algorithm of the NTRU cryptosystem, using the public-key polynomial as a key, and using the blind-value polynomial to randomize the seed-value polynomial, to generate an encryption seed-value polynomial as the public-key cipher text, and the encryption unit may generate the encryption information that includes the encryption seed-value polynomial as the public-key cipher text and the second functional value. [0069] According to this construction, an NTRU encryption algorithm may be adopted as the public-key encryption algorithm. [0070] Here, the shared-key generating unit may perform a one-way function on the seed value, to generate a functional value, and generate a verification value, the blind value, and the shared key, from the functional value, the encryption unit may include: a public-key obtaining subunit operable to obtain a public key; a first encryption subunit operable to perform a public-key encryption algorithm on the verification value, using the public key and the blind value, to generate a first cipher text; and a second encryption subunit operable to perform, on the seed value, a computation algorithm different from the public-key encryption algorithm, to generate a second cipher text, and the encryption unit may generate the encryption information that includes the first cipher text and the second cipher text. [0071] According to this construction, the shared-key generation apparatus performs a public-key encryption algorithm on the generated verification value, using the obtained public key and the generated blind value, to generate a first cipher text, performs a different computation algorithm on the generated seed value, based on the generated verification value, to generate a second cipher text, and transmits the encryption information that includes the first cipher text and the second cipher text. Since the construction uses a two-phase algorithm as such, chances of the first and second cipher texts being attacked and decrypted will be lowered. [0072] Here, the public-key encryption algorithm may conform to an NTRU cryptosystem, the public-key obtaining subunit may obtain a public-key polynomial generated according to a key-generation algorithm of the NTRU cryptosystem, as the public key, the first encryption subunit may generate a verification-value polynomial from the verification value, generate a blind-value polynomial from the blind value, and encrypt the verification-value polynomial according to an encryption algorithm of the NTRU cryptosystem, using the public-key polynomial as a key, and using the blind-value polynomial to randomize the verification-value polynomial, to generate an encryption verification-value polynomial as the first cipher text, and the encryption unit may generate the encryption information that includes the encryption verification-value polynomial as the first cipher text and the second cipher text. [0073] According to this construction, an NTRU encryption algorithm may be adopted as the public-key encryption algorithm. [0074] Here, the different computation algorithm may be a symmetric key encryption algorithm, and the second encryption subunit may perform the symmetric key encryption algorithm on the seed value using the verification value as a key, to generate the second cipher text. [0075] Alternatively, the different computation algorithm may be bitwise exclusive-or, and the second encryption subunit may perform the bitwise exclusive-or on the verification value and the seed value, to generate the second cipher text. [0076] Still alternatively, the different computation algorithm may be addition, and the second encryption subunit may perform the addition on the verification value and the seed value, to generate the second cipher text. [0077] Still alternatively, the different computation algorithm may be multiplication, and the second encryption subunit may perform the multiplication on the verification value and the seed value, to generate the second cipher text. [0078] According to these constructions, symmetric key encryption algorithm, bitwise exclusive-or, addition, and multiplication may be adopted as the different computation algorithm. [0079] Here, the seed-value generating unit may generate a random number, as the seed value. [0080] According to this construction, the shared-key generation apparatus generates a random number, and sets the generated random number as the seed value. This realizes generation of a seed value which is different from another seed value that has been generated first during a series of processes as follows: generating a seed value, generating a blind value and a shared key, generating encryption information, and transmitting the encryption information. Accordingly, the encryption information will be different each time of transmission from the shared-key generation apparatus. Therefore, even if an unauthorized third party illegally intercepts and records the encryption information, it is quite difficult for him to guess an original seed value, from the recorded encryption information. [0081] Here, the shared-key generating unit may perform a one-way function on the seed value, to generate a functional value, and generate the blind value and the shared key from the functional value. [0082] According to this construction, the shared-key generation apparatus performs a one-way function on the seed value to generate a functional value, and generates the blind value and the shared key from the generated functional value. Therefore it can be expected that the destination apparatus that receives the encryption information generates a blind value and a shared key respectively being the same as the blind value and the shared key, from the seed value that the destination apparatus has decrypted using the same method. [0083] Here, the one-way function may be a hash function, and the shared-key generating unit may perform the hash function on the seed value. [0084] According to this construction, the shared-key generation apparatus performs a hash function on the seed value. Therefore a functional value is a ssuredly obtained. [0085] Here, the shared-key generating unit may generate the blind value by setting a part of the functional value as the blind value, and generate the shared key by setting another part of the functional value as the shared key. [0086] According to this construction, the shared-key generation apparatus sets a part of the generated functional value as the blind value, and sets another part thereof as the shared key. Therefore the blind value and the shared key will be obtained assuredly from a functional value. [0087] Here, the shared-key generation apparatus may further include: an obtaining unit operable to obtain a content; and an encryption unit operable to encrypt the obtained content using the shared key, to generate an encrypted content, where the transmitting unit further transmits the encrypted content. [0088] According to this construction, the shared-key generation apparatus encrypts the obtained content using the generated shared key to generate an encrypted content, and transmits the encrypted content. Accordingly, there is an effect of transmitting a content decryptable by the destination apparatus, in secrecy. [0089] In addition, the present invention is a shared-key recovery apparatus that receives a shared key from a shared-key generation apparatus in secrecy, the shared-key generation apparatus generating a seed value, generating a blind value and a shared key from the seed value, encrypting the seed value based on the blind value to generate encryption information, and transmitting the encryption information, the shared-key recovery apparatus including: a receiving unit operable to receive the encryption information; a decryption unit operable to decrypt the encryption information, to generate a decryption seed value; a shared-key generating unit operable to generate a decryption blind value and a decryption shared key, using the decryption seed value and according to a same shared-key generating method used in the shared-key generation apparatus; a re-encryption unit operable to encrypt the decryption seed value based on the decryption blind value, to generate re-encryption information; a judging unit operable-to judge, based on the encryption information and the re-encryption information, whether the decryption shared key should be outputted; and an outputting unit operable, when the judging unit has judged affirmatively, to output the decryption shared key. [0090] According to this construction, the shared-key recovery apparatus generates a decryption seed value from the received encryption information, re-encrypts the generated decryption seed value, to generate re-encryption information, and judges, based on the received encryption information and the re-encryption information newly generated, whether to output the decryption shared key. Therefore, a decryption shared key will be outputted if a shared key generated in the shared-key generation apparatus is identical to a decryption shared key generated in the shared-key recovery apparatus. In other words, the construction has an effect of preventing a decryption shared key from being outputted, if a shared key generated in the shared-key generation apparatus is not identical to a decryption shared key generated in the shared-key recovery apparatus. [0091] Here, the shared-key generation apparatus may perform a one-way function on the seed value to generate a functional value, generate the blind value and the shared key from the functional value, obtain a public key, perform a public-key encryption algorithm on the seed value using the public key and the blind value, to generate an encryption seed value as the encryption information, and transmit the encryption seed value, the receiving unit may receive the encryption seed value as the encryption information, the decryption unit may include: a secret-key obtaining subunit operable to obtain a secret key that corresponds to the public key; and a public-key decryption subunit operable to perform, on the received encryption seed value, a public-key decryption algorithm that corresponds to the public-key encryption algorithm, using the obtained secret key, to generate the decryption seed value, the shared-key generating unit may perform the one-way function on the decryption seed value to generate a decryption functional value, and generate the decryption blind value and the decryption shared key from the decryption functional value, the re-encryption unit may include: a public-key obtaining subunit operable to obtain the public key; and a re-encryption subunit operable to perform the public-key encryption algorithm on the decryption seed value using the public key and the decryption blind value, to generate a re-encryption seed value as the re-encryption information, and the judging unit may judge whether the encryption seed value is identical to the re-encryption seed value, and when judging affirmatively, determine that the decryption shared key should be outputted. [0092] According to this construction, the shared-key recovery apparatus judges whether the encryption seed value is identical to the re-encryption seed value. Since judgment as to whether to output a decryption shared key results in affirmative when these values are identical, there is an effect of assuredly performing the outputting judgment. [0093] Here, the public-key encryption algorithm and the public-key decryption algorithm may conform to an NTRU cryptosystem, the shared-key generation apparatus may obtain a public-key polynomial generated according to a key-generation algorithm of the NTRU cryptosystem, as the public key, generate a seed-value polynomial from the seed value, generate a blind-value polynomial from the blind value, encrypt the seed-value polynomial according to an encryption algorithm of the NTRU cryptosystem, using the public-key polynomial as a key, and using the blind-value polynomial to randomize the seed-value polynomial, to generate an encryption seed-value polynomial as the encryption seed value, and transmit the encryption seed-value polynomial as the encryption seed value, the receiving unit may receive the encryption seed-value polynomial as the encryption seed value, the secret-key obtaining subunit may obtain a secret-key polynomial generated according to the key-generation algorithm of the NTRU cryptosystem, as the secret key, the public-key decryption subunit may decrypt the received encryption seed-value polynomial according to a decryption algorithm of the NTRU cryptosystem and using the obtained secret-key polynomial as a key, to generate a decryption seed-value polynomial, and generates the decryption seed value from the decryption seed-value polynomial, the public-key obtaining subunit may obtain the public-key polynomial as the public key, the re-encryption subunit may generate a seed-value polynomial from the decryption seed value, generate a blind-value polynomial from the decryption blind value, and encrypt the seed-value polynomial according to the encryption algorithm of the NTRU cryptosystem, using the public-key polynomial as a key, and using the blind-value polynomial to randomize the seed-value polynomial, to generate a re-encryption seed-value polynomial, and the judging unit may judge whether the encryption seed-value polynomial is identical to the re-encryption seed-value polynomial. [0094] According to this construction, an NTRU encryption algorithm may be adopted as the public-key encryption algorithm and the public-key decryption algorithm. [0095] Here, the shared-key generation apparatus may obtain a public key, generate a blind value, perform a public-key encryption algorithm on the seed value using the public key and the blind value to generate a public-key cipher text, perform a second one-way function on at least one of the seed value, the blind value, and the shared key to generate a second functional value, generate the encryption information that includes the public-key cipher text and the second functional value, and transmit the encryption information, the receiving unit may receive the encryption information that includes the public-key cipher text and the second functional value, the decryption unit may include: a secret-key obtaining subunit operable to obtain a secret key that corresponds to the public key; a public-key decryption subunit operable to perform, on the public-key cipher text included in the received encryption information, a public-key decryption algorithm that corresponds to the public-key encryption algorithm, to generate a decryption seed value; and a function subunit operable to perform the second one-way function on at least one of the decryption seed value, the decryption blind value, and the decryption shared key, to generate a decryption second functional value, and the judging unit may judge whether the second functional value included in the received encryption information is identical to the decryption second functional value instead of performing judging based on the encryption information and the re-encryption information, and when judging affirmatively, determine that the decryption shared key should be outputted. [0096] According to this construction, instead of basing the encryption information and the re-encryption information, the judgment is performed as to whether the second functional value included in the received encryption information is identical to the generated decryption second functional value, then decides to output the decryption shared key when the mentioned values are judged to be identical. Therefore, there is an effect of assuredly performing the outputting judgment. [0097] Here, the shared-key generation apparatus may perform a one-way function on the seed value to generate a functional value, and generate the blind value and the shared key from the functional value, and the shared-key generating unit may perform the first one-way function on the decryption seed value to generate a decryption functional value, and generate the decryption blind value and the decryption shared key from the decryption functional value. [0098] According to this construction, the decryption shared key is generated from the decryption functional value obtained by performing the first one-way function on the decryption seed value. Therefore even when the decryption seed value is revealed, it is still difficult to estimate the decryption shared key. [0099] Here, the shared-key generation apparatus may perform a first one-way function on the seed value to generate a first functional value, and generate the shared key from the first functional value, instead of generating the blind value and the shared key, and the shared-key generating unit may perform the first one-way function on the decryption seed value to generate a decryption functional value, and generate the decryption shared key from the decryption functional value, instead of generating the decryption blind value and the decryption shared key. [0100] According to this construction, the decryption shared key is generated from the decryption functional value obtained by performing the first one-way function on the decryption seed value. Therefore even when the decryption seed value is revealed, it is still difficult to estimate the decryption shared key. [0101] Here, the public-key encryption algorithm and the public-key decryption algorithm may conform to an NTRU cryptosystem, the shared-key generation apparatus may obtain a public-key polynomial generated according to a key-generation algorithm of the NTRU cryptosystem, as the public key, generate a seed-value polynomial from the seed value, generate a blind-value polynomial from the blind value, encrypt the seed-value polynomial according to an encryption algorithm of the NTRU cryptosystem using the public-key polynomial as a key and using the blind-value polynomial to randomize the seed-value polynomial, to generate an encryption seed-value polynomial as the public-key cipher text, and generate the encryption information that includes the encryption seed-value polynomial as the public-key cipher text and the second functional value, the secret-key obtaining subunit may obtain a secret-key polynomial generated according to the key-generation algorithm of the NTRU cryptosystem, as the secret key, and the public-key decryption subunit may generate a public-key cipher-text polynomial from the public-key ciphertext, decrypts the public-key cipher-text polynomial according to a decryption algorithm of the NTRU cryptosystem using the secret-key polynomial as a key to generate a decryption seed-value polynomial, and generate the decryption seed value from the decryption seed-value polynomial. [0102] According to this construction, an NTRU encryption algorithm may be adopted as the public-key encryption algorithm and the public-key decryption algorithm. [0103] Here, the shared-key generation apparatus may perform a one-way function on the seed value to generate a functional value, generate a verification value, the blind value, and the shared key from the functional value, obtain a public key, perform a public-key encryption algorithm on the verification value using the public key and the blind value to generate a first cipher text, performs, based on the verification value, a computation algorithm different from the public-key encryption algorithm on the seed value, to generate a second cipher text, generate the encryption information that includes the first cipher text and the second cipher text, and transmit the encryption information, the receiving unit may receive the encryption information that includes the first cipher text and the second cipher text, the decryption unit may include: a secret-key obtaining subunit operable to obtain a secret key that corresponds to the public key; a public-key decryption subunit operable to perform, on the first cipher text included in the received encryption information, a public-key decryption algorithm that corresponds to the public-key encryption algorithm, to generate a decryption verification value; and a computation decryption subunit operable to perform, on the second cipher text included in the received encryption information, a computation algorithm for performing an inverse computation of the different computation algorithm, to generate a decryption seed value, the shared-key generating unit may perform the one-way function on the decryption seed value to generate a decryption functional value, and generate a decryption verification value, the decryption blind value, and the decryption shared key, from the decryption functional value, the re-encryption unit may include: a public-key obtaining subunit operable to obtain the public key; and a re-encryption subunit operable to perform, on the decryption verification value, the public-key encryption algorithm using the public key and the decryption blind value, to generate the re-encryption information, and the judging unit may judge whether the first cipher text included in the encryption information is identical to the re-encryption information, and when judging affirmatively, determine that the decryption shared key should be outputted. [0104] According to this construction, a decryption verification value is generated by performing, on the first cipher text, a public-key decryption algorithm that corresponds to the public-key encryption algorithm, and based on thus generated decryption verification value, the computation algorithm is performed on the second cipher text, to generate a decryption seed value. Since the construction uses a two-phase algorithm as such, chances of the first and second cipher texts being attacked and decrypted will be lowered. [0105] Here, the public-key encryption algorithm and the public-key decryption algorithm may conform to an NTRU cryptosystem, the shared-key generation apparatus may obtain a public-key polynomial generated according to a key-generation algorithm of the NTRU cryptosystem, as the public key, generate a verification-value polynomial from the verification value, generate a blind-value polynomial from the blind value, encrypt the verification-value polynomial according to an encryption algorithm of the NTRU cryptosystem, using the public-key polynomial as a key, and using the blind-value polynomial to randomize the verification-value polynomial, to generate an encryption verification-value polynomial as the first cipher text, generate the encryption information that includes the encryption verification-value polynomial as the first cipher text and the second cipher text, and transmit the encryption information, the receiving unit may receive the encryption information that includes the encryption verification-value polynomial and the second cipher text, the secret-key obtaining subunit may obtain a secret-key polynomial generated according to the key-generation algorithm of the NTRU cryptosystem, as the secret key, the public-key decryption subunit may generate a first cipher-text polynomial from the first cipher text, decrypt the first cipher-text polynomial according to a decryption algorithm of the NTRU cryptosystem using the secret-key polynomial as a key, to generate a decryption verification polynomial, and generate the decryption verification value from the decryption verification-value polynomial, the public-key obtaining subunit may obtain the public-key polynomial, the re-encryption subunit may generate a decryption verification-value polynomial from the decryption verification value, generate a blind-value polynomial from the decryption blind value, and encrypt the decryption verification-value polynomial according to the encryption algorithm of the NTRU cryptosystem, using the public-key polynomial as a key, and using the blind-value polynomial to randomize the decryption verification-value polynomial, to generate a re-encryption verification-value polynomial as the re-encryption information, and the judging unit may judge whether the encryption verification-value polynomial as the first cipher text is identical to the re-encryption verification-value polynomial as the re-encryption information. [0106] According to this construction, an NTRU encryption algorithm may be adopted as the public-key encryption algorithm and the public-key decryption algorithm. [0107] Here, the different computation algorithm may be a symmetric key encryption algorithm, and the computation algorithm for performing the inverse computation may be a corresponding symmetric key decryption algorithm, and the computation decryption subunit may perform the symmetric key decryption algorithm on the second cipher text, using the decryption verification value as a key, to generate the decryption seed value. [0108] Alternatively, the different computation algorithm and the computation algorithm for performing the inverse computation may be bitwise exclusive-or, and the computation decryption subunit may perform the bitwise exclusive-or on the decryption verification value and the second cipher text, to generate the decryption seed value. [0109] Still alternatively, the different computation algorithm may be addition and the computation algorithm for performing the inverse computation be subtraction, and the computation decryption subunit may perform the subtraction on the decryption verification value and the second cipher text, to generate the decryption seed value. [0110] Still alternatively, the different calculation algorithm may be multiplication and the computation algorithm for performing the inverse computation be division, and the computation decryption subunit may perform the division on the decryption verification value and the second cipher text, to generate the decryption seed value. [0111] According to these constructions, symmetric key decryption algorithm, bitwise exclusive-or, subtraction, and division may be adopted as the computation algorithm for performing the inverse computation. [0112] Here, the shared-key generating unit may perform a one-way function on the decryption seed value to generate a functional value, and generate the decryption blind value and the decryption shared key from the functional value. [0113] According to this construction, a one-way function is performed on a seed value to generate a functional value, and a decryption blind value and a decryption shared key are generated from the generated functional value. Therefore, the same method as used in the shared-key generation apparatus is used. [0114] Here, the one-way function may be a hash function, and the shared-key generating unit may perform the hash function on the decryption seed value. [0115] According to this construction, a hash function is performed on the decryption seed value. Therefore, a functional value will be assuredly obtained. [0116] Here, the shared-key generating unit may generate the decryption blind value by setting a part of the functional value as the decryption blind value, and generate the decryption shared key by setting another part of the functional value as the decryption shared key. [0117] According to this construction, a part of the generated functional value is set as the decryption blind value, and another part thereof as the decryption shared key. Therefore, the decryption blind value and the decryption shared key will be assuredly obtained from the functional value. [0118] Here, the shared- key generation apparatus may further obtain a content, encrypt the obtained content using the shared key to generate an encrypted content, and transmit the encrypted content, and the shared-key recovery apparatus may further include: a content receiving unit operable to receive the encrypted content; a decryption unit operable to decrypt the received encrypted content using the outputted decryption shared key, to generate a decrypted content; and a playback unit operable to playback the decrypted content. [0119] According to this construction, the shared-key recovery apparatus decrypts the received encrypted content, using the outputted decryption shared key, to generate a decrypted content. Therefore, there is an effect that a content is received from the shared-key generation apparatus, in secrecy. [0120] These and other objects, advantages and features of the invention will become apparent from the following description thereof taken in conjunction with the accompanying drawings which illustrate a specific embodiment of the invention. In the drawings: [0121]FIG. 1 is a conceptual diagram showing the structure of a content distribution system [0122]FIG. 2 is a block diagram showing the structure of an encryption apparatus [0123]FIG. 3 is a block diagram showing the structure of a decryption apparatus [0124]FIG. 4 is a process-block diagram showing the operations of the encryption apparatus [0125]FIG. 5 is a flowchart showing the operations of the encryption apparatus [0126]FIG. 6 is a block diagram showing the structure of an encryption apparatus [0127]FIG. 7 is a block diagram showing the structure of a decryption apparatus [0128]FIG. 8 is a process-block diagram showing the operations of the encryption apparatus [0129]FIG. 9 is a block diagram showing the structure of an encryption apparatus [0130]FIG. 10 is a block diagram showing the structure of a decryption apparatus [0131]FIG. 11 is a process-block diagram showing the operations of the encryption apparatus [0132]FIG. 12 is a process-block diagram showing the operations of a modification example for the encryption apparatus [0133]FIG. 13 is a block diagram showing the structure of an encryption apparatus [0134]FIG. 14 is a block diagram showing the structure of a decryption apparatus [0135]FIG. 15 is a flowchart showing the operations of the encryption apparatus [0136]FIG. 16 is a process-block diagram showing the operations of the encryption apparatus [0137]FIG. 17 is a block diagram showing the structure of an encryption apparatus [0138]FIG. 18 is a block diagram showing the structure of a decryption apparatus [0139]FIG. 19 is a process-block diagram showing the operations of the encryption apparatus [0140]FIG. 20 is a process-block diagram showing the operations of a modification example for the encryption apparatus [0141] 1. First Embodiment [0142] The following describes a content distribution system [0143] 1.1 NTRU Cryptosystem [0144] As follows, the NTRU cryptosystem used in the content distribution system [0145] Note that the NTRU cryptosystem and the method that the NTRU cryptosystem adopts for generating public key and secret key are detailed in the non-patent reference 2. [0146] (1) System Parameter of NTRU Cryptosystem [0147] In the NTRU cryptosystem, system parameters N, p, q (that are integers) exist, and the encryption apparatus and the decryption apparatus, which are detailed later, have these system parameters. [0148] In the mentioned reference, three examples of system parameters are listed, namely, (N, p, q)=(107, 3, 64), (N, p, q)=(167, 3, 128), and (N, p, q)=(503, 3, 256). [0149] Hereinafter in this embodiment, the system parameter N=167 is used for description. [0150] (2) Polynomial Operation in NTRU Cryptosystem [0151] As aforementioned, the NTRU cryptosystem is a public-key cryptosystem that performs encryption/decryption using polynomial operation. [0152] The polynomial used in the NTRU cryptosystem is N−1 degrees for the system parameter N. When, for example, N=5, the polynomial is X [0153] Furthermore, a public key h, a secret key f, a plaintext m, a random number r, and a cipher text c, which are used in encryption or decryption, are expressed as polynomial that is N−1 degree or below. (hereinafter, each are referred to as “public-key polynomial h”, “secret-key polynomial f”, “plaintext polynomial m”, “random-number polynomial r”, and “cipher text polynomial c”.) [0154] The polynomial operation is arranged to yield a result being a polynomial at N−1 degree or below, by using the relational expression X [0155] For example, when N=5, the product of X ( = = =2 [0156] As in the above, the polynomial operation is arranged always to yield a polynomial at N−1 degrees or below. [0157] (3) Encryption in NTRU Cryptosystem [0158] The encryption apparatus, which will be described later, performs encryption according to the NTRU cryptosystem, as described as follows. [0159] In encryption, the encryption algorithm E, which is a polynomial computation, is performed on the plaintext polynomial m, using a random-number polynomial r and a public-key polynomial h (which are detailed later), to generate a cipher text polynomial c=E(m, r, h). [0160] This E(m, r, h) is a result of the polynomial operation, which is obtained by inputting, in the NTRU cryptographic encryption algorithm E, the plaintext polynomial m, the random-number polynomial r, and the public-key polynomial h. The encryption algorithm E is detailed in the non-patent reference 2, therefore is not described here. [0161] Note that in the NTRU cryptosystem, a parameter d for generating the random polynomial r is determined in advance. The random polynomial r is selected so that, among the terms constituting the random-number polynomial r, the coefficient for d terms is 1, the coefficient for other d terms is −1, and the coefficient for the rest of the terms is 0. [0162] To summarize, the random-number polynomial r is a polynomial being N−1 degrees or below, and N coefficients exit for N terms from the degree 0 (constant term) to the degree N−1. The random-number polynomial r is selected so that, out of these N coefficients, d coefficients are 1, other d coefficients are −1, and (N−2d) coefficients are 0. [0163] In the non-patent reference [0164] (4) Decryption in NTRU Cryptosystem [0165] The decryption apparatus, which will be described later, performs decryption according to the NTRU cryptosystem, as described as follows. [0166] In decryption, the decryption algorithm D, which is a polynomial calculation, is performed on the cipher text polynomial c, using a secret-key polynomial f, to generate a decrypted text polynomial m′=D(c, f). [0167] This D (c, f)is a result of the polynomial operation, which is obtained by inputting, in the NTRU cryptographic decryption algorithm D, the cipher text polynomial c and the secret-key polynomial f. The decryption algorithm D is detailed in the non-patent reference 2, therefore is not described here. [0168] (5) Decryption Error in NTRU Cryptosystem [0169] In this NTRU cryptosystem, it sometimes happens that the generated decrypted text polynomial m′ is different from the plaintext polynomial m. In such a case, the correct plaintext m will not be obtained in decryption. This occurrence is called “decryption error”. [0170] 1.2 Structure of Content Distribution System [0171] The content distribution system [0172] The content server apparatus [0173] The encryption apparatus [0174] 1.3 Structure of Content Server Apparatus [0175] The content server apparatus [0176] The content server apparatus [0177] 1.4 Structure of Memory Card [0178] The memory card [0179] Meanwhile, the memory card [0180] Here, the secret-key polynomial f and the public-key polynomial h are generated according to the NTRU cryptosystem, and correspond to each other. [0181] 1.5 Structure of Encryption Apparatus [0182] The encryption apparatus [0183] The encryption apparatus [0184] (1) Public-Key Input Unit [0185] The public-key input unit [0186] (2) Random-Number Generating Unit [0187] The random-number generating unit [0188] (3) First Function Unit [0189] The first function unit [0190] (4) Encryption Unit [0191] The encryption unit [0192] The encryption unit [0193] For example, the encryption unit [0194] Next, the encryption unit [0195] Next, the encryption unit [0196] The first cipher text c [0197] Next, the encryption unit [0198] Note that in FIG. 2, each block representing a respective constituting part of the encryption apparatus [0199] (5) First Transmitting Unit [0200] The first transmitting unit [0201] (6) Shared-Key Encryption Unit [0202] The shared-key encryption unit [0203] Generally, in the symmetric key cryptography, an apparatus at the encryption side performs a symmetric key cryptographic algorithm Sym on a plaintext m, using an encryption key K, to generate a cipher text=Sym(m, K), while an apparatus at the decryption side performs a symmetric key cryptographic algorithm Sym on the cipher text c, using an encryption key K, to generate a decrypted text m′=Sym(c, K) Here, if the encryption key K used in generation of the cipher text is identical to the encryption key K used in generation of the decrypted text, then m′=m holds. Note that the symmetric key cryptography and the DES cryptosystem are detailed in the non-patent reference 1, therefore detailed description thereof is omitted here. [0204] Next, the shared-key encryption unit [0205] (7) Second Transmitting Unit [0206] The second transmitting unit [0207] 1.6 Structure of Decryption Apparatus [0208] The decryption apparatus [0209] The decryption apparatus [0210] (1) Secret-Key Input Unit [0211] The secret-key input unit [0212] (2) First Receiving Unit [0213] The first receiving unit [0214] (3) Decryption Unit [0215] The decryption unit [0216] The decryption unit [0217] Concretely, when the decryption random-number polynomial sp′=X [0218] Next, the decryption unit [0219] (4) Second Function Unit [0220] The second function unit [0221] The second function unit [0222] (5) Comparison Unit [0223] The comparison unit [0224] The encryption unit [0225] The comparison computation unit [0226] (6) Second Receiving Unit [0227] The second receiving unit [0228] (7) Shared-Key Decryption Unit [0229] The shared-key decryption unit [0230] The shared-key decryption unit [0231] Next, the shared-key decryption unit [0232] 1.7 Playback Apparatus [0233] The playback apparatus [0234] The monitor [0235] 1.8 Operation Performed by Encryption Apparatus [0236] The operations performed by the encryption apparatus [0237] The public-key input unit [0238] Then, the random-number generating unit [0239] The first function unit [0240] Next, the encryption unit [0241] The first transmitting unit [0242] Next, the secret-key input unit [0243] The first receiving unit [0244] Next, the decryption unit [0245] The second function unit [0246] Next, the comparison unit [0247] The shared-key encryption unit [0248] Next, the second transmitting unit [0249] If the first cipher text c [0250] The shared-key decryption unit [0251] 1.9 Operation Verification of Content Distribution System [0252] As follows, the entire operation performed by the content distribution system [0253] First, the encryption apparatus [0254] Specifically, this encryption apparatus [0255] Generate a random number s. [0256] Generate G(s), and generate u and K, from the G(s). [0257] Generate a first cipher text c [0258] Output the shared key K and the first cipher text c [0259] Next, the encryption apparatus [0260] On the other hand, the decryption apparatus [0261] Specifically, this decryption apparatus [0262] Decrypt the first cipher text c [0263] Generate G (s′), and generate u′ and K′ from the G (s′). [0264] Generate a first re-cipher text c [0265] Check to see if c [0266] Here, if the decryption apparatus [0267] Next, the decryption apparatus [0268] Note that if a decryption error has occurred, the decryption random number s′ and the random number s are not identical. The random-number value u′ and the shared key K′ that are derived from the G(s′) will be respectively different from u and k, too. In this case however, s′ and u′ will be respectively different from s and u, too. Therefore, the first re-cipher text c [0269] 1.10 Effect of First Embodiment [0270] In the conventional RSA-KEM algorithm, an element s will be inputted into the hash function G to derive a shared key K, the element s being unable to be derived from the cipher text C unless the secret key is known. However, there is a possibility of decryption error if a shared key is attempted to be distributed, using the NTRU cryptosystem and applying the RSA-KEM algorithm that is a key encapsulation mechanism. This means that occasionally the elements cannot be derived even using the secret key, thereby deriving an incorrect shared key K′. [0271] However the content distribution system, the encryption/decryption apparatuses that relate to the first embodiment are able to prevent derivation of different keys between the encryption apparatus and the decryption apparatus even when a decryption error occurs. This is realized by the processes of the first embodiment. In this process, in addition to a shared key, a random-number value u is generated from the hash functional value G(s) of the random number s, and the decryption apparatus re-encrypts the decryption random number s′ using the random-number value u and the public-key polynomial h, to generate a first re-cipher text c [0272] In addition, according to the system of the present invention, the security can be logically verified using the same method as the verification method described in the non-patent reference 3. [0273] 1.11 Modification Example [0274] The first embodiment described above is one example of carrying out the present invention. Needless to say, the present invention is not limited to this particular embodiment, and can be carried with various modifications as long as they are within the scope of the present invention. In light of this, the following cases are included in the present invention. [0275] (1) The Parameter N to be Used in NTRU Cryptosystem May Take Other Value than [0276] (2) The conversion method between the element of each bit in the bit sequence and the coefficient of each term in the polynomial, which is performed in the encryption unit [0277] For example, the conversion of the random number s to the random-number polynomial sp may be performed using a function that corresponds the element of each bit in the bit sequence to the coefficient of each term in the polynomial, in one-to-one relation. Alternatively, the mentioned conversion may be performed using a functional-value table that stores the element of each bit in the bit sequence and the coefficient of each term in the polynomial in one-to-one relation. [0278] Moreover, the conversion from the random-number value u to the random-number polynomial r may be performed in other methods, as long as the following conditions are held: r is uniquely obtained from u, and among r, the coefficient of d terms of degree is 1, the coefficient of d terms of degree is −1, and the coefficient of other terms of degree is 0. For example, the conversion may be performed using a function or a functional-value table, which correspond a random-number value u to a polynomial. [0279] (3) The public-key cryptosystem, used in the encryption unit [0280] Accordingly, the public-key cryptosystem used in the encryption unit [0281] For example, if the E [0282] Note that the E [0283] (4) In the first embodiment, the first function unit [0284] For example, the k/2 highest-order bits of the functional value G(s) may be set as a random-number value u, and the k*3/2 lowest-order bits may be set as a shared key K. Alternatively, as a random-number value u, k bits may be selected so that every other bit in the 2 k bits of the functional value G(s) is selected, and the other k bits may be set as a shared key K. [0285] (5) In the first embodiment, the random-number value u is generated in the first function unit [0286] For example, u=Func(s) may be used with respect to an arbitrary function Func, so that the encryption apparatus [0287] generate G(s), and generate K from the G(s), and [0288] generate Func(s), and sets u=Func(s). [0289] (6) Further, the random-number value u is generated in the first function unit [0290] To be more specific, the first cipher text c [0291] The encryption apparatus [0292] generates G(s), and generates K from the G(s), and [0293] transmits the random-number value u separately, from the encryption apparatus [0294] The decryption apparatus [0295] receives the random-number value u, and [0296] generates a first re-cipher text c [0297] At this time, it may be arranged that the encryption apparatus [0298] (7) As for the random-number value u, the condition is that the encryption apparatus [0299] For instance, the encryption apparatus [0300] The encryption apparatus [0301] (a) generates G (s), and generates K, u1 from the G (s), [0302] (b) generates the random-number value u2, and separately transmit the random-number value u2 to the decryption apparatus [0303] (c) generates a random-number value u from u=u1 xor u2, and [0304] (d) generates a first cipher text c [0305] The decryption apparatus [0306] (e) receives the random-number value u2, [0307] (f) generates G(s′), and generates K′, and u [0308] (g) generates a random-number value u′ from u′=u1′ xor u2, and [0309] (h) generates a first re-cipher text c [0310] At this time, the encryption apparatus [0311] In (c) and (g), other computation may be performed in place of bitwise exclusive-or. For example, in (c) and (g), addition and subtraction may be used respectively. Alternatively, multiplication and division may be used. [0312] (8) In the first embodiment, the shared key K′ is outputted when the first re-cipher text c [0313] In this case, the first function unit [0314] A concrete example therefor is described as follows. [0315] The content distribution system [0316] The encryption apparatus [0317] Next, the encryption unit [0318] The first transmitting unit [0319] Next, in the decryption apparatus [0320] The decryption unit [0321] Then, the second function unit [0322] In the comparison unit [0323] In this case, for further heightening security, the method disclosed in the patent reference 1 may be used where encryption is performed on a random number s added additional information, so as to generate a first cipher text c [0324] In addition, as shown in the patent reference 1, the value of an invertible conversion of s and Ra, namely F(s, Ra), may be used instead of the value of s||Ra. [0325] 2. Second Embodiment [0326] The following describes a content distribution system [0327] The content distribution system [0328] The following description focuses on the differences mentioned above. [0329] 2.1 Structure of Content Distribution System [0330] The content distribution system [0331] 2.2 Structure of Encryption Apparatus [0332] The encryption apparatus [0333] The following describes the random-number generating unit [0334] (1) Random-Number Generating Unit [0335] The random-number generating unit [0336] (2) First Function Unit [0337] The first function unit [0338] Here, the function G is a hash function having output length of 3 k bits. The first function unit [0339] Next, the first function unit [0340] (3) Encryption Unit [0341] The encryption unit [0342] The encryption unit [0343] Next, the encryption unit [0344] Next, the encryption unit [0345] Next, the encryption unit [0346] (4) Random-Number Mask Unit [0347] The random-number mask unit [0348] Here, xor is an operator representing bitwise exclusive-or. [0349] Note that the random-number mask unit [0350] (5) First Transmitting Unit [0351] The first transmitting unit [0352] 2.2 Structure of Decryption Apparatus [0353] The decryption apparatus [0354] Here, the first receiving unit [0355] (1) First Receiving Unit [0356] The first receiving unit [0357] (2) Decryption Unit [0358] The decryption unit [0359] The decryption unit [0360] Next, the decryption unit [0361] (3) Random-Number Mask Removal Unit [0362] The random-number mask removal unit [0363] Note that when the random-number mask unit [0364] (4) Second Function Unit [0365] The second function unit [0366] The second function unit [0367] (5) Comparison Unit [0368] The comparison unit [0369] The encryption unit [0370] Furthermore, the comparison computation unit [0371] 2.3 Operation Performed by Content Distribution System [0372] As follows, the whole operation performed by the content distribution system [0373] The encryption apparatus [0374] Specifically, this encryption apparatus [0375] (a) Generate a random number s. [0376] (b) Generate G(s), and generate a, K, and u from the G(s). [0377] (c) Generate a first cipher text c [0378] (d) Generate c [0379] Next, the encryption apparatus [0380] On the other hand, the decryption apparatus [0381] Specifically, this decryption apparatus [0382] (a) Decrypt a first cipher text c [0383] (b) Generate s′=c [0384] (c) Generate G(s′), and generate a″, K′, u′ from the G(s′) [0385] (d) Generate a first re-cipher text c [0386] (e) Check to see if c [0387] Here, if the decryption apparatus [0388] Next, the decryption apparatus [0389] Here, since the encryption key K (used for generation of shared-key cipher text) is identical to the encryption key K′ (used for generation of decrypted text), the decryption apparatus [0390] Note that if a decryption error has occurred, the decryption verification value a′ and the verification value a are not identical. The decryption random number s' obtained from the second cipher text c [0391] 2.4 Effect of Second Embodiment [0392] In the conventional RSA-KEM algorithm, an element s will be inputted into the hash function G to derive a shared key K, the element s being unable to be derived from the cipher text C unless the secret key is known. However, there is a possibility of decryption error if a shared key is attempted to be distributed, using the NTRU cryptosystem and applying the RSA-KEM algorithm that is a key encapsulation mechanism. This means that occasionally the element s cannot be derived even using the secret key, thereby deriving an incorrect shared key K′. [0393] However the content distribution system, the encryption/decryption apparatuses that relate to the second embodiment are able to prevent derivation of different key between the encryption apparatus and the decryption apparatus even when a decryption error occurs. This is realized by the process of the second embodiment, as follows. In this process, in addition to a shared key, a verification value a and a random-number value u are generated from the hash functional value G(s) of the random number s, and the decryption apparatus re-encrypts the decryption verification value a′ using the random-number value u and the public-key polynomial h, to generate a first re-cipher text c [0394] In addition, according to the method of the present invention, the security can be logically verified using the same method as the verification method described in the non-patent reference 3. [0395] 2.5 Modification Example [0396] The second embodiment described above is one example of carrying out the present invention. However, the present invention is not limited to this particular embodiment, and can be carried with various modifications as long as they are within the scope of the present invention. Needless to say, the same modifications as those in the first embodiment can be applied hereto, but the following cases are also included in the present invention. [0397] (1) The conversion from the verification value a to the verification-value polynomial ap may be other methods. For example, the conversion may be performed using a function that corresponds the element of each bit in the bit sequence to the coefficient of each term in the polynomial, in one-to-one relation. Alternatively, the mentioned conversion may be performed using a functional-value table that stores the element of each bit in the bit sequence and the coefficient of each term in the polynomial in one-to-one relation. [0398] In addition, the conversion from the random-number value u to the random-number polynomial r may be performed in other methods, as long as the following conditions are held: r is uniquely obtained from r, and the coefficient of d terms of degree is 1, the coefficient of d terms of degree is −1, and the coefficient of other terms of degree is 0. For example, the conversion may be performed using a function or a functional-value table, which correspond a random-number value u to a polynomial. [0399] (2) The public-key cryptosystem, used in the encryption unit [0400] For example, if the E [0401] (3) In the second embodiment, the random-number value u is generated in the first function unit [0402] For example, u=Func(s) may be used with respect to an arbitrary function Func, so that the encryption apparatus [0403] Generate G(s), and generate a and K from the G(s). [0404] Generate Func(s), and sets u=Func(s). [0405] (4) Moreover, the random-number value u is generated in the first function unit [0406] More specifically, the encryption apparatus [0407] Generate G(s), and generate a, and K from the G(s). [0408] The encryption apparatus [0409] (5) As for the random-number value u, the condition is that the encryption apparatus [0410] For instance, the encryption apparatus [0411] Generate G(s), and generate a, K, u1, from the G(s). [0412] The encryption apparatus [0413] The encryption apparatus [0414] (6) The decryption apparatus [0415] (7) The decryption apparatus [0416] (8) In the second embodiment, the shared key K′ is outputted when the first re-cipher text c [0417] 3. Summary of First and Second Embodiments [0418] As described so far, the present invention is a shared-key generation apparatus, which outputs shared-key data, and encryption shared-key data resulting from encrypting the shared-key data based on predetermined public-key data. The shared-key generation apparatus specifically includes: a secret-number data generating unit operable to generate secret-number data; a shared-key derivation unit operable to convert the secret-number data into random-number data and the shared-key data, based on a predetermined process; and a first encryption unit operable to encrypt the secret-number data based on the public-key data and the random-number data, to generate encryption shared-key data. [0419] In addition, the present invention is a shared-key generation apparatus, which outputs shared-key data, and encryption shared-key data resulting from encrypting the shared-key data based on predetermined public-key data. The shared-key generation apparatus specifically includes: a secret-number generating unit operable to generate secret-number data; a shared-key derivation unit operable to convert the secret-number data into verification-value data, random-number data, and the shared-key data; a first encryption unit operable to encrypt the verification-value data based on the public-key data and the random-number data, to generate first encryption preliminary data; and a second encryption unit operable to encrypt the secret-number data based on the verification-value data, to generate second encryption preliminary data, where the encryption shared-key data is made up of the first encryption preliminary data and the second encryption preliminary data. [0420] Here, the second encryption unit may perform bitwise exclusive-or on the secret-number data and the verification-value data, to generate the second encryption preliminary data. [0421] Here, the second encryption unit may encrypt the secret-number data using the verification-value data as a cryptographic key and according to the symmetric key cryptography, to generate the second encryption preliminary data. [0422] Here, the second encryption unit may add the verification-value data to the secret-number data, to generate the second encryption preliminary data. [0423] Here, the second encryption unit may multiply the secret-number data by the verification-value data, to generate the second encryption preliminary data. [0424] Here, the encryption shared-key data may be bit connecting data between the first encryption preliminary data and the second encryption preliminary data. [0425] Here, the first encryption unit may perform NTRU cryptographic encryption, to generate the encryption shared-key data. [0426] Here, the first encryption unit may perform NTRU cryptographic encryption, to generate the first encryption preliminary data. [0427] Here, the secret-number data may be a random number having been randomly generated. [0428] Here, the shared-key derivation unit may use a one-way hash function, as the predetermined process. [0429] Furthermore, the present invention is a shared-key recovery apparatus, which decrypts encryption shared-key data based on secret-key data and public-key data that are predetermined, to generate shared-key data, and outputs the generated shared-key data. The shared-key recovery apparatus includes: a first decryption unit operable to decrypt the encryption shared-key data based on the secret-key data, to generate secret-number data; a shared-key derivation unit operable to convert the secret-number data into random-number data and the shared-key data, based on a predetermined process; and a third encryption unit operable to encrypt the secret-number data based on the public-key data and the random-number data, to generate re-encryption shared-key data, where the shared-key recovery apparatus outputs the shared-key data when the encryption shared-key data is equal to the re-encryption shared-key data. [0430] In addition, the present invention is a shared-key recovery apparatus, which decrypts encryption shared-key data based on secret-key data and public-key data that are predetermined, to generate shared-key data, and outputs the generated shared-key data, the encryption shared-key data being made up of first encryption preliminary data and second encryption preliminary data. The shared-key recovery apparatus includes: a first decryption unit operable to decrypt the first encryption preliminary data based on the secret-key data, to generate verification-value data; a second decryption unit operable to decrypt the second encryption preliminary data based on the verification-value data, to generate secret-number data; a shared-key derivation unit operable to convert, based on a predetermined process, the secret-number data into verification-value verification data, random-number data, and the shared-key data; and a third encryption unit operable to encrypt the verification-value verification data based on the public-key data and the random-number data, to generate third encryption preliminary data, where the shared-key recovery apparatus outputs the shared-key data when the first encryption preliminary data is equal to the third encryption preliminary data. [0431] In addition, the present invention is a shared-key recovery apparatus, which decrypts encryption shared-key data based on secret-key data and public-key data that are predetermined, to generate shared-key data, and outputs the generated shared-key data, the encryption shared-key data being made up of first encryption preliminary data and second encryption preliminary data. The shared-key recovery apparatus includes: a first decryption unit operable to decrypt the first encryption preliminary data based on the secret-key data, to generate verification-value data; a second decryption unit operable to decrypt the second encryption preliminary data based on the verification-value data, to generate secret-number data; a shared-key derivation unit operable to convert, based on a predetermined process, the secret-number data into verification-value verification data, random-number data, and the shared-key data; and a third encryption unit operable to encrypt the verification-value data based on the public-key data and the random-number data, to generate third encryption preliminary data, where the shared-key recovery apparatus outputs the shared-key data when the first encryption preliminary data is equal to the third encryption preliminary data. [0432] Here, the second decryption unit may perform bitwise exclusive-or on the second encryption preliminary data and on the verification-value data, to generate the secret-number data. [0433] Here, the second decryption unit may decrypt the second encryption preliminary data using the verification-value data as a cryptographic key and according to the symmetric key cryptography, to generate the secret-number data. [0434] Here, the second decryption unit may subtract the verification-value data from the second encryption preliminary data, to generate the secret-number data. [0435] Here, the second decryption unit may divide the second encryption preliminary data by the verification -value data, to generate the secret-number data. [0436] Here, the first decryption unit may perform NTRU cryptographic decryption, to generate the shared-key data. [0437] Here, the first decryption unit may perform NTRU cryptographic decryption, to generate the verification-value data. [0438] Here, the shared-key derivation unit may use a one-way hash function, as the predetermined process. [0439] Furthermore, the present invention is an encryption apparatus that encrypts plaintext data based on predetermined public-key data, to generate cipher-text data. The encryption apparatus includes: a secret-number data generating unit operable to generate secret-number data; a shared-key derivation unit operable to convert, based on a predetermined process, the secret-number data into random-number data and shared-key data; a first encryption unit operable to encrypt the secret-number data based on the public-key data and the random-number data, to generate first encryption preliminary data; a second encryption unit operable to encrypt the plaintext data based on the shared-key data, to generate second encryption preliminary data, where the cipher-text data is made up of the first encryption preliminary data and the second encryption preliminary data. [0440] Further, the present invention is a decryption apparatus that decrypts cipher-text data made up of first encryption preliminary data and second encryption preliminary data, based on secret-key data and public-key data that are predetermined, to generate decrypted-text data, and outputs the decrypted-text data. The decryption apparatus includes: a first decryption unit operable to decrypt the first encryption preliminary data based on the secret-key data, to generate secret-number data; a shared-key derivation unit operable to convert, based on a predetermined process, the secret-number data into random-number data and shared-key data; a third encryption unit operable to encrypt the secret-number data based on the public-key data and the random-number data, to generate third encryption preliminary data; and a decryption unit operable, when the first encryption preliminary data is equal to the third encryption preliminary data, to decrypt the second encryption preliminary data based on the shared-key data, to generate the decrypted-text data. [0441] In addition, the present invention is a cryptosystem comprised of an encryption apparatus and a decryption apparatus, the encryption apparatus encrypting plaintext data based on predetermined public-key data to generate cipher-text data, and the decryption apparatus decrypting the cipher-text data based on the public-key data and predetermined secret-key data and outputting resulting decrypted-text data. The encryption apparatus includes: a secret-number data generating unit operable to generate secret-number data; a shared-key derivation unit operable to convert, based on a predetermined process, the secret-number data into random-number data and shared-key data; a first encryption unit operable to encrypt the secret-number data based on the public-key data and the random-number data, to generate first encryption preliminary data; a second encryption unit operable to encrypt the plaintext data based on the shared-key data, to generate second encryption preliminary data, where the cipher-text data is made up of the first encryption preliminary data, the second encryption preliminary data, and third encryption preliminary data. The decryption apparatus includes: a first decryption unit operable to decrypt the first encryption preliminary data based on the secret-key data, to generate secret-number data; a shared-key derivation unit operable to convert, based on a predetermined process, the secret-number data into random-number data and shared-key data; a third encryption unit operable to encrypt the secret-number data based on the public-key data and the random-number data, to generate the third encryption preliminary data; and a decryption unit operable, when the first encryption preliminary data is equal to the third encryption preliminary data, to decrypt the second encryption preliminary data based on the shared-key, to generate the decrypted-text data. [0442] As described above, the present invention has been conceived in view of the problems that the conventional system has, and constructs in a cryptosystem a new encapsulation mechanism to which NTRU cryptosystem can be applied to, thereby preventing derivation of different keys between its encryption apparatus and decryption apparatus, and realizing assured cryptographic communication from the transmission apparatus to the reception apparatus, with use of a key derived from the key encapsulation mechanism. [0443] As clear from the above, the present invention provides a cryptosystem that the conventional technologies were not able to provide, therefore is very valuable. [0444] 4. Third Embodiment [0445] The following describes a content distribution system [0446] The content distribution system [0447] 4.1 Structure of content distribution system [0448] The content distribution system [0449] The content distribution system [0450] 4.2 Structure of Encryption Apparatus [0451] The encryption apparatus [0452] The encryption apparatus [0453] (1) Public-Key Input Unit [0454] The public-key input unit [0455] (2) Random-Number Generating Unit [0456] The random-number generating unit [0457] (3) First Function Unit [0458] The first function unit [0459] Next, the first function unit [0460] (4) Encryption Unit [0461] The encryption unit [0462] The encryption unit [0463] The first cipher text c [0464] Next, the encryption unit [0465] (5) Second Junction Unit [0466] The second function unit [0467] Here, the function H is a hash function, and is one of the one-way functions. [0468] The first cipher text c [0469] Next, the second function unit [0470] Next, the second function unit [0471] (6) Random-Number Mask Unit [0472] The random-number mask unit [0473] Note that the random-number mask unit [0474] (7) First Transmitting Unit [0475] The first transmitting unit [0476] (8) Shared-Key Encryption Unit [0477] The shared-key encryption unit [0478] The shared-key encryption unit [0479] 4.3 Structure of Decryption Apparatus [0480] The decryption apparatus [0481] The decryption apparatus [0482] Note that the shared-key decryption unit [0483] (1) Secret-Key Input Unit [0484] The secret-key input unit [0485] (2) First Receiving Unit [0486] The first receiving unit [0487] Note that when the random-number mask unit [0488] (3) Decryption Unit [0489] The decryption unit [0490] The decryption unit [0491] Next, the decryption unit [0492] (4) Third Function Unit [0493] The third function unit [0494] The third function unit [0495] (5) Random-Number Mask Removal Unit [0496] The random-number mask removal unit [0497] (6) Fourth Function Unit [0498] The fourth function unit [0499] The fourth function unit [0500] (7) Comparison Unit [0501] The comparison unit [0502] (8) Shared-Key Decryption Unit [0503] The shared-key decryption unit [0504] For other points, the shared-key decryption unit [0505] In addition, the second receiving unit [0506] 4.4 Operation of Content Distribution System [0507] The operations performed by the content distribution system [0508] The public-key input unit [0509] Next, the random-number generating unit [0510] The first function unit [0511] Next, the encryption unit [0512] Next, the second function unit [0513] The random-number mask unit [0514] Next, the first transmitting unit [0515] Next, the shared-key encryption unit [0516] The second transmitting unit [0517] On the other hand, the secret-key input unit [0518] The first receiving unit [0519] Next, the decryption unit [0520] Next, the third function unit [0521] The random-number mask removal unit [0522] The fourth function unit [0523] Next, the comparison unit [0524] If the decryption verification value a′ and the verification value a″ are equal (Step S [0525] Next, the second receiving unit [0526] The shared-key decryption unit [0527] 4.5 Operation Verification of Content Distribution System [0528] As follows, the entire operation performed by the content distribution system [0529] Specifically, this encryption apparatus [0530] Generate a random number s. [0531] Generate G(s), and generate a and K, from the G(s). [0532] Generate a first cipher text c [0533] Generate c [0534] Output the shared key K and the cipher text C=(c [0535] Next, the encryption apparatus [0536] On the other hand, the decryption apparatus [0537] Specifically, this decryption apparatus [0538] Decrypts the first cipher text c [0539] Generate s′=c [0540] Generate G(s′), and generate a″ and K′ from the G(s′). [0541] Check to see if a″=a′ holds. If it holds, output the shared key K′. [0542] Here, if the decryption apparatus [0543] Next, the decryption apparatus [0544] Since the encryption key K (used for generation of the shared-key cipher text) is identical to the encryption key K′ (used for generation of decrypted text), the decryption apparatus can obtain the correct mi′=mi (1=<i=<n). [0545] 4.6 Effect of Third Embodiment [0546] The conventional RSA-KEM algorithm uses a*P and a*W as input of a hash function H, and uses the Diffie-Hellman problem in the final stage of deriving the shared key K, with which the derivation of the shared key K is difficult unless the secret key is known. Therefore, other public-key cryptosystems that do not use the Diffie-Hellman problem, such as the NTRU cryptography, cannot take advantage of the PSEC-KEM algorithm, since these cryptosystems do not have inputs that correspond to a*P, and a*W of the Diffie-Hellman problem. [0547] However in the present invention, the content distribution system, the encryption apparatus, and the decryption apparatus have a verification value a and its cipher text c [0548] Note that in the NTRU cryptosystem, there is a possibility that the resulting decrypted text is different from an original plaintext, even if a public key is used to encrypt a plaintext to generate a cipher text, and the cipher text is decrypted using the secret key (e.g. refer to the non-patent reference 2). If such a decryption error has occurred, an incorrect decryption verification value a′ will be obtained. However, the decryption apparatus of the present invention will not output the shared key K′, since a′ will not be equal to the verification value a″ obtained from G(s′). Therefore, the present invention has an effect of preventing different keys to be established between the encryption apparatus and the decryption apparatus, even if a decryption error has occurred. [0549] In addition, the decryption apparatus will not perform operation for generating a re-cipher text. Therefore, the computation amount will be reduced, compared to the conventional technology. [0550] According to this, key encapsulation mechanism can be constructed using the NTRU cryptography, and so the key distribution is realized between the encryption apparatus and decryption apparatus using the NTRU cryptography. [0551] In addition, according to the system of the present invention, the security can be logically verified using the same method as the verification method described in the non-patent reference 3. [0552] 4.7 Modification Example [0553] The third embodiment described above is one example of carrying out the present invention. Needless to say, the present invention is not limited to this particular embodiment, and can be carried with various modifications as long as they are within the scope of the present invention. In light of this, the following cases are included in the present invention. [0554] (1) The parameter N to be used in NTRU cryptosystem may Take Other Value Than 167. [0555] (2) The conversion from a bit sequence to polynomial, performed in the encryption unit [0556] For example, the conversion may be performed using a function or a functional-value table, which correspond bit sequence and polynomial in one-to-one relation. [0557] Alternatively, the conversion method stated in the modification example (1) for the second embodiment may also be used. [0558] (3) The public-key cryptosystem, used in the encryption unit [0559] Accordingly, the public-key cryptosystem used in the encryption unit [0560] For example, if the RSA cryptosystem is to be used, h and f may be respectively set as a public key and a secret key of the RSA cryptosystem. Then, in the encryption unit [0561] In addition, if the E [0562] Note that the RSA cryptosystem and the E [0563] (4) In the third embodiment, the first function unit [0564] (5) The second function unit [0565] For example, with respect to a two term operation #, a#c [0566] (6) Furthermore, the Method Used in the Second Function unit [0567] For example, the second function unit [0568] making c [0569] making c [0570] In such cases, the third function unit [0571] H(a′), or [0572] a′. [0573] (7) In the third embodiment, the random-number mask unit [0574] For example, the random-number mask unit [0575] making c [0576] making s*H(a, c [0577] 5. Fourth Embodiment [0578] The following describes a content distribution system [0579] The content distribution system [0580] The following description focuses on the differences mentioned above. [0581] 5.1 Structure of Content Distribution System [0582] The content distribution system [0583] The content distribution system [0584] 5.2 Structure of Encryption Apparatus [0585] The encryption apparatus [0586] Among the mentioned components, the public-key input unit [0587] (1) First Function Unit [0588] The first function unit [0589] Here, the function G is a hash function having output length of 3 k bits. The first function unit [0590] Next, the first function unit [0591] (2) Encryption Unit [0592] The encryption unit [0593] The encryption unit [0594] Specifically, the encryption unit [0595] Next, in the same manner as the encryption unit [0596] Next, the encryption unit [0597] 5.3 Structure of Decryption Apparatus [0598] The decryption apparatus [0599] Here, among the mentioned components, the third function unit [0600] (1) Secret-Key Input Unit [0601] The secret-key input unit [0602] (2) Decryption Unit [0603] The decryption unit [0604] (3) Fourth Function Unit [0605] The fourth function unit [0606] The fourth function unit [0607] (4) Comparison Unit [0608] The comparison unit [0609] The encryption unit [0610] The comparison computation unit [0611] 5.4 Operation Verification of Content Distribution System [0612] As follows, the entire operation performed by the content distribution system [0613] The encryption apparatus [0614] Specifically, this encryption apparatus [0615] (a) Generate a random number s. [0616] (b) Generate G(s), and generate a, K, and u, from the G(s). [0617] (c) Generate a first cipher text c [0618] (d)Generate c [0619] Next, the encryption apparatus [0620] On the other hand, the decryption apparatus [0621] Specifically, the decryption apparatus [0622] (a) Decrypt the first cipher text c [0623] (b) Generate s′=c [0624] (c) Generate G(s′), and generate a″, K′, and u′ from the G(s′). [0625] (d) Generate a first re-cipher text c [0626] (e) Check to see if c [0627] Here, if the decryption apparatus [0628] Next, the decryption apparatus [0629] Since the encryption key K (used for generation of the shared-key cipher text) is identical to the encryption key K′ (used for generation of decrypted text), the decryption apparatus can obtain the correct mi′=mi (1=<i=<n). [0630] 5.5 Effect of Content Distribution System [0631] The conventional RSA-KEM algorithm uses a*P and a*W as input of a hash function H, and uses the Diffie-Hellman problem in the final stage of deriving the shared key K, with which the derivation of the shared key K is difficult unless the secret key is known. Therefore, other public-key cryptosystems that do not use the Diffie-Hellman problem, such as the NTRU cryptography, cannot take advantage of the PSEC-KEM algorithm, since these cryptosystems do not have inputs that correspond to a*P, and a*W of the Diffie-Hellman problem. [0632] However in the present invention, the content distribution system, the encryption apparatus, and the decryption apparatus have a verification value a and its cipher text c [0633] If a decryption error has occurred, an incorrect decryption verification value a′ will be obtained. However, the decryption apparatus of the present invention will not output the shared key K′, since c [0634] According to this, key encapsulation mechanism can be constructed using the NTRU cryptosystem, and so the key distribution is realized between the encryption apparatus and decryption apparatus using the NTRU cryptosystem. [0635] In addition, according to the system of the present invention, the security can be logically verified using the same method as the verification method described in the non-patent reference 3. [0636] 5.6 Modification Example [0637] The fourth embodiment described above is one example of carrying out the present invention. The present invention is not limited to this particular embodiment, and can be carried with various modifications as long as they are within the scope of the present invention. Needless to say, the same modification examples for the third embodiment can be provided for the fourth embodiment. However, the following cases are also included in the present invention. [0638] (1) The method of converting the random-number value u to the random-number polynomial r, performed in the encryption unit [0639] Alternatively, the conversion method stated in the modification example (1) for the second embodiment may also be used. [0640] (2) The public-key cryptosystem, used in the encryption unit [0641] If the E [0642] (3) In the fourth embodiment, the random-number value u is generated in the first function unit [0643] For example, u=Func(s) may be used with respect to an arbitrary function Func, so that the encryption apparatus [0644] generate G(s), and generate a, and K from the G(s), and [0645] generate Func(s), and sets u=Func(s). [0646] (4) Further, the random-number value u is generated in the first function unit [0647] Specifically, the cipher text C and the random-number value u may be transmitted to the decryption apparatus [0648] Generate G(s), and generate a and K from the G(s). [0649] The encryption apparatus [0650] At this time, it may be arranged that the encryption apparatus [0651] (5) Furthermore, as for the random-number value u, the condition is that the encryption apparatus [0652] For instance, the cipher text C and the random-number value u2 may be transmitted to the decryption apparatus [0653] Generate G(s), and generate a, K, and u1, from the G(s) [0654] The encryption apparatus [0655] Generate a random-number value u, from u=u1 xor u2. [0656] At this time, the encryption apparatus [0657] (6) The decryption apparatus [0658] Specifically, as the process-block diagram of FIG. 20 shows, the check may be performed using the decryption unit [0659] (a) Decrypt the first cipher text c [0660] (b) Generate s′=c [0661] (c) Generate G(s′) (Step S [0662] (d) Check to see if a″=a′ holds (Step S [0663] In addition, in this process, it may check whether the first cipher text c [0664] 7. Summary of Third and Fourth Embodiments [0665] As described so far, the present invention is a shared-key generation apparatus, which outputs shared-key data, and encryption shared-key data resulting from encrypting the shared-key data based on predetermined public-key data. The shared-key generation apparatus specifically includes: a secret-number data generating unit operable to generate secret-number data; a shared-key derivation unit operable to convert the secret-number data into verification-value data and the shared-key data, based on a predetermined process; and a first encryption unit operable to encrypt the verification-value data based on the public-key data, to generate first encryption preliminary data; a verification-value conversion unit operable to convert the verification-value data into conversion verification-value data, based on a predetermined process; and a second encryption unit operable to encrypt the secret-number data based on the conversion verification-value data, to generate second encryption preliminary data, where the encryption shared-key data is made up of the first encryption preliminary data and the second encryption preliminary data. [0666] In addition, the present invention is a shared-key generation apparatus, which outputs shared-key data, and encryption shared-key data resulting from encrypting the shared-key data based on predetermined public-key data. The shared-key generation apparatus specifically includes: a secret-number data generating unit operable to generate secret-number data; a shared-key derivation unit operable to convert the secret-number data and first encryption preliminary data into verification-value data and the shared-key data, based on a predetermined process; and a first encryption unit operable to encrypt the verification-value data based on the public-key data, to generate the first encryption preliminary data; a verification-value conversion unit operable to convert the verification-value data into conversion verification-value data, based on a predetermined process; and a second encryption unit operable to encrypt the secret-number data based on the conversion verification-value data, to generate second encryption preliminary data, where the encryption shared-key data is made up of the first encryption preliminary data and the second encryption preliminary data. [0667] In addition, the present invention is a shared-key generation apparatus, which outputs shared-key data, and encryption shared-key data resulting from encrypting the shared-key data based on predetermined public-key data. The shared-key generation apparatus specifically includes: a secret-number data generating unit operable to generate secret-number data; a shared-key derivation unit operable to convert the secret-number data into verification-value data, random-number data, and the shared-key data, based on a predetermined process; a first encryption unit operable to encrypt the verification-value data based on the public-key data and the random-number data, to generate first encryption preliminary data; a verification-value conversion unit operable to convert the verification-value data into conversion verification-value data, based on a predetermined process; and a second encryption unit operable to encrypt the secret-number data based on the conversion verification-value data, to generate second encryption preliminary data, where the encryption shared-key data is made up of the first encryption preliminary data and the second encryption preliminary data. [0668] In addition, the present invention is a shared-key generation apparatus, which outputs shared-key data, and encryption shared-key data resulting from encrypting the shared-key data based on predetermined public-key data. The shared-key generation apparatus specifically includes: a secret-number data generating unit operable to generate secret-number data; a shared-key derivation unit operable to convert the secret-number data into verification-value data, random-number data, and the shared-key data, based on a predetermined process; a first encryption unit operable to encrypt the verification-value data based on the public-key data and the random-number data, to generate first encryption preliminary data; a verification-value conversion unit operable to convert the verification-value data and the first encryption preliminary data into conversion verification-value data, based on a predetermined process; and a second encryption unit operable to encrypt the secret-number data based on the conversion verification-value data, to generate second encryption preliminary data, where the encryption shared-key data is made up of the first encryption preliminary data and the second encryption preliminary data. [0669] Here, the secret-number data may be a random number having been randomly generated. [0670] Here, the shared-key derivation unit may use a one-way hash function, as the predetermined process. [0671] Here, the first encryption unit may perform an NTRU cryptographic encryption, to generate the first encryption preliminary data. [0672] Here, the verification-value conversion unit may use a one-way hash function, as the predetermined process. [0673] Here, the predetermined process preformed by the verification-value conversion unit may be to set the verification-value data as it is, as the conversion verification-value data. [0674] Here, the second encryption unit may perform bitwise exclusive-or on the secret-number data and the conversion verification-value data, to generate the second encryption preliminary data. [0675] Here, the second encryption unit may encrypt the secret-number data using the conversion verification-value data as a cryptographic key and according to the symmetric key cryptography, to generate the second encryption preliminary data. [0676] Here, the second encryption unit may add the conversion verification-value data to the secret-number data, to generate the second encryption preliminary data. [0677] Here, the second encryption unit may multiply the secret-number data by the conversion verification-value data, to generate the second encryption preliminary data. [0678] Here, the encryption shared-key data may be bit connecting data between the first encryption preliminary data and the second encryption preliminary data. [0679] Furthermore, the present invention is a shared-key recovery apparatus, which decrypts encryption shared-key data based on predetermined secret-key data, to generate shared-key data, and outputs the generated shared-key data, the encryption shared-key data being made up of first encryption preliminary data and second encryption preliminary data. The shared-key recovery apparatus includes: a first decryption unit operable to decrypt the first encryption preliminary data based on the secret-key data, to generate verification-value data; a verification-value conversion unit operable to convert, based on a predetermined process, the verification-value data into conversion verification-value data; a second decryption unit operable to decrypt the second encryption preliminary data based on the conversion verification-value data, to generate secret-number data; and a shared-key derivation unit operable to convert, based on a predetermined process, the secret-number data into verification-value verification data and the shared-key data, where the shared-key recovery apparatus outputs the shared-key data when the verification-value data is equal to the verification-value verification data. [0680] In addition, the present invention is a shared-key recovery apparatus, which decrypts encryption shared-key data based on predetermined secret-key data, to generate shared-key data, and outputs the generated shared-key data, the encryption shared-key data being made up of first encryption preliminary data and second encryption preliminary data. The shared-key recovery apparatus includes: a first decryption unit operable to decrypt the first encryption preliminary data based on the secret-key data, to generate verification-value data; a verification-value conversion unit operable to convert, based on a predetermined process, the verification-value data and the first encryption preliminary data into conversion verification-value data; a second decryption unit operable to decrypt the second encryption preliminary data based on the conversion verification-value data, to generate secret-number data; and a shared-key derivation unit operable to convert, based on a predetermined process, the secret-number data into verification-value verification data and the shared-key data, where the shared-key recovery apparatus outputs the shared-key data when the verification-value data is equal to the verification-value verification data. [0681] In addition, the present invention is a shared-key recovery apparatus, which decrypts encryption shared-key data based on predetermined secret-key data, to generate shared-key data, and outputs the generated shared-key data, the encryption shared-key data being made up of first encryption preliminary data and second encryption preliminary data. The shared-key recovery apparatus includes: a first decryption unit operable to decrypt the first encryption preliminary data based on the secret-key data, to generate verification-value data; a verification-value conversion unit operable to convert, based on a predetermined process, the verification-value data into conversion verification-value data; a second decryption unit operable to decrypt, based on the conversion verification-value data, the second encryption preliminary data into secret-number data; and a shared-key derivation unit operable to convert, based on a predetermined process, the secret-number data into verification-value verification data, random-number data, and the shared-key data, where the shared-key recovery apparatus outputs the shared-key data when the verification-value data is equal to the verification-value verification data. [0682] In addition, the present invention is a shared-key recovery apparatus, which decrypts encryption shared-key data based on predetermined secret-key data, to generate shared-key data, and outputs the generated shared-key data, the encryption shared-key data being made up of first encryption preliminary data and second encryption preliminary data. The shared-key recovery apparatus includes: a first decryption unit operable to decrypt the first encryption preliminary data based on the secret-key data, to generate verification-value data; a verification-value conversion unit operable to convert, based on a predetermined process, the verification-value data and the first encryption preliminary data into conversion verification-value data; a second decryption unit operable to decrypt, based on the conversion verification-value data, the second encryption preliminary data into secret-number data; a shared-key derivation unit operable to convert, based on a predetermined process, the secret-number data into verification-value verification data, random-number data, and the shared-key data, where the shared-key recovery apparatus outputs the shared-key data when the verification-value data is equal to the verification-value verification data. [0683] In addition, the present invention is a shared-key recovery apparatus, which decrypts encryption shared-key data based on secret-key data and public key data that are predetermined, to generate shared-key data, and outputs the generated shared-key data, the encryption shared-key data being made up of first encryption preliminary data and second encryption preliminary data. The shared-key recovery apparatus includes: a first decryption unit operable to decrypt the first encryption preliminary data based on the secret-key data, to generate verification-value data; a verification-value conversion unit operable to convert, based on a predetermined process, the verification-value data into conversion verification-value data; a second decryption unit operable to decrypt, based on the conversion verification-value data, the second encryption preliminary data into secret-number data; a shared-key derivation unit operable to convert, based on a predetermined process, the secret-number data into verification-value verification data, random-number data, and the shared-key data; and a third encryption unit operable to encrypt the verification-value verification data based on the public-key data and the random-number data, to generate third encryption preliminary data, where the shared-key recovery apparatus outputs the shared-key data when the first encryption preliminary data is equal to the third encryption preliminary data. [0684] In addition, the present invention is a shared-key recovery apparatus, which decrypts encryption shared-key data based on secret-key data and public key data that are predetermined, to generate shared-key data, and outputs the generated shared-key data, the encryption shared-key data being made up of first encryption preliminary data and second encryption preliminary data. The shared-key recovery apparatus includes: a first decryption unit operable to decrypt the first encryption preliminary data based on the secret-key data, to generate verification-value data; a verification-value conversion unit operable to convert, based on a predetermined process, the verification-value data into conversion verification-value data; a second decryption unit operable to decrypt, based on the conversion verification-value data, the second encryption preliminary data into secret-number data; a shared-key derivation unit operable to convert, based on a predetermined process, the secret-number data into verification-value verification data, random-number data, and the shared-key data; and a third encryption unit operable to encrypt the verification-value data based on the public-key data and the random-number data, to generate third encryption preliminary data, where the shared-key recovery apparatus outputs the shared-key data when the first encryption preliminary data is equal to the third encryption preliminary data. [0685] In addition, the present invention is a shared-key recovery apparatus, which decrypts encryption shared-key data based on secret-key data and public key data that are predetermined, to generate shared-key data, and outputs the generated shared-key data, the encryption shared-key data being made up of first encryption preliminary data and second encryption preliminary data. The shared-key recovery apparatus includes: a first decryption unit operable to decrypt the first encryption preliminary data based on the secret-key data, to generate verification-value data; a verification-value conversion unit operable to convert, based on a predetermined process, the verification-value data and the first encryption preliminary data into conversion verification-value data; a second decryption unit operable to decrypt, based on the conversion verification-value data, the second encryption preliminary data into secret-number data; a shared-key derivation unit operable to convert, based on a predetermined process, the secret-number data into verification-value verification data, random-number data, and the shared-key data; and a third encryption unit operable to encrypt the verification-value verification data based on the public-key data and the random-number data, to generate third encryption preliminary data, where the shared-key recovery apparatus outputs the shared-key data when the first encryption preliminary data is equal to the third encryption preliminary data. [0686] In addition, the present invention is a shared-key recovery apparatus, which decrypts encryption shared-key data based on secret-key data and public key data that are predetermined, to generate shared-key data, and outputs the generated shared-key data, the encryption shared-key data being made up of first encryption preliminary data and second encryption preliminary data. The shared-key recovery apparatus includes: a first decryption unit operable to decrypt the first encryption preliminary data based on the secret-key data, to generate verification-value data; a verification-value conversion unit operable to convert, based on a predetermined process, the verification-value data and the first encryption preliminary data into conversion verification-value data; a second decryption unit operable to decrypt, based on the conversion verification-value data, the second encryption preliminary data into secret-number data; a shared-key derivation unit operable to convert, based on a predetermined process, the secret-number data into verification-value verification data, random-number data, and the shared-key data; and a third encryption unit operable to encrypt the verification-value data based on the public-key data and the random-number data, to generate third encryption preliminary data, where the shared-key recovery apparatus outputs the shared-key data when the first encryption preliminary data is equal to the third encryption preliminary data. [0687] Here, the shared-key derivation unit may use a one-way hash function, as the predetermined process. [0688] Here, the first decryption unit may perform NTRU cryptographic decryption, to generate the verification-value data. [0689] Here, the verification-value conversion unit may use a one-way hash function, as the predetermined process. [0690] Here, the predetermined process preformed by the verification-value conversion unit may be to set the verification-value data as it is, as the conversion verification-value data. [0691] Here, the second decryption unit may perform bitwise exclusive-or on the second encryption preliminary data and the conversion verification-value data, to generate the secret-number data. [0692] Here, the second decryption unit may decrypt the second encryption preliminary data using the conversion verification-value data as a cryptographic key and according to the symmetric key cryptography, to generate the secret-number data. [0693] Here, the second decryption unit may subtract the conversion verification-value data from the second encryption preliminary data, to generate the secret-number data. [0694] Here, the second decryption unit may divide the second encryption preliminary data by the conversion verification-value data, to generate the secret-number data. [0695] In addition, the present invention is an encryption apparatus that encrypts data based on predetermined public-key data, to generate cipher-text data. The encryption apparatus includes: a secret-number data generating unit operable to generate secret-number data; a shared-key derivation unit operable to convert, based on a predetermined process, the secret-number data into verification-value data and shared-key data; a first encryption unit operable to encrypt the verification-value data based on the public-key data, to generate first encryption preliminary data; a verification-value conversion unit operable to convert, based on a predetermined process, the verification-value data into conversion verification-value data; a second encryption unit operable to encrypt the secret-number data based on the conversion verification-value data, to generate second encryption preliminary data; and a third encryption unit operable to encrypt the plaintext data based on the shared-key data, to generate third encryption preliminary data, where the cipher-text data is made up of the first encryption preliminary data, the second encryption preliminary data, and the third encryption preliminary data. [0696] Further, the present invention is a decryption apparatus that decrypts, based on predetermined secret-key data, cipher-text data made up of first encryption preliminary data, second encryption preliminary data, and third encryption preliminary data, to generate decrypted-text data, and outputs the decrypted-text data. The decryption apparatus includes: a first decryption unit operable to decrypt the first encryption preliminary data based on the secret-key data, to generate verification-value data; a verification-value conversion unit operable to convert, based on a predetermined process, the verification-value data into conversion verification-value data; a second decryption unit operable to decrypt the second encryption preliminary data based on the conversion verification-value data, to generate secret-number data; and a shared-key derivation unit operable to convert, based on a predetermined process, the secret-number data into verification-value verification data and shared-key data; and a decryption unit operable, when the verification-value data is identical to the verification-value verification data, to decrypt the third encryption preliminary data based on the shared-key, to generate the decrypted-text data. [0697] In addition, the present invention is a cryptosystem comprised of an encryption apparatus and a decryption apparatus, the encryption apparatus encrypting plaintext data based on predetermined public-key data to generate cipher-text data, and the decryption apparatus decrypting the cipher-text data based on predetermined secret-key data and outputting resulting decrypted-text data. The encryption apparatus includes: a secret-number data generating unit operable to generate secret-number data; a shared-key derivation unit operable to convert, based on a predetermined process, the secret-number data into verification-value data and shared-key data; a first encryption unit operable to encrypt the verification-value data based on the public-key data, to generate first encryption preliminary data; a verification-value conversion unit operable to convert, based on a predetermined process, the verification-value data into conversion verification-value data; a second encryption unit operable to encrypt the secret-number data based on the conversion verification-value data, to generate second encryption preliminary data, and a third encryption unit operable to encrypt the plaintext data based on the shared-key data, to generate third encryption preliminary data, where the cipher-text data is made up of the first encryption preliminary data, the second encryption preliminary data, and the third encryption preliminary data. The decryption apparatus includes: a first decryption unit operable to decrypt the first encryption preliminary data based on the secret-key data, to generate verification-value data; a verification-value conversion unit operable to convert, based on a predetermined process, the verification-value data into the conversion verification-value data; a second decryption unit operable to decrypt the second encryption preliminary data based on the conversion verification-value data, to generate the secret-number data; a shared-key derivation unit operable to convert, based on a predetermined process, the secret-number data into verification-value verification data and shared-key data; and a decryption unit operable, when the verification-value data is identical to the verification-value verification data, to decrypt the third encryption preliminary data based on the shared-key data, to generate the decrypted-text data. [0698] As described above, the present invention has been conceived in view of the problems that the conventional system has, and constructs in a cryptosystem an encapsulation mechanism to which NTRU cryptosystem can be applied, thereby realizing key distribution between its encryption apparatus and decryption apparatus using NTRU cryptography. [0699] As clear from the above, the present invention provides a cryptosystem that the conventional technologies were not able to provide, therefore is very valuable. [0700] 8. Other Modification Examples [0701] So far, the present invention has been described by way of the aforementioned embodiments. However, needless to say, the present invention is not limited to the aforementioned embodiments, and includes the following cases. [0702] (1) Instead of transmitting each of cipher texts to the decryption apparatus via the Internet, the encryption apparatus may alternatively write each cipher text in a recording medium such as a DVD, and the decryption apparatus may accordingly read each cipher text from the recording medium. [0703] (2) The NTRU cryptosystem used in the present invention may be, instead of in the type described in the non-patent reference 3, in an EESS (efficient embedded security standard) type. The detail of the EESS-type NTRU cryptosystem is described in “EESS: Consortium for efficient embedded security, efficient embedded security standards #1: Implementation aspects of NTRU encrypt and NTRU sign, Version 2.0,” available at http://www.ceesstandards.org, May 2003. Therefore, the following only briefly discusses the EESS-type NTRU cryptosystem. [0704] In the EESS-type NTRU cryptosystem, a random-number polynomial r is either a polynomial expression that has d coefficients of 1, and (N-d) coefficients of 0, or a polynomial expression obtained using a plurality of such polynomial expressions. Therefore, if the random polynomial r in the above-described embodiments is generated to yield such polynomial expressions, the EESS-type NTRU cryptosystem may be alternatively used, instead of the NTRU cryptosystem, with a similar effect. [0705] (3) The content distribution system may be structured as follows. [0706] That is, the content distribution system may be comprised of a content server apparatus, an encryption apparatus, a broadcast apparatus, a reception apparatus, a decryption apparatus, a playback apparatus, and a monitor. [0707] Here, the encryption apparatus and the decryption apparatus respectively correspond to the encryption apparatus [0708] The content server apparatus and the encryption apparatus are connected to each other via a dedicated circuit, and the content server apparatus transmits contents such as movie, made up of image and audio, to the encryption apparatus via this dedicated circuit. The encryption apparatus and the broadcast apparatus are connected with each other via a dedicated circuit. The encryption apparatus transmits each of cipher texts to the broadcast apparatus, and the broadcast apparatus performs multiplexing on the cipher texts, and broadcasts them over a digital broadcast wave. [0709] The reception apparatus and the decryption apparatus are connected to each other, and likewise, the decryption apparatus and the playback apparatus are connected to each other too. The reception apparatus receives a digital broadcast wave, extracts each of cipher texts from the received digital broadcast wave, and transmits extracted cipher texts to the decryption apparatus. The decryption apparatus receives the cipher texts, generates a playback content using the received cipher texts, and outputs the generated playback content to the playback apparatus. The playback apparatus is connected to the decryption apparatus and to the monitor that includes therein a speaker. The playback apparatus receives the playback content, and generates an image signal and an audio signal, from the received playback content, and the monitor displays an image and outputs an audio. [0710] (4) The content server apparatus and the encryption apparatus may be integrated into one apparatus. The decryption apparatus and the playback apparatus may be also integrated into one apparatus. [0711] (5) In each of the aforementioned embodiments, the memory card [0712] Alternatively, the encryption apparatus [0713] In addition, the key management apparatus may generate a secret-key polynomial and a public-key polynomial, and transmit the secret-key polynomial and the public-key polynomial secretly and securely, to the decryption apparatus [0714] (6) The contents to be distributed in the content distribution system is not limited to contents such as movie, comprised of image and audio. Alternatively, the contents may include a database generated by moving images, still images, audio, music, document, novel, DB software, and the like. Further, electric spreadsheet-data and computer program generated using spreadsheet software, and other kinds of data for computer may be included therein. [0715] Furthermore, the contents may, instead of being the mentioned work, may alternatively be key information used for such as encryption, decryption, digital signature, and signature verification. [0716] For example, the following arrangement is possible. As described in each of the above embodiments, the encryption apparatus and the decryption apparatus share a same shared-key. On this premise, the encryption apparatus encrypts a content key using the shared key to generate an encrypted content key, encrypts a content using the content key to generate an encrypted content, and transmits the encrypted content key and the encrypted content to the decryption apparatus. Then the decryption apparatus receives the encrypted content key and the encrypted content, decrypts the encrypted content key using the shared key to generate the content key, and decrypts the encrypted content using thus obtained content key, to finally obtain the content. [0717] (7) The present invention may be methods of the above description. Moreover, the present invention may be a computer program that realizes these methods using a computer, or may be a digital signal comprised of the computer program. [0718] In addition, the present invention may be a computer-readable recording medium storing the mentioned computer program or the mentioned digital signal. The computer-readable recording medium includes: a flexible disc, a hard disc, a CD-ROM, an MO, a DVD, a DVD-ROM, a DVD-RAM, a BD(blu-ray disc), and a semiconductor memory. [0719] In addition, the present invention may be the computer program and the digital signal, in a form recorded in these recording mediums. [0720] In addition, the present invention may be to transmit the computer program or the digital signal, such as via a network and a data broadcast and the like, the network being represented by an electric communication circuit, a radio circuit, a cable communication circuit, and the Internet. [0721] In addition, the present invention may be a computer system equipped with a microprocessor and a memory, where the memory stores the computer program, and the microprocessor operates according to the computer program. [0722] In addition, another computer system that is independent may execute the present invention, by transmitting the computer program or the digital signal in a form stored in the recording medium, or by transmitting the computer program or the digital signal via the described network, and the like. [0723] (8) The present invention may be a combination of some of the described embodiments and the modification examples. [0724] Although the present invention has been fully described by way of examples with reference to accompanying drawings, it is to be noted that various changes and modifications will be apparent to those skilled in the art. Therefore, unless such changes and modifications depart from the scope of the present invention, they should be construed as being included therein. [0725] ( PCT )[0726] The content distribution system described above is used managerially, repeatedly, and continuously, in an industry where a content supplier provides users with digital work such as music, movie, and novel. Furthermore, the encryption apparatus and the decryption apparatus that constitute the content distribution system are produced and sold, in the electric-appliance industry selling electric appliances. [0727] In particular, the present invention is preferable in industries that supply digital work in recorded form in a recording medium, either by distribution in the market, via the network, or by the broadcast. [0728] Although the present invention has been fully described by way of examples with reference to accompanying drawings, it is to be noted that various changes and modifications will be apparent to those skilled in the art. Therefore, unless such changes and modifications depart from the scope of the present invention, they should be construed as being included therein. Referenced by
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