Cryptographic system, re-encryption key generation device, re-encryption device, cryptographic method, and cryptographic program

A re-encryption key generation device of a cryptographic system that implements a proxy re-encryption function in a cryptographic scheme according to which when two pieces of information correspond to each other, a ciphertext in which is set one of the two pieces of information can be decrypted with a decryption key in which is set the other one of the two pieces of information, the re-encryption key generation device comprising:

• a decryption key k*rk generation part that generates a decryption key k*rk by converting, using conversion information W1, a decryption key k* in which is set one of attribute information x and attribute information v corresponding to each other;
• a conversion information W1 encryption part that generates encrypted conversion information ψrk by encrypting the conversion information W1 with one of attribute information x′ and attribute information v′ corresponding to each other being set;
• and
• a re-encryption key transmission part that transmits to a re-encryption device the decryption key k*rk and the encrypted conversion information ψrk, as a re-encryption key rk.

A re-encryption device of a cryptographic system that implements a proxy re-encryption function in a cryptographic scheme according to which when two pieces of information correspond to each other, a ciphertext in which is set one of the two pieces of information can be decrypted with a decryption key in which is set the other one of the two pieces of information, the re-encryption device comprising:

• a re-encryption key receiving part that receives, as a re-encryption key rk, a decryption key k*rk which is generated by converting, using conversion information W1, a decryption key k* in which is set one of attribute information x and attribute information v corresponding to each other, and encrypted conversion information ψrk which is generated by encrypting the conversion information W1 with one of attribute information x′ and attribute information v′ corresponding to each other being set;
• a ciphertext receiving part that receives a ciphertext cenc in which is set the other one of the attribute information x and the attribute information v;
• a ciphertext crenc generation part that generates a ciphertext crenc by setting at least one of additional information H and additional information Θ corresponding to each other in the ciphertext cenc received by the ciphertext receiving part;
• a decryption key k*renc generation part that generates a decryption key k*renc by setting at least the other one of the additional information H and the additional information Θ in the decryption key k*rk included in the re-encryption key rk;
• and
• a re-encrypted ciphertext transmission part that transmits the ciphertext crenc, the decryption key k*renc, and the encrypted conversion information ψrk, as a re-encrypted ciphertext CT.

A cryptographic method for implementing a proxy re-encryption function in a cryptographic scheme according to which when two pieces of information correspond to each other, a ciphertext in which is set one of the two pieces of information can be decrypted with a decryption key in which is set the other one of the two pieces of information, the cryptographic method comprising:

• generating a decryption key k*rk by converting, using conversion information W1, a decryption key k* in which is set one of attribute information x and attribute information v corresponding to each other, by a re-encryption key generation device;
• generating encrypted conversion information ψrk by encrypting the conversion information W1 with one of attribute information x′ and attribute information v′ corresponding to each other being set, by the re-encryption key generation device;
• transmitting to a re-encryption device the decryption key k*rk and the encrypted conversion information ψrk, as a re-encryption key rk, by the re-encrypted key generation device;
• receiving step of receiving a ciphertext cenc in which is set the other one of the attribute information x and the attribute information v, by the re-encryption device;
• generating a ciphertext crenc by setting at least one of additional information H and additional information Θ corresponding to each other in the ciphertext cenc, by the re-encryption device;
• generating a decryption key k*renc by setting at least the other one of the additional information H and the additional information Θ in the decryption key k*rk included in the re-encryption key rk, by the re-encryption device;
• and
• transmitting the ciphertext crenc, the decryption key k*renc, and the encrypted conversion information ψrk, as a re-encrypted ciphertext CT, by the re-encryption device.

A cryptographic program that implements a proxy re-encryption function in a cryptographic scheme according to which when two pieces of information correspond to each other, a ciphertext in which is set one of the two pieces of information can be decrypted with a decryption key in which is set the other one of the two pieces of information, the cryptographic program comprising a re-encryption key generation program and a re-encryption program,

• the re-encryption key generation program causing a computer to execute
• a decryption key k*rk generation process of generating a decryption key k*rk by converting, using conversion information W1, a decryption key k* in which is set one of attribute information x and attribute information v corresponding to each other;
• a conversion information W1 encryption process of generating encrypted conversion information ψrk by encrypting the conversion information W1 with one of attribute information x′ and attribute information v′ corresponding to each other being set;
• and
• a re-encryption key transmission process of transmitting to the re-encryption program the decryption key k*rk and the encrypted conversion information ψrk, as a re-encryption key rk,
• the re-encryption program causing a computer to execute
• a ciphertext receiving process of receiving a ciphertext cenc in which is set the other one of the attribute information x and the attribute information v;
• a ciphertext crenc generation process of generating a ciphertext crenc by setting at least one of additional information H and additional information Θ corresponding to each other in the ciphertext cenc received in the ciphertext receiving process;
• a decryption key k*renc generation step of generating a decryption key k*renc by setting at least the other one of the additional information H and the additional information Θ in the decryption key k*rk included in the re-encryption key rk;
• and
• a re-encrypted ciphertext transmission process of transmitting the ciphertext crenc, the decryption key k*renc, and the encrypted conversion information ψrk, as a re-encrypted ciphertext CT.