Ciphertext conversion system, ciphertext conversion method, and non-transitory computer readable medium
A ciphertext conversion system ( 100 ) includes a conversion key generation device ( 600 ) and a conversion device ( 700 ). The conversion key generation device ( 600 ) performs encryption of a public-key encryption scheme using a public key and a first decryption enabling condition, so as to generate a first converted public-key ciphertext and a key corresponding to the first converted public-key ciphertext, performs encryption of a symmetric-key encryption scheme using a second symmetric-key encryption secret key as a plaintext and the key corresponding to the first converted public-key ciphertext as a secret key, so as to generate a first partial conversion key, and calculates, as a second partial conversion key, an exclusive-OR of a result of performing encryption using a first symmetric-key encryption secret key and first auxiliary information and a result of performing encryption using the second symmetric-key encryption secret key and second auxiliary information. The conversion device ( 700 ) calculates, as at least part of a converted symmetric-key ciphertext, an exclusive-OR of a symmetric-key ciphertext and the second partial conversion key, where the symmetric-key ciphertext is an exclusive-OR of a plaintext and a result of performing encryption using the first symmetric-key encryption secret key and the first auxiliary information.
1 . A ciphertext conversion system comprising:
a conversion key generation device including
processing circuitry to
perform encryption of a public-key encryption scheme using a public key and a first decryption enabling condition, so as to generate a first converted public-key ciphertext and a key corresponding to the first converted public-key ciphertext,
perform encryption of a symmetric-key encryption scheme using a second symmetric-key encryption secret key as a plaintext and the key corresponding to the first converted public-key ciphertext as a secret key, so as to generate a first partial conversion key, and
calculate, as a second partial conversion key, an exclusive-OR of a value calculated by performing encryption in a counter mode of a block cipher using a first symmetric-key encryption secret key and first auxiliary information and a value calculated by performing encryption in the counter mode of the block cipher using the second symmetric-key encryption secret key and second auxiliary information; and
a conversion device including
processing circuitry to
calculate, as at least part of a converted symmetric-key ciphertext to be used to decrypt a plaintext used in generating a symmetric-key ciphertext, an exclusive-OR of the symmetric-key ciphertext and the second partial conversion key, the symmetric-key ciphertext being an exclusive-OR of the plaintext and a value calculated by performing encryption in the counter mode of the block cipher using the first symmetric-key encryption secret key and the first auxiliary information.
2 . The ciphertext conversion system according to claim 1 ,
wherein the converted symmetric-key ciphertext is composed of at least part of the converted symmetric-key ciphertext and the second auxiliary information.
3 . The ciphertext conversion system according to claim 1 ,
wherein the public-key encryption scheme is an attribute-based encryption scheme.
4 . The ciphertext conversion system according to claim 2 ,
wherein the public-key encryption scheme is an attribute-based encryption scheme.
5 . The ciphertext conversion system according to claim 3 ,
wherein the processing circuitry of the conversion device performs delegation conversion of the attribute-based encryption scheme using the public key, the first converted public-key ciphertext, and a second decryption enabling condition, so as to generate a second converted public-key ciphertext, the second decryption enabling condition being a condition that is more restrictive than the first decryption enabling condition.
6 . The ciphertext conversion system according to claim 4 ,
wherein the processing circuitry of the conversion device performs delegation conversion of the attribute-based encryption scheme using the public key, the first converted public-key ciphertext, and a second decryption enabling condition, so as to generate a second converted public-key ciphertext, the second decryption enabling condition being a condition that is more restrictive than the first decryption enabling condition.
7 . The ciphertext conversion system according to claim 5 , further comprising
a decryption device including
processing circuitry to
perform decryption of attribute-based encryption using a user secret key corresponding to the first decryption enabling condition and the second converted public-key ciphertext, so as to decrypt the key corresponding to the first converted public-key ciphertext,
perform decryption of symmetric-key encryption using the decrypted key corresponding to the first converted public-key ciphertext and the first partial conversion key, so as to decrypt the second symmetric-key encryption secret key, and
calculate, as the plaintext, an exclusive-OR of at least part of the converted symmetric-key ciphertext and a value calculated by performing encryption in the counter mode of the block cipher using the decrypted second symmetric-key encryption secret key and the second auxiliary information.
8 . The ciphertext conversion system according to claim 6 , further comprising
a decryption device including
processing circuitry to
perform decryption of attribute-based encryption using a user secret key corresponding to the first decryption enabling condition and the second converted public-key ciphertext, so as to decrypt the key corresponding to the first converted public-key ciphertext,
perform decryption of symmetric-key encryption using the decrypted key corresponding to the first converted public-key ciphertext and the first partial conversion key, so as to decrypt the second symmetric-key encryption secret key, and
calculate, as the plaintext, an exclusive-OR of at least part of the converted symmetric-key ciphertext and a value calculated by performing encryption in the counter mode of the block cipher using the decrypted second symmetric-key encryption secret key and the second auxiliary information.
9 . A ciphertext conversion method comprising:
performing encryption of a public-key encryption scheme using a public key and a first decryption enabling condition, so as to generate a first converted public-key ciphertext and a key corresponding to the first converted public-key ciphertext, by a conversion key generation device, which is a computer;
performing encryption of a symmetric-key encryption scheme using a second symmetric-key encryption secret key as a plaintext and the key corresponding to the first converted public-key ciphertext as a secret key, so as to generate a first partial conversion key, by the conversion key generation device;
calculating, as a second partial conversion key, an exclusive-OR of a value calculated by performing encryption in a counter mode of a block cipher using a first symmetric-key encryption secret key and first auxiliary information and a value calculated by performing encryption in the counter mode of the block cipher using the second symmetric-key encryption secret key and second auxiliary information, by the conversion key generation device; and
calculating, as at least part of a converted symmetric-key ciphertext to be used to decrypt a plaintext used in generating a symmetric-key ciphertext, an exclusive-OR of the symmetric-key ciphertext and the second partial conversion key, the symmetric-key ciphertext being an exclusive-OR of the plaintext and a value calculated by performing encryption in the counter mode of the block cipher using the first symmetric-key encryption secret key and the first auxiliary information, by a conversion device, which is a computer.
10 . A non-transitory computer readable medium storing a ciphertext conversion program that causes a conversion key generation device, which is a computer, to execute a conversion destination setting process and a key generation process, and causes a conversion device, which is a computer to execute a conversion process,
the conversion destination setting process being a process of performing encryption of a public-key encryption scheme using a public key and a first decryption enabling condition, so as to generate a first converted public-key ciphertext and a key corresponding to the first converted public-key ciphertext,
the key generation process being a process of performing encryption of a symmetric-key encryption scheme using a second symmetric-key encryption secret key as a plaintext and the key corresponding to the first converted public-key ciphertext as a secret key, so as to generate a first partial conversion key, and
calculating, as a second partial conversion key, an exclusive-OR of a value calculated by performing encryption in a counter mode of a block cipher using a first symmetric-key encryption secret key and first auxiliary information and a value calculated by performing encryption in the counter mode of the block cipher using the second symmetric-key encryption secret key and second auxiliary information,
the conversion process being a process of calculating, as at least part of a converted symmetric-key ciphertext to be used to decrypt a plaintext used in generating a symmetric-key ciphertext, an exclusive-OR of the symmetric-key ciphertext and the second partial conversion key, the symmetric-key ciphertext being an exclusive-OR of the plaintext and a value calculated by performing encryption in the counter mode of the block cipher using the first symmetric-key encryption secret key and the first auxiliary information.