Network storage for processing cryptographic files while keeping private key secret in key terminal
In a storage system ( 101 ), a key terminal ( 141 ) secretly records a primary private key included together with a primary public key in a primary key pair. An upload terminal ( 111 ) encrypts an object file into an encrypted file with a generated common key, encrypts the common key into a first cipher with the primary public key, and stores the encrypted file and the first cipher in a storage server ( 131 ). A download terminal ( 121 ) generates a temporary key pair including a temporary public key and a temporary private key, secretly records the temporary private key, transmits the temporary public key to the key terminal ( 141 ), and signs in to the storage server ( 131 ). Then, the key terminal ( 141 ) and the storage server ( 131 ) in cooperation with each other generate a second cipher acquired by encrypting the common key with the temporary public key while the common key is kept secret from the storage server ( 131 ), and transmit the second cipher to the download terminal ( 121 ). The download terminal ( 121 ) decrypts the shared key from the second cipher with the temporary private key, and decrypts, with the common key, the object file from the encrypted file acquired from the storage server ( 131 ).
1 . A storage system comprising:
an upload terminal;
a download terminal;
a storage server; and
a key terminal that secretly records a primary private key included together with a primary public key in a primary key pair, wherein
(a) the upload terminal
generates a common key,
generates an encrypted file by encrypting an object file with the generated common key,
generates a first cipher by encrypting the generated common key with the primary public key, and
stores the generated encrypted file and the generated first cipher in the storage server,
(b) the download terminal
generates a temporary key pair including a temporary public key and a temporary private key,
secretly records the temporary private key,
transmits the temporary public key to the key terminal, and
attempts a sign-in to the storage server,
(c) when the sign-in is successful, the key terminal and the storage server in cooperation with each other
generate a second cipher acquired by encrypting the common key with the temporary public key from the recorded primary private key, the transmitted temporary public key, and the stored first cipher while the common key is kept secret from the storage server, and
transmit the generated second cipher to the download terminal, and
(d) the download terminal
decrypts the common key from the transmitted second cipher with the temporary private key,
acquires the stored encrypted file from the storage server, and
decrypts the object file from the acquired encrypted file with the decrypted common key.
2 . The storage system according to claim 1 , wherein
the key terminal
generates a proxy key from the recorded primary private key and the transmitted temporary public key, and
transmits the generated proxy key to the storage server, and the storage server
generates the second cipher by reencrypting the stored first cipher with the transmitted proxy key, and
transmits the generated second cipher to the download terminal.
3 . The storage system according to claim 1 , wherein the key terminal
acquires the stored first cipher from the storage server,
decrypts the common key from the first cipher with the recorded primary private key,
generates the second cipher by encrypting the decrypted common key with the transmitted temporary public key, and
transmits the generated second cipher to the download terminal via the storage server.
4 . The storage system according to claim 3 , wherein whether the sign-in is successful is determined by multi-factor authentication in which use of the key terminal is one of factors.
5 . The storage system according to claim 4 , wherein
an attempt at the sign-in to the storage server is accompanied by the temporary public key, and
the key terminal acquires, from the storage server, the temporary public key accompanying the attempt at the sign-in, and thus receives the temporary public key.
6 . The storage system according to claim 4 , wherein
the download terminal displays, on a screen, a code acquired by encoding the generated temporary public key in a machine recognizable manner, and
the key terminal performs scanning and machine recognition on the displayed code, and thus receives the temporary public key.
7 . The storage system according to claim 6 , wherein
an attempt at the sign-in to the storage server is accompanied by the temporary public key,
the key terminal reports the temporary public key subjected to the machine recognition to the storage server, and
a fact that the temporary public key accompanying the attempt at the sign-in and the temporary public key reported from the key terminal coincide with each other is set as a necessary condition for success in the sign-in.
8 . The storage system according to claim 7 , wherein
the key terminal provides an electronic signature with the recorded primary private key to the temporary public key subjected to the machine recognition, and reports the temporary public key together with the electronic signature to the storage server,
the storage server verifies the reported electronic signature with the primary public key, and
a fact that the verification is successful is set as a necessary condition for success in the sign-in.
9 . The storage system according to claim 1 , wherein
the upload terminal and the download terminal are achieved by the same computer used by a key owner who owns the primary key pair, and
the storage system is configured to back up and restore the object file by the key owner.
10 . The storage system according to claim 1 , wherein
the upload terminal is achieved by a computer used by a file owner who owns the object file,
the download terminal is achieved by another computer used by a key owner who owns the primary key pair, and
the storage system is configured to transmit the object file from the file owner to the key owner.
11 . A storage method to be executed by an upload terminal, a download terminal, a storage server, and a key terminal that secretly records a primary private key included together with a primary public key in a primary key pair, the storage method comprising:
(a) by the upload terminal,
generating a common key;
generating an encrypted file by encrypting an object file with the generated common key;
generating a first cipher by encrypting the generated common key with the primary public key; and
storing the generated encrypted file and the generated first cipher in the storage server;
(b) by the download terminal,
generating a temporary key pair including a temporary public key and a temporary private key;
secretly recording the temporary private key;
transmitting the temporary public key to the key terminal; and
attempting a sign-in to the storage server;
(c) when the sign-in is successful, by the key terminal and the storage server in cooperation with each other,
generating a second cipher acquired by encrypting the common key with the temporary public key from the recorded primary private key, the transmitted temporary public key, and the stored first cipher while the common key is kept secret from the storage server; and
transmitting the generated second cipher to the download terminal; and
(d) by the download terminal,
decrypting the common key from the transmitted second cipher with the temporary private key;
acquiring the stored encrypted file from the storage server; and
decrypting the object file from the acquired encrypted file with the decrypted common key.
12 . A non-transitory computer-readable information recording medium recording at least one of a upload program, a download program, a key program, and a storage program comprising:
the upload program executed in an upload terminal;
the download program executed in a download terminal;
the storage program executed in a storage server; and
the key program executed in a key terminal that secretly records a primary private key included together with a primary public key in a primary key pair, wherein
(a) the upload program causes the upload terminal to execute processing of
generating a common key,
generating an encrypted file by encrypting an object file with the generated common key,
generating a first cipher by encrypting the generated common key with the primary public key, and
storing the generated encrypted file and the generated first cipher in the storage server,
(b) the download program causes the download terminal to execute processing of
generating a temporary key pair including a temporary public key and a temporary private key,
secretly recording the temporary private key,
transmitting the temporary public key to the key terminal, and
attempting a sign-in to the storage server,
(c) when the sign-in is successful, the key program and the storage program respectively cause the key terminal and the storage server in cooperation with each other to execute processing of
generating a second cipher acquired by encrypting the common key with the temporary public key from the recorded primary private key, the transmitted temporary public key, and the stored first cipher while the common key is kept secret from the storage server, and
transmitting the generated second cipher to the download terminal, and
(d) the download program causes the download terminal to execute processing of decrypting the common key from the transmitted second cipher with the temporary private key,
acquiring the stored encrypted file from the storage server, and
decrypting the object file from the acquired encrypted file with the decrypted common key.