At present, digital-data recordable media, such as digital audio tapes (DATs), compact discs (CDs), and digital video discs (DVDs), are distributed, and various types of content such as music data and picture data are recorded as digital data in the media and are widely distributed.
Such digital data differs from analog data, and is free from data deterioration due to data copying between recording media. If copying is limitlessly permitted, there is a possibility that the rights of a content-copyright holder and other content-related-right holders may be violated. For protecting the right of the digital data, there is the SCMS (Serial Copy Management System) as a copyright protection technology.
The SCMS (Serial Copy Management System) is a digital-data-copying restriction system. It allows only-one-generation (1-Generation) copying, and prohibits digital copying for two or more generations. Specifically, by recording, in digital-data-recorded media, a code representing copying only once, restriction of copying is performed based on the code.
Nevertheless, in one-generation copying control using the SCMS, based on the recorded code in the media which represents copying only once, that is, the bit state, it is determined whether or not copying is allowed. Accordingly, by using a device that can freely operate the bit, rewriting of the code is made possible, and many copies identical to the original data can be made. Therefore, in particular, PC-used (personal computer-used) copying of digital data such as CDs, which is free from restrictions of law, is actually free.
In addition, for systems for the purpose of protecting copyright that records/plays back content such as pictures and music, a system has been proposed in which content is encrypted and provided to a user and in which a key for decryption is provided to a normal user.
By way of example, there is a system configuration in which various types of content, such as music data, picture data, and game programs which are encrypted, are distributed to users by using the Internet or media such as CDs and DVDs and in which only a person identified as a normal user is provided with a means for decrypting the encrypted content, that is, a decryption key.
The encrypted data can be returned to usable decrypted data (plaintext) by decryption processing based on a predetermined procedure. Such a data encryption/decryption method is conventionally known in which an encryption key is used for information-encrypting processing and a decryption key is used for decrypting processing.
Among various types of examples of data encryption/decryption methods using an encryption key and a decryption key, there is a method as an example that is a so-called a common key cryptosystem. In the common key cryptosystem, by setting an encryption key for data-encrypting processing and a decryption key for data decryption to be common, and providing a normal user with a common key for the encryption processing and decryption, data accessing by a user having no key is excluded. A typical of this system is the DES (Data encryption standard)
The encryption key and the decryption key for the above encryption processing and decryption can be obtained by using a unidirectional function, such as the Hash function, based on, for example, a password or the like. The unidirectional function is a function in which reverse finding of its input from its output is very difficult. For example, by using a user-decided password as an input in an application of the unidirectional function, an encryption key and a decryption key are generated based on the output. It is substantially impossible to perform reverse finding of the password as the original data from the encryption key and the decryption key obtained as described above.
A system in which a process using the encryption key for use in encryption and a process using the decryption key for use in decryption have different algorithms is a so-called public key cryptosystem. The public key cryptosystem is a system in which unspecified users use an usable public key, and an encrypted document for a specified person is encryption-processed by using a public key issued by the specified person. The document encrypted by the public key becomes able to be decryption-processed by using only a secret key corresponding to the public key used in the encryption process. Since a secret key is possessed by a person who issues a public key, a document encrypted by the public key can be decrypted by only the person who possesses the secret key. One typical public key cryptosystem is the RSA (Rivest-Shamir-Adelman) cryptography. Use of such a cryptosystem enables a system in which encrypted content can be decrypted only for a normal user.
In this system, for example, a 2-bit EMI (Encryption Mode Indicator) is defined as copy control information. When the EMI is 00B (B indicates that the value before it is a binary number), it indicates that content is of a Copy-freely type, and when the EMI is 01B, it indicates that content is of a No-more-copies type in which the content may further not be copied. When the EMI is 10B, it indicates that content is of a Copy-one-generation type in which copying only once is allowed, and when the EMI is 11B, it indicates that content is a Copy-never type in which copying is prohibited.
When the EMI represents the Copy-freely or Copy-one-generation type, it is determined that content can be copied. Alternatively, when the EMI represents the No-more-copies or Copy-never type, it is determined that content cannot be copied. If management of the copy rule information is appropriately executed, copyright protection is realized.
However, even in the content providing system using encryption, if information on copying rules which is recorded on a medium such as a CD or a DVD is rewritten by an invalid user, a problem occurs in that copying ignoring the original copying rules becomes executable.