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Identity-based conditional proxy re-encryption (IBCPRE) is a type of proxy re-encryption (PRE) scheme in the identity-based public key cryptographic setting. An IBCPRE scheme is a natural extension of proxy re-encryption on two aspects. The first aspect is to extend the proxy re-encryption notion to the identity-based public key cryptographic setting. The second aspect is to extend the feature set of proxy re-encryption to support conditional proxy re-encryption. By conditional proxy re-encryption, a proxy can use an IBCPRE scheme to re-encrypt a ciphertext but the ciphertext would only be well-formed for decryption if a condition applied onto the ciphertext together with the re-encryption key is satisfied. This allows fine-grained proxy re-encryption and can be useful for applications such as secure sharing over encrypted cloud data storage. == Introduction == A public-key encryption scheme allows anyone who has the public key of a receiver to encrypt messages to the receiver using the public key in such a way that only the corresponding private key known only to the receiver can decrypt and recover the messages. The public key of a user, therefore, can be published for allowing everyone to use it for encrypting messages to the user while the private key of the user has to be kept secret for the decryption purpose. Both the public key and the corresponding private key of the user are generated by the user in general .〔(【引用サイトリンク】url=http://www.emc.com/emc-plus/rsa-labs/standards-initiatives/what-is-a-digital-envelope.htm )〕 Under the identity-based cryptographic setting, the public key of the user can be an arbitrary string of bits provided that the string can uniquely identify the user in the system. The unique string, for example, can be an email address, a phone number, and a staff ID (if used only internally within an organization). However, the corresponding private key is no longer generated by the user. From the public key, which is a unique binary string, there is a key generation center (KGC), which generates and issues the private key to the user. The KGC has a public key, which is assumed to be publicly known, and the encryption and decryption then work under the unique binary string defined public key and the corresponding private key, respectively, with respect to the KGC’s public key. Proxy Re-encryption allows a ciphertext, which originally can only be decrypted by a user, to be transformed by a public entity, called proxy, to another ciphertext so that another user can also decrypt. Suppose the two users are Alice and Bob. Alice has some messages: . She intends to encrypt them under her public key, and then upload the encrypted messages to some server. Now when Alice wants to share these n encrypted messages with Bob, Alice can use a proxy re-encryption scheme to allow the server to re-encrypt these n encrypted messages so that Bob can decrypt these re-encrypted messages directly using his own private key. To do so in the proxy re-encryption scheme, Alice uses her private key and the public key of Bob to generate a re-encryption key. Alice then sends the re-encryption key to the server. Upon receiving this re-encryption key, the server uses the key to transform all the n encrypted messages to a new form denoted as D1, D2, …, Dn. Bob can then download , decrypt them, and recover the messages using his private key. In an identity-based conditional proxy re-encryption (IBCPRE) system, users set their public keys as unique identities of the users. One of the main advantages of using identity-based cryptographic algorithms is the elimination of public key certificates which can help enhance the usability of the target security applications. The term ‘Conditional’ in IBCPRE refers to an additional feature, which allows each encrypted message to have a ‘tag’ associated with. In addition to the tag, each re-encryption key also has a ‘tag’ attached. The IBCPRE is designed so that only if the tag of an encrypted message matches with the tag of a re-encryption key can the encrypted message be re-encrypted. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Identity-based conditional proxy re-encryption」の詳細全文を読む スポンサード リンク
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