TECHNICAL FIELD

The present invention relates to a searchable encryption technique capable of executing search processing in a state where data is encrypted.

BACKGROUND ART

In recent years, there has been cloud computing technology that uses computing resources in a network, particularly the Internet, to execute and provide various services. As an example of such services, a service may be possible in which various types of data are stored on a network, and only a searcher who is permitted to use the data downloads and uses the data.

However, among the pieces of data stored on the network, there may be data such as personal information of the user that must be kept confidential so as not to leak to a third party. Such data can be kept confidential by encryption such as private key encryption or public key encryption.

As described above, by storing encrypted data on the network, it is possible to achieve both confidentiality of data and the use of cloud computing. However, there is a problem that data that is encrypted cannot be searched for. A searchable encryption technique is available as a technique for solving this problem. With the searchable encryption technique, encrypted data can be searched for by use of a special encryption method.

In the searchable encryption technique, it is essential that a searching user can access only searchable information.

Patent Literature 1 and Non-Patent Literature 1 describe searchable encryption techniques that can control access by using a public key encryption technique such as ID-based encryption or inner-product predicate encryption. Non-Patent Literature 2 describes a searchable encryption technique that uses a common key

CITATION LIST

Patent Literature

Patent Literature 1: Japanese Patent Application No. 2011-549822

Non-Patent Literature

Non-Patent Literature 1: Nori MATSUDA, Takashi ITO, Hideya SHIBATA, Mitsuhiro HATTORI, and Takato HIRANO. “Efficient Searchable Encryption and Its Application to Web Services”, In DICOMO 2013 (DS-2), 2013.Non-Patent Literature 2: R. Curtmola, J. Garay, S. Kamara, and R. Ostrovsky. “Searchable Symmetric Encryption: Improved Definitions and Efficient Constructions”. ACM CCS 2006.

SUMMARY OF INVENTION

Technical Problem

The searchable encryption techniques described in Patent Literature 1 and Non-Patent Literature 1 are known for requiring a long search processing time. Although the searchable encryption technique described in Non-Patent Literature 2 is faster in search processing than the searchable encryption techniques described in Patent Literature 1 and Non-Patent Literature 1, the technique of Non-Patent Literature 2 is not capable of cryptographic access control.

An object of the present invention is to make it possible to shorten processing time required for search while realizing flexible access control.

Solution to Problem

A search device according to the present invention includes:

an auxiliary collation unit to determine whether or not an auxiliary tag corresponding to an auxiliary query obtained by conversion of a keyword exists, among auxiliary tags stored in an auxiliary tag storage unit and obtained by conversion of a search word; and

a main collation unit to specify, when it is determined by the auxiliary collation unit that the auxiliary tag corresponding to the auxiliary query exists, an encryption tag corresponding to a search query being set with attribute information indicating an attribute of a user, and the keyword, among encryption tags stored in an encryption tag storage unit and being set with an access condition indicating an accessible attribute, and the search word.

Advantageous Effects of Invention

In the present invention, before searching for an encryption tag corresponding to a search query, it is determined whether or not an auxiliary tag corresponding to an auxiliary query exists. Hence, in cases where an auxiliary tag corresponding to the auxiliary query does not exist, it is unnecessary to execute time-consuming processing of searching for the encryption tag corresponding to the search query, so that a processing time for search can be shortened. Also, as for the encryption tag corresponding to the search query, flexible access control can be performed.

