# Technique for Searching Data in a Cryptographically Protected SQL Database
## 2 Related Works
Their solution is based on the searchable encryption technique developed by [PERSON] et al. [29] and applied to unstructured documents. This mechanism creates an inverted search index of keywords in the database to enable keyword search queries over encrypted data. The practicality of this solution is achieved through the use of the cuckoo hashing technique, which makes the search in the index efficient. The proposed solution supports Boolean and range queries. [PERSON] et al. [27] propose a system (called Acra) which allows, among other things, to provide a search for encrypted data in SQL databases. The proposed Acra Searchable Encryption (Acra SE) solution is based on a blind indexing approach that develops the original idea of the CipherSweet project [30]. The main component of the Acra SE scheme is the so-called Acra Server, which works as a reverse proxy (transparent encryption/decryption proxy server). It sits between the application and the database. The application does not know that the data are encrypted before it gets into the database, the database does not know that someone encrypted the data. It is worth noting that the encryption and secure search functions of Acra Server can be configured for each column. This means that every table in the database can be fully encrypted (every column), partially encrypted (some columns are encrypted, some not), or fully unencrypted. All Acra's searchable encryption security properties are very similar to the security properties of CipherSweet, which poses the risk of partially known plaintext attacks. In this connection, [PERSON] et al. [27] provide practical recommendations to ensure security. However, despite certain solutions aimed at ensuring the security of storing and searching for sensitive data, Acra, like CipherSweet, which was taken as a prototype of a searchable encryption scheme, supports the minimum functionality of queries, namely, only for equality. Various DBMSs are characterized by the so-called technology of \"transparent data encryption\" (TDI) [31], which allows you to selectively encrypt sensitive data stored in database files, as well as in files related to data recovery, such as redo logs, archive logs, backup tapes. The essence of transparent encryption is that a combination of two keys is used: a key for each database table, which is unique, a master key that is stored outside the database in the so-called \"wallet\". Data stored on disk are encrypted; however, they are automatically decrypted for the legitimate user to process queries. That is, when the user selects encrypted columns, the DBMS quietly extracts the key from the \"wallet\", decrypts the columns and shows them to the user. As a result, the server must have access to the decryption keys, and an attacker who has compromised the DBMS software can gain access to all data. Therefore, the main goal of TDE is to protect sensitive data located in the corresponding files of the operating system. TDE is not a full blown encryption system and it should not be used in this capacity. In addition, attention should be paid to the fact that the ability to perform search operations over encrypted databases leads to the complexity of systems and an increase in the amount of memory required and query execution time. At that, some searchable encryption schemes when performing certain queries do not provide sufficient data confidentiality. That is, with long-term observation, an attacker can obtain a significant part of the information about sensitive data. All this testifies to the openness of the secure search problem and the need for further research in this direction to ensure secure work with remote databases and data storages.