125 encrypted with a respective encryption key. The client device 110 is configured to determine whether data to be stored in the database 125 of the data storage server 120 comprises security sensitive data by parsing the data to be stored in the relational database 125.

In the following detailed embodiments of the client device 110 and the data storage server 120 will be described, where the database 125 is implemented as a relational database 125, for instance, a SQL based relational database 125. In these embodiments, as will be described in more detail below the data storage server 120 is configured to store the security sensitive data in one or more encrypted columns of one or more tables of the relational database 125. As used herein, an encrypted column of a table of the relational database 125 is a column having column elements being encrypted with the same encryption key. FIG. 1b shows an example of a table 127 of the relational database 125 comprising two plaintext column 127a and 127c as well as one encrypted column 127b of security sensitive data.

It will be appreciated, however, that the data storage server 120 may comprise a non-relational database 125 and the client device 110 may be configured to operate therewith, wherein the security sensitive data are stored in an encrypted data structure other than an encrypted column of a table. For instance, in an embodiment, the database 125 may be implemented as a MongoDB database 125. For such an embodiment, the data storage server 120 may be configured to store the security sensitive data in one or more encrypted fields of the MongoDB database 125.

In an embodiment, database configuration information for identifying the rows and/or columns of the relational database 125 encrypted with a respective encryption key may be stored in the memory 115 of the client device 110. In an embodiment, the database configuration information may comprise one or more metadata tables. In an embodiment, such a metadata table could represent every encrypted row and/or column 127b of the relational database 125 using a first value, such as “1”, while representing every non-encrypted row and/or column 127a, 127c of the relational database 125 with a second value different from the first value, such as “0”. Alternatively or additionally, the database configuration information may comprise a list of the encrypted rows and/or columns of the relational database and their respective encryption keys. For instance, a first column element of a table of the relational database 125 may be encrypted with the encryption key of a first client device 110, while a second column element may be encrypted with the encryption key of a second client device 110. For the case of an encrypted column 127b the data storage server 120 is configured to determine based on the identifier of the encryption key the row of the encrypted column 127b to store the encrypted security sensitive data.

On the basis of the database configuration information the client device 110 is configured to identify the encrypted rows and/or columns 127b of the tables of the relational database 125 and, thus, to determine whether the data to be stored in the relational database 125 comprises security sensitive data to be stored in one of the encrypted rows and/or columns 127b of the table 127 of the relational database 125 identified by means of the database configuration information.

In an embodiment, the client device 101 may be configured to periodically poll the database configuration information from the remote data storage server 120. In case of changes of the relational database 125, such as new or changed encrypted rows and/or columns of the relational database 125, the data storage server 120 may be configured to provide an update of the database configuration information to the client device 110.

In an embodiment, the client device 110 is configured to parse the data to be stored in the relational database 125 of the data storage server 120 using a concrete syntax tree, i.e. a parse tree.

If the client device 110 determines that the data to be stored in the relational database 125 of the data storage server 120 comprises security sensitive data to be stored in an encrypted column 127b of the relational database 125, the client device 110 is further configured to encrypt the security sensitive data with an encryption key, such as the encryption key 115b illustrated in FIG. 1a, associated with the encrypted column 127b of the table 127 of the relational database 125. In an embodiment, the client device 101 may be configured to encrypt the security sensitive data with the encryption key 115b using a random encryption scheme (where the same plain text results in different cypher texts) or a deterministic random encryption scheme (where the same plain text results in the same cypher text).

Moreover, the client device 110 is configured to provide the data, including the encrypted security sensitive data, and an identifier of the encryption key 115b to the remote storage server 120. On the basis of the identifier of the encryption key 115b, the data storage server 120 is configured to identify the corresponding encrypted column 127b of the table 127 of the relational database 125 and to store the security sensitive data therein.

In an embodiment, the memory 115 of the client device 110 is configured to store the respective encryption key, such as the encryption key 115b, for encrypting the security sensitive data to be stored in the encrypted column 127b of the table 127 of the relational database 125 persistently. In another embodiment, the memory 115 of the client device 110 is configured to store the respective encryption key, such as the encryption key 115b, temporarily.

In an embodiment, the client device 110 is further configured to obtain the respective encryption key, such as the encryption key 115b, for encrypting the security sensitive data to be stored in the encrypted column 127b of the relational database 125 from the remote data storage server 120 on the basis of the database configuration information.

In another embodiment, the client device 110 is further configured to obtain the respective encryption key, such as the encryption key 115b, in encrypted form from the data storage server 120 and to decrypt the respective encrypted encryption key, such as the encryption key 115b, with a client device master key 115a. In an embodiment, the client device 110 is configured to obtain the client device master key 115a from the key management server 130. In an embodiment, the key management server 130 is operated by a trusted third party and configured to manage the client device master keys of all client devices 110 of the client storage system 100, including the client device master key 115a of the client device 110.

In an embodiment, the client device 110 is further configured to convert any security sensitive data to be stored in the relational database 125 from a human-readable format into a binary format and to encrypt the security sensitive data in the binary format with the respective encryption key 115b for encrypting the security sensitive data to be stored in the encrypted column 127b of the table 127 of the relational database 125. Likewise, for extracting the security sensitive data from the relational database 125 the client device 110 may be configured to reconvert (after decryption) the security sensitive data from the binary format into the human-readable format. In case the client device 110 requests the data in binary format, no format conversion may be necessary.

Complementary to the client device 110, the data storage server 120 is configured to store security sensitive data in encrypted columns of one or more tables, such as the encrypted column 127b of the table 127 shown in FIG. 1b, of the relational database 125 encrypted (by the client device(s) 110) with a respective encryption key, such as the encryption key 115b. The data storage server 120 is configured to receive data, including the encrypted security sensitive data encrypted with the encryption key 115b, and an identifier of the encryption key 115b from the client device 110 for storing the encrypted security sensitive data in an encrypted column of the relational database 125. As already described above, on the basis of the identifier of the encryption key 115b the data storage server 120 is configured to identify the corresponding encrypted column 127b of the relational database 125 and to store the security sensitive data therein. In an embodiment, the data storage server 120 is configured to implement a data definition language (DDL), which enables the management of encrypted rows and/or columns of the relational database 125, encryption keys, and client device master keys.

In an embodiment, the relational database 125 is configured to store the security sensitive data in encrypted columns of the relational database 125 encrypted with a respective encryption key using a binary data type. In an embodiment, the data storage server 120 is further configured to convert, in response to a data request query from the client device 110, the security sensitive data stored in encrypted rows and/or columns of the relational database 125 using the binary data type to a data type defined by the data request query. In an embodiment, the binary data type is the “bytea” data type defined in PostgreSQL. In an embodiment, the binary data type is an extension of the “bytea” data type, which in comparison to the “bytea” data type has an increased error checking of the inserted data and forbids certain types of operations on columns of this data type. For example, attempting to concatenate two encrypted strings of this data type by the data storage server may return an error message to the client device 110.

As already describe above, in an embodiment, the data storage server 120 is further configured to provide database configuration information to each of the client devices 110. The database configuration information allows each client device 110 identifying the columns of the relational database 125 encrypted with a respective encryption key, such as the encryption key 115b. In an embodiment, the database configuration information comprises a metadata table comprising for each encrypted column of the relational database 125 the identifier(s) of each of the respective encryption keys used for encrypting the data of the respective rows of the encrypted column of the relational database 125.

FIG. 2 illustrates an embodiment of the interactions between the client device 110, the remote data storage server (referred to as database server) 120, and the remote key management server (KMS) 130 in the process of creating an encrypted column, i.e. a column of encrypted data stored in the relational database 125 of the remote data storage server 120, including the following steps.

- Step 201: A client driver 111a of the client device 110 (which may be a software executed by the processor 111 of the client device 110) sends a request to the KMS 130 to create a client master key 115a.
- Step 203: If a client master key 115a has been successfully created, metadata is saved in the remote data storage server 120.
- Step 205: The client driver 111a of the client device 110 generates a column encryption key 115b on the basis of the client master key 115a.
- Step 207: The client driver 111a of the client device 110 requests the KMS 130 to encrypt the column encryption key 115b using the client master key 115a.
- Step 209: The KMS 130 returns the encrypted column encryption key 115b back to the client device 110.
- Step 211: The client driver 111a of the client device 110 sends the encrypted column encryption key 115b to the remote data storage server 120.
- Step 212: The client driver 111a of the client device 110 creates a table with an encrypted column re-written to also include the data type to be used by the remote data storage server 120.
- Step 213: The client driver 111a of the client device 110 instructs the remote data storage server 120 to create the table 127 with the encrypted column 127b of security sensitive data. For instance, a “CREATE TABLE” command may be send by the client driver 111a identifying two data types, namely the data type to be used by the remote data storage server 120 and the data type expected by a user querying the data.
- Step 215: The remote data storage server 120 stores the configuration information (referred to as metadata in FIG. 2) and reports the successful operation to the client device 110.

Thus, as illustrated by FIG. 2, in an embodiment, the client device 110 may be configured to define the columns of the relational database 125 that store security sensitive data in encrypted form. This may happen, for instance, during table creation or when adding a new column to a table of the relational database 125. As already described above, the client device 110 may be configured to detect which columns of the relational database 125 accept only encrypted data during parsing of a new query including SQL statements for creating a table or adding a new column and to encrypt the data before it is sent to the data storage server 120.

FIG. 3 illustrates an embodiment of the interactions between the client device 110, the remote data storage server (referred to as database server) 120, and the remote key management server (KMS) 130 in the process of inserting security sensitive data 125a, i.e. to add security sensitive data in an already existing encrypted column, such as column 127b of table 127 shown in FIG. 1b, of the relational database 125 of the remote data storage server 120.

- Step 301: An application 111b running on the client device 110 informs the client driver 111a to execute a query to insert further data in the relational database 125 of the remote data storage server 120.
- Step 303: In response to step 301, the client driver 111a parses the query from the application 111b for the column names associated with the data to be stored in the relational database 125 of the remote data storage server 120.
- Step 305: The client driver 111a identifies the column names stored in the memory 115 of the client device 110 on the basis of the parsed query.
- Step 307: The client driver 111a of the client device 110 requests the KMS 130 to decrypt the respective encryption key 115b for each of the columns identified in step 305 of the relational database 125 storing encrypted security sensitive data. This step and the following step 309 may be omitted, if the respective encryption key(s) 115b is cached in the memory 115 of the client device 110.
- Step 309: The KMS 130 returns the requested decrypted encryption key(s) 115b to the client device 110.
- Step 311: The client driver 111a of the client device 110 encrypts the security sensitive data of the further data to be inserted into the relational database 125 using the encryption key(s) 115b.
- Step 313: The client driver 111a of the client device 110 deletes the encryption key(s) 115b. According to a variant, the client driver 111a of the client device 110 may cache the encryption key(s) 115b in the memory 115 for future use.
- Step 315: The client driver 111a of the client device 110 sends the further data, including the encrypted security sensitive data, along with one or more identifiers of the encryption key(s) 115b used by the client device 110 for encrypting the security sensitive data to the remote data storage server 120. On the basis of the one or more identifiers provided by the client device 110, the remote data storage server 120 stores the further data, including the encrypted security sensitive data, in the respective column(s) 127b of the relational database 125 identified by the one or more identifiers and reports this to the client device 110.

Thus, as illustrated in FIG. 3, in an embodiment, the application 111b of the client device 110 is configured to generate the data to be stored in the relational database 125 of the remote data storage server 120 and to request storage of the data in the remote data storage server 120 using a first type of data query comprising the data. In response to this query, the client device 110, more specifically the client driver 111a, is configured to replace the security sensitive data in the first type of data query with the encrypted security sensitive data.

FIG. 4 illustrates an embodiment of the interactions between the client device 110, the remote data storage server (referred to as database server) 120, and the remote key management server (KMS) 130 in the process of an updating query, i.e. in the process of replacing security sensitive data in the relational database 125 of the remote data storage server 120.

- Step 401: The application 111b informs the client driver 111a to execute an update query for data in the relational database 125 of the remote data storage server 120.
- Step 403: In response to step 401, the client driver 111a parses the update query from the application 111b for the column names associated with data to be updated in the relational database 125 of the remote data storage server 120.
- Step 405: The client driver 111a identifies the column names stored in the memory 115 of the client device 110 on the basis of the parsed update query.
- Step 407: The client driver 111a of the client device 110 requests the KMS 130 to decrypt the respective encryption key 115b for each of the columns identified in step 405 of the relational database 125 storing encrypted security sensitive data. This step and the following step 409 may be omitted, if the respective encryption key(s) 115b is cached in the memory 115 of the client device 110.
- Step 409: The KMS 130 returns the requested decrypted encryption key(s) 115b to the client device 110.
- Step 411: The client driver 111a of the client device 110 encrypts the values of the update query for updating the security sensitive data of the relational database 125 using the encryption key(s) 115b.
- Step 413: The client driver 111a of the client device 110 encrypts the conditions of the update query for updating the security sensitive data of the relational database 125 using the encryption key(s) 115b.
- Step 415: The client driver 111a of the client device 110 sends the update query, including the encrypted values and the encrypted conditions, along with one or more identifiers of the encryption key(s) 115b used by the client device 110 for encrypting the values and the conditions of the update query to the remote data storage server 120. On the basis of the one or more identifiers provided by the client device 110, the remote data storage server 120 updates the data, including the encrypted security sensitive data, in the respective column(s) of the relational database 125 identified by the one or more identifiers and reports this to the client device 110.
- Step 417: The client driver 111a of the client device 110 deletes the encryption key(s)