Can you discuss the advanced security features in SQL Server, such as Always Encrypted, Data Masking, and Row-Level Security, and their implementation strategies?

SQL Server provides several advanced security features to enable secure data access and protection. In this answer, we will discuss three of these features: Always Encrypted, Data Masking, and Row-Level Security.

### Always Encrypted

Always Encrypted is a feature that enables encryption of sensitive data at rest and in transit. It provides end-to-end encryption by ensuring that sensitive data is encrypted both on the client-side and when stored in the database. Even database administrators and others who have access to the database do not have access to the encryption keys that are required to decrypt sensitive data.

To use Always Encrypted, you need to create an encryption key hierarchy that includes a column master key and a column encryption key. The column master key is stored in a key store, while the column encryption key is used to encrypt and decrypt the data. You can create a column encryption key using various algorithms, such as AES and RSA.

Here is an example of how to enable Always Encrypted for a column in a table:

--create column master key
CREATE COLUMN MASTER KEY MyCMK 
WITH 
    (
        KEY_STORE_PROVIDER_NAME = 'MSSQL_CERTIFICATE_STORE',
        KEY_PATH = 'CurrentUser/My/AEColumnMasterKey',
        ENCRYPTION_TYPE = RSA_OAEP,
        CREATE_NEW = TRUE
    );

--create column encryption key
CREATE COLUMN ENCRYPTION KEY MyCEK
WITH 
    (
        COLUMN_MASTER_KEY = MyCMK,
        ALGORITHM = 'RSA_OAEP',
        ENCRYPTED_VALUE = <encrypted_value>
    );

--create a table with an encrypted column
CREATE TABLE Employee 
( 
    EmpID INT PRIMARY KEY,
    Name NVARCHAR(50),
    Salary NVARCHAR(20) COLLATE Latin1_General_BIN2 ENCRYPTED WITH (COLUMN_ENCRYPTION_KEY = MyCEK, ENCRYPTION_TYPE = RANDOMIZED, ALGORITHM = 'AEAD_AES_256_CBC_HMAC_SHA_256')
);

In the above example, a column master key and a column encryption key are created. Then we create a table with an encrypted column using the column encryption key. The ‘ENCRYPTION_TYPE‘ parameter specifies the type of encryption used, and the ‘ALGORITHM‘ parameter specifies the encryption algorithm.

### Data Masking

Data Masking is a security feature that helps protect sensitive data by obscuring it from non-privileged users. It is used to hide sensitive data from unauthorized personnel while still allowing privileged users to access it.

To use Data Masking, you can define a masking rule for a column that replaces the sensitive data with non-sensitive data. You can use different types of masking functions, such as random data, or a custom string.

Here is an example of how to use data masking to mask a column in a table:

--add a new column with masked data to the existing table
ALTER TABLE Employee
ADD MaskedSalary NVARCHAR(20) MASKED WITH (FUNCTION = 'default()') NULL;

In the above example, a new column named ‘MaskedSalary‘ is added to the table. The ‘MASKED WITH‘ parameter specifies the masking function that should be used. In this case, we use the ’default()’ function to mask the data .

### Row-Level Security

Row-Level Security (RLS) is a feature that restricts access to rows in a table based on user-specific criteria. It enables you to define security policies that are applied to rows in a table, so that users can only view or modify the rows that are associated with them.

To use RLS, you can define a security predicate for a table that identifies which rows should be visible to a specific user based on the user’s credentials or attributes. You can also define a security function that performs a more complex calculation to determine which rows should be visible.

Here is an example of how to use row-level security to restrict access to rows in a table:

--create a security policy 
CREATE SECURITY POLICY EmployeeFilter  
ADD FILTER PREDICATE EmployeeFilterPredicate(EmployeeID) 
ON Employee WITH (STATE = ON);

--create a security function that does the filtering
CREATE FUNCTION EmployeeFilterPredicate(@empid int)  
RETURNS TABLE  
WITH SCHEMABINDING  
AS  
    RETURN 
        SELECT 1 AS fn_securitypredicate_result  
        WHERE @empid = CAST(SESSION_CONTEXT(N'empid') AS int);

In the above example, we create a security policy that restricts access to rows in the ‘Employee‘ table. We then create a security function that does the filtering based on the employee id defined in the session parameters. The ‘SESSION_CONTEXT‘ function is used to retrieve the employee id from the session parameters.

To summarize, Always Encrypted, Data Masking, and Row-Level Security are advanced security features in SQL Server that enable secure data access and protection. Each of these features has its own implementation strategies and can be used in combination to provide an even more secure environment.