Column-Level Encryption in SQL Server

Published: October 26, 2023

Version: SQL Server 2022

Column-level encryption in SQL Server allows you to encrypt sensitive data at the column level, providing an additional layer of security for your databases. This feature is crucial for protecting personally identifiable information (PII), financial data, and other confidential data stored in your SQL Server instances.

Introduction to Column-Level Encryption

SQL Server offers several mechanisms for data encryption, including Transparent Data Encryption (TDE) for encrypting the entire database files and Always Encrypted for client-side encryption. Column-level encryption provides a granular approach, enabling you to encrypt specific data types within a table, such as social security numbers, credit card details, or passwords.

Key Concepts

Data Encryption Standard (DES): A symmetric encryption algorithm used for encrypting data within a column.
Column Master Key (CMK): A key that protects the Data Encryption Key (DEK). It can be stored securely in a trusted key store, such as SQL Server's certificate store or an external key management system.
Data Encryption Key (DEK): A symmetric key used to encrypt the actual data in the column. The DEK itself is encrypted by the CMK.
Deterministic Encryption: Provides the ability to generate equality and join conditions on encrypted columns. However, it allows attackers to gain information about the encrypted data by observing multiple rows with the same ciphertext.
Randomized Encryption: A more secure option that encrypts data through a more complex transformation that is not repeatable. Randomized encryption is not searchable and is suitable for data where searchability is not a requirement.

Implementing Column-Level Encryption

1. Creating Encryption Keys

Before encrypting columns, you need to create the necessary keys. This involves creating a Column Master Key and then a Column Encryption Key (which is analogous to the DEK).

Creating a Column Master Key (CMK)

The CMK should be stored securely. Here's an example of creating a CMK using a certificate:

                
-- First, create a certificate if you don't have one
CREATE CERTIFICATE MyColumnMasterKeyCert WITH SUBJECT = 'Column Master Key for Encryption';

-- Then, create the Column Master Key using the certificate
CREATE COLUMN MASTER KEY MyColumnMasterKey
WITH (KEY_STORE_PROVIDER_NAME = 'CERTIFICATE_STORE',
      KEY_PATH = 'LocalMachine/My/MyColumnMasterKeyCert');
                
            

Creating a Column Encryption Key (CEK)

The CEK is used to encrypt the actual data. It is encrypted by the CMK.

                
-- Create a Column Encryption Key
CREATE COLUMN ENCRYPTION KEY MyColumnEncryptionKey
WITH VALUES (
    COLUMN_MASTER_KEY = MyColumnMasterKey,
    KEY_STORE_PROVIDER_NAME = 'CERTIFICATE_STORE',
    ENCRYPTED_VALUE = (BLOB = varbinary_blob_value), -- You need to obtain this blob value
    ADDITIONAL_SALTS = (array_of_salt_values) -- Optional salts
);
                
            

Note: The `varbinary_blob_value` needs to be generated by encrypting a new symmetric key with your CMK. This is typically done using PowerShell or other client tools.

2. Encrypting a Column

Once your keys are in place, you can encrypt a column in an existing table or during table creation.

Encrypting an Existing Column

                
ALTER TABLE MyTable
ALTER COLUMN SensitiveData_Column VARBINARY(256)
ENCRYPTED WITH (ENCRYPTION_TYPE = 'RANDOMIZED',
               ALGORITHM_NAME = 'AEAD_AES_256_CBC',
               COLUMN_ENCRYPTION_KEY = MyColumnEncryptionKey);
                
            

Encrypting a Column During Table Creation

                
CREATE TABLE SensitiveDataTable (
    ID INT PRIMARY KEY,
    NormalData VARCHAR(100),
    SecretMessage VARCHAR(256) ENCRYPTED WITH (ENCRYPTION_TYPE = 'DETERMINISTIC',
                                                ALGORITHM_NAME = 'AEAD_AES_256_CBC',
                                                COLUMN_ENCRYPTION_KEY = MyColumnEncryptionKey)
);
                
            

Searching Encrypted Columns

Searching encrypted columns depends on the encryption type:

Deterministic Encryption: You can perform direct comparisons and joins on the encrypted column.
Randomized Encryption: You cannot directly search or join on these columns. You would typically need to decrypt the data first or use other techniques like searching indexed views on the decrypted data.

Example: Searching a Deterministically Encrypted Column

                
SELECT ID, NormalData
FROM SensitiveDataTable
WHERE SecretMessage = 'some specific value';

Decrypting Encrypted Data

To view the plaintext of an encrypted column, you can use the `DecryptByKey` function (for deterministic encryption) or `DecryptByKeyAuto` (which handles both types but requires the appropriate keys to be available in the session).

                
SELECT
    ID,
    NormalData,
    DecryptByKey(SecretMessage) AS DecryptedMessage
FROM SensitiveDataTable;
                
            

Considerations and Best Practices

Key Management: Securely managing your Column Master Keys is paramount. Use robust key management solutions.
Performance Impact: Encryption and decryption operations consume CPU resources and can impact query performance, especially for large datasets or complex queries. Choose the appropriate encryption type based on your requirements.
Data Type Compatibility: Ensure the data type of the column is compatible with the encryption algorithm. For example, `VARBINARY` is often used for encrypted data.
Application Changes: Applications that access encrypted data might need to be updated to handle the encrypted format or to perform decryption on the client side.
Auditing: Implement auditing to track access to sensitive data and key management operations.