Always Encrypted Columns - Rishan Solutions

Always Encrypted Columns in SQL Server: A Comprehensive Guide

In today’s world, data protection and privacy are more important than ever. Organizations are increasingly looking for ways to secure sensitive data, especially in a world where data breaches and compliance regulations are a growing concern. SQL Server’s Always Encrypted feature is an essential tool that helps organizations ensure that sensitive data stored in their databases is protected both at rest and during transmission. The Always Encrypted feature encrypts sensitive data in a way that makes it invisible to unauthorized users and SQL Server itself, ensuring that even database administrators or anyone with access to the database can’t read the encrypted data.

This comprehensive guide will delve into Always Encrypted columns in SQL Server, providing a detailed explanation of how they work, how to implement them, best practices, and how they fit into the larger security and compliance landscape. We will explore everything from encryption keys and certificates to setting up Always Encrypted and troubleshooting issues.

1. Introduction to Always Encrypted

Always Encrypted is a feature introduced in SQL Server 2016 that provides end-to-end encryption for sensitive data in SQL Server. The primary goal of Always Encrypted is to allow for the encryption of data without exposing the keys to SQL Server. This ensures that even if an attacker gains access to the database, they won’t be able to read the sensitive data.

SQL Server offers two types of encryption:

Transparent Data Encryption (TDE): Encrypts the entire database at rest.
Always Encrypted: Encrypts specific columns in the database, ensuring that data is encrypted both at rest and in use.

Unlike TDE, which encrypts the entire database, Always Encrypted works at the column level. This means that only the sensitive data is encrypted, making it more suitable for protecting personally identifiable information (PII), credit card details, or any other sensitive data that needs to be encrypted while being used by authorized applications.

2. How Always Encrypted Works

Always Encrypted uses two encryption keys to protect data:

Column Encryption Keys (CEKs): These are the keys used to encrypt and decrypt the data in specific columns. The CEK is stored in the database itself, but its access is restricted and tightly controlled.
Column Master Keys (CMKs): These keys protect the CEKs. CMKs are stored outside the database and typically reside in a secure location, such as a Key Management Service (KMS), hardware security modules (HSMs), or Windows Certificate Store. CMKs provide an additional layer of security by ensuring that the CEK is never exposed directly.

In Always Encrypted, the encryption process occurs on the client side, which means that the application encrypts and decrypts the data before sending it to SQL Server. SQL Server itself never sees the unencrypted data. This allows the data to remain encrypted at all times, even when it is being processed by SQL Server.

2.1. Steps in the Encryption Process

Encryption: When data is inserted or updated into the database, it is first encrypted by the client application using the CEK. SQL Server stores the encrypted data in the specified column.
Decryption: When data is retrieved from the database, the client application decrypts it using the corresponding CEK. This process occurs on the client side, and SQL Server only returns the encrypted data to the client.

2.2. Key Management in Always Encrypted

Column Master Keys (CMK): These are the keys used to encrypt the CEKs. The CMKs can be managed externally to SQL Server (e.g., in HSMs or KMS) or stored in the Windows Certificate Store.
Column Encryption Keys (CEK): These are the keys that directly encrypt and decrypt the data in Always Encrypted columns. These keys are stored in the SQL Server database, but access is tightly controlled to prevent unauthorized users from accessing the keys.

3. Types of Always Encrypted Columns

SQL Server allows for two types of encryption in Always Encrypted:

Deterministic Encryption: This type of encryption always encrypts the same plaintext value to the same ciphertext. This is useful for scenarios where the encrypted data must be searchable or used for joining tables. Example: If the value “123-45-6789” (a Social Security Number) is encrypted, it will always be encrypted to the same ciphertext value.
Randomized Encryption: This type of encryption encrypts the same plaintext value to a different ciphertext each time it is encrypted. This is more secure because it prevents attackers from inferring any pattern from the encrypted data. However, it cannot be used for operations that require the data to be compared or sorted, such as equality or range searches. Example: If the value “123-45-6789” is encrypted, it may produce a different ciphertext value each time it is encrypted.

4. Setting Up Always Encrypted Columns

Setting up Always Encrypted involves several key steps, which we will detail below:

4.1. Prerequisites

Before setting up Always Encrypted, ensure the following prerequisites are met:

SQL Server 2016 or later (Enterprise, Standard, or Developer editions).
A client application that supports Always Encrypted, such as SQL Server Management Studio (SSMS) or a .NET application.
Access to a Column Master Key (CMK). This could be a key stored in a Windows Certificate Store, Key Management Service (KMS), or Hardware Security Module (HSM).

4.2. Create the Column Master Key (CMK)

The first step in setting up Always Encrypted is to create a Column Master Key (CMK). The CMK is used to encrypt the Column Encryption Key (CEK).

Using Windows Certificate Store: If you’re using a Windows Certificate Store, the CMK can be created by generating a certificate and storing it in the store.

-- Create a certificate for CMK
CREATE CERTIFICATE CMKCertificate
WITH SUBJECT = 'Always Encrypted CMK';

Using Azure Key Vault or HSM: You can use Azure Key Vault or a hardware security module (HSM) for CMK management. For example, to use Azure Key Vault:

-- Add Azure Key Vault CMK
CREATE COLUMN MASTER KEY MyCMK
WITH
    PROVIDER = 'AZURE_KEY_VAULT',
    KEY_VAULT_URI = 'https://<Your-Key-Vault-URL>/';

4.3. Create the Column Encryption Key (CEK)

Next, create the Column Encryption Key (CEK). This key is used to encrypt and decrypt the data within Always Encrypted columns.

-- Create a CEK and encrypt it using CMK
CREATE COLUMN ENCRYPTION KEY MyCEK
WITH VALUES
(
    COLUMN_MASTER_KEY = MyCMK,
    ALGORITHM = 'RSA_OAEP',
    ENCRYPTED_VALUE = <ENCRYPTED-CMK-VALUE>
);

4.4. Create a Table with Always Encrypted Columns

Now that the keys are created, you can create a table that will contain Always Encrypted columns. Here’s an example:

CREATE TABLE SensitiveData (
    EmployeeID INT PRIMARY KEY,
    Name NVARCHAR(100),
    SSN NVARCHAR(11) ENCRYPTED WITH (COLUMN_ENCRYPTION_KEY = MyCEK, ENCRYPTION_TYPE = DETERM, ALGORITHM = 'AEAD_AES_256_CBC_HMAC_SHA_512'),
    CreditCardNumber NVARCHAR(19) ENCRYPTED WITH (COLUMN_ENCRYPTION_KEY = MyCEK, ENCRYPTION_TYPE = RAND, ALGORITHM = 'AEAD_AES_256_CBC_HMAC_SHA_512')
);

In this example, the SSN column is encrypted using deterministic encryption, while the CreditCardNumber column uses randomized encryption.

4.5. Inserting Encrypted Data

When you insert data into a table with Always Encrypted columns, the data will automatically be encrypted. However, it must be encrypted on the client side before it is inserted.

Here’s an example:

-- Insert data into the table (client-side encryption handled by application)
INSERT INTO SensitiveData (EmployeeID, Name, SSN, CreditCardNumber)
VALUES (1, 'John Doe', '123-45-6789', '4111-1111-1111-1111');

The data in the SSN and CreditCardNumber columns will be automatically encrypted on the client side before it is inserted into the database.

4.6. Querying Encrypted Data

To query the encrypted data, use SQL Server Management Studio (SSMS) or an application that supports Always Encrypted. The client application will decrypt the data after retrieving it from SQL Server.

-- Query the encrypted data (decryption happens client-side)
SELECT EmployeeID, Name, SSN, CreditCardNumber FROM SensitiveData;

The decrypted data will be returned to the client, but SQL Server will never see the unencrypted values.

5. Best Practices for Always Encrypted

Key Management: Always ensure that the Column Master Keys and Column Encryption Keys are securely managed. Use a Key Management System (KMS) or HSM to store these keys and ensure they are rotated periodically.
Least Privilege Access: Limit access to encryption keys to only the users or services that need them. Implementing least privilege access is critical in maintaining the security of the encrypted data.
Client-Side Encryption: Make sure that your client application is responsible for encrypting and decrypting the data. This ensures that sensitive data is protected before being sent to SQL Server.
Testing and Monitoring: Regularly test the setup to ensure that encryption is working as expected. Use SQL Server monitoring tools to keep track of encryption-related performance.

6. Troubleshooting Always Encrypted

Decryption Failures: If decryption fails, check that the correct encryption keys are being used and that the client has the proper access rights.
Column Encryption Key Mismatch: Ensure that the CEKs used in your queries match the ones defined for the Always Encrypted columns.
Performance Considerations: Always Encrypted can have a performance overhead, especially when using randomized encryption. Carefully monitor performance and optimize queries where necessary.

SQL Server’s Always Encrypted feature is a powerful tool for protecting sensitive data in the database. By encrypting data on the client side, Always Encrypted ensures that data is protected at all times, even during query execution. Implementing Always Encrypted requires careful planning, especially around key management and encryption algorithms, but it provides organizations with a robust solution for meeting data privacy and compliance regulations.

By following best practices for encryption key management, access control, and monitoring, organizations can successfully implement Always Encrypted and safeguard their most sensitive data from unauthorized access.