How do you encrypt data in SQL databases?

Encrypting data in SQL databases involves protecting sensitive information at rest, in transit, and during processing. Most databases provide built-in tools or support external methods to achieve this. The primary approaches include using Transparent Data Encryption (TDE), column-level encryption, and application-layer encryption. Each method addresses specific security needs and requires careful implementation to balance performance and protection.

For encrypting data at rest, Transparent Data Encryption (TDE) is a common solution. TDE encrypts the entire database file, including backups, without requiring changes to the application code. For example, SQL Server uses TDE by enabling it via the CREATE DATABASE ENCRYPTION KEY command and specifying an encryption algorithm like AES-256. Similarly, PostgreSQL uses the pgcrypto extension or third-party tools like pg_tde for tablespace encryption. TDE relies on a database master key and certificates stored separately from the data, ensuring that even if physical storage is compromised, the data remains unreadable without the key. However, TDE doesn’t protect data in memory or during queries, so additional measures are needed for end-to-end security.

To encrypt specific columns or fields, column-level encryption allows granular control. For instance, MySQL’s AES_ENCRYPT() and AES_DECRYPT() functions can encrypt individual columns during inserts or updates. In SQL Server, using ENCRYPTBYKEY with symmetric keys ensures only authorized users can decrypt the data. This approach is useful for protecting highly sensitive fields like credit card numbers or social security numbers. However, it requires application logic to handle encryption and decryption, which can complicate queries and indexing. Additionally, keys must be managed securely, often through a key vault like Azure Key Vault or AWS KMS, to prevent unauthorized access.

For data in transit, SSL/TLS encryption is critical. Configuring the database to require encrypted connections ensures that data moving between the application and database isn’t intercepted. For example, PostgreSQL uses sslmode=verify-full in connection strings to enforce TLS, while SQL Server uses certificates validated during connection handshakes. Application-layer encryption, where data is encrypted before being sent to the database, adds another layer of security. Libraries like OpenSSL or framework-specific tools (e.g., Django’s Fernet) can handle this. Combining these methods—encrypting data at rest, in transit, and selectively at the column level—provides a robust defense against breaches while maintaining usability for developers.