TDE - Transparent Data Encryption

TDE stands for "Transparent Data Encryption." It's a security feature primarily used in databases to encrypt data at rest. Here's a simple breakdown of what TDE is and how it works:

Purpose: The main goal of TDE is to protect data at rest by encrypting database files. This means that if someone were to gain unauthorized access to the physical files, they wouldn't be able to read the data without the encryption keys.

How it works:

1. Encryption: TDE encrypts the entire database. It doesn't encrypt individual rows or columns, but rather the storage files (like the data files, log files, and backup files). 

2. Transparency: As the name suggests, the encryption process is transparent to the applications and users accessing the database. When the database is in use, the data is decrypted as it's read into memory, and applications can interact with it as if it were unencrypted. When data is written back to the disk, it's automatically encrypted.

3. Keys: TDE uses a hierarchy of encryption keys to secure the data. The primary one is the database encryption key (DEK), which is stored in the database boot record for easy retrieval during database startup. The DEK is protected by another key, typically a certificate-based key stored outside the database or a key managed by a hardware security module (HSM).

Advantages:

1. No Application Changes Needed: Since TDE operates at the storage layer, there's no need to make any changes to the applications that access the database.

2. Performance: TDE usually has a lower performance overhead compared to column-level encryption or other application-level encryption methods.

3. Protection Against Physical Threats: TDE protects against threats where raw database files might be accessed, such as stolen backups or unauthorized disk access.

Considerations:

1. Doesn't Encrypt Data in Transit: TDE only encrypts data at rest. If you need to secure data as it travels over a network, you'll need additional security measures like SSL/TLS.

2. Key Management: Proper management of encryption keys is critical. Losing the keys means losing access to the data.

3. Performance Overhead: While the overhead is generally lower than other encryption methods, there's still some performance impact.

Many modern relational database management systems (RDBMS), like Microsoft SQL Server, Oracle Database, and others, offer TDE as a feature. Given the rising concerns about data breaches and security, TDE has become an essential tool for many organizations to protect sensitive data.