TDE protects database storage when it is offline; field-level encryption can keep selected values hidden from the database engine itself. They address different threats. TDE is useful against someone who obtains covered database files or backups, while client-side field encryption can also reduce what a database operator can see—if the encryption keys stay outside the database environment. Neither protects plaintext from an authorized application or endpoint that can decrypt it.
How the protection boundaries compare
| Question | Transparent database encryption (TDE) | Field-level or client-side encryption |
|---|---|---|
| What is encrypted? | Database storage, typically data and log files; coverage of other stored copies depends on the product and configuration. Microsoft SQL Server TDE | Chosen values, such as selected columns. The specific boundary depends on where encryption happens and which component holds the keys. Microsoft Always Encrypted client development |
| Who can see plaintext during normal operation? | The running database engine decrypts data for authorized queries, so a principal allowed to query the data can ordinarily receive plaintext. | In a client-side design, the client encrypts values before sending them to the database and decrypts results after receiving them. The database handles ciphertext, but the client and any other component with decryption access can see plaintext. |
| Does it protect against a live database account or DBA? | Not by itself: TDE is an at-rest control, not a restriction on queries made through the running engine. | Potentially, for selected values, if keys and plaintext remain beyond the database operator’s reach. This is not guaranteed by the label “field-level encryption.” |
| Can the database search or process the data? | Queries operate on decrypted data inside the engine as usual. | Usually with restrictions. Supported operations depend on the scheme; Always Encrypted’s deterministic and randomized modes have different trade-offs, and secure-enclave support is platform- and version-dependent. Microsoft Always Encrypted query limitations |
| What about backups? | Coverage is product-specific. Azure SQL TDE covers associated backups and transaction logs at rest; AWS RDS storage encryption covers automated backups, read replicas, and snapshots according to its guidance. Check the exact service and backup path. Azure SQL TDE overview; AWS RDS encryption best practices | Encrypted column values remain encrypted in copies that preserve them, but exports, application-generated files, and other copies need separate review. Coverage depends on the implementation and data path. |
| Typical implementation impact | Often broad storage protection with little or no application change; key backup and recovery still matter. | Can require changes to drivers or application code, queries, reporting, and every path that reads or writes the protected values. |
What TDE protects—and what it does not
TDE encrypts database files and logs at rest. In SQL Server, it uses a database encryption key within a key hierarchy; the SQL Server documentation also makes backing up and recovering the associated certificate or key material an operational requirement. Microsoft’s SQL Server TDE documentation describes that model. Azure SQL describes TDE as real-time encryption and decryption of database files, associated backups, and transaction logs at rest, aimed at malicious offline activity and designed to work without application changes. Azure SQL TDE overview
That makes TDE relevant when an attacker can obtain storage media or covered database files but does not also have the necessary keys and access to the running service. It does not make a live database query return ciphertext to an otherwise authorized user: the engine decrypts data to perform normal operations. A stolen disk and a compromised database account are different threat scenarios, and TDE addresses the former rather than controlling the latter.
Do not assume “TDE” means every copy is covered. Azure SQL documents coverage for associated backups and transaction logs, and AWS RDS describes storage-encryption coverage for its database storage, automated backups, read replicas, and snapshots. These are platform-specific statements, not a guarantee that every database product encrypts every export, external copy, or backup in the same way. On AWS RDS, storage encryption and engine-level TDE are separate layers; TDE support is engine-specific, with AWS listing it for RDS for SQL Server and Oracle. Check the current constraints for the exact engine and version before relying on it. AWS RDS encryption best practices
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What field-level encryption adds
Field-level encryption is an architectural category, not one universal feature. It means encrypting selected values rather than relying only on whole-storage protection. Its value against a database operator depends on the encryption boundary: a database-side function whose keys or plaintext are accessible to database administrators offers a different separation from client-side encryption whose keys are kept outside the database environment.
Always Encrypted as a client-side example
Microsoft Always Encrypted illustrates the stronger client-side boundary. An enabled client driver encrypts sensitive parameters before they are sent to SQL Server or Azure SQL and decrypts results on the client. Microsoft describes the feature as keeping sensitive data and related encryption keys from being revealed to SQL Server or Azure SQL Database. That is Microsoft’s description of Always Encrypted, not a claim about every field-encryption design. Develop applications using Always Encrypted
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This separation can reduce what a database administrator can see, but it does not hide values from an application process that is authorized and able to decrypt them. If an attacker compromises that application while it has access to the keys or plaintext, field encryption alone may not prevent exposure.
Can the database query encrypted fields?
It depends on the encryption method and the operation. In standard Always Encrypted, deterministic encryption produces the same ciphertext for the same plaintext. That permits selected equality-based operations, including point lookups, equality joins, grouping, and indexing, but repeated ciphertext reveals that values match and can expose patterns—particularly when the possible values come from a small set.
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Randomized encryption produces different ciphertext for repeated instances of the same plaintext, reducing that equality leakage. In return, standard database operations on the encrypted values are much more limited. Always Encrypted secure enclaves enable some additional computations, including pattern matching and comparisons, but supported operations depend on the SQL Server or Azure SQL platform and version. Confirm those limits against the deployment and driver versions you will use. Microsoft Always Encrypted with secure enclaves
These are Always Encrypted behaviors, not rules for every field-encryption system. Application-side cryptography may require redesigning search, joins, uniqueness checks, indexing, or reports; some designs use keyed lookup tokens or other mechanisms that need their own security analysis. Test representative queries and all application read/write paths before choosing a scheme.
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Key custody is part of the security boundary
For client-side encryption to keep keys from the database, the key architecture must actually separate them. In Always Encrypted, column master keys are held in a trusted external key store; examples documented by Microsoft include the Windows Certificate Store, Azure Key Vault, and hardware security modules (HSMs). The database stores key metadata and encrypted column encryption keys, not plaintext column master keys. Microsoft Always Encrypted key management
Decide which roles may provision, use, rotate, back up, and recover keys. Separating key administration from database administration can support a design intended to keep selected plaintext out of DBAs’ reach, but it also creates availability and recovery obligations. If one team or compromised environment controls both the application and key store, the intended separation may not hold.
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Should you use both?
Often, yes—when the threat model includes both exposed storage and database operators who should not see a small set of sensitive values. TDE can provide broad protection for covered database storage and backups, while client-side field encryption can create an additional boundary around selected data. Layering does not remove the need to verify backup and export paths or control access to clients and keys.
Choose based on who might gain access and where the data is when that happens:
- Stolen storage or covered backup: TDE or the managed service’s documented storage-encryption feature is relevant; verify coverage for the exact copy and recovery workflow.
- Live database credentials or privileged database access: TDE alone is not the control for hiding query results. Consider client-side encryption for selected values if the database must not see their plaintext.
- Compromised application or authorized endpoint: Neither approach prevents a component with decryption access from exposing plaintext. Reduce that risk through application security, least privilege, endpoint controls, and careful key access.
- Search-heavy data or complex reporting: Validate field-encryption query limits against actual workloads before committing; a stronger confidentiality boundary may require query or reporting compromises.
Plan deployment and recovery before enabling encryption
- Define the attacker and data state. Distinguish an offline copy of storage, a live database account, a privileged operator, and a compromised application. The controls needed are not interchangeable.
- Map every copy and path. Inventory database files, logs, backups, replicas, snapshots, exports, temporary files, reporting flows, and application caches. Verify coverage with the relevant database or cloud-service documentation rather than assuming one setting protects every copy.
- Select the encryption boundary. Use TDE for broad at-rest protection; use field-level encryption only for chosen values whose plaintext should be unavailable at the database layer. Identify which processes must decrypt those values.
- Test workload compatibility. With the actual engine, edition, service tier, client-driver versions, schema, and queries, test search, joins, sorting, indexing, reporting, migrations, and restore procedures. Always Encrypted and enclave capabilities vary by platform and version.
- Document key operations. Assign permissions for key use, rotation, backup, and recovery; test restoration and define how the service remains available if a key store is unavailable.
- Keep complementary controls. Apply least privilege, authentication, auditing, secure network connections, application security, and endpoint protection. Encryption protects particular data states and boundaries; it does not replace access control or prevent every authorized disclosure.
Platform features are not interchangeable. SQL Server and Azure SQL offer TDE and Always Encrypted, but availability and capabilities can depend on edition, version, service tier, driver, and enclave support. For PostgreSQL, the official encryption-options documentation discusses application-level, file-system/block-level, and network encryption; it should not be read as establishing a universal built-in upstream TDE capability. Managed services and extensions may provide additional approaches. PostgreSQL Encryption Options
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