| name | selective-encrypted-storage-pattern |
| description | Security pattern for field-level encryption at rest. Use when encrypting specific sensitive data fields before storage, implementing application-level encryption for databases, or when only certain data elements need encryption at rest. Addresses "Leak data at rest" problem. |
Selective Encrypted Storage Security Pattern
Application explicitly encrypts specific sensitive data elements before persisting them to storage. Application controls which data is encrypted and manages encryption operations.
Problem Addressed
Leak data at rest: Sensitive data stored in databases, files, or other storage could be accessed by unauthorized parties (database breach, stolen backups, etc.).
Core Components
| Role | Type | Responsibility |
|---|
| Application | Entity | Decides what to encrypt, invokes encryption |
| Cryptographer | Cryptographic Primitive | Performs encryption/decryption |
| Storage | Storage | Persists data (encrypted and plaintext) |
Data Elements
- d: Plaintext sensitive data
- {d}_k: Ciphertext
- keyInfo: Key identification/material
- config: Cipher configuration
Pattern Flow
Storage
Application → [encrypt(d, keyInfo, config)] → Cryptographer
Cryptographer → [{d}_k] → Application
Application → [store({d}_k)] → Storage
Retrieval
Application → [retrieve] → Storage
Storage → [{d}_k] → Application
Application → [decrypt({d}_k, keyInfo, config)] → Cryptographer
Cryptographer → [d] → Application
Key Characteristics
Application-Controlled
- Application decides WHAT data to encrypt
- Application invokes encryption before storage
- Application invokes decryption after retrieval
Field-Level Granularity
- Encrypt specific fields (SSN, credit cards, etc.)
- Non-sensitive data stored plaintext
- Enables partial data access
Key Per Data Type
- Different keys for different sensitivity levels
- Key compromise limits exposure
- Supports key rotation per data category
When to Use
Use Selective Encryption When:
- Only specific fields are sensitive
- Different data needs different protection levels
- Need to query non-sensitive fields
- Application must control encryption
Consider Transparent Encryption When:
- All data equally sensitive
- Simpler implementation preferred
- Database/filesystem encryption sufficient
Security Considerations
Key Management Critical
- Keys separate from encrypted data
- Use Key Management Service (KMS) or HSM
- Implement key rotation
- Audit key access
Algorithm Selection
- AES-256-GCM (authenticated encryption)
- RSA-3072+ for key encryption
- Follow Encryption pattern guidelines
What to Encrypt
Typically encrypt:
- Personally Identifiable Information (PII)
- Payment card data
- Health information
- Authentication credentials
- Cryptographic keys
Index/Search Challenges
Encrypted data cannot be:
- Searched directly
- Indexed efficiently
- Sorted
Solutions:
- Blind indexes (hash-based)
- Searchable encryption (advanced)
- Encrypt only display fields, index separately
Data Flow Analysis
Trace plaintext through entire flow:
- Application memory
- Logs (never log plaintext!)
- Caches
- Temporary files
- Error messages
- Backups
Performance Impact
- Encryption/decryption adds latency
- Consider caching decrypted values (securely)
- Batch operations where possible
Implementation Approaches
Application-Level
// Before storage
encryptedSSN = encrypt(ssn, ssnKey)
db.store(record with encryptedSSN)
// After retrieval
record = db.retrieve()
ssn = decrypt(record.encryptedSSN, ssnKey)
ORM/Framework Integration
Many frameworks support field-level encryption:
- Django encrypted fields
- Hibernate encryption
- ActiveRecord attr_encrypted
Database Features
Some databases offer column-level encryption:
- SQL Server Always Encrypted
- Oracle TDE column encryption
- PostgreSQL pgcrypto
Key Rotation Strategy
- Generate new key
- Re-encrypt data with new key (background)
- Update key reference
- Deprecate old key
- Eventually delete old key
Consider:
- Dual-key period during rotation
- Performance impact of mass re-encryption
- Backup/restore implications
Implementation Checklist
Related Patterns
- Transparent encrypted storage (alternative: encrypt everything)
- Encryption (underlying operations)
- Cryptographic key management (key handling)
- Selective encrypted transmission (encryption in transit)
References
