---

name: selective-encrypted-storage 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

Data Elements

• d: Plaintext sensitive data
• {d}_k: Ciphertext
• keyInfo: Key identification/material
• config: Cipher configuration

Pattern Flow

Storage

CopyApplication → [encrypt(d, keyInfo, config)] → Cryptographer
Cryptographer → [{d}_k] → Application
Application → [store({d}_k)] → Storage

Retrieval

CopyApplication → [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

Copy// 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

1. Generate new key
2. Re-encrypt data with new key (background)
3. Update key reference
4. Deprecate old key
5. Eventually delete old key

Consider:

• Dual-key period during rotation
• Performance impact of mass re-encryption
• Backup/restore implications

Implementation Checklist

• Identified all sensitive data fields
• Using strong algorithm (AES-256-GCM)
• Keys stored separately from data
• Key management system in place
• Key rotation procedure defined
• Plaintext never logged
• Caches secured
• Backup encryption addressed
• Search/index strategy defined
• Performance tested

Related Patterns

• Transparent encrypted storage (alternative: encrypt everything)
• Encryption (underlying operations)
• Cryptographic key management (key handling)
• Selective encrypted transmission (encryption in transit)

References

• Source: https://securitypatterns.distrinet-research.be/patterns/07_01_001__selective_encrypted_storage/
• OWASP Cryptographic Storage Cheat Sheet