FHE Key Management:
Best Practices for Production Systems

FHE key management is critical for security. The encryption is only as strong as the protection of your keys. This guide covers best practices for managing FHE keys in production.

FHE Key Types

FHE systems use several key types:

• Secret key: Used for encryption and decryption—must be kept private
• Public key: Used for encryption only—can be shared
• Evaluation keys: Enable specific operations on ciphertexts
• Rotation keys: Enable SIMD slot rotations
• Relinearization keys: Reduce ciphertext size after multiplication

Secret Key Protection

The secret key is the crown jewel. Protect it with:

• Hardware Security Modules (HSMs): Store keys in tamper-resistant hardware
• Key encryption: Encrypt keys at rest with separate master keys
• Access controls: Strict permissions on key access
• Audit logging: Track all key usage

// Never do this
const secretKey = "abc123...";  // Hardcoded key

// Better: Load from secure storage
const secretKey = await hsm.getKey('fhe-secret-key');
// Or
const secretKey = await keyVault.getSecret('fhe-secret-key');

Key Generation

Proper key generation is essential:

• Use cryptographically secure random number generators
• Generate keys in a secure environment (air-gapped if possible)
• Verify key integrity before use
• Consider key ceremonies for high-value applications

Key Rotation

Regular key rotation limits exposure from potential compromise:

• Define rotation schedule based on risk assessment
• Re-encrypt data with new keys (can be done homomorphically)
• Maintain old keys for decryption until all data is rotated
• Securely destroy old keys after rotation complete

Evaluation Key Distribution

Evaluation keys must reach the computing party:

• Transmit over encrypted channels (TLS)
• Consider key caching to avoid repeated transmission
• Validate key integrity on receipt
• Evaluation keys don't need same protection as secret keys

Multi-Party Scenarios

When multiple parties are involved:

• Each party may hold their own secret key
• Threshold schemes split keys across parties
• Multi-key FHE allows computation on data encrypted under different keys
• Define clear key ownership and responsibilities

Backup and Recovery

Key loss means data loss. Plan for recovery:

• Secure backups with geographic distribution
• Key escrow for critical applications
• Recovery procedures tested regularly
• Consider Shamir's Secret Sharing for key backup

Proper FHE key management ensures your encrypted data remains both secure and accessible. Invest in robust key infrastructure from the start.