Encryption and key management - protecting backups at rest

Backups often contain sensitive data. Encryption protects them at rest, but key management is the discipline that keeps them recoverable.

Encrypt backups at rest

Tools:

• BorgBackup: client-side encryption with AES-OCB.
• age: modern encryption tool.
• openssl enc: standard tool for symmetric encryption.
• gpg: PGP encryption.

BorgBackup example:

# Init with encryption
borg init --encryption repokey-blake2 /backup/repo

# Create encrypted backup
borg create /backup/repo::backup-2026-08-09 /var/data

# Borg automatically encrypts before sending to /backup/repo

The encryption key is derived from the passphrase. Lose the passphrase, lose the backups.

Key management

Keys must be:

• Backed up: at least 2 independent copies.
• Access-controlled: only key custodians can read.
• Versioned: when rotated, old keys are kept (until decryptions of old backups are no longer needed).
• Audited: who accesses the key, when.

Storage options:

• Vault (HashiCorp, AWS KMS, GCP Secret Manager): managed secret storage.
• Hardware security module (HSM): physical device.
• Paper backup in safe: for the most critical keys.
• Multi-person: split the key across multiple people.

For a single key:

# Generate a strong passphrase
openssl rand -base64 32 > /etc/borg/passphrase
chmod 600 /etc/borg/passphrase

# Backup the passphrase
gpg --encrypt --recipient ops@example.com /etc/borg/passphrase
# Or store in vault

Key recovery in DR

If you cannot find the key, you cannot decrypt the backups. The DR plan must include key recovery.

DR procedure:
1. Find the vault operator.
2. Retrieve the key from the vault.
3. Decrypt the passphrase file.
4. Use the passphrase to unlock BorgBackup.
5. Restore data.

Test this. A DR plan that has not been tested is a hope.

Rotation

Rotate keys periodically:

• Quarterly for high-value keys.
• Annually for standard keys.
• Immediately on personnel change.

When you rotate:

• Old key is kept until old backups are no longer needed.
• New key is used for new backups.
• Document the rotation in the key log.

Knowledge check