Securely Store PKA Keys on a Server

TL;DR

Don’t store your Private Key directly in files or code! Use a Hardware Security Module (HSM) or a dedicated key management system. If those aren’t options, encrypt the key with strong encryption and restrict access tightly.

Securely Storing PKA Keys

Public-Key Cryptography (PKA) relies on keeping your Private Key secret. Compromising it means anyone can decrypt your data or forge signatures. Here’s how to store it safely on a server, from best to worst options:

1. Hardware Security Module (HSM)

1. What is it? A dedicated physical device designed to protect cryptographic keys. It never exposes the private key in plain text.
2. How it works: You perform cryptographic operations *within* the HSM. The key stays inside, and only signed data or encrypted results come out.
3. Benefits: Highest level of security, tamper-resistant, often FIPS 140-2 certified.
4. Cost: Most expensive option.

2. Key Management System (KMS)

1. What is it? Software or a service that manages the lifecycle of cryptographic keys, including generation, storage, rotation, and access control. Cloud providers (AWS KMS, Azure Key Vault, Google Cloud KMS) offer these as services.
2. How it works: Keys are encrypted at rest and in transit. Access is controlled through permissions and auditing.
3. Benefits: Good security, easier to manage than HSMs, often integrates with other cloud services.
4. Cost: Moderate cost, typically pay-per-use.

3. Encrypted Key Storage (If HSM/KMS aren’t feasible)

This is the least secure option but better than storing the key in plain text. It requires careful implementation.

1. Choose a Strong Encryption Algorithm: Use AES-256 or ChaCha20 with appropriate modes (e.g., GCM).
2. Generate a Key Encryption Key (KEK): This key will encrypt your PKA Private Key. This KEK is the new thing you need to protect!
3. Securely Store the KEK: See section 4 below for KEK storage options.
4. Encrypt the Private Key: Use a library or command-line tool to encrypt your private key with the KEK.

   openssl enc -aes-256-cbc -salt -in private.key -out encrypted_private.key

5. Restrict Access: Limit file permissions on the encrypted key file to only the necessary users and processes (e.g., 600 or 400).
6. Regular Rotation: Rotate both your PKA Private Key *and* the KEK periodically.

4. Securing the Key Encryption Key (KEK)

The security of your encrypted private key depends entirely on how well you protect the KEK.

• Don’t hardcode it: Never store the KEK directly in code or configuration files.
• Environment Variables: Store the KEK as an environment variable, accessible only by the application that needs it.

  export KEK=your_very_secret_key

• Vault Solutions (HashiCorp Vault): Use a secrets management tool like HashiCorp Vault to store and manage the KEK.
• Operating System Keyring: Store the KEK in the operating system’s secure keyring (e.g., macOS Keychain, Windows Credential Manager). This provides some protection against unauthorized access.

5. Things to *Never* Do

• Store the Private Key in Plain Text: This is the worst possible thing you can do.
• Commit the Private Key to Version Control (Git, etc.): Even accidentally committing it can lead to compromise. Use .gitignore and pre-commit hooks to prevent this.
• Email or Share the Private Key: Obvious security risk.
• Use Weak Encryption Algorithms: DES, RC4, and MD5 are all outdated and insecure.