Email Signing: Private Key Security

TL;DR

Signing an email should not expose your private key. However, misconfiguration or malicious software can create vulnerabilities. This guide explains how to keep your private key safe during email signing.

Understanding Email Signing

Email signing uses cryptography to verify the sender’s identity and ensure the message hasn’t been altered in transit. It works with a pair of keys: a private key (which you must keep secret) and a public key (which you share). When you sign an email, your private key creates a digital signature. Recipients use your public key to verify this signature.

Potential Risks & How to Avoid Them

1. Key Storage: The biggest risk is how your private key is stored.
   • Never store your private key in plain text.
   • Use a secure key management system (KMS). This could be hardware-based (like a YubiKey) or software-based (like OpenSSL with a strong password, or a dedicated email security tool).
   • Avoid storing keys on easily compromised devices. Laptops and phones are more vulnerable than dedicated servers.
2. Software Vulnerabilities: Email clients and signing software can have bugs.
   • Keep your email client and signing software up to date. Updates often include security patches.
   • Be cautious about plugins or extensions. Only install trusted ones from reputable sources.
3. Malware: Malware can steal your private key.
   • Use anti-virus and anti-malware software. Keep it updated.
   • Be careful about opening attachments or clicking links in emails, especially from unknown senders.
4. Compromised Agent: If the signing agent (the program doing the signing) is compromised, your key could be stolen.
   • Regularly audit the security of your email server and signing infrastructure.
   • Use strong authentication for access to your email server. Multi-factor authentication (MFA) is highly recommended.

Practical Steps to Protect Your Private Key

1. Secure Your Key File: If you’re managing the key file yourself, restrict access.
   • On Linux/macOS:

     chmod 600 /path/to/your/private.key

     This command sets permissions so only the owner can read and write the file.

   • On Windows, ensure only your user account has access to the key file.
2. Use a Password: When generating or importing your private key, always use a strong password.

   For example, using OpenSSL:

   openssl genrsa -out private.pem 2048

   openssl pkcs8 -topk8 -inform PEM -in private.pem -outform DER -nocrypt -out private.der

3. Hardware Security Modules (HSMs): Consider using an HSM or a YubiKey for the highest level of security.
   • These devices store your private key securely and require physical presence to authorize signing operations.
4. Email Client Configuration: Configure your email client correctly.
   • Ensure it’s using a secure connection (TLS/SSL).
   • Verify the client is properly configured to use your key management system.

Monitoring and Auditing

1. Log Analysis: Regularly review logs for suspicious activity related to email signing.
2. Key Rotation: Periodically rotate your private key (generate a new one and revoke the old one). This limits the impact of a potential compromise.