Secure Shell (SSH) is a cryptographic network protocol for securely accessing and managing systems over an unsecured network. While most people are familiar with SSH keys for authentication, there’s another powerful feature of SSH that’s less commonly used but offers enhanced security and scalability: SSH Certificates.
What are SSH Certificates?
SSH certificates are an evolution of the traditional SSH key system. Instead of using individual keys for authentication (a public and a private key pair), SSH certificates introduce a third component: a certificate authority (CA). This CA can sign other SSH keys, vouching for their authenticity.
An SSH certificate is a public key that a CA has signed. The certificate contains information about the key, owner, and the CA’s signature. When a system sees this certificate, it can trust it if it trusts the CA that signed it.
Why Use SSH Certificates?
- Scalability: Managing individual SSH keys can become a nightmare in large organizations with hundreds or thousands of servers. With SSH certificates, you only need to trust the CA on each server. Any key signed by that CA will be trusted, eliminating the need to distribute public keys to every server.
- Short-lived Certificates: One of the advantages of SSH certificates is that they can be short-lived. Instead of keys that last indefinitely (until they’re manually revoked), SSH Certificates can expire after a set period, like a day or even an hour. This reduces the risk associated with lost or compromised keys.
- Enhanced Security: With the traditional SSH Key method, if an attacker gains access to a system and adds their public key, they can maintain access. With SSH certificates, even if an attacker obtains a certificate, it will expire after a short time, limiting potential unauthorized access.
How Do SSH Certificates Work?
- Setting up a Certificate Authority (CA): The first step to creating a CA is to generate its Private Key. This special SSH key pair is used exclusively for signing other keys. It’s crucial to keep the private key of the CA extremely secure.
- Signing Keys: The CA can sign other public SSH keys once set up. This process attaches the CA’s signature to the public key, turning it into an SSH certificate.
- Distributing the CA’s Public Key: For servers to trust the certificates, they need to trust the CA. This is done by adding the CA’s public key to a trusted list on the server. Once done, the server will trust any certificate signed by the CA.
- Authentication: When users try to SSH into a server, they present their SSH certificate. The server checks the certificate’s signature against the CA’s public key. Access is granted if it matches and the certificate hasn’t expired.
Conclusion
SSH certificates offer a powerful and scalable way to manage SSH access in large environments. They provide enhanced security features, like short-lived certificates, that can significantly reduce risks. While the initial setup might be a bit more complex than traditional SSH keys, the benefits in terms of security and manageability make it a worthy investment.
If you’re looking to bolster your organization’s security posture or simplify SSH key management in a large environment, it might be time to consider switching to SSH certificates.