What are the differences between SSH certificates and TLS certificates?
Before delve into too much detail, we need to start with the primary reason why you would select an SSH certificate over a TLS certificate because at the core of this question, is purpose.
What is the purpose of SSH Certificates?
SSH and TLS have two hugely different purposes. SSH (Secure Shell) for example is really designed for authentication and will be utilised in SSH connections, aswell as other technologies/protocols that piggy back off of it, such as SCP (secure copy) and SFTP (secure file transfer). All of these SSH, SCP, and SFTP require authentication and this is where SSH certificates come into play because they provide a stronger authentication solution than the standard username and password.
What is the purpose of TLS certificates?
Now TLS (Transport Layer Security) certificates are utilised most commonly in securing website traffic, and changing that little http in your website url over to a https. I want to be clear at this part, and emphasize that securing web traffic is only one usage, it can be used for other network communication as well.
How do SSH Certificates differ from TLS certificates technically?
Each certificate approach has a different formatting, and this is because they’re held to different standards. SSH Certificates are formatted to an OpenSSH implementation standard and TLS certificates are formatted to X.509 standard.
Are there any other key differences?
Absolutely, lets explore them now.
- Common use cases for each of the certificate approaches are different, for example:
- SSH Certificates: Automated server access, DevOps infrastructure, secure remote administration
- TLS Certificates: HTTPS websites, API endpoints, email servers, VPN endpoints
- Both types of certificate can be used for authentication, however they operate differently at this point, for example;
- SSH certificates are usually utilised with authentication occurring in both directions. Therefore the server authenticates the client, and the client authenticates the server.
- TLS certificates are primarily used for authenticating a server to a client, however similar in concept to the SSH certificates you can implement mutual TLS for authentication going both ways (client to server and server to client).
- The certification authorities used can often be different due to the purposes of whats being achieved.
- TLS certificates will usually use a public certificate authority because the server is trying to prove it can be trusted to the public.
- SSH certificates will most likely use a private certificate authority because they’re normally managed by organisations for only internal purposes.
Validation Process
- SSH Certificates: Validated through trust on first use or through trusted CA keys configured on SSH clients
- TLS Certificates: Validated through certificate chains that lead to trusted root certificates pre-installed in browsers/operating systems
Key Differences:
| Feature | SSH Certificate | X.509 Certificate |
|---|---|---|
| Purpose | SSH authentication | TLS, encryption, digital signatures |
| Format | OpenSSH-specific | X.509 (DER/PEM) |
| Signing Authority | SSH CA (public key) | PKI CA hierarchy |
| Common Use | SSH key authentication | HTTPS, VPNs, S/MIME, etc. |
| Verification Model | Trusted SSH CA keys | Chain of trust |
SSH vs X.509 Certificates Summary
While both certificates involve cryptographic signing and authentication, SSH certificates are specifically designed for SSH access, whereas X.509 certificates are widely used in broader PKI applications, including TLS/SSL encryption and identity verification.