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Random freezes, missing sound and display glitches usually trace back to one bad driver. Find and replace yours safely.Free scan · under a minuteAn OpenSSH certificate authority (CA) lets you authenticate many users and hosts through a small set of trusted CA public keys instead of copying every individual key into authorized_keys and known_hosts. The CA signs ordinary SSH public keys, adding principals, validity dates, a key identifier and policy extensions. Servers trust a user CA with TrustedUserCAKeys; clients trust a host CA with an @cert-authority entry; and servers present host certificates through HostCertificate.
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Create separate CA key pairs for users and hosts, keep each private CA key offline or in a tightly controlled signing system, and distribute only the public CA keys. The procedures below use standard OpenSSH tools and show the complete trust path in both directions.
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What an OpenSSH CA actually does
An OpenSSH certificate is a signed wrapper around an existing public key. It does not replace the corresponding private key and it is not an X.509 certificate. The certificate contains the public key, the CA signature, a key identifier, one or more principals, validity times, and signed options or extensions.
A relying party needs only the CA public key and a local policy. It verifies that the certificate was signed by that CA, that the embedded public key is being used by its matching private key, that the certificate is currently valid, and that its principals and options satisfy policy. This compresses trust: a server can trust one user CA instead of hundreds of user keys, while clients can trust one host CA instead of maintaining a key entry for every server.
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OpenSSH has two certificate roles:
- User (client) certificates: authenticate a person or service to an SSH server.
- Host certificates: authenticate an SSH server to a client.
Ubuntu’s ssh-keygen manual describes the distinction directly: user certificates authenticate users to servers, whereas host certificates authenticate server hosts to users. A certificate created for one role should not be used as the other role.
Design the CA hierarchy before generating keys
Use separate host and user CAs
Generate one CA key pair for user certificates and another for host certificates, for example ca_user and ca_host. ANSSI OpenSSH guidance recommends distinct CAs when their roles differ. Separating them limits the effect of a compromise and lets you rotate one trust domain without replacing the other.
OpenSSH does not support a certification chain in which a root CA delegates to subordinate CAs. Each relying party must trust the CA that signs the certificates it accepts. If you need organizational separation, use separate CA keys and configure the appropriate public key at each server or client rather than trying to build an X.509-style hierarchy.
Protect the CA private keys
The CA private key can sign arbitrary certificates for its role, so it is more sensitive than any individual host or user key. Store it away from ordinary production servers where practical, restrict file ownership and permissions, and protect it with a unique, robust passphrase. Keep an issuance record for every certificate, including its owner, asset, principal, validity interval and identifier.
The signer can be a file on a controlled administrative system. OpenSSH also documents optional integrations with a PKCS#11 token using ssh-keygen -D and an ssh-agent using ssh-keygen -U. These mechanisms can keep the private key from being directly exposed to the signing command, but neither is required for a basic deployment.
Choose names and policy fields consistently
Use a unique -I key identifier that includes an issuance date, ticket or asset reference. OpenSSH logs this identifier when the certificate is used, making it valuable during audits. Principals must be narrow: a user certificate should name only the account or service identity it is allowed to assume, and a host certificate should list every DNS name or address clients are expected to use.
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Generate the two CA key pairs
Run the following on a protected signing system. The umask prevents newly created private-key files from being world-readable.
umask 077
ssh-keygen -t ed25519 -f ca_user -C 'OpenSSH user CA'
ssh-keygen -t ed25519 -f ca_host -C 'OpenSSH host CA'ssh-keygen prompts for a passphrase for each private key. The exact algorithm and file permissions should follow the OpenSSH versions and cryptographic policy used in your environment. The public files are ca_user.pub and ca_host.pub; distribute those files, never the private files.
Issue a user certificate for a client key
Start with an ordinary user public key. The private key remains on the user’s workstation or in its normal agent. The following example signs id_ed25519.pub for the account principal alice:
ssh-keygen -s ./ca_user
-I alice-laptop-2026-10
-V +52w
-n alice
./id_ed25519.pubThe options have specific meanings:
-s ./ca_userselects the user CA private key.-I alice-laptop-2026-10sets the certificate key identifier recorded in logs.-V +52wsets a validity interval beginning at the current time and ending 52 weeks later. Choose a lifetime that matches your local threat model and reissuance capability; OpenSSH documentation does not mandate one universal lifetime.-n alicerestricts the certificate’s principals toalice.- The final argument is the public key being certified.
The command writes id_ed25519-cert.pub beside the input public key. It does not alter id_ed25519, and possession of the certificate alone is useless without the matching private key.
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Offer the certificate from the SSH client
Tell the client to use the private key and offer its certificate alongside it:
Host bastion.example.com
IdentityFile ~/.ssh/id_ed25519
CertificateFile ~/.ssh/id_ed25519-cert.pubIdentityFile points to the private key. CertificateFile points to the signed public certificate. The certificate file is safe to install with the private key because it contains public material, but keep normal restrictive permissions on the entire ~/.ssh directory.
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Configure servers to trust user certificates
Copy ca_user.pub to each destination server through a controlled channel. Store it as a root-owned file that unprivileged users cannot modify, for example /etc/ssh/ca_user.pub. Add this directive to sshd_config:
TrustedUserCAKeys /etc/ssh/ca_user.pubWith only TrustedUserCAKeys configured, the login username must appear in the certificate’s principal list. In the example, a connection as alice succeeds only when the certificate contains the alice principal and the client proves possession of the matching private key.
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If certificates use role names, employee IDs or service identities rather than Unix account names, add an AuthorizedPrincipalsFile policy:
TrustedUserCAKeys /etc/ssh/ca_user.pub
AuthorizedPrincipalsFile /etc/ssh/auth_principals/%uFor an account named alice, create /etc/ssh/auth_principals/alice with one accepted principal per line, for example:
alice
engineering-adminThe principal in the certificate must match one of the lines in the file. This lets an account accept a controlled role without allowing every certificate that names an unrelated principal. Make the directory and files root-owned and non-writable by ordinary users.
Validate and reload safely
- Check the complete daemon configuration before changing the running service:
sshd -t. - Correct ownership, permissions or path errors reported by the test.
- Reload the SSH service using the service manager appropriate to the host. A reload preserves existing sessions while applying the new configuration.
- Keep an existing administrative session open and test a new connection with verbose output before closing it.
Issue a host certificate
Use each server’s existing host public key as the certificate subject. This example signs an Ed25519 host key and includes both a DNS name and an address:
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ssh-keygen -s ./ca_host
-I web-01-2026-10
-V +52w
-h
-n web-01.example.com,10.0.2.15
/etc/ssh/ssh_host_ed25519_key.pubHere, -h marks the certificate as a host certificate, while -n lists the host principals. Include every name clients will actually use: a short hostname, fully qualified DNS name and address are different principals. The output is /etc/ssh/ssh_host_ed25519_key-cert.pub.
The private host key is not replaced. The certificate must match the public key derived from the private key named in the server configuration. If you sign a different host key, sshd will not be able to use the certificate with that HostKey.
Configure sshd to present the host certificate
Install the certificate with root ownership and restrictive permissions, then pair it explicitly with its private host key:
HostKey /etc/ssh/ssh_host_ed25519_key
HostCertificate /etc/ssh/ssh_host_ed25519_key-cert.pubOpenSSH does not load host certificates automatically merely because a file has the expected suffix. The HostCertificate directive is required, and the certificate public key must correspond to a configured HostKey. Run sshd -t, reload the daemon, and test a fresh connection after installing it.
Make clients trust the host CA
Distribute ca_host.pub to client machines through configuration management or another authenticated administrative channel. Add an @cert-authority line to a user’s ~/.ssh/known_hosts or to the system-wide ssh_known_hosts file:
@cert-authority *.example.com ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAA... host-caThe host pattern limits where this CA is trusted. Choose a pattern that covers the names you actually operate without unnecessarily extending trust to unrelated domains. ANSSI guidance identifies the @cert-authority marker as the client-side declaration that a CA may sign host certificates.
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Verify the CA fingerprint through an authenticated channel before distributing it. A CA public key is not self-authenticating; an attacker who substitutes a different public key could make a client trust a fraudulent host certificate. After the CA key is trusted, the host certificate still needs a principal matching the name or address used in the SSH command. If a client connects to web-01.example.com, that exact name must be among the certificate principals; a certificate containing only 10.0.2.15 will not satisfy the hostname check.
Inspect every certificate before deployment
Use ssh-keygen -L to display the signer, serial information, principals, validity interval and extensions:
ssh-keygen -L -f id_ed25519-cert.pub
ssh-keygen -L -f /etc/ssh/ssh_host_ed25519_key-cert.pubCheck each item before placing a certificate into production:
- The CA fingerprint is the expected fingerprint for the role.
- The certificate key type and embedded public key match the intended private key.
- User principals contain only the required account or role names.
- Host principals contain every DNS name or address clients will use, and no unnecessary names.
- Validity start and end times match the issuance record and the clocks on the participating systems.
- The user principal maps to the login name or to an entry in
AuthorizedPrincipalsFile. - The identifier from
-Iidentifies the person, device or asset that received the certificate. - Server configuration passes
sshd -t. - A test connection succeeds with verbose client logging before broad rollout.
Control principals, extensions and lifetime
Do not rely on broad defaults. Specify principals with -n and set an explicit validity interval with -V. A certificate that names every possible user or host defeats the policy benefit of a CA. Keep each role’s options and extensions no broader than necessary, and review them with ssh-keygen -L.
Short-lived certificates reduce the period in which a stolen private key or certificate remains useful, but they increase issuance and renewal work. Long-lived certificates reduce operational overhead but leave more time for misuse. The appropriate interval depends on how quickly your organization can reissue certificates, revoke trust and respond to a compromise; OpenSSH itself does not prescribe a universal value or revocation schedule.
CA deployment compared with individual key files
| Operational concern | Per-host keys and known_hosts entries | OpenSSH CA |
|---|---|---|
| Trust entries | Every user public key must be authorized on each server, and every host key must be distributed to clients. | Servers trust a user CA public key; clients trust a host CA public key. Individual certificates carry the changing identities. |
| Issuance and rotation | Adding, replacing or removing an identity requires editing many files and distributing each new key. | Issue or renew certificates from the signer while the trust anchor remains stable; rotate the CA itself when necessary. |
| Principal policy | Account and host names are implicit in file placement or known-hosts patterns. | Principals and validity are explicit certificate fields, optionally checked against AuthorizedPrincipalsFile. |
| Auditability | Individual key comments can help, but there is no certificate-use identifier by default. | The -I key identifier is logged when a certificate is used and can carry an issuance or asset reference. |
| Compromise impact | A stolen individual key generally affects the accounts or hosts that trust that key. | A compromised CA private key can authorize many identities in its role, so it requires stronger protection and an incident plan. |
| Compatibility | Works with virtually every SSH implementation that supports public keys. | Requires OpenSSH certificate support and correct configuration on both sides; ordinary OpenSSH clients and servers support these mechanisms. |
Rotation and compromise response
Routine certificate renewal
Reissue certificates before their validity ends, preserving the same principal policy and assigning a new identifier that points to the new issuance record. Install the replacement certificate alongside the existing private key, test it, and remove the expired file when the transition is complete.
Rotating a CA key
Because clients and servers trust CA public keys directly, CA rotation is a trust-anchor change. Introduce the new public key at every relying party, issue replacement certificates from the new private key, verify them, and remove the old public key only after certificates signed by it are no longer needed. Keep the old CA private key disabled and protected for the duration required by your incident and audit policy.
Responding to a compromised CA
- Stop issuing certificates from the compromised private key.
- Remove or replace that CA public key from every server or client that trusts it.
- Generate a replacement CA key pair with a distinct protected private key.
- Distribute the replacement public key through an authenticated channel.
- Reissue affected user or host certificates and test both trust directions.
- Review logs using certificate identifiers to determine which identities may have been exposed.
OpenSSH’s certificate format makes short-lived credentials and CA-key rotation practical, but it does not provide one universal revocation protocol. Your local policy must define how quickly trust anchors are removed and certificates are reissued.
Troubleshooting common failures
The server rejects a valid-looking user certificate
- Confirm the client is offering both the private key and its
CertificateFile; a certificate without its matching private key cannot authenticate. - Check that
TrustedUserCAKeyspoints to the intended, readable CA public key. - Verify the certificate principal. Without
AuthorizedPrincipalsFile, it must match the requested login username exactly. - If an principals file is configured, confirm the file path expands to the correct account and contains the principal on its own line.
- Inspect validity times and system clocks with
ssh-keygen -L. - Run the client with verbose logging and inspect the server authentication log for the certificate identifier.
The client reports host-key verification failure
- Confirm the host CA public key is in the effective
known_hostsfile and is preceded by@cert-authority. - Check that the host pattern matches the name used in the SSH command.
- Inspect the host certificate and ensure that same name appears as a principal.
- Verify the distributed CA fingerprint through your authenticated administrative channel.
- Ensure the server is actually presenting the certificate through a matching
HostKeyandHostCertificatepair.
sshd refuses to start after installation
Run sshd -t before restarting. Typical causes are a misspelled path, incorrect permissions, a certificate signed for a different host public key, or a directive placed in a configuration file that is not being included. Keep an existing session open until a new session has authenticated successfully.
Quick Recap
Final implementation checklist
- Create independent, passphrase-protected user and host CA keys.
- Store private CA keys away from ordinary servers whenever practical.
- Sign user public keys with narrow account or role principals and explicit validity intervals.
- Configure clients with the matching private key and
CertificateFile. - Install the user CA public key and configure
TrustedUserCAKeyson every destination server. - Use
AuthorizedPrincipalsFilewhen certificate roles should not equal Unix account names. - Sign each server host public key with the host CA, including every client-facing DNS name and address.
- Configure the matching
HostKeyandHostCertificatedirectives. - Install the host CA public key with an
@cert-authorityentry in client known-hosts policy. - Inspect certificates, validate with
sshd -t, test verbose connections, and record each-Iidentifier. - Document renewal, CA rotation and compromise procedures before relying on the CA for production access.
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