Support wildcard principal names in SSH host certificates

[tc-tlv]

Description

SSH host certificate verification currently requires an exact match between the requested hostname and a certificate principal. Host certificate principals may also contain wildcard characters, so a certificate principal such as *.example.com should match host.example.com.

According to the SSH documentation (ssh-keygen.1):

For host certificates, the principals specified using the -n argument are hostnames and may contain wildcard characters.

This behavior is supported by OpenSSH since version 10.3.

Current behavior

OpenSshCertificateHostKeyVerifier lowercases the hostname and checks it with:

principals.contains(principalName)

As a result, wildcard principals are rejected even when they match the hostname.

Expected behavior

Each allowed principal should be evaluated using the existing host wildcard matching logic. For example:

• *.example.com should match host.example.com.
• *.example.com should not match example.com.
• Existing exact principal matching should continue to work.

Proposed fix

Reuse HostKey.matchesWildcardPattern when checking certificate principals. I have the following patch, but I cannot push it:

support-wildcard-host-certificate-principals.patch

Acceptance criteria

• Host certificates with wildcard hostname principals are accepted when the requested hostname matches.
• Non-matching hostnames remain rejected.
• Exact hostname and IP-address principals continue to behave as before.
• Tests cover matching and non-matching wildcard principals.

[norrisjeremy]

Hi @tc-tlv,

Can you open a PR with your proposed patch?
That will allow us to more easily review it.

Thanks!
Jeremy