[Bug 1952421] Re: Issue on sshd finds correct private key for a certificate when using ssh-agent

[Launchpad Bug Tracker]

** Merge proposal linked:
   https://code.launchpad.net/~utkarsh/ubuntu/+source/openssh/+git/openssh/+merge/412456

-- 
You received this bug notification because you are a member of Ubuntu
Foundations Bugs, which is subscribed to openssh in Ubuntu.
https://bugs.launchpad.net/bugs/1952421

Title:
  Issue on sshd finds correct private key for a certificate when using
  ssh-agent

Status in openssh package in Ubuntu:
  Fix Released
Status in openssh source package in Focal:
  New
Status in openssh source package in Hirsute:
  New
Status in openssh source package in Impish:
  New

Bug description:
  Reported as https://bugzilla.mindrot.org/show_bug.cgi?id=3254
  upstream:

  Please take a look at line 1936 in main() function in sshd.c.

  /* Find matching private key */
  	for (j = 0; j < options.num_host_key_files; j++) {
  		if (sshkey_equal_public(key,
  			sensitive_data.host_keys[j])) {
  			sensitive_data.host_certificates[j] = key;
  				break;
  			}
  	}

  the sshkey_equal_public() is trying to compare a cert's pub with a private key, and it never find a match which makes sshd cannot use this certificate even though its private key is in ssh-agent.
  I believe it should be comparing a cert's public key with a public key in sensitive_data as follow.

  /* Find matching private key */
  	for (j = 0; j < options.num_host_key_files; j++) {
  		if (sshkey_equal_public(key,
  			sensitive_data.host_pubkeys[j])) {
  			sensitive_data.host_certificates[j] = key;
  				break;
  			}
  	}

  https://github.com/openssh/openssh-portable/blob/V_8_4/sshd.c#L1936

  Due to this HostCertificate and HostKeyAgent not working together in
  sshd and this affects every version of openssh back till Focal, at
  least.

To manage notifications about this bug go to:
https://bugs.launchpad.net/ubuntu/+source/openssh/+bug/1952421/+subscriptions