Bug 2088916

Description of problem:
If I configure the ssh client to use host-based authentication with
ssh-keysign, it seems to be unable to use the rsa key and gives a fairly
incomprehensible error message saying:
ssh-keysign: sshkey_sign failed: error in libcrypto
ssh_keysign: no reply
Other key types like ecdsa seem to work OK. Was ssh-rsa key support disabled?
If so, that should be documented more clearly. Ideally, there would be a
clearer error message.

Version-Release number of selected component (if applicable):
openssh-8.7p1-8.el9

How reproducible:
always

Steps to Reproduce:
1. In a freshly installed CentOS Stream 9 qemu kvm, create /etc/ssh/sshd_config.d/99-local.conf with these contents:
HostKey /etc/ssh/ssh_host_rsa_key
HostKey /etc/ssh/ssh_host_ecdsa_key
HostbasedAuthentication yes
PasswordAuthentication no
Then create /etc/ssh/ssh_config.d/99-local.conf with these contents:
HostbasedAuthentication yes
EnableSSHKeysign yes
2. systemctl restart sshd
3. ssh -n localhost uptime

Actual results:
[root@localhost ssh]# ssh -n localhost uptime
ssh-keysign: sshkey_sign failed: error in libcrypto
ssh_keysign: no reply
sign using hostkey ssh-rsa SHA256:RUgzE2VqS2tcUzf1UM+x6UQX2GzQF25mL+4HPn+lScU failed
root@localhost: Permission denied (publickey,gssapi-keyex,gssapi-with-mic,hostbased).


Expected results:
Either accept the rsa key or give a clearer error message.

Additional info:
With -v flags, you can see what's going on:
[root@localhost ssh]# ssh -v -n localhost uptime |& egrep 'hostbased|keysign'
debug1: Authentications that can continue: publickey,gssapi-keyex,gssapi-with-mic,hostbased
debug1: Next authentication method: hostbased
debug1: userauth_hostbased: trying hostkey ssh-ed25519 SHA256:u1vXVwHOTDXzgvJ419yue2in9fqtxxJw2lVJJu8sARI
debug1: Authentications that can continue: publickey,gssapi-keyex,gssapi-with-mic,hostbased
debug1: userauth_hostbased: trying hostkey ecdsa-sha2-nistp256 SHA256:R1JJaMNtqtfTqG/cb7neXwjw7q277TruFudn2HE9oa8
debug1: Authentications that can continue: publickey,gssapi-keyex,gssapi-with-mic,hostbased
debug1: userauth_hostbased: trying hostkey ssh-rsa SHA256:RUgzE2VqS2tcUzf1UM+x6UQX2GzQF25mL+4HPn+lScU
ssh-keysign: sshkey_sign failed: error in libcrypto
ssh_keysign: no reply
root@localhost: Permission denied (publickey,gssapi-keyex,gssapi-with-mic,hostbased).

It looks like ssh_keysign is able to use the ecdsa and ed25519 keys, but it chockes on the ssh-rsa key. I can understand that maybe ssh-rsa was disabled for security reasons, but there should be a clearer error message explaining what's going on.