Created attachment 3288 [details]
use correct algorithm identification in ssh-agent

If I read the certkeys specification [1] and the source code correctly, the ssh-agent does not generate correct signatures when the certificate is loaded in agent.

The agent_decode_alg() function [2] looks only on the actual key type and for certificate key types, does not generate correct alg argument for the sshkey_sign() function (ending with raw ssh-rsa signature).

This is not the same what happens in sshconnect2.c in key_sig_algorithm() [3], where also the certificate types are taken into the account and the alg argument to sshkey_sign() is properly formatted.

I do not have separate reproducer inside of OpenSSH since I was using OpenSSH agent, libssh client and openSSH server, which failed to verify these signatures (when the base rsa-ssh algorithms were disabled).

The attached patch resolved the issue for me in the above setup. I am not sure whether there is some more elegant way how to achieve the same.

[1] https://github.com/openssh/openssh-portable/blob/master/PROTOCOL.certkeys#L41
[2] https://github.com/openssh/openssh-portable/blob/master/ssh-agent.c#L265
[3] https://github.com/openssh/openssh-portable/blob/master/sshconnect2.c#L1102