Contribution description

When SSH public key authentication is attempted right after ssh_connect(), there's a chance SSH_MSG_EXT_INFO message will not be received by the client in timely manner. In case if an ssh-rsa public key is being used, this in turn may lead to libssh picking SHA1 digest algorithm, which is disabled by default:
https://github.com/canonical/libssh/blob/f23d1454e50d0dbb314edd9bf4227ab72303484b/src/pki.c#L443-L454 This may cause intermittent SSH public key authentication failures with no clear reason as SSH logging is disabled in rtrlib.
The fix is based on this suggestion:
https://archive.libssh.org/libssh/2025-05/0000023.html

It is possible to work around the problem by using PubkeyAcceptedAlgorithms in /etc/ssh/config or ~/.ssh/config, but there are a couple of issues with this approach:

• with libssh, you can't use PubkeyAcceptedAlgorithms +ssh-rsa and you must list all the algos instead
• adding ssh-rsa to the list of pubkey algos weakens security as it also enables SHA1 digests, weakening SSH security

Support for ssh-rsa public keys is required by RFC8210: https://datatracker.ietf.org/doc/html/rfc8210#section-9.1

Cache servers supporting SSH transport MUST accept RSA authentication and SHOULD accept Elliptic Curve Digital Signature Algorithm (ECDSA) authentication.

Testing procedure

In order to reproduce the issue, you need to use ssh-rsa key for RPKI SSH authentication.
We tested it using downstream FRR version, with StayRTR being used as the server.
It is not easy to trigger reliably b/c the issue is very time-dependent. The issue with libssh ssh-rsa public key authentication is rather known though, it started causing problems after libssh tightened their security defaults and disabled SHA1 digest for auth.