Connection failed to Fedora 33 SSH/SFTP server

[Vitaly Zaitsev]

Description of problem:
Cannot connect to the OpenSSH server on Fedora 33 due to recent crypto-policy changes with using standard RSA-4096 keys (curve25519 works).

I can also reproduce this issue with FileZilla on Windows 10 and even in PuTTY client.

Version-Release number of selected component (if applicable):
filezilla-3.51.0-1.fc33
libfilezilla-0.25.0-2.fc33

How reproducible:
Always.

Steps to Reproduce:

1. Install Fedora 33.
2. Install OpenSSH server: sudo dnf install openssh-server
3. Enable it: sudo systemctl enable --now sshd.service
4. Disable password authentication:

echo "PasswordAuthentication no" | sudo tee /etc/ssh/sshd_config.d/00-disablepass.conf
sudo chmod 0600 /etc/ssh/sshd_config.d/00-disablepass.conf

5. Restart OpenSSH server: sudo systemctl restart sshd.service
6. Create SFTP connection in FileZilla to 127.0.0.1.
7. Try to connection with RSA key authentication (id_rsa should be added to the SSH agent).

Actual results:
Connection failure:
FATAL ERROR: No supported authentication methods available (server sent: publickey)

Expected results:
Successful connection.

Additional info:
Logs attached:

• filezilla.log - internal FileZilla log file with full debug enabled;
• sshd-filezilla.log - OpenSSH Server log with debug for the FileZilla connection;
• sshd-console-client.log - OpenSSH Server log with debug for the default console SSH client.