Dual Authentication Not Supported - AuthenticationMethods publickey,password

The ssh2 crate currently does not support SSH authentication when the server enforces both public key and password authentication in the same session using:

AuthenticationMethods publickey,password

This configuration requires the client to first authenticate with a public key and then provide a password before authentication is considered successful. However, ssh2-rs only supports using one method at a time and does not allow chaining authentication methods in a single session.

Steps to Reproduce:

1. Configure the SSH server (/etc/ssh/sshd_config) with:

PasswordAuthentication yes
PubkeyAuthentication yes
AuthenticationMethods publickey,password

Restart the SSH service:

sudo systemctl restart ssh

2. Attempt to authenticate using ssh2-rs with both a key and a password:

let tcp = TcpStream::connect("your.server.com:22")?;
let mut sess = Session::new().unwrap();
sess.set_tcp_stream(tcp);
sess.handshake()?;

// Public key authentication
sess.userauth_pubkey_file("your-user", None, Path::new("/path/to/private_key"), None)?; // fails here

// Password authentication
sess.userauth_password("your-user", "your-password")?;

assert!(sess.authenticated());

3. The code fails with the error:

{ code: Session(-19), msg: "Invalid signature for supplied public key, or bad username/public key combination" }

4. Running SSH manually on the same machine works fine:

ssh -i /path/to/private_key user@example.com

Expected Behavior:

The ssh2 crate should allow chaining authentication methods in the same session when the server enforces publickey,password.

Actual Behavior:

• The first authentication method succeeds (userauth_pubkey_file()), but userauth_password() fails.
• If userauth_password() is called first, userauth_pubkey_file() is never executed.
• The crate does not provide a way to handle publickey,password authentication.

Environment:

• ssh2 crate version: 0.10
• Rust version: rustc 1.82.0
• OS: Ubuntu 24.04 LTS
• SSH Server: OpenSSH_8.9p1

Additional Context:

• The issue is not related to key format (PEM is used and works with manual SSH).
• The issue only occurs when publickey,password is enforced by the SSH server.

Start by reproducing the configured AuthenticationMethods publickey,password flow using the userauth_pubkey_file() and userauth_password() entry points described in the issue. Inspect the ssh2-rs and underlying libssh2 authentication behavior to determine how sequential methods are exposed; done means both methods can complete in one session and authenticated() succeeds.