Authentication

Overview

Authentication is performed by calling the Authenticate() method on your SshSession instance with an appropriate SshCredential object. The session must be in the Connected state before authenticating.

using NullOpsDevs.LibSsh;
using NullOpsDevs.LibSsh.Credentials;

var session = new SshSession();
session.Connect("example.com", 22);

// Authenticate using your chosen method
bool success = session.Authenticate(credential);

if (success)
{
    // Session is now in LoggedIn state
    // You can execute commands, transfer files, etc.
}

Password Authentication

Password authentication is the simplest method but is generally less secure than key-based authentication. It's suitable for development environments or when key-based authentication isn't available.

var credential = SshCredential.FromPassword("username", "password");
session.Authenticate(credential);

Public Key Authentication (File-based)

Public key authentication is the recommended method for most use cases. It uses asymmetric cryptography where you authenticate with a private key that corresponds to a public key registered on the server.

Basic Usage

var credential = SshCredential.FromPublicKeyFile(
    username: "username",
    publicKeyPath: "~/.ssh/id_rsa.pub",
    privateKeyPath: "~/.ssh/id_rsa",
    passphrase: null  // or provide passphrase if key is encrypted
);

session.Authenticate(credential);

With Encrypted Private Key

var credential = SshCredential.FromPublicKeyFile(
    username: "username",
    publicKeyPath: "~/.ssh/id_ed25519.pub",
    privateKeyPath: "~/.ssh/id_ed25519",
    passphrase: "my-secure-passphrase"
);

session.Authenticate(credential);

Public Key Authentication (Memory-based)

When you need to load keys from sources other than the filesystem (e.g., databases, configuration systems, or encrypted stores), you can use memory-based authentication.

byte[] publicKeyData = LoadPublicKeyFromSecureStore();
byte[] privateKeyData = LoadPrivateKeyFromSecureStore();

var credential = SshCredential.FromPublicKeyMemory(
    username: "username",
    publicKeyData: publicKeyData,
    privateKeyData: privateKeyData,
    passphrase: null  // or provide passphrase if key is encrypted
);

session.Authenticate(credential);

SSH Agent Authentication

SSH agent authentication delegates key management to an SSH agent (ssh-agent on Linux/macOS, Pageant on Windows). This method is convenient when you have multiple keys or want to avoid storing private keys in your application.

var credential = SshCredential.FromAgent("username");
session.Authenticate(credential);

The SSH agent will:

Try each available identity in the agent
Return success when a valid key is found
Return failure if no keys authenticate successfully

Host-based Authentication

Host-based authentication allows a trusted client host to authenticate users without requiring individual credentials. This method is rarely used in modern deployments and is typically restricted to tightly controlled environments.

var credential = SshCredential.FromHostBased(
    username: "username",
    publicKeyPath: "/etc/ssh/ssh_host_rsa_key.pub",
    privateKeyPath: "/etc/ssh/ssh_host_rsa_key",
    passphrase: null,
    hostname: "client-hostname.example.com",
    localUsername: "local-username"  // optional, defaults to username
);
session.Authenticate(credential);

Error Handling

Authentication failures can occur for various reasons. Always check the return value and handle failures appropriately:

try
{
    var credential = SshCredential.FromPassword("username", "wrong-password");
    bool success = session.Authenticate(credential);

    if (!success)
    {
        Console.WriteLine("Authentication failed. Check your credentials.");
        return;
    }
}
catch (SshException ex)
{
    Console.WriteLine($"SSH error during authentication: {ex.Message}");
}

Common authentication failures:

Invalid credentials: Wrong password or key not authorized on server
Key format issues: Unsupported key type or corrupted key file
Permission denied: Server configuration doesn't allow the authentication method
Agent not available: SSH agent not running when using agent authentication

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