docs: example using multiple authentication methods

Users may prefer one authentication
(`-o PreferredAuthentications=password` for example), or the user might
not have a public key, or anything else.

This example will try 3 auth methods, usually the order is: public-key,
keyboard-interactive, password.

Closes #225

Signed-off-by: Carlos Alexandro Becker <caarlos0@users.noreply.github.com>

examples/multi-auth/main.go

@@ -0,0 +1,77 @@
+package main
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"os"
+	"os/signal"
+	"syscall"
+	"time"
+
+	"github.com/charmbracelet/log"
+	"github.com/charmbracelet/ssh"
+	"github.com/charmbracelet/wish"
+	"github.com/charmbracelet/wish/logging"
+	gossh "golang.org/x/crypto/ssh"
+)
+
+const (
+	host          = "localhost"
+	port          = 23234
+	carlosPubkey  = "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAILxWe2rXKoiO6W14LYPVfJKzRfJ1f3Jhzxrgjc/D4tU7"
+	validPassword = "asd123"
+)
+
+func main() {
+	s, err := wish.NewServer(
+		wish.WithAddress(fmt.Sprintf("%s:%d", host, port)),
+		wish.WithHostKeyPath(".ssh/term_info_ed25519"),
+		wish.WithPublicKeyAuth(func(_ ssh.Context, key ssh.PublicKey) bool {
+			log.Info("public-key")
+			carlos, _, _, _, _ := ssh.ParseAuthorizedKey([]byte(carlosPubkey))
+			return ssh.KeysEqual(carlos, key)
+		}),
+		wish.WithPasswordAuth(func(_ ssh.Context, password string) bool {
+			log.Info("password")
+			return password == validPassword
+		}),
+		wish.WithKeyboardInteractiveAuth(func(_ ssh.Context, challenger gossh.KeyboardInteractiveChallenge) bool {
+			log.Info("keyboard-interactive")
+			answers, err := challenger("", "", []string{"how much is 2+3: "}, []bool{true})
+			if err != nil {
+				return false
+			}
+			return len(answers) == 1 && answers[0] == "5"
+		}),
+		wish.WithMiddleware(
+			logging.Middleware(),
+			func(h ssh.Handler) ssh.Handler {
+				return func(s ssh.Session) {
+					wish.Println(s, "authorized!")
+				}
+			},
+		),
+	)
+	if err != nil {
+		log.Error("could not start server", "error", err)
+	}
+
+	done := make(chan os.Signal, 1)
+	signal.Notify(done, os.Interrupt, syscall.SIGINT, syscall.SIGTERM)
+	log.Info("Starting SSH server", "host", host, "port", port)
+	go func() {
+		if err = s.ListenAndServe(); err != nil && !errors.Is(err, ssh.ErrServerClosed) {
+			log.Error("could not start server", "error", err)
+			done <- nil
+		}
+	}()
+
+	<-done
+	log.Info("Stopping SSH server")
+	ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
+	defer func() { cancel() }()
+	if err := s.Shutdown(ctx); err != nil && !errors.Is(err, ssh.ErrServerClosed) {
+		log.Error("could not stop server", "error", err)
+	}
+}