feat: implement SSH honeypot server with auth and config

Add core SSH server with password authentication, per-IP failure
tracking, credential memory with TTL, and static credential support.
Includes TOML config loading with validation, Ed25519 host key
auto-generation, and a Nix package output.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/config/config.go

@@ -0,0 +1,98 @@
+package config
+
+import (
+	"fmt"
+	"os"
+	"time"
+
+	"github.com/BurntSushi/toml"
+)
+
+type Config struct {
+	SSH      SSHConfig  `toml:"ssh"`
+	Auth     AuthConfig `toml:"auth"`
+	LogLevel string     `toml:"log_level"`
+}
+
+type SSHConfig struct {
+	ListenAddr  string `toml:"listen_addr"`
+	HostKeyPath string `toml:"host_key_path"`
+}
+
+type AuthConfig struct {
+	AcceptAfter       int          `toml:"accept_after"`
+	CredentialTTL     string       `toml:"credential_ttl"`
+	StaticCredentials []Credential `toml:"static_credentials"`
+
+	// Parsed duration, not from TOML directly.
+	CredentialTTLDuration time.Duration `toml:"-"`
+}
+
+type Credential struct {
+	Username string `toml:"username"`
+	Password string `toml:"password"`
+}
+
+func Load(path string) (*Config, error) {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return nil, fmt.Errorf("reading config: %w", err)
+	}
+
+	cfg := &Config{}
+	if err := toml.Unmarshal(data, cfg); err != nil {
+		return nil, fmt.Errorf("parsing config: %w", err)
+	}
+
+	applyDefaults(cfg)
+
+	if err := validate(cfg); err != nil {
+		return nil, fmt.Errorf("validating config: %w", err)
+	}
+
+	return cfg, nil
+}
+
+func applyDefaults(cfg *Config) {
+	if cfg.SSH.ListenAddr == "" {
+		cfg.SSH.ListenAddr = ":2222"
+	}
+	if cfg.SSH.HostKeyPath == "" {
+		cfg.SSH.HostKeyPath = "oubliette_host_key"
+	}
+	if cfg.Auth.AcceptAfter == 0 {
+		cfg.Auth.AcceptAfter = 10
+	}
+	if cfg.Auth.CredentialTTL == "" {
+		cfg.Auth.CredentialTTL = "24h"
+	}
+	if cfg.LogLevel == "" {
+		cfg.LogLevel = "info"
+	}
+}
+
+func validate(cfg *Config) error {
+	d, err := time.ParseDuration(cfg.Auth.CredentialTTL)
+	if err != nil {
+		return fmt.Errorf("invalid credential_ttl %q: %w", cfg.Auth.CredentialTTL, err)
+	}
+	if d <= 0 {
+		return fmt.Errorf("credential_ttl must be positive, got %s", d)
+	}
+	cfg.Auth.CredentialTTLDuration = d
+
+	if cfg.Auth.AcceptAfter < 1 {
+		return fmt.Errorf("accept_after must be at least 1, got %d", cfg.Auth.AcceptAfter)
+	}
+
+	for i, cred := range cfg.Auth.StaticCredentials {
+		if cred.Username == "" {
+			return fmt.Errorf("static_credentials[%d]: username must not be empty", i)
+		}
+		if cred.Password == "" {
+			return fmt.Errorf("static_credentials[%d]: password must not be empty", i)
+		}
+	}
+
+	return nil
+}