gomail/internal/tlsutil/selfsigned.go

// Package tlsutil provides certificate loading: LoadOrGenerate for the
// file/self-signed paths (this file), and ACMEManager (acme_manager.go) for
// real Let's Encrypt-style issuance via internal/acme. The self-signed
// generator here remains the fallback for tls.mode "off" or "file" without
// a cert on disk yet — genuinely necessary for local dev/testing, not a
// placeholder for a missing feature.
package tlsutil

import (
	"crypto/rand"
	"crypto/rsa"
	"crypto/tls"
	"crypto/x509"
	"crypto/x509/pkix"
	"fmt"
	"log/slog"
	"math/big"
	"net"
	"time"
)

// LoadOrGenerate returns a tls.Config for the given mode:
//   - "file": load cert/key from disk paths
//   - anything else ("acme" not yet implemented, "off"): generate a self-signed
//     cert so STARTTLS/IMAPS/etc. still work during development. Logs a loud
//     warning since this is never appropriate for production.
func LoadOrGenerate(mode, hostname, certFile, keyFile string, minVersion uint16) (*tls.Config, error) {
	var cert tls.Certificate
	var err error

	switch mode {
	case "file":
		cert, err = tls.LoadX509KeyPair(certFile, keyFile)
		if err != nil {
			return nil, fmt.Errorf("loading TLS cert/key: %w", err)
		}
	case "acme":
		// Reaching here (rather than the real ACMEManager path in main.go)
		// means mode=="acme" but no acme_domains were configured — a real
		// ACME client exists (internal/acme, wired in main.go), it's just
		// not usable without knowing which domain(s) to request a cert for.
		slog.Warn("TLS mode is 'acme' but no acme_domains are configured — "+
			"generating a SELF-SIGNED certificate instead. Set tls.acme_domains "+
			"in config.yaml to enable real Let's Encrypt issuance.",
			"hostname", hostname)
		cert, err = generateSelfSigned(hostname)
		if err != nil {
			return nil, fmt.Errorf("generating self-signed cert: %w", err)
		}
	default:
		slog.Warn("TLS mode is 'off' — generating a SELF-SIGNED certificate. "+
			"This is fine for local testing but MUST NOT be used in production; "+
			"set tls.mode to 'acme' (with acme_domains configured) or 'file'.",
			"mode", mode, "hostname", hostname)
		cert, err = generateSelfSigned(hostname)
		if err != nil {
			return nil, fmt.Errorf("generating self-signed cert: %w", err)
		}
	}

	return &tls.Config{
		Certificates: []tls.Certificate{cert},
		MinVersion:   minVersion,
		ServerName:   hostname,
	}, nil
}

// ParseMinVersion converts the config string ("TLS12"/"TLS13") to the
// crypto/tls constant.
func ParseMinVersion(s string) uint16 {
	if s == "TLS13" {
		return tls.VersionTLS13
	}
	return tls.VersionTLS12
}

func generateSelfSigned(hostname string) (tls.Certificate, error) {
	priv, err := rsa.GenerateKey(rand.Reader, 2048)
	if err != nil {
		return tls.Certificate{}, err
	}

	serial, err := rand.Int(rand.Reader, new(big.Int).Lsh(big.NewInt(1), 128))
	if err != nil {
		return tls.Certificate{}, err
	}

	template := x509.Certificate{
		SerialNumber: serial,
		Subject:      pkix.Name{CommonName: hostname, Organization: []string{"GoMail (self-signed, dev only)"}},
		NotBefore:    time.Now().Add(-time.Hour),
		NotAfter:     time.Now().AddDate(1, 0, 0),
		KeyUsage:     x509.KeyUsageKeyEncipherment | x509.KeyUsageDigitalSignature | x509.KeyUsageCertSign,
		ExtKeyUsage:  []x509.ExtKeyUsage{x509.ExtKeyUsageServerAuth},
		IsCA:         true,
		BasicConstraintsValid: true,
	}

	if ip := net.ParseIP(hostname); ip != nil {
		template.IPAddresses = []net.IP{ip}
	} else {
		template.DNSNames = []string{hostname}
	}

	derBytes, err := x509.CreateCertificate(rand.Reader, &template, &template, &priv.PublicKey, priv)
	if err != nil {
		return tls.Certificate{}, err
	}

	return tls.Certificate{
		Certificate: [][]byte{derBytes},
		PrivateKey:  priv,
	}, nil
}