smallprox/cert.go

package main

import (
	"crypto/ecdsa"
	"crypto/elliptic"
	"crypto/rand"
	"crypto/tls"
	"crypto/x509"
	"crypto/x509/pkix"
	"fmt"
	"math/big"
	"net"
	"os"
	"time"
)

// generateSelfSignedCert creates an in-memory self-signed TLS certificate.
func generateSelfSignedCert(extraHosts []string) (tls.Certificate, error) {
	priv, err := ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
	if err != nil {
		return tls.Certificate{}, fmt.Errorf("failed to generate private key: %w", err)
	}

	serialNumberLimit := new(big.Int).Lsh(big.NewInt(1), 128)
	serialNumber, err := rand.Int(rand.Reader, serialNumberLimit)
	if err != nil {
		return tls.Certificate{}, fmt.Errorf("failed to generate serial number: %w", err)
	}

	dnsNames := []string{"localhost"}
	ipAddresses := []net.IP{net.ParseIP("127.0.0.1"), net.ParseIP("::1")}

	if hostname, err := os.Hostname(); err == nil && hostname != "" {
		dnsNames = append(dnsNames, hostname)
	}

	for _, host := range extraHosts {
		if host == "" {
			continue
		}
		if ip := net.ParseIP(host); ip != nil {
			ipAddresses = append(ipAddresses, ip)
		} else {
			dnsNames = append(dnsNames, host)
		}
	}

	template := x509.Certificate{
		SerialNumber: serialNumber,
		Subject: pkix.Name{
			Organization: []string{"smallprox"},
			CommonName:   "smallprox-localhost",
		},
		NotBefore:             time.Now().Add(-1 * time.Hour),
		NotAfter:              time.Now().Add(365 * 24 * time.Hour),
		KeyUsage:              x509.KeyUsageKeyEncipherment | x509.KeyUsageDigitalSignature,
		ExtKeyUsage:           []x509.ExtKeyUsage{x509.ExtKeyUsageServerAuth},
		BasicConstraintsValid: true,
		DNSNames:              dnsNames,
		IPAddresses:           ipAddresses,
	}

	derBytes, err := x509.CreateCertificate(rand.Reader, &template, &template, &priv.PublicKey, priv)
	if err != nil {
		return tls.Certificate{}, fmt.Errorf("failed to create certificate: %w", err)
	}

	cert := tls.Certificate{
		Certificate: [][]byte{derBytes},
		PrivateKey:  priv,
	}

	return cert, nil
}