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// Package ca manages mitmux's root CA: generating it on first run,
// loading it on subsequent runs, and signing per-host leaf certificates
// on demand for TLS interception.
package ca
import (
"crypto/ecdsa"
"crypto/elliptic"
"crypto/rand"
"crypto/tls"
"crypto/x509"
"crypto/x509/pkix"
"encoding/pem"
"errors"
"fmt"
"math/big"
"net"
"os"
"path/filepath"
"sync"
"time"
)
const (
certFileName = "ca.pem"
keyFileName = "ca-key.pem"
)
// CA holds the root certificate and key used to sign per-host leaf
// certificates during TLS interception.
type CA struct {
Cert *x509.Certificate
Key *ecdsa.PrivateKey
CertPEM []byte
KeyPEM []byte
leafMu sync.Mutex
leafCache map[string]*tls.Certificate
}
// leafLifetime is kept well under the ~398 day limit modern browsers
// enforce on leaf certificates.
const leafLifetime = 300 * 24 * time.Hour
// LeafFor returns a TLS certificate for host (a DNS name or IP address,
// no port), signed by the CA. Certificates are generated once and cached
// in memory for the life of the process.
func (c *CA) LeafFor(host string) (*tls.Certificate, error) {
c.leafMu.Lock()
defer c.leafMu.Unlock()
if c.leafCache == nil {
c.leafCache = make(map[string]*tls.Certificate)
}
if cert, ok := c.leafCache[host]; ok {
return cert, nil
}
cert, err := c.signLeaf(host)
if err != nil {
return nil, err
}
c.leafCache[host] = cert
return cert, nil
}
func (c *CA) signLeaf(host string) (*tls.Certificate, error) {
key, err := ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
if err != nil {
return nil, err
}
serial, err := rand.Int(rand.Reader, new(big.Int).Lsh(big.NewInt(1), 128))
if err != nil {
return nil, err
}
tmpl := &x509.Certificate{
SerialNumber: serial,
Subject: pkix.Name{
CommonName: host,
Organization: []string{"mitmux"},
},
NotBefore: time.Now().Add(-time.Hour),
NotAfter: time.Now().Add(leafLifetime),
KeyUsage: x509.KeyUsageDigitalSignature,
ExtKeyUsage: []x509.ExtKeyUsage{x509.ExtKeyUsageServerAuth},
BasicConstraintsValid: true,
IsCA: false,
}
if ip := net.ParseIP(host); ip != nil {
tmpl.IPAddresses = []net.IP{ip}
} else {
tmpl.DNSNames = []string{host}
}
der, err := x509.CreateCertificate(rand.Reader, tmpl, c.Cert, &key.PublicKey, c.Key)
if err != nil {
return nil, err
}
leaf, err := x509.ParseCertificate(der)
if err != nil {
return nil, err
}
return &tls.Certificate{
Certificate: [][]byte{der, c.Cert.Raw},
PrivateKey: key,
Leaf: leaf,
}, nil
}
// Dir returns the directory mitmux stores its CA material in
// (XDG config dir, e.g. ~/.config/mitmux).
func Dir() (string, error) {
cfg, err := os.UserConfigDir()
if err != nil {
return "", fmt.Errorf("resolve config dir: %w", err)
}
return filepath.Join(cfg, "mitmux"), nil
}
// EnsureCA loads the CA from dir, generating and persisting a new one if
// none exists yet.
func EnsureCA(dir string) (*CA, error) {
certPath := filepath.Join(dir, certFileName)
keyPath := filepath.Join(dir, keyFileName)
certPEM, certErr := os.ReadFile(certPath)
keyPEM, keyErr := os.ReadFile(keyPath)
if certErr == nil && keyErr == nil {
return load(certPEM, keyPEM)
}
if !errors.Is(certErr, os.ErrNotExist) && certErr != nil {
return nil, fmt.Errorf("read %s: %w", certPath, certErr)
}
if !errors.Is(keyErr, os.ErrNotExist) && keyErr != nil {
return nil, fmt.Errorf("read %s: %w", keyPath, keyErr)
}
ca, err := generate()
if err != nil {
return nil, fmt.Errorf("generate CA: %w", err)
}
if err := persist(dir, ca); err != nil {
return nil, fmt.Errorf("persist CA: %w", err)
}
return ca, nil
}
func generate() (*CA, error) {
key, err := ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
if err != nil {
return nil, err
}
serial, err := rand.Int(rand.Reader, new(big.Int).Lsh(big.NewInt(1), 128))
if err != nil {
return nil, err
}
tmpl := &x509.Certificate{
SerialNumber: serial,
Subject: pkix.Name{
CommonName: "mitmux local CA",
Organization: []string{"mitmux"},
},
NotBefore: time.Now().Add(-time.Hour),
NotAfter: time.Now().AddDate(10, 0, 0),
KeyUsage: x509.KeyUsageCertSign | x509.KeyUsageDigitalSignature | x509.KeyUsageCRLSign,
BasicConstraintsValid: true,
IsCA: true,
MaxPathLenZero: true,
}
der, err := x509.CreateCertificate(rand.Reader, tmpl, tmpl, &key.PublicKey, key)
if err != nil {
return nil, err
}
cert, err := x509.ParseCertificate(der)
if err != nil {
return nil, err
}
keyDER, err := x509.MarshalECPrivateKey(key)
if err != nil {
return nil, err
}
return &CA{
Cert: cert,
Key: key,
CertPEM: pem.EncodeToMemory(&pem.Block{Type: "CERTIFICATE", Bytes: der}),
KeyPEM: pem.EncodeToMemory(&pem.Block{Type: "EC PRIVATE KEY", Bytes: keyDER}),
}, nil
}
func load(certPEM, keyPEM []byte) (*CA, error) {
certBlock, _ := pem.Decode(certPEM)
if certBlock == nil {
return nil, errors.New("no PEM block found in CA certificate")
}
cert, err := x509.ParseCertificate(certBlock.Bytes)
if err != nil {
return nil, fmt.Errorf("parse CA certificate: %w", err)
}
keyBlock, _ := pem.Decode(keyPEM)
if keyBlock == nil {
return nil, errors.New("no PEM block found in CA key")
}
key, err := x509.ParseECPrivateKey(keyBlock.Bytes)
if err != nil {
return nil, fmt.Errorf("parse CA key: %w", err)
}
return &CA{Cert: cert, Key: key, CertPEM: certPEM, KeyPEM: keyPEM}, nil
}
func persist(dir string, ca *CA) error {
if err := os.MkdirAll(dir, 0o700); err != nil {
return err
}
if err := os.WriteFile(filepath.Join(dir, certFileName), ca.CertPEM, 0o644); err != nil {
return err
}
if err := os.WriteFile(filepath.Join(dir, keyFileName), ca.KeyPEM, 0o600); err != nil {
return err
}
return nil
}
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