feat: implement multi CA serving

README.md

@@ -1,17 +1,61 @@
 # OCSP Server
 
-OCSP Server is a minimal implementation of both a OCSP and CRL server in Golang, using a single CRL as the source for both interfaces.
-Originally created by Florian Bauer and now adapted for Spacebar. It provides the following http endpoints:
+OCSP Server is a minimal implementation of both an OCSP and CRL server in Golang, using a CRL as the source for both interfaces.
+Originally created by Florian Bauer and now adapted for Spacebar. A single instance can serve any number of CAs (e.g. a root CA and several intermediates).
 
-| Endpoint   | Description                                              |
-|------------|----------------------------------------------------------|
-| `/ocsp`    | OCSP responder supporting both `GET` and `POST` requests |
-| `/crl`     | CRL responder in DER format                              |
-| `/crl.pem` | CRL responder in PEM format                              |
-| `/ca`      | Issuer CA certificate in DER format                      |
-| `/ca.pem`  | Issuer CA certificate in PEM format                      |
+## Configuration
 
-You need to provide a CRL file, the root certificate and cert/key with extendedKeyUsage `OCSPSigning` to allow the OCSP server to sign the OCSP responses.
-When using OCSP, the certificate is checked against the CRL for validity.
+Point the server at a single directory that contains one subdirectory per CA. The subdirectory name is used as the route prefix.
 
-Synchronization of the CAs CRL is out of scope of this project. You can use any mechanism to update the CRL file. Just notify the ocspcrl server process via `SIGHUP` signal to reload the CRL file.
+```
+cas/
+├── root/
+│   ├── ca.crt
+│   ├── responder.crt
+│   ├── key.pem
+│   └── crl.pem
+├── hr/
+│   ├── ca.crt
+│   ├── responder.crt
+│   ├── key.pem
+│   └── crl.pem
+└── it/
+    ├── ca.crt
+    ├── responder.crt
+    ├── key.pem
+    └── crl.pem
+```
+
+Each CA subdirectory must contain:
+
+| File            | Description                                                              |
+|-----------------|--------------------------------------------------------------------------|
+| `ca.crt`        | The CA certificate (PEM)                                                 |
+| `responder.crt` | The OCSP responder certificate with `extendedKeyUsage = OCSPSigning`     |
+| `key.pem`       | The private key for the responder certificate (PEM)                      |
+| `crl.pem`       | The current CRL issued by the CA (PEM or DER)                            |
+
+Run the server with:
+
+```
+ocspcrl --cas-directory /path/to/cas
+```
+
+## Endpoints
+
+For every CA subdirectory `<route-prefix>`, the following endpoints are exposed:
+
+| Endpoint                    | Content-Type                | Description                                              |
+|-----------------------------|-----------------------------|----------------------------------------------------------|
+| `<route-prefix>/ocsp`       | (OCSP)                      | OCSP responder supporting both `GET` and `POST` requests |
+| `<route-prefix>/ocsp/`      | (OCSP)                      | OCSP responder supporting both `GET` and `POST` requests |
+| `<route-prefix>/crl`        | `application/pkix-cert`     | CRL in DER form                                          |
+| `<route-prefix>/crl.pem`    | `application/pkix-crl`      | CRL in PEM form                                          |
+| `<route-prefix>/ca`         | `application/pkix-cert`     | CA certificate in DER form                               |
+| `<route-prefix>/ca.pem`     | `application/x-x509-ca-cert`| CA certificate in PEM form                               |
+
+For example, with the layout above, the OCSP endpoint for the `hr` intermediate is `https://<host>/hr/ocsp` and its CRL is at `https://<host>/hr/crl.pem`.
+
+## Reloading the CRLs
+
+Synchronization of the CAs' CRLs is out of scope of this project. You can use any mechanism to update the CRL files. Notify the `ocspcrl` server process via `SIGHUP` to reload every CA's CRL from disk.

internal/metrics/metrics.go

@@ -26,11 +26,11 @@ var (
 		Buckets: prometheus.ExponentialBuckets(0.0001, 2, 10),
 	}, []string{labelPath})
 
-	CrlEntries = prometheus.NewGauge(prometheus.GaugeOpts{
+	CrlEntries = prometheus.NewGaugeVec(prometheus.GaugeOpts{
 		Namespace: "ocspcrl",
 		Name:      "crl_entries_total",
 		Help:      "Number of entries in the CRL",
-	})
+	}, []string{"ca"})
 )
 
 func init() {

main.go

@@ -11,6 +11,10 @@ import (
 	"net/http"
 	"os"
 	"os/signal"
+	"path/filepath"
+	"sort"
+	"strings"
+	"sync"
 	"syscall"
 
 	"github.com/alecthomas/kingpin/v2"
@@ -21,180 +25,320 @@ import (
 	"ocspcrl/internal/ocsp_source"
 )
 
-type loadCrlFunction func() error
-
-func loadCrlFromFile(path string) (*x509.RevocationList, error) {
-	crlContent, openCrlError := os.ReadFile(path)
-	if openCrlError != nil {
-		return nil, openCrlError
-	}
-
-	// if the file contains a pem block, decode it
-	// otherwise, assume it is a DER encoded CRL
-	crlBlock := &pem.Block{}
-	if bytes.Contains(crlContent, []byte("BEGIN")) {
-		block, rest := pem.Decode(crlContent)
-		if len(rest) > 0 {
-			return nil, fmt.Errorf("failed to decode crl")
-		}
-		crlBlock = block
-	} else {
-		crlBlock = &pem.Block{Type: "X509 CRL", Bytes: crlContent}
-	}
-
-	crl, parseCrlError := x509.ParseRevocationList(crlBlock.Bytes)
-	if parseCrlError != nil {
-		return nil, parseCrlError
-	}
-	return crl, nil
-}
-
-func reloadCrlWorker(signal chan os.Signal, loadCrlFunc loadCrlFunction) {
-	defer log.Println("reload crl worker stopped")
-	for {
-		select {
-		case _, ok := <-signal:
-			if !ok {
-				return
-			}
-			loadCrlError := loadCrlFunc()
-			if loadCrlError != nil {
-				log.Printf("failed to reload crl: %v", loadCrlError)
-			} else {
-				log.Println("reloaded crl")
-			}
-		}
-	}
-}
-
-type responder struct {
-	certificatePath string
-	keyPath         string
-}
-
-type crlSourceFile struct {
-	path string
-}
+const (
+	crlFileName       = "crl.pem"
+	keyFileName       = "key.pem"
+	responderFileName = "responder.crt"
+	caFileName        = "ca.crt"
+)
 
 type configuration struct {
-	responder                *responder
-	caCrtPath                string
-	crlSourceType            string
-	crlSourceFile            *crlSourceFile
+	casDirectory             string
 	applicationListenAddress string
 	metricsListenAddress     string
 }
 
+func parseConfiguration(args []string) *configuration {
+	config := &configuration{}
+	app := kingpin.New("ocspcrl", "OCSP responder / CRL server (multi-CA)")
+	app.HelpFlag.Short('h')
+	app.Flag("cas-directory", "Path to a directory containing one subdirectory per CA. Each subdirectory must contain ca.crt, responder.crt, key.pem and crl.pem. The subdirectory name is used as the route prefix.").
+		Envar("CAS_DIRECTORY").Required().ExistingDirVar(&config.casDirectory)
+	app.Flag("web.listen-address", "Address for application endpoint").
+		Envar("WEB_LISTEN_ADDRESS").Default(":8080").StringVar(&config.applicationListenAddress)
+	app.Flag("metrics.listen-address", "Address for metrics endpoint").
+		Envar("METRICS_LISTEN_ADDRESS").Default("[::1]:8081").StringVar(&config.metricsListenAddress)
+	kingpin.MustParse(app.Parse(args))
+	return config
+}
+
+func decodeCrlBytes(content []byte) ([]byte, error) {
+	if !bytes.Contains(content, []byte("BEGIN")) {
+		return content, nil
+	}
+	block, rest := pem.Decode(content)
+	if block == nil {
+		return nil, fmt.Errorf("crl pem block could not be decoded")
+	}
+	if len(bytes.TrimSpace(rest)) > 0 {
+		return nil, fmt.Errorf("crl file contains trailing data")
+	}
+	return block.Bytes, nil
+}
+
+func loadCrlFromFile(path string) (*x509.RevocationList, error) {
+	content, readError := os.ReadFile(path)
+	if readError != nil {
+		return nil, readError
+	}
+	derBytes, decodeError := decodeCrlBytes(content)
+	if decodeError != nil {
+		return nil, fmt.Errorf("%s: %w", path, decodeError)
+	}
+	return x509.ParseRevocationList(derBytes)
+}
+
+func loadCertificateFromFile(path string) (*x509.Certificate, error) {
+	content, readError := os.ReadFile(path)
+	if readError != nil {
+		return nil, readError
+	}
+	block, rest := pem.Decode(content)
+	if block == nil {
+		return nil, fmt.Errorf("%s: certificate pem block could not be decoded", path)
+	}
+	if len(bytes.TrimSpace(rest)) > 0 {
+		return nil, fmt.Errorf("%s: certificate file contains trailing data", path)
+	}
+	return x509.ParseCertificate(block.Bytes)
+}
+
+type caInstance struct {
+	name          string
+	crlPath       string
+	caCertificate *x509.Certificate
+	source        *ocsp_source.CrlSource
+
+	crlMutex sync.RWMutex
+	crl      *x509.RevocationList
+}
+
+func (c *caInstance) reloadCrl() error {
+	crl, loadError := loadCrlFromFile(c.crlPath)
+	if loadError != nil {
+		return loadError
+	}
+	c.crlMutex.Lock()
+	c.crl = crl
+	c.crlMutex.Unlock()
+	metrics.CrlEntries.WithLabelValues(c.name).Set(float64(len(crl.RevokedCertificateEntries)))
+	c.source.UseCrl(*crl)
+	return nil
+}
+
+func (c *caInstance) currentCrl() *x509.RevocationList {
+	c.crlMutex.RLock()
+	defer c.crlMutex.RUnlock()
+	return c.crl
+}
+
+type caFiles struct {
+	key       string
+	responder string
+	ca        string
+	crl       string
+}
+
+func newCaFiles(directory string) caFiles {
+	return caFiles{
+		key:       filepath.Join(directory, keyFileName),
+		responder: filepath.Join(directory, responderFileName),
+		ca:        filepath.Join(directory, caFileName),
+		crl:       filepath.Join(directory, crlFileName),
+	}
+}
+
+func (f caFiles) ensureExist(caName string) error {
+	for _, path := range []string{f.key, f.responder, f.ca, f.crl} {
+		if _, statError := os.Stat(path); statError != nil {
+			return fmt.Errorf("ca %q: %w", caName, statError)
+		}
+	}
+	return nil
+}
+
+func verifyResponderIssuedByCa(responder tls.Certificate, ca *x509.Certificate) error {
+	if responder.Leaf == nil {
+		return fmt.Errorf("responder leaf certificate could not be parsed")
+	}
+	if !bytes.Equal(ca.RawSubject, responder.Leaf.RawIssuer) {
+		return fmt.Errorf("responder certificate issuer does not match ca certificate subject; %+q != %+q",
+			ca.Subject.String(), responder.Leaf.Issuer.String())
+	}
+	return nil
+}
+
+func loadCa(name, directory string) (*caInstance, error) {
+	files := newCaFiles(directory)
+	if existsError := files.ensureExist(name); existsError != nil {
+		return nil, existsError
+	}
+
+	responderKeyPair, loadResponderError := tls.LoadX509KeyPair(files.responder, files.key)
+	if loadResponderError != nil {
+		return nil, fmt.Errorf("ca %q: failed to load responder key pair: %w", name, loadResponderError)
+	}
+
+	caCertificate, loadCaError := loadCertificateFromFile(files.ca)
+	if loadCaError != nil {
+		return nil, fmt.Errorf("ca %q: failed to load ca certificate: %w", name, loadCaError)
+	}
+
+	if verifyError := verifyResponderIssuedByCa(responderKeyPair, caCertificate); verifyError != nil {
+		return nil, fmt.Errorf("ca %q: %w", name, verifyError)
+	}
+
+	instance := &caInstance{
+		name:          name,
+		crlPath:       files.crl,
+		caCertificate: caCertificate,
+		source:        ocsp_source.NewCrlSource(caCertificate, responderKeyPair),
+	}
+
+	if reloadError := instance.reloadCrl(); reloadError != nil {
+		return nil, fmt.Errorf("ca %q: failed to load crl: %w", name, reloadError)
+	}
+
+	return instance, nil
+}
+
+func listCaSubdirectories(rootDir string) ([]string, error) {
+	entries, readDirError := os.ReadDir(rootDir)
+	if readDirError != nil {
+		return nil, fmt.Errorf("failed to read cas directory: %w", readDirError)
+	}
+	names := []string{}
+	for _, entry := range entries {
+		if !entry.IsDir() {
+			continue
+		}
+		if strings.HasPrefix(entry.Name(), ".") {
+			continue
+		}
+		names = append(names, entry.Name())
+	}
+	sort.Strings(names)
+	return names, nil
+}
+
+func discoverCas(rootDir string) ([]*caInstance, error) {
+	names, listError := listCaSubdirectories(rootDir)
+	if listError != nil {
+		return nil, listError
+	}
+	if len(names) == 0 {
+		return nil, fmt.Errorf("no ca subdirectories found in %s", rootDir)
+	}
+
+	cas := make([]*caInstance, 0, len(names))
+	for _, name := range names {
+		instance, loadError := loadCa(name, filepath.Join(rootDir, name))
+		if loadError != nil {
+			return nil, loadError
+		}
+		cas = append(cas, instance)
+	}
+	return cas, nil
+}
+
+func writeBinary(w http.ResponseWriter, contentType string, body []byte) {
+	w.Header().Set("Content-Type", contentType)
+	w.Write(body)
+}
+
+func writePem(w http.ResponseWriter, contentType, blockType string, body []byte) {
+	w.Header().Set("Content-Type", contentType)
+	pem.Encode(w, &pem.Block{Type: blockType, Bytes: body})
+}
+
+func registerOcspRoutes(router *http.ServeMux, prefix string, ca *caInstance) {
+	responder := cfocsp.NewResponder(ca.source, nil)
+	router.Handle(prefix+"/ocsp", responder)
+	router.Handle(prefix+"/ocsp/", http.StripPrefix(prefix+"/ocsp/", responder))
+}
+
+func registerCrlRoutes(router *http.ServeMux, prefix string, ca *caInstance) {
+	router.HandleFunc(prefix+"/crl", func(w http.ResponseWriter, r *http.Request) {
+		writeBinary(w, "application/pkix-cert", ca.currentCrl().Raw)
+	})
+	router.HandleFunc(prefix+"/crl.pem", func(w http.ResponseWriter, r *http.Request) {
+		writePem(w, "application/pkix-crl", "X509 CRL", ca.currentCrl().Raw)
+	})
+}
+
+func registerCaCertificateRoutes(router *http.ServeMux, prefix string, ca *caInstance) {
+	router.HandleFunc(prefix+"/ca", func(w http.ResponseWriter, r *http.Request) {
+		writeBinary(w, "application/pkix-cert", ca.caCertificate.Raw)
+	})
+	router.HandleFunc(prefix+"/ca.pem", func(w http.ResponseWriter, r *http.Request) {
+		writePem(w, "application/x-x509-ca-cert", "CERTIFICATE", ca.caCertificate.Raw)
+	})
+}
+
+func registerCaRoutes(router *http.ServeMux, ca *caInstance) {
+	prefix := "/" + ca.name
+	registerOcspRoutes(router, prefix, ca)
+	registerCrlRoutes(router, prefix, ca)
+	registerCaCertificateRoutes(router, prefix, ca)
+}
+
+func buildApplicationRouter(cas []*caInstance) *http.ServeMux {
+	router := http.NewServeMux()
+	for _, ca := range cas {
+		registerCaRoutes(router, ca)
+		log.Printf("registered ca %q with routes under /%s/", ca.name, ca.name)
+	}
+	return router
+}
+
+func reloadAllCrls(cas []*caInstance) {
+	for _, ca := range cas {
+		if reloadError := ca.reloadCrl(); reloadError != nil {
+			log.Printf("failed to reload crl for ca %q: %v", ca.name, reloadError)
+		} else {
+			log.Printf("reloaded crl for ca %q", ca.name)
+		}
+	}
+}
+
+func runReloadWorker(signalChan <-chan os.Signal, cas []*caInstance) {
+	defer log.Println("reload crl worker stopped")
+	for {
+		_, ok := <-signalChan
+		if !ok {
+			return
+		}
+		reloadAllCrls(cas)
+	}
+}
+
+func startServer(server *http.Server, label string) <-chan struct{} {
+	closed := make(chan struct{})
+	go func() {
+		log.Printf("starting %s server on %+q", label, server.Addr)
+		if listenError := server.ListenAndServe(); !errors.Is(listenError, http.ErrServerClosed) {
+			log.Printf("%s server error: %v", label, listenError)
+		}
+		close(closed)
+	}()
+	return closed
+}
+
 func main() {
 	log.SetFlags(log.Lshortfile)
-	config := &configuration{
-		responder:     &responder{},
-		crlSourceFile: &crlSourceFile{},
-	}
-	app := kingpin.New("ocspcrl", "OCSP responder / CRL server")
-	app.HelpFlag.Short('h')
-	app.Flag("responder.certificate-path", "Path to the responder certificate").Envar("RESPONDER_CERTIFICATE_PATH").Required().ExistingFileVar(&config.responder.certificatePath)
-	app.Flag("responder.key-path", "Path to the responder key").Envar("RESPONDER_KEY_PATH").Required().ExistingFileVar(&config.responder.keyPath)
-	app.Flag("ca-crt-path", "Path to the CA certificate").Envar("CA_CRT_PATH").Required().ExistingFileVar(&config.caCrtPath)
-	app.Flag("crl-source-type", "Type of CRL source").Envar("CRL_SOURCE").Default("file").EnumVar(&config.crlSourceType, "file")
-	app.Flag("source.file.path", "Path to the CRL file").Envar("SOURCE_FILE_PATH").ExistingFileVar(&config.crlSourceFile.path)
-	app.Flag("web.listen-address", "Address for application endpoint").Envar("WEB_LISTEN_ADDRESS").Default(":8080").StringVar(&config.applicationListenAddress)
-	app.Flag("metrics.listen-address", "Address for metrics endpoint").Envar("METRICS_LISTEN_ADDRESS").Default("[::1]:8081").StringVar(&config.metricsListenAddress)
-	kingpin.MustParse(app.Parse(os.Args[1:]))
+	config := parseConfiguration(os.Args[1:])
 
-	responderKeyPair, loadResponderKeyPairError := tls.LoadX509KeyPair(config.responder.certificatePath, config.responder.keyPath)
-	if loadResponderKeyPairError != nil {
-		log.Fatalf("failed to load responder key pair: %v", loadResponderKeyPairError)
+	cas, discoverError := discoverCas(config.casDirectory)
+	if discoverError != nil {
+		log.Fatalf("failed to load cas: %v", discoverError)
 	}
 
-	caCrtContent, openCaCrtError := os.ReadFile(config.caCrtPath)
-	if openCaCrtError != nil {
-		log.Fatalf("failed to open ca certificate: %v", openCaCrtError)
-	}
-	block, rest := pem.Decode(caCrtContent)
-	if len(rest) > 0 {
-		log.Fatalln("failed to decode ca certificate")
-	}
-	caCertificate, loadCaCertificateError := x509.ParseCertificate(block.Bytes)
-	if loadCaCertificateError != nil {
-		log.Fatalf("failed to parse ca certificate: %v", loadCaCertificateError)
-	}
+	applicationRouter := buildApplicationRouter(cas)
 
-	if !bytes.Equal(caCertificate.RawSubject, responderKeyPair.Leaf.RawIssuer) {
-		log.Fatalf("responder certificate issuer does not match ca certificate subject; %+q != %+q", caCertificate.Subject.String(), responderKeyPair.Leaf.Issuer.String())
-	}
+	terminationChan := make(chan os.Signal, 1)
+	signal.Notify(terminationChan, syscall.SIGINT, syscall.SIGTERM)
 
-	source := ocsp_source.NewCrlSource(caCertificate, responderKeyPair)
-
-	crl := &x509.RevocationList{}
-
-	loadCrl := func() error {
-		curlCandidate, loadCrlError := loadCrlFromFile(config.crlSourceFile.path)
-		if loadCrlError != nil {
-			return loadCrlError
-		}
-		metrics.CrlEntries.Set(float64(len(curlCandidate.RevokedCertificateEntries)))
-		source.UseCrl(*curlCandidate)
-		crl = curlCandidate
-		return nil
-	}
-
-	signalChan := make(chan os.Signal, 1)
-	signal.Notify(signalChan, syscall.SIGINT, syscall.SIGTERM)
-
-	// initial load of the CRL
-	if loadCrlError := loadCrl(); loadCrlError != nil {
-		log.Fatalf("failed to load crl: %v", loadCrlError)
-	}
-
-	// on HUP reload the CRL
 	hupChan := make(chan os.Signal, 1)
 	signal.Notify(hupChan, syscall.SIGHUP)
-	go reloadCrlWorker(hupChan, loadCrl)
-
-	responder := cfocsp.NewResponder(source, nil)
-
-	applicationRouter := http.NewServeMux()
-	applicationRouter.Handle("/ocsp", responder)
-	applicationRouter.Handle("/ocsp/", http.StripPrefix("/ocsp/", responder))
-	applicationRouter.HandleFunc("/crl", func(w http.ResponseWriter, r *http.Request) {
-		w.Header().Set("Content-Type", "application/pkix-crl")
-		w.Write(crl.Raw)
-	})
-	applicationRouter.HandleFunc("/crl.pem", func(w http.ResponseWriter, r *http.Request) {
-		w.Header().Set("Content-Type", "application/pkix-crl")
-		pem.Encode(w, &pem.Block{Type: "X509 CRL", Bytes: crl.Raw})
-	})
-	applicationRouter.HandleFunc("/ca", func(w http.ResponseWriter, r *http.Request) {
-		w.Header().Set("Content-Type", "application/pkix-cert")
-		w.Write(caCertificate.Raw)
-	})
-	applicationRouter.HandleFunc("/ca.pem", func(w http.ResponseWriter, r *http.Request) {
-		w.Header().Set("Content-Type", "application/x-x509-ca-cert")
-		pem.Encode(w, &pem.Block{Type: "CERTIFICATE", Bytes: caCertificate.Raw})
-	})
+	go runReloadWorker(hupChan, cas)
 
 	applicationServer := &http.Server{Addr: config.applicationListenAddress, Handler: metrics.Middleware(applicationRouter)}
 	metricsServer := &http.Server{Addr: config.metricsListenAddress, Handler: promhttp.Handler()}
 
-	applicationServerClosed := make(chan any)
-	metricsServerClosed := make(chan any)
-	go func() {
-		log.Printf("starting application server on %+q", config.applicationListenAddress)
-		if listenError := applicationServer.ListenAndServe(); !errors.Is(listenError, http.ErrServerClosed) {
-			log.Printf("application error: %v", listenError)
-		}
-		close(applicationServerClosed)
-	}()
-	go func() {
-		log.Printf("starting metrics server on %+q", config.metricsListenAddress)
-		if listenError := metricsServer.ListenAndServe(); !errors.Is(listenError, http.ErrServerClosed) {
-			log.Printf("metrics server error: %v", listenError)
-		}
-		close(metricsServerClosed)
-	}()
+	applicationServerClosed := startServer(applicationServer, "application")
+	metricsServerClosed := startServer(metricsServer, "metrics")
 
-	<-signalChan
+	<-terminationChan
 	close(hupChan)
 	applicationServer.Shutdown(nil)
 	metricsServer.Shutdown(nil)