API のための相互 TLS（mTLS）：クライアント証明書の実践

Acceptable client certificate CA names
O=Example, CN=Example Internal Root
Requested Signature Algorithms: id-ml-dsa-65:id-ml-dsa-87:id-ml-dsa-44:ECDSA+SHA256:ECDSA+SHA384:ECDSA+SHA512:ed25519:ed448:...

[issuing]
basicConstraints = critical, CA:TRUE, pathlen:0
keyUsage = critical, keyCertSign, cRLSign
extendedKeyUsage = serverAuth, clientAuth
subjectKeyIdentifier = hash
authorityKeyIdentifier = keyid
[worker7]
basicConstraints = critical, CA:FALSE
keyUsage = critical, digitalSignature
extendedKeyUsage = clientAuth
subjectAltName = URI:spiffe://example.test/ns/prod/sa/worker-7
authorityKeyIdentifier = keyid

openssl genpkey -algorithm EC -pkeyopt ec_paramgen_curve:P-256 -out worker-7.key
openssl req -new -key worker-7.key -subj "/O=Example/CN=worker-7" -out worker-7.csr
openssl x509 -req -in worker-7.csr -CA issuing.pem -CAkey issuing.key -extfile ext.cnf -extensions worker7 -days 1 -out worker-7.pem
cat worker-7.pem issuing.pem > worker-7-chain.pem

server {
    listen 8443 ssl;
    server_name api.internal.test;
    error_log /var/log/nginx/mtls.log info;   # client certificate errors are logged at "info"
    ssl_certificate     /w/pki/server-chain.pem;
    ssl_certificate_key /w/pki/server.key;

    ssl_client_certificate /w/pki/root.pem;   # trust anchor for client certificates
    ssl_verify_client      on;
    ssl_verify_depth       2;

    location / {
        default_type text/plain;
        return 200 "verify=$ssl_client_verify subject=$ssl_client_s_dn issuer=$ssl_client_i_dn\n";
    }
}

--- no client certificate
<head><title>400 No required SSL certificate was sent</title></head>
--- worker-7, leaf only
<head><title>400 The SSL certificate error</title></head>
--- worker-7 with its intermediate
verify=SUCCESS subject=CN=worker-7,O=Example issuer=CN=Example Issuing CA 1,O=Example
--- certificate from another CA
<head><title>400 The SSL certificate error</title></head>
--- expired certificate
<head><title>400 The SSL certificate error</title></head>

[info] client sent no required SSL certificate while reading client request headers
[info] client SSL certificate verify error: (21:unable to verify the first certificate) while reading client request headers
[info] client SSL certificate verify error: (21:unable to verify the first certificate) while reading client request headers
[info] client SSL certificate verify error: (10:certificate has expired) while reading client request headers

transport_socket:
  name: envoy.transport_sockets.tls
  typed_config:
    "@type": type.googleapis.com/envoy.extensions.transport_sockets.tls.v3.DownstreamTlsContext
    require_client_certificate: true
    common_tls_context:
      tls_params: { tls_minimum_protocol_version: TLSv1_3 }
      tls_certificates:
      - certificate_chain: { filename: /w/pki/server-chain.pem }
        private_key: { filename: /w/pki/server.key }
      validation_context:
        trusted_ca: { filename: /w/pki/root.pem }
        match_typed_subject_alt_names:
        - san_type: URI
          matcher: { exact: "spiffe://example.test/ns/prod/sa/worker-7" }
# in the http_connection_manager of the same listener:
forward_client_cert_details: SANITIZE_SET
set_current_client_cert_details: { uri: true }

--- worker-7
HTTP/1.1 200 OK
x-backend-saw-xfcc: Hash=048c219f601a877b19c77fc8834750cdc6b1394b9080a50fa8eaeaf6f3793bd0;URI=spiffe://example.test/ns/prod/sa/worker-7
--- worker-7 sending its own x-forwarded-client-cert
HTTP/1.1 200 OK
x-backend-saw-xfcc: Hash=048c219f601a877b19c77fc8834750cdc6b1394b9080a50fa8eaeaf6f3793bd0;URI=spiffe://example.test/ns/prod/sa/worker-7
--- billing (valid certificate, SAN not allowed)
curl: (56) OpenSSL SSL_read: error:0A000416:SSL routines::sslv3 alert certificate unknown, errno 0
--- no client certificate
curl: (56) OpenSSL SSL_read: error:0A00045C:SSL routines::tlsv13 alert certificate required, errno 0
--- certificate from another CA
curl: (56) OpenSSL SSL_read: error:0A000418:SSL routines::tlsv1 alert unknown ca, errno 0

listener.127.0.0.1_10443.ssl.fail_verify_error: 1
listener.127.0.0.1_10443.ssl.fail_verify_no_cert: 1
listener.127.0.0.1_10443.ssl.fail_verify_san: 1
listener.127.0.0.1_10443.ssl.handshake: 2

var allowed = map[string]string{"spiffe://example.test/ns/prod/sa/worker-7": "/jobs"}

func handler(w http.ResponseWriter, r *http.Request) {
	// With RequireAndVerifyClientCert, VerifiedChains[0][0] is the verified client leaf.
	leaf := r.TLS.VerifiedChains[0][0]
	if len(leaf.URIs) != 1 {
		http.Error(w, "client certificate must carry exactly one URI SAN", http.StatusForbidden)
		return
	}
	id := leaf.URIs[0].String()
	if allowed[id] != r.URL.Path {
		http.Error(w, id+" may not call "+r.URL.Path, http.StatusForbidden)
		return
	}
	fmt.Fprintf(w, "hello %s, cert valid until %s\n", id, leaf.NotAfter.Format(time.RFC3339))
}

func server(ln net.Listener) {
	cert, err := tls.LoadX509KeyPair(pki+"server-chain.pem", pki+"server.key")
	if err != nil {
		log.Fatal(err)
	}
	srv := &http.Server{
		Handler: http.HandlerFunc(handler),
		TLSConfig: &tls.Config{
			Certificates: []tls.Certificate{cert},
			ClientAuth:   tls.RequireAndVerifyClientCert,
			ClientCAs:    pool("root.pem"),
			MinVersion:   tls.VersionTLS13,
		},
		ErrorLog: log.New(os.Stdout, "server log: ", 0),
	}
	srv.ServeTLS(ln, "", "")
}

server log: http: TLS handshake error from 127.0.0.1:PORT: tls: client didn't provide a certificate
no client certificate        client error: Get "https://127.0.0.1:PORT/jobs": remote error: tls: certificate required
worker-7                     200 hello spiffe://example.test/ns/prod/sa/worker-7, cert valid until 2026-10-21T15:17:06Z
billing                      403 spiffe://example.test/ns/prod/sa/billing may not call /jobs
server log: http: TLS handshake error from 127.0.0.1:PORT: tls: client didn't provide a certificate
certificate from another CA  client error: Get "https://127.0.0.1:PORT/jobs": remote error: tls: certificate required
server log: http: TLS handshake error from 127.0.0.1:PORT: tls: failed to verify certificate: x509: certificate signed by unknown authority
same, sent anyway            client error: Get "https://127.0.0.1:PORT/jobs": remote error: tls: unknown certificate authority
server log: http: TLS handshake error from 127.0.0.1:PORT: tls: failed to verify certificate: x509: certificate has expired or is not yet valid: current time 2026-10-20T15:18:44Z is after 2026-09-02T00:00:00Z
expired certificate          client error: Get "https://127.0.0.1:PORT/jobs": remote error: tls: expired certificate

function authorize(req, res) {
  // With rejectUnauthorized: false the handshake succeeds for any certificate (or none),
  // so this check is the only thing standing between the request and the handler.
  if (!req.socket.authorized) {
    res.writeHead(401).end(`client certificate rejected: ${req.socket.authorizationError}\n`);
    return;
  }
  const cert = req.socket.getPeerCertificate();
  // subjectaltname is a string such as "URI:spiffe://..." — parse it, do not substring-match it.
  const uris = (cert.subjectaltname ?? "").split(", ").filter((n) => n.startsWith("URI:")).map((n) => n.slice(4));
  const id = uris.length === 1 ? uris[0] : null;
  if (!id || ALLOWED.get(id) !== req.url) {
    res.writeHead(403).end(`${id ?? "no single URI SAN"} may not call ${req.url}\n`);
    return;
  }
  res.end(`hello ${id}, serial ${cert.serialNumber}\n`);
}

const serverOptions = (rejectUnauthorized) => ({
  key: pki("server.key"),
  cert: pki("server-chain.pem"),
  ca: pki("root.pem"), // trust anchor for client certificates
  requestCert: true, // send CertificateRequest
  rejectUnauthorized, // true: drop connections whose client certificate does not verify
  minVersion: "TLSv1.3",
});

node v24.22.0, OpenSSL 3.5.8
### rejectUnauthorized: true
  server tlsClientError: ERR_SSL_PEER_DID_NOT_RETURN_A_CERTIFICATE
no client certificate        client error: ERR_SSL_TLSV13_ALERT_CERTIFICATE_REQUIRED: ...:tlsv13 alert certificate required:...:SSL alert number 116
worker-7                     200 hello spiffe://example.test/ns/prod/sa/worker-7, serial 7D2333CCB4C609E16E5592C959C71250AD4703BA
billing                      403 spiffe://example.test/ns/prod/sa/billing may not call /jobs
  server tlsClientError: ECONNRESET
certificate from another CA  client error: ECONNRESET: socket hang up
  server tlsClientError: ECONNRESET
expired certificate          client error: ECONNRESET: socket hang up
### rejectUnauthorized: false
no client certificate        401 client certificate rejected: UNABLE_TO_GET_ISSUER_CERT
worker-7                     200 hello spiffe://example.test/ns/prod/sa/worker-7, serial 7D2333CCB4C609E16E5592C959C71250AD4703BA
billing                      403 spiffe://example.test/ns/prod/sa/billing may not call /jobs
certificate from another CA  401 client certificate rejected: UNABLE_TO_VERIFY_LEAF_SIGNATURE
expired certificate          401 client certificate rejected: CERT_HAS_EXPIRED