optimize expiry periods of certificates (#21)

p2p/transport/webtransport/cert_manager.go

@@ -15,11 +15,19 @@ import (
 	"github.com/multiformats/go-multihash"
 )
 
+// Allow for a bit of clock skew.
+// When we generate a certificate, the NotBefore time is set to clockSkewAllowance before the current time.
+// Similarly, we stop using a certificate one clockSkewAllowance before its expiry time.
+const clockSkewAllowance = time.Hour
+
 type certConfig struct {
 	tlsConf *tls.Config
 	sha256  [32]byte // cached from the tlsConf
 }
 
+func (c *certConfig) Start() time.Time { return c.tlsConf.Certificates[0].Leaf.NotBefore }
+func (c *certConfig) End() time.Time   { return c.tlsConf.Certificates[0].Leaf.NotAfter }
+
 func newCertConfig(start, end time.Time) (*certConfig, error) {
 	conf, err := getTLSConf(start, end)
 	if err != nil {
@@ -32,22 +40,17 @@ func newCertConfig(start, end time.Time) (*certConfig, error) {
 }
 
 // Certificate renewal logic:
-// 0. To simplify the math, assume the certificate is valid for 10 days (in real life: 14 days).
-// 1. On startup, we generate the first certificate (1).
-// 2. After 4 days, we generate a second certificate (2).
-//    We don't use that certificate yet, but we advertise the hashes of (1) and (2).
-//    That allows clients to connect to us using addresses that are 4 days old.
-// 3. After another 4 days, we now actually start using (2).
-//    We also generate a third certificate (3), and start advertising the hashes of (2) and (3).
-//    We continue to remember the hash of (1) for validation during the Noise handshake for another 4 days,
-//    as the client might be connecting with a cached address.
+// 1. On startup, we generate one cert that is valid from now (-1h, to allow for clock skew), and another
+//    cert that is valid from the expiry date of the first certificate (again, with allowance for clock skew).
+// 2. Once we reach 1h before expiry of the first certificate, we switch over to the second certificate.
+//    At the same time, we stop advertising the certhash of the first cert and generate the next cert.
 type certManager struct {
 	clock     clock.Clock
 	ctx       context.Context
 	ctxCancel context.CancelFunc
 	refCount  sync.WaitGroup
 
-	mx            sync.Mutex
+	mx            sync.RWMutex
 	lastConfig    *certConfig // initially nil
 	currentConfig *certConfig
 	nextConfig    *certConfig // nil until we have passed half the certValidity of the current config
@@ -61,64 +64,71 @@ func newCertManager(clock clock.Clock) (*certManager, error) {
 		return nil, err
 	}
 
-	t := m.clock.Ticker(certValidity * 4 / 9) // make sure we're a bit faster than 1/2
-	m.refCount.Add(1)
-	go func() {
-		defer m.refCount.Done()
-		defer t.Stop()
-		if err := m.background(t); err != nil {
-			log.Fatal(err)
-		}
-	}()
+	m.background()
 	return m, nil
 }
 
 func (m *certManager) init() error {
-	start := m.clock.Now()
-	end := start.Add(certValidity)
-	cc, err := newCertConfig(start, end)
+	start := m.clock.Now().Add(-clockSkewAllowance)
+	var err error
+	m.nextConfig, err = newCertConfig(start, start.Add(certValidity))
 	if err != nil {
 		return err
 	}
-	m.currentConfig = cc
-	return m.cacheAddrComponent()
+	return m.rollConfig()
 }
 
-func (m *certManager) background(t *clock.Ticker) error {
-	for {
-		select {
-		case <-m.ctx.Done():
-			return nil
-		case start := <-t.C:
-			end := start.Add(certValidity)
-			cc, err := newCertConfig(start, end)
+func (m *certManager) rollConfig() error {
+	// We stop using the current certificate clockSkewAllowance before its expiry time.
+	// At this point, the next certificate needs to be valid for one clockSkewAllowance.
+	nextStart := m.nextConfig.End().Add(-2 * clockSkewAllowance)
+	c, err := newCertConfig(nextStart, nextStart.Add(certValidity))
 	if err != nil {
 		return err
 	}
-			m.mx.Lock()
-			if m.nextConfig != nil {
 	m.lastConfig = m.currentConfig
 	m.currentConfig = m.nextConfig
+	m.nextConfig = c
+	return m.cacheAddrComponent()
+}
+
+func (m *certManager) background() {
+	d := m.currentConfig.End().Add(-clockSkewAllowance).Sub(m.clock.Now())
+	log.Debugw("setting timer", "duration", d.String())
+	t := m.clock.Timer(d)
+	m.refCount.Add(1)
+
+	go func() {
+		defer m.refCount.Done()
+		defer t.Stop()
+
+		for {
+			select {
+			case <-m.ctx.Done():
+				return
+			case now := <-t.C:
+				m.mx.Lock()
+				if err := m.rollConfig(); err != nil {
+					log.Errorw("rolling config failed", "error", err)
 				}
-			m.nextConfig = cc
-			if err := m.cacheAddrComponent(); err != nil {
-				m.mx.Unlock()
-				return err
-			}
+				d := m.currentConfig.End().Add(-clockSkewAllowance).Sub(now)
+				log.Debugw("rolling certificates", "next", d.String())
+				t.Reset(d)
 				m.mx.Unlock()
 			}
 		}
+	}()
 }
 
 func (m *certManager) GetConfig() *tls.Config {
-	m.mx.Lock()
-	defer m.mx.Unlock()
+	m.mx.RLock()
+	defer m.mx.RUnlock()
 	return m.currentConfig.tlsConf
 }
 
 func (m *certManager) AddrComponent() ma.Multiaddr {
-	m.mx.Lock()
-	defer m.mx.Unlock()
+	m.mx.RLock()
+	defer m.mx.RUnlock()
 	return m.addrComp
 }
 

p2p/transport/webtransport/cert_manager_test.go

@@ -46,14 +46,15 @@ func TestInitialCert(t *testing.T) {
 	conf := m.GetConfig()
 	require.Len(t, conf.Certificates, 1)
 	cert := conf.Certificates[0]
-	require.Equal(t, cl.Now().UTC(), cert.Leaf.NotBefore)
-	require.Equal(t, cl.Now().Add(certValidity).UTC(), cert.Leaf.NotAfter)
+	require.Equal(t, cl.Now().Add(-clockSkewAllowance).UTC(), cert.Leaf.NotBefore)
+	require.Equal(t, cert.Leaf.NotBefore.Add(certValidity), cert.Leaf.NotAfter)
 	addr := m.AddrComponent()
 	components := splitMultiaddr(addr)
-	require.Len(t, components, 1)
+	require.Len(t, components, 2)
 	require.Equal(t, ma.P_CERTHASH, components[0].Protocol().Code)
 	hash := certificateHashFromTLSConfig(conf)
 	require.Equal(t, hash[:], certHashFromComponent(t, components[0]))
+	require.Equal(t, ma.P_CERTHASH, components[1].Protocol().Code)
 }
 
 func TestCertRenewal(t *testing.T) {
@@ -63,21 +64,39 @@ func TestCertRenewal(t *testing.T) {
 	defer m.Close()
 
 	firstConf := m.GetConfig()
-	require.Len(t, splitMultiaddr(m.AddrComponent()), 1)
+	first := splitMultiaddr(m.AddrComponent())
+	require.Len(t, first, 2)
+	require.NotEqual(t, first[0].Value(), first[1].Value(), "the hashes should differ")
 	// wait for a new certificate to be generated
-	cl.Add(certValidity / 2)
-	require.Eventually(t, func() bool { return len(splitMultiaddr(m.AddrComponent())) > 1 }, 200*time.Millisecond, 10*time.Millisecond)
-	// the actual config used should still be the same, we're just advertising the hash of the next config
-	components := splitMultiaddr(m.AddrComponent())
-	require.Len(t, components, 2)
-	for _, c := range components {
-		require.Equal(t, ma.P_CERTHASH, c.Protocol().Code)
+	cl.Add(certValidity - 2*clockSkewAllowance - time.Second)
+	require.Never(t, func() bool {
+		for i, c := range splitMultiaddr(m.AddrComponent()) {
+			if c.Value() != first[i].Value() {
+				return true
+			}
 		}
-	require.Equal(t, firstConf, m.GetConfig())
-	cl.Add(certValidity / 2)
+		return false
+	}, 100*time.Millisecond, 10*time.Millisecond)
+	cl.Add(2 * time.Second)
 	require.Eventually(t, func() bool { return m.GetConfig() != firstConf }, 200*time.Millisecond, 10*time.Millisecond)
-	newConf := m.GetConfig()
-	// check that the new config now matches the second component
-	hash := certificateHashFromTLSConfig(newConf)
-	require.Equal(t, hash[:], certHashFromComponent(t, components[1]))
+	secondConf := m.GetConfig()
+
+	second := splitMultiaddr(m.AddrComponent())
+	require.Len(t, second, 2)
+	for _, c := range second {
+		require.Equal(t, ma.P_CERTHASH, c.Protocol().Code)
+	}
+	// check that the 2nd certificate from the beginning was rolled over to be the 1st certificate
+	require.Equal(t, first[1].Value(), second[0].Value())
+	require.NotEqual(t, first[0].Value(), second[1].Value())
+
+	cl.Add(certValidity - 2*clockSkewAllowance + time.Second)
+	require.Eventually(t, func() bool { return m.GetConfig() != secondConf }, 200*time.Millisecond, 10*time.Millisecond)
+	third := splitMultiaddr(m.AddrComponent())
+	require.Len(t, third, 2)
+	for _, c := range third {
+		require.Equal(t, ma.P_CERTHASH, c.Protocol().Code)
+	}
+	// check that the 2nd certificate from the beginning was rolled over to be the 1st certificate
+	require.Equal(t, second[1].Value(), third[0].Value())
 }

p2p/transport/webtransport/transport_test.go

@@ -140,7 +140,16 @@ func TestHashVerification(t *testing.T) {
 
 	t.Run("fails using only a wrong hash", func(t *testing.T) {
 		// replace the certificate hash in the multiaddr with a fake hash
-		addr, _ := ma.SplitLast(ln.Multiaddr())
+		addr := ln.Multiaddr()
+		// strip off all certhash components
+		for {
+			a, comp := ma.SplitLast(addr)
+			if comp.Protocol().Code != ma.P_CERTHASH {
+				break
+			}
+			addr = a
+		}
+
 		addr = addr.Encapsulate(foobarHash)
 
 		_, err := tr2.Dial(context.Background(), addr, serverID)
@@ -224,7 +233,7 @@ func TestListenerAddrs(t *testing.T) {
 	ln2, err := tr.Listen(ma.StringCast("/ip4/127.0.0.1/udp/0/quic/webtransport"))
 	require.NoError(t, err)
 	hashes1 := extractCertHashes(ln1.Multiaddr())
-	require.Len(t, hashes1, 1)
+	require.Len(t, hashes1, 2)
 	hashes2 := extractCertHashes(ln2.Multiaddr())
 	require.Equal(t, hashes1, hashes2)
 }