diff --git a/cmd/ntpresponder/main.go b/cmd/ntpresponder/main.go
index e98e6a1d..e9d3cf80 100644
--- a/cmd/ntpresponder/main.go
+++ b/cmd/ntpresponder/main.go
@@ -27,6 +27,7 @@ import (
 	"os/signal"
 	"runtime"
 
+	"github.com/facebook/time/ntp/ntske"
 	"github.com/facebook/time/ntp/responder/announce"
 	"github.com/facebook/time/ntp/responder/checker"
 	"github.com/facebook/time/ntp/responder/server"
@@ -41,8 +42,10 @@ func main() {
 	s := server.Server{}
 
 	var (
-		debugger bool
-		logLevel string
+		debugger        bool
+		logLevel        string
+		enableNTS       bool // master switch for the NTS path
+		ntsKeystoreKeys int  // size of the cookie master-key ring
 	)
 
 	flag.StringVar(&logLevel, "loglevel", "info", "Set a log level. Can be: debug, info, warning, error")
@@ -58,6 +61,8 @@ func main() {
 	flag.DurationVar(&s.Config.ExtraOffset, "extraoffset", 0, "Extra offset to return to clients")
 	flag.BoolVar(&s.Config.ManageLoopback, "manage-loopback", true, "Add/remove IPs. If false, these must be managed elsewhere")
 	flag.TextVar(&s.Config.TimestampType, "timestamptype", timestamp.SWRX, fmt.Sprintf("Timestamp type. Can be: %s, %s", timestamp.HWRX, timestamp.SWRX))
+	flag.BoolVar(&enableNTS, "enable-nts", false, "Enable NTS (Network Time Security) authenticated NTP")
+	flag.IntVar(&ntsKeystoreKeys, "nts-keystore-keys", 2, "Number of NTS cookie master keys kept in the rotating ring")
 
 	flag.Parse()
 	s.Config.IPs.SetDefault()
@@ -79,6 +84,21 @@ func main() {
 		log.Fatalf("Config is invalid: %v", err)
 	}
 
+	if enableNTS {
+		if ntsKeystoreKeys < 1 {
+			log.Fatalf("nts-keystore-keys must be >= 1, got %d", ntsKeystoreKeys)
+		}
+		ks, err := ntske.NewInMemoryKeystore(ntske.InMemoryKeystoreOptions{
+			MaxKeys:    uint32(ntsKeystoreKeys), // #nosec G115 -- ntsKeystoreKeys guarded >= 1
+			InitialKey: ntske.SharedTestMasterKey,
+		})
+		if err != nil {
+			log.Fatalf("Failed to set up NTS keystore: %v", err)
+		}
+		s.Config.Keystore = ks
+		log.Info("NTS enabled: NTP requests carrying extension fields will be authenticated")
+	}
+
 	if debugger {
 		log.Warningf("Staring profiler on %s", pprofHTTP)
 		go func() {
diff --git a/ntp/ntske/keystore.go b/ntp/ntske/keystore.go
index 275ce18c..6bf3b590 100644
--- a/ntp/ntske/keystore.go
+++ b/ntp/ntske/keystore.go
@@ -76,6 +76,24 @@ const (
 	// deterministic AEAD whose 16-octet synthetic IV doubles as the auth tag.
 	masterAEADID = protocol.AEADAESSIVCMAC512
 )
+
+// SharedTestMasterKey is a fixed 64-octet cookie master key used ONLY to bridge
+// the gap until a Keychain-backed keystore lets every responder in the fleet
+// share sealing keys. Seeding both the NTP responder and the standalone NTS-KE
+// server with it (via InMemoryKeystoreOptions.InitialKey) lets cookies sealed
+// by one open on the other.
+//
+// NOT for production: anyone holding these bytes can forge cookies. Delete this
+// once the fleet-wide (Keychain) keystore lands.
+var SharedTestMasterKey = []byte{
+	0xad, 0x17, 0x97, 0x8b, 0x12, 0x2b, 0x0c, 0xc1, 0x66, 0x81, 0x04, 0x55,
+	0xb4, 0xbb, 0x9a, 0xca, 0x0e, 0x85, 0x9b, 0xf0, 0x2d, 0x19, 0xe1, 0xdd,
+	0xb8, 0x1a, 0x85, 0xdb, 0x41, 0xd7, 0x48, 0x5f, 0xe4, 0x4a, 0x27, 0x06,
+	0xaa, 0x2d, 0x1b, 0x01, 0x7f, 0xab, 0x86, 0xaa, 0xf9, 0xe9, 0x01, 0xee,
+	0x8b, 0x3c, 0x2d, 0x58, 0x90, 0xdd, 0xa4, 0xc3, 0x46, 0x63, 0xb8, 0xa4,
+	0x2b, 0x55, 0xe5, 0x0c,
+}
+
 const (
 	cookieKeyIDLen = 4  // big-endian master-key identifier
 	cookieNonceLen = 16 // random per-cookie nonce, mixed into the SIV as AD
@@ -131,6 +149,10 @@ var _ Keystore = (*InMemoryKeystore)(nil)
 type InMemoryKeystoreOptions struct {
 	MaxKeys uint32
 	Rand    io.Reader
+	// InitialKey, if set, seeds the ring with this exact master key instead of
+	// generating a random one, letting separate processes share a sealing key.
+	// Must be masterKeyLen octets.
+	InitialKey []byte
 }
 
 // NewInMemoryKeystore returns a keystore seeded with one freshly generated
@@ -148,12 +170,34 @@ func NewInMemoryKeystore(opts InMemoryKeystoreOptions) (*InMemoryKeystore, error
 	if opts.Rand != nil {
 		ks.rand = opts.Rand
 	}
+	if len(opts.InitialKey) > 0 {
+		if err := ks.seedKey(opts.InitialKey); err != nil {
+			return nil, err
+		}
+		return ks, nil
+	}
 	if err := ks.Rotate(); err != nil {
 		return nil, err
 	}
 	return ks, nil
 }
 
+// seedKey installs key as the initial sealing key (Key ID 1). Unlike Rotate it
+// takes a caller-supplied key rather than generating a random one.
+func (ks *InMemoryKeystore) seedKey(key []byte) error {
+	if len(key) != masterKeyLen {
+		return fmt.Errorf("ntske: initial key must be %d octets, got %d", masterKeyLen, len(key))
+	}
+	ks.mu.Lock()
+	defer ks.mu.Unlock()
+	ks.nextID++
+	id := ks.nextID
+	ks.ring[id] = bytes.Clone(key)
+	ks.order = append(ks.order, id)
+	ks.current = id
+	return nil
+}
+
 // Rotate generates a new master key, makes it the sealing key, and ages out the
 // oldest key if the ring is over capacity. Cookies sealed with a still-present
 // key remain openable.
diff --git a/ntp/ntske/keystore_test.go b/ntp/ntske/keystore_test.go
index d689e41e..00fab022 100644
--- a/ntp/ntske/keystore_test.go
+++ b/ntp/ntske/keystore_test.go
@@ -243,3 +243,30 @@ func TestConcurrentSealOpen(t *testing.T) {
 		require.NoError(t, err)
 	}
 }
+
+// TestInitialKeyEnablesCrossKeystoreOpen verifies the shared-key bridge: two
+// independent keystores seeded with the same InitialKey can open each other's
+// cookies, which is what lets the standalone NTS-KE server and the NTP
+// responder interoperate.
+func TestInitialKeyEnablesCrossKeystoreOpen(t *testing.T) {
+	sealer, err := NewInMemoryKeystore(InMemoryKeystoreOptions{InitialKey: SharedTestMasterKey})
+	require.NoError(t, err)
+	opener, err := NewInMemoryKeystore(InMemoryKeystoreOptions{InitialKey: SharedTestMasterKey})
+	require.NoError(t, err)
+
+	c2s, s2c := sessionKeys(64)
+	cookie, err := sealer.SealCookie(protocol.AEADAESSIVCMAC512, c2s, s2c)
+	require.NoError(t, err)
+
+	_, gotC2S, gotS2C, err := opener.OpenCookie(cookie)
+	require.NoError(t, err)
+	require.Equal(t, c2s, gotC2S)
+	require.Equal(t, s2c, gotS2C)
+}
+
+// TestInitialKeyWrongLength checks that a master key of the wrong size is
+// rejected rather than silently producing a broken keystore.
+func TestInitialKeyWrongLength(t *testing.T) {
+	_, err := NewInMemoryKeystore(InMemoryKeystoreOptions{InitialKey: make([]byte, masterKeyLen-1)})
+	require.Error(t, err)
+}