ipn/ipnlocal: discard node keys that have been rotated out

A non-signing node can be allowed to re-sign its new node keys following
key renewal/rotation (e.g. via `tailscale up --force-reauth`). To be
able to do this, node's TLK is written into WrappingPubkey field of the
initial SigDirect signature, signed by a signing node.

The intended use of this field implies that, for each WrappingPubkey, we
typically expect to have at most one active node with a signature
tracing back to that key. Multiple valid signatures referring to the
same WrappingPubkey can occur if a client's state has been cloned, but
it's something we explicitly discourage and don't support:
https://tailscale.com/s/clone

This change propagates rotation details (wrapping public key, a list
of previous node keys that have been rotated out) to netmap processing,
and adds tracking of obsolete node keys that, when found, will get
filtered out.

Updates tailscale/corp#19764

Signed-off-by: Anton Tolchanov <anton@tailscale.com>

control/controlclient/direct.go

@@ -558,7 +558,7 @@ func (c *Direct) doLogin(ctx context.Context, opt loginOpt) (mustRegen bool, new
 
 	var nodeKeySignature tkatype.MarshaledSignature
 	if !oldNodeKey.IsZero() && opt.OldNodeKeySignature != nil {
-		if nodeKeySignature, err = resignNKS(persist.NetworkLockKey, tryingNewKey.Public(), opt.OldNodeKeySignature); err != nil {
+		if nodeKeySignature, err = tka.ResignNKS(persist.NetworkLockKey, tryingNewKey.Public(), opt.OldNodeKeySignature); err != nil {
 			c.logf("Failed re-signing node-key signature: %v", err)
 		}
 	} else if isWrapped {
@@ -729,45 +729,6 @@ func (c *Direct) doLogin(ctx context.Context, opt loginOpt) (mustRegen bool, new
 	return false, resp.AuthURL, nil, nil
 }
 
-// resignNKS re-signs a node-key signature for a new node-key.
-//
-// This only matters on network-locked tailnets, because node-key signatures are
-// how other nodes know that a node-key is authentic. When the node-key is
-// rotated then the existing signature becomes invalid, so this function is
-// responsible for generating a new wrapping signature to certify the new node-key.
-//
-// The signature itself is a SigRotation signature, which embeds the old signature
-// and certifies the new node-key as a replacement for the old by signing the new
-// signature with RotationPubkey (which is the node's own network-lock key).
-func resignNKS(priv key.NLPrivate, nodeKey key.NodePublic, oldNKS tkatype.MarshaledSignature) (tkatype.MarshaledSignature, error) {
-	var oldSig tka.NodeKeySignature
-	if err := oldSig.Unserialize(oldNKS); err != nil {
-		return nil, fmt.Errorf("decoding NKS: %w", err)
-	}
-
-	nk, err := nodeKey.MarshalBinary()
-	if err != nil {
-		return nil, fmt.Errorf("marshalling node-key: %w", err)
-	}
-
-	if bytes.Equal(nk, oldSig.Pubkey) {
-		// The old signature is valid for the node-key we are using, so just
-		// use it verbatim.
-		return oldNKS, nil
-	}
-
-	newSig := tka.NodeKeySignature{
-		SigKind: tka.SigRotation,
-		Pubkey:  nk,
-		Nested:  &oldSig,
-	}
-	if newSig.Signature, err = priv.SignNKS(newSig.SigHash()); err != nil {
-		return nil, fmt.Errorf("signing NKS: %w", err)
-	}
-
-	return newSig.Serialize(), nil
-}
-
 // newEndpoints acquires c.mu and sets the local port and endpoints and reports
 // whether they've changed.
 //

ipn/ipnlocal/network-lock.go

@@ -18,6 +18,7 @@ import (
 	"net/netip"
 	"os"
 	"path/filepath"
+	"slices"
 	"time"
 
 	"tailscale.com/health/healthmsg"
@@ -27,10 +28,12 @@ import (
 	"tailscale.com/tailcfg"
 	"tailscale.com/tka"
 	"tailscale.com/types/key"
+	"tailscale.com/types/logger"
 	"tailscale.com/types/netmap"
 	"tailscale.com/types/persist"
 	"tailscale.com/types/tkatype"
 	"tailscale.com/util/mak"
+	"tailscale.com/util/set"
 )
 
 // TODO(tom): RPC retry/backoff was broken and has been removed. Fix?
@@ -66,6 +69,7 @@ func (b *LocalBackend) tkaFilterNetmapLocked(nm *netmap.NetworkMap) {
 		return // TKA not enabled.
 	}
 
+	tracker := rotationTracker{logf: b.logf}
 	var toDelete map[int]bool // peer index => true
 	for i, p := range nm.Peers {
 		if p.UnsignedPeerAPIOnly() {
@@ -76,21 +80,32 @@ func (b *LocalBackend) tkaFilterNetmapLocked(nm *netmap.NetworkMap) {
 			b.logf("Network lock is dropping peer %v(%v) due to missing signature", p.ID(), p.StableID())
 			mak.Set(&toDelete, i, true)
 		} else {
-			if err := b.tka.authority.NodeKeyAuthorized(p.Key(), p.KeySignature().AsSlice()); err != nil {
+			details, err := b.tka.authority.NodeKeyAuthorizedWithDetails(p.Key(), p.KeySignature().AsSlice())
+			if err != nil {
 				b.logf("Network lock is dropping peer %v(%v) due to failed signature check: %v", p.ID(), p.StableID(), err)
 				mak.Set(&toDelete, i, true)
+				continue
+			}
+			if details != nil {
+				// Rotation details are returned when the node key is signed by a valid SigRotation signature.
+				tracker.addRotationDetails(p.Key(), details)
 			}
 		}
 	}
 
+	obsoleteByRotation := tracker.obsoleteKeys()
+
 	// nm.Peers is ordered, so deletion must be order-preserving.
-	if len(toDelete) > 0 {
+	if len(toDelete) > 0 || len(obsoleteByRotation) > 0 {
 		peers := make([]tailcfg.NodeView, 0, len(nm.Peers))
-		filtered := make([]ipnstate.TKAFilteredPeer, 0, len(toDelete))
+		filtered := make([]ipnstate.TKAFilteredPeer, 0, len(toDelete)+len(obsoleteByRotation))
 		for i, p := range nm.Peers {
-			if !toDelete[i] {
+			if !toDelete[i] && !obsoleteByRotation.Contains(p.Key()) {
 				peers = append(peers, p)
 			} else {
+				if obsoleteByRotation.Contains(p.Key()) {
+					b.logf("Network lock is dropping peer %v(%v) due to key rotation", p.ID(), p.StableID())
+				}
 				// Record information about the node we filtered out.
 				fp := ipnstate.TKAFilteredPeer{
 					Name:         p.Name(),
@@ -122,6 +137,84 @@ func (b *LocalBackend) tkaFilterNetmapLocked(nm *netmap.NetworkMap) {
 	}
 }
 
+// rotationTracker determines the set of node keys that are made obsolete by key
+// rotation.
+//   - for each SigRotation signature, all previous node keys referenced by the
+//     nested signatures are marked as obsolete.
+//   - if there are multiple SigRotation signatures tracing back to the same
+//     wrapping pubkey (e.g. if a node is cloned with all its keys), we keep
+//     just one of them, marking the others as obsolete.
+type rotationTracker struct {
+	// obsolete is the set of node keys that are obsolete due to key rotation.
+	// users of rotationTracker should use the obsoleteKeys method for complete results.
+	obsolete set.Set[key.NodePublic]
+
+	// byWrappingKey keeps track of rotation details per wrapping pubkey.
+	byWrappingKey map[string][]sigRotationDetails
+
+	logf logger.Logf
+}
+
+// sigRotationDetails holds information about a node key signed by a SigRotation.
+type sigRotationDetails struct {
+	np          key.NodePublic
+	numPrevKeys int
+}
+
+// addRotationDetails records the rotation signature details for a node key.
+func (r *rotationTracker) addRotationDetails(np key.NodePublic, d *tka.RotationDetails) {
+	r.obsolete.Make()
+	r.obsolete.AddSlice(d.PrevNodeKeys)
+	rd := sigRotationDetails{
+		np:          np,
+		numPrevKeys: len(d.PrevNodeKeys),
+	}
+	if r.byWrappingKey == nil {
+		r.byWrappingKey = make(map[string][]sigRotationDetails)
+	}
+	wp := string(d.WrappingPubkey)
+	r.byWrappingKey[wp] = append(r.byWrappingKey[wp], rd)
+}
+
+// obsoleteKeys returns the set of node keys that are obsolete due to key rotation.
+func (r *rotationTracker) obsoleteKeys() set.Set[key.NodePublic] {
+	for _, v := range r.byWrappingKey {
+		// If there are multiple rotation signatures with the same wrapping
+		// pubkey, we need to decide which one is the "latest", and keep it.
+		// The signature with the largest number of previous keys is likely to
+		// be the latest, unless it has been marked as obsolete (rotated out) by
+		// another signature (which might happen in the future if we start
+		// compacting long rotated signature chains).
+		slices.SortStableFunc(v, func(a, b sigRotationDetails) int {
+			// Group all obsolete keys after non-obsolete keys.
+			if ao, bo := r.obsolete.Contains(a.np), r.obsolete.Contains(b.np); ao != bo {
+				if ao {
+					return 1
+				}
+				return -1
+			}
+			// Sort by decreasing number of previous keys.
+			return b.numPrevKeys - a.numPrevKeys
+		})
+		// If there are several signatures with the same number of previous
+		// keys, we cannot determine which one is the latest, so all of them are
+		// rejected for safety.
+		if len(v) >= 2 && v[0].numPrevKeys == v[1].numPrevKeys {
+			r.logf("at least two nodes (%s and %s) have equally valid rotation signatures with the same wrapping pubkey, rejecting", v[0].np, v[1].np)
+			for _, rd := range v {
+				r.obsolete.Add(rd.np)
+			}
+		} else {
+			// The first key in v is the one with the longest chain of previous
+			// keys, so it must be the newest one. Mark all older keys as obsolete.
+			for _, rd := range v[1:] {
+				r.obsolete.Add(rd.np)
+			}
+		}
+	}
+	return r.obsolete
+}
+
 // tkaSyncIfNeeded examines TKA info reported from the control plane,
 // performing the steps necessary to synchronize local tka state.
 //

ipn/ipnlocal/network-lock_test.go

@@ -13,8 +13,11 @@ import (
 	"net/http/httptest"
 	"os"
 	"path/filepath"
+	"reflect"
 	"testing"
 
+	go4mem "go4.org/mem"
+
 	"github.com/google/go-cmp/cmp"
 	"tailscale.com/control/controlclient"
 	"tailscale.com/health"
@@ -30,6 +33,7 @@ import (
 	"tailscale.com/types/persist"
 	"tailscale.com/types/tkatype"
 	"tailscale.com/util/must"
+	"tailscale.com/util/set"
 )
 
 type observerFunc func(controlclient.Status)
@@ -563,18 +567,32 @@ func TestTKAFilterNetmap(t *testing.T) {
 	}
 	n4Sig.Signature[3] = 42 // mess up the signature
 	n4Sig.Signature[4] = 42 // mess up the signature
-	n5GoodSig, err := signNodeKey(tailcfg.TKASignInfo{NodePublic: n5.Public()}, nlPriv)
+
+	n5nl := key.NewNLPrivate()
+	n5InitialSig, err := signNodeKey(tailcfg.TKASignInfo{NodePublic: n5.Public(), RotationPubkey: n5nl.Public().Verifier()}, nlPriv)
 	if err != nil {
 		t.Fatal(err)
 	}
 
+	resign := func(nl key.NLPrivate, currentSig tkatype.MarshaledSignature) (key.NodePrivate, tkatype.MarshaledSignature) {
+		nk := key.NewNode()
+		sig, err := tka.ResignNKS(nl, nk.Public(), currentSig)
+		if err != nil {
+			t.Fatal(err)
+		}
+		return nk, sig
+	}
+
+	n5Rotated, n5RotatedSig := resign(n5nl, n5InitialSig.Serialize())
+
 	nm := &netmap.NetworkMap{
 		Peers: nodeViews([]*tailcfg.Node{
 			{ID: 1, Key: n1.Public(), KeySignature: n1GoodSig.Serialize()},
-			{ID: 2, Key: n2.Public(), KeySignature: nil},                   // missing sig
-			{ID: 3, Key: n3.Public(), KeySignature: n1GoodSig.Serialize()}, // someone elses sig
-			{ID: 4, Key: n4.Public(), KeySignature: n4Sig.Serialize()},     // messed-up signature
-			{ID: 5, Key: n5.Public(), KeySignature: n5GoodSig.Serialize()},
+			{ID: 2, Key: n2.Public(), KeySignature: nil},                       // missing sig
+			{ID: 3, Key: n3.Public(), KeySignature: n1GoodSig.Serialize()},     // someone elses sig
+			{ID: 4, Key: n4.Public(), KeySignature: n4Sig.Serialize()},         // messed-up signature
+			{ID: 50, Key: n5.Public(), KeySignature: n5InitialSig.Serialize()}, // rotated
+			{ID: 51, Key: n5Rotated.Public(), KeySignature: n5RotatedSig},
 		}),
 	}
 
@@ -586,12 +604,39 @@ func TestTKAFilterNetmap(t *testing.T) {
 
 	want := nodeViews([]*tailcfg.Node{
 		{ID: 1, Key: n1.Public(), KeySignature: n1GoodSig.Serialize()},
-		{ID: 5, Key: n5.Public(), KeySignature: n5GoodSig.Serialize()},
+		{ID: 51, Key: n5Rotated.Public(), KeySignature: n5RotatedSig},
 	})
 	nodePubComparer := cmp.Comparer(func(x, y key.NodePublic) bool {
 		return x.Raw32() == y.Raw32()
 	})
-	if diff := cmp.Diff(nm.Peers, want, nodePubComparer); diff != "" {
+	if diff := cmp.Diff(want, nm.Peers, nodePubComparer); diff != "" {
+		t.Errorf("filtered netmap differs (-want, +got):\n%s", diff)
+	}
+
+	// Create two more node signatures using the same wrapping key as n5.
+	// Since they have the same rotation chain, both will be filtered out.
+	n7, n7Sig := resign(n5nl, n5RotatedSig)
+	n8, n8Sig := resign(n5nl, n5RotatedSig)
+
+	nm = &netmap.NetworkMap{
+		Peers: nodeViews([]*tailcfg.Node{
+			{ID: 1, Key: n1.Public(), KeySignature: n1GoodSig.Serialize()},
+			{ID: 2, Key: n2.Public(), KeySignature: nil},                       // missing sig
+			{ID: 3, Key: n3.Public(), KeySignature: n1GoodSig.Serialize()},     // someone elses sig
+			{ID: 4, Key: n4.Public(), KeySignature: n4Sig.Serialize()},         // messed-up signature
+			{ID: 50, Key: n5.Public(), KeySignature: n5InitialSig.Serialize()}, // rotated
+			{ID: 51, Key: n5Rotated.Public(), KeySignature: n5RotatedSig},      // rotated
+			{ID: 7, Key: n7.Public(), KeySignature: n7Sig},                     // same rotation chain as n8
+			{ID: 8, Key: n8.Public(), KeySignature: n8Sig},                     // same rotation chain as n7
+		}),
+	}
+
+	b.tkaFilterNetmapLocked(nm)
+
+	want = nodeViews([]*tailcfg.Node{
+		{ID: 1, Key: n1.Public(), KeySignature: n1GoodSig.Serialize()},
+	})
+	if diff := cmp.Diff(want, nm.Peers, nodePubComparer); diff != "" {
 		t.Errorf("filtered netmap differs (-want, +got):\n%s", diff)
 	}
 }
@@ -1130,3 +1175,85 @@ func TestTKARecoverCompromisedKeyFlow(t *testing.T) {
 		t.Errorf("NetworkLockSubmitRecoveryAUM() failed: %v", err)
 	}
 }
+
+func TestRotationTracker(t *testing.T) {
+	newNK := func(idx byte) key.NodePublic {
+		// single-byte public key to make it human-readable in tests.
+		raw32 := [32]byte{idx}
+		return key.NodePublicFromRaw32(go4mem.B(raw32[:]))
+	}
+	n1, n2, n3, n4, n5 := newNK(1), newNK(2), newNK(3), newNK(4), newNK(5)
+
+	pk1, pk2, pk3 := []byte{1}, []byte{2}, []byte{3}
+	type addDetails struct {
+		np      key.NodePublic
+		details *tka.RotationDetails
+	}
+	tests := []struct {
+		name       string
+		addDetails []addDetails
+		want       set.Set[key.NodePublic]
+	}{
+		{
+			name: "empty",
+			want: nil,
+		},
+		{
+			name: "single_prev_key",
+			addDetails: []addDetails{
+				{np: n1, details: &tka.RotationDetails{PrevNodeKeys: []key.NodePublic{n2}, WrappingPubkey: pk1}},
+			},
+			want: set.SetOf([]key.NodePublic{n2}),
+		},
+		{
+			name: "several_prev_keys",
+			addDetails: []addDetails{
+				{np: n1, details: &tka.RotationDetails{PrevNodeKeys: []key.NodePublic{n2}, WrappingPubkey: pk1}},
+				{np: n3, details: &tka.RotationDetails{PrevNodeKeys: []key.NodePublic{n4}, WrappingPubkey: pk2}},
+				{np: n2, details: &tka.RotationDetails{PrevNodeKeys: []key.NodePublic{n3, n4}, WrappingPubkey: pk1}},
+			},
+			want: set.SetOf([]key.NodePublic{n2, n3, n4}),
+		},
+		{
+			name: "several_per_pubkey_latest_wins",
+			addDetails: []addDetails{
+				{np: n2, details: &tka.RotationDetails{PrevNodeKeys: []key.NodePublic{n1}, WrappingPubkey: pk3}},
+				{np: n3, details: &tka.RotationDetails{PrevNodeKeys: []key.NodePublic{n1, n2}, WrappingPubkey: pk3}},
+				{np: n4, details: &tka.RotationDetails{PrevNodeKeys: []key.NodePublic{n1, n2, n3}, WrappingPubkey: pk3}},
+				{np: n5, details: &tka.RotationDetails{PrevNodeKeys: []key.NodePublic{n4}, WrappingPubkey: pk3}},
+			},
+			want: set.SetOf([]key.NodePublic{n1, n2, n3, n4}),
+		},
+		{
+			name: "several_per_pubkey_same_chain_length_all_rejected",
+			addDetails: []addDetails{
+				{np: n2, details: &tka.RotationDetails{PrevNodeKeys: []key.NodePublic{n1}, WrappingPubkey: pk3}},
+				{np: n3, details: &tka.RotationDetails{PrevNodeKeys: []key.NodePublic{n1, n2}, WrappingPubkey: pk3}},
+				{np: n4, details: &tka.RotationDetails{PrevNodeKeys: []key.NodePublic{n1, n2}, WrappingPubkey: pk3}},
+				{np: n5, details: &tka.RotationDetails{PrevNodeKeys: []key.NodePublic{n1, n2}, WrappingPubkey: pk3}},
+			},
+			want: set.SetOf([]key.NodePublic{n1, n2, n3, n4, n5}),
+		},
+		{
+			name: "several_per_pubkey_longest_wins",
+			addDetails: []addDetails{
+				{np: n2, details: &tka.RotationDetails{PrevNodeKeys: []key.NodePublic{n1}, WrappingPubkey: pk3}},
+				{np: n3, details: &tka.RotationDetails{PrevNodeKeys: []key.NodePublic{n1, n2}, WrappingPubkey: pk3}},
+				{np: n4, details: &tka.RotationDetails{PrevNodeKeys: []key.NodePublic{n1, n2}, WrappingPubkey: pk3}},
+				{np: n5, details: &tka.RotationDetails{PrevNodeKeys: []key.NodePublic{n1, n2, n3}, WrappingPubkey: pk3}},
+			},
+			want: set.SetOf([]key.NodePublic{n1, n2, n3, n4}),
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			r := &rotationTracker{logf: t.Logf}
+			for _, ad := range tt.addDetails {
+				r.addRotationDetails(ad.np, ad.details)
+			}
+			if got := r.obsoleteKeys(); !reflect.DeepEqual(got, tt.want) {
+				t.Errorf("rotationTracker.obsoleteKeys() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}

tka/sig.go

@@ -304,3 +304,78 @@ func (s *NodeKeySignature) verifySignature(nodeKey key.NodePublic, verificationK
 		return fmt.Errorf("unhandled signature type: %v", s.SigKind)
 	}
 }
+
+// RotationDetails holds additional information about a nodeKeySignature
+// of kind SigRotation.
+type RotationDetails struct {
+	// PrevNodeKeys is a list of node keys which have been rotated out.
+	PrevNodeKeys []key.NodePublic
+
+	// WrappingPubkey is the public key which has been authorized to sign
+	// this rotating signature.
+	WrappingPubkey []byte
+}
+
+// rotationDetails returns the RotationDetails for a SigRotation signature.
+func (s *NodeKeySignature) rotationDetails() (*RotationDetails, error) {
+	if s.SigKind != SigRotation {
+		return nil, nil
+	}
+
+	sri := &RotationDetails{}
+	nested := s.Nested
+	for nested != nil {
+		if len(nested.Pubkey) > 0 {
+			var nestedPub key.NodePublic
+			if err := nestedPub.UnmarshalBinary(nested.Pubkey); err != nil {
+				return nil, fmt.Errorf("nested pubkey: %v", err)
+			}
+			sri.PrevNodeKeys = append(sri.PrevNodeKeys, nestedPub)
+		}
+		if nested.SigKind != SigRotation {
+			break
+		}
+		nested = nested.Nested
+	}
+	sri.WrappingPubkey = nested.WrappingPubkey
+	return sri, nil
+}
+
+// ResignNKS re-signs a node-key signature for a new node-key.
+//
+// This only matters on network-locked tailnets, because node-key signatures are
+// how other nodes know that a node-key is authentic. When the node-key is
+// rotated then the existing signature becomes invalid, so this function is
+// responsible for generating a new wrapping signature to certify the new node-key.
+//
+// The signature itself is a SigRotation signature, which embeds the old signature
+// and certifies the new node-key as a replacement for the old by signing the new
+// signature with RotationPubkey (which is the node's own network-lock key).
+func ResignNKS(priv key.NLPrivate, nodeKey key.NodePublic, oldNKS tkatype.MarshaledSignature) (tkatype.MarshaledSignature, error) {
+	var oldSig NodeKeySignature
+	if err := oldSig.Unserialize(oldNKS); err != nil {
+		return nil, fmt.Errorf("decoding NKS: %w", err)
+	}
+
+	nk, err := nodeKey.MarshalBinary()
+	if err != nil {
+		return nil, fmt.Errorf("marshalling node-key: %w", err)
+	}
+
+	if bytes.Equal(nk, oldSig.Pubkey) {
+		// The old signature is valid for the node-key we are using, so just
+		// use it verbatim.
+		return oldNKS, nil
+	}
+
+	newSig := NodeKeySignature{
+		SigKind: SigRotation,
+		Pubkey:  nk,
+		Nested:  &oldSig,
+	}
+	if newSig.Signature, err = priv.SignNKS(newSig.SigHash()); err != nil {
+		return nil, fmt.Errorf("signing NKS: %w", err)
+	}
+
+	return newSig.Serialize(), nil
+}

tka/sig_test.go

@@ -5,6 +5,7 @@ package tka
 
 import (
 	"crypto/ed25519"
+	"reflect"
 	"testing"
 
 	"github.com/google/go-cmp/cmp"
@@ -298,3 +299,143 @@ func TestSigSerializeUnserialize(t *testing.T) {
 		t.Errorf("unmarshalled version differs (-want, +got):\n%s", diff)
 	}
 }
+
+func TestNodeKeySignatureRotationDetails(t *testing.T) {
+	// Trusted network lock key
+	pub, priv := testingKey25519(t, 1)
+	k := Key{Kind: Key25519, Public: pub, Votes: 2}
+
+	// 'credential' key (the one being delegated to)
+	cPub, cPriv := testingKey25519(t, 2)
+
+	n1, n2, n3 := key.NewNode(), key.NewNode(), key.NewNode()
+	n1pub, _ := n1.Public().MarshalBinary()
+	n2pub, _ := n2.Public().MarshalBinary()
+	n3pub, _ := n3.Public().MarshalBinary()
+
+	tests := []struct {
+		name    string
+		nodeKey key.NodePublic
+		sigFn   func() NodeKeySignature
+		want    *RotationDetails
+	}{
+		{
+			name:    "SigDirect",
+			nodeKey: n1.Public(),
+			sigFn: func() NodeKeySignature {
+				s := NodeKeySignature{
+					SigKind: SigDirect,
+					KeyID:   pub,
+					Pubkey:  n1pub,
+				}
+				sigHash := s.SigHash()
+				s.Signature = ed25519.Sign(priv, sigHash[:])
+				return s
+			},
+			want: nil,
+		},
+		{
+			name:    "SigWrappedCredential",
+			nodeKey: n1.Public(),
+			sigFn: func() NodeKeySignature {
+				nestedSig := NodeKeySignature{
+					SigKind:        SigCredential,
+					KeyID:          pub,
+					WrappingPubkey: cPub,
+				}
+				sigHash := nestedSig.SigHash()
+				nestedSig.Signature = ed25519.Sign(priv, sigHash[:])
+
+				sig := NodeKeySignature{
+					SigKind: SigRotation,
+					Pubkey:  n1pub,
+					Nested:  &nestedSig,
+				}
+				sigHash = sig.SigHash()
+				sig.Signature = ed25519.Sign(cPriv, sigHash[:])
+				return sig
+			},
+			want: &RotationDetails{
+				WrappingPubkey: cPub,
+			},
+		},
+		{
+			name:    "SigRotation",
+			nodeKey: n2.Public(),
+			sigFn: func() NodeKeySignature {
+				nestedSig := NodeKeySignature{
+					SigKind:        SigDirect,
+					Pubkey:         n1pub,
+					KeyID:          pub,
+					WrappingPubkey: cPub,
+				}
+				sigHash := nestedSig.SigHash()
+				nestedSig.Signature = ed25519.Sign(priv, sigHash[:])
+
+				sig := NodeKeySignature{
+					SigKind: SigRotation,
+					Pubkey:  n2pub,
+					Nested:  &nestedSig,
+				}
+				sigHash = sig.SigHash()
+				sig.Signature = ed25519.Sign(cPriv, sigHash[:])
+				return sig
+			},
+			want: &RotationDetails{
+				WrappingPubkey: cPub,
+				PrevNodeKeys:   []key.NodePublic{n1.Public()},
+			},
+		},
+		{
+			name:    "SigRotationNestedTwice",
+			nodeKey: n3.Public(),
+			sigFn: func() NodeKeySignature {
+				initialSig := NodeKeySignature{
+					SigKind:        SigDirect,
+					Pubkey:         n1pub,
+					KeyID:          pub,
+					WrappingPubkey: cPub,
+				}
+				sigHash := initialSig.SigHash()
+				initialSig.Signature = ed25519.Sign(priv, sigHash[:])
+
+				prevRotation := NodeKeySignature{
+					SigKind: SigRotation,
+					Pubkey:  n2pub,
+					Nested:  &initialSig,
+				}
+				sigHash = prevRotation.SigHash()
+				prevRotation.Signature = ed25519.Sign(cPriv, sigHash[:])
+
+				sig := NodeKeySignature{
+					SigKind: SigRotation,
+					Pubkey:  n3pub,
+					Nested:  &prevRotation,
+				}
+				sigHash = sig.SigHash()
+				sig.Signature = ed25519.Sign(cPriv, sigHash[:])
+
+				return sig
+			},
+			want: &RotationDetails{
+				WrappingPubkey: cPub,
+				PrevNodeKeys:   []key.NodePublic{n2.Public(), n1.Public()},
+			},
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			sig := tt.sigFn()
+			if err := sig.verifySignature(tt.nodeKey, k); err != nil {
+				t.Fatalf("verifySignature(node) failed: %v", err)
+			}
+			got, err := sig.rotationDetails()
+			if err != nil {
+				t.Fatal(err)
+			}
+			if !reflect.DeepEqual(got, tt.want) {
+				t.Errorf("rotationDetails() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}

tka/tka.go

@@ -668,25 +668,36 @@ func (a *Authority) Inform(storage Chonk, updates []AUM) error {
 // NodeKeyAuthorized checks if the provided nodeKeySignature authorizes
 // the given node key.
 func (a *Authority) NodeKeyAuthorized(nodeKey key.NodePublic, nodeKeySignature tkatype.MarshaledSignature) error {
+	_, err := a.NodeKeyAuthorizedWithDetails(nodeKey, nodeKeySignature)
+	return err
+}
+
+// NodeKeyAuthorized checks if the provided nodeKeySignature authorizes
+// the given node key, and returns RotationDetails if the signature is
+// a valid rotation signature.
+func (a *Authority) NodeKeyAuthorizedWithDetails(nodeKey key.NodePublic, nodeKeySignature tkatype.MarshaledSignature) (*RotationDetails, error) {
 	var decoded NodeKeySignature
 	if err := decoded.Unserialize(nodeKeySignature); err != nil {
-		return fmt.Errorf("unserialize: %v", err)
+		return nil, fmt.Errorf("unserialize: %v", err)
 	}
 	if decoded.SigKind == SigCredential {
-		return errors.New("credential signatures cannot authorize nodes on their own")
+		return nil, errors.New("credential signatures cannot authorize nodes on their own")
 	}
 
 	kID, err := decoded.authorizingKeyID()
 	if err != nil {
-		return err
+		return nil, err
 	}
 
 	key, err := a.state.GetKey(kID)
 	if err != nil {
-		return fmt.Errorf("key: %v", err)
+		return nil, fmt.Errorf("key: %v", err)
 	}
 
-	return decoded.verifySignature(nodeKey, key)
+	if err := decoded.verifySignature(nodeKey, key); err != nil {
+		return nil, err
+	}
+	return decoded.rotationDetails()
 }
 
 // KeyTrusted returns true if the given keyID is trusted by the tailnet