tka,types/key: implement NLPrivate glue for tailnet key authority keys

Signed-off-by: Tom DNetto <tom@tailscale.com>

tka/builder.go

@@ -8,6 +8,11 @@ import (
 	"fmt"
 )
 
+// Types implementing Signer can sign update messages.
+type Signer interface {
+	SignAUM(*AUM) error
+}
+
 // UpdateBuilder implements a builder for changes to the tailnet
 // key authority.
 //
@@ -15,7 +20,7 @@ import (
 // must then be applied to all Authority objects using Inform().
 type UpdateBuilder struct {
 	a      *Authority
-	signer func(*AUM) error
+	signer Signer
 
 	state  State
 	parent AUMHash
@@ -29,7 +34,7 @@ func (b *UpdateBuilder) mkUpdate(update AUM) error {
 	update.PrevAUMHash = prevHash
 
 	if b.signer != nil {
-		if err := b.signer(&update); err != nil {
+		if err := b.signer.SignAUM(&update); err != nil {
 			return fmt.Errorf("signing failed: %v", err)
 		}
 	}
@@ -101,7 +106,7 @@ func (b *UpdateBuilder) Finalize() ([]AUM, error) {
 // Updates are specified by calling methods on the returned UpdatedBuilder.
 // Call Finalize() when you are done to obtain the specific update messages
 // which actuate the changes.
-func (a *Authority) NewUpdater(signer func(*AUM) error) *UpdateBuilder {
+func (a *Authority) NewUpdater(signer Signer) *UpdateBuilder {
 	return &UpdateBuilder{
 		a:      a,
 		signer: signer,

tka/builder_test.go

@@ -5,11 +5,19 @@
 package tka
 
 import (
+	"crypto/ed25519"
 	"testing"
 
 	"github.com/google/go-cmp/cmp"
 )
 
+type signer25519 ed25519.PrivateKey
+
+func (s signer25519) SignAUM(update *AUM) error {
+	update.sign25519(ed25519.PrivateKey(s))
+	return nil
+}
+
 func TestAuthorityBuilderAddKey(t *testing.T) {
 	pub, priv := testingKey25519(t, 1)
 	key := Key{Kind: Key25519, Public: pub, Votes: 2}
@@ -17,7 +25,7 @@ func TestAuthorityBuilderAddKey(t *testing.T) {
 	a, _, err := Create(&Mem{}, State{
 		Keys:               []Key{key},
 		DisablementSecrets: [][]byte{disablementKDF([]byte{1, 2, 3})},
-	}, priv)
+	}, signer25519(priv))
 	if err != nil {
 		t.Fatalf("Create() failed: %v", err)
 	}
@@ -25,10 +33,7 @@ func TestAuthorityBuilderAddKey(t *testing.T) {
 	pub2, _ := testingKey25519(t, 2)
 	key2 := Key{Kind: Key25519, Public: pub2, Votes: 1}
 
-	b := a.NewUpdater(func(update *AUM) error {
-		update.sign25519(priv)
-		return nil
-	})
+	b := a.NewUpdater(signer25519(priv))
 	if err := b.AddKey(key2); err != nil {
 		t.Fatalf("AddKey(%v) failed: %v", key2, err)
 	}
@@ -56,15 +61,12 @@ func TestAuthorityBuilderRemoveKey(t *testing.T) {
 	a, _, err := Create(&Mem{}, State{
 		Keys:               []Key{key, key2},
 		DisablementSecrets: [][]byte{disablementKDF([]byte{1, 2, 3})},
-	}, priv)
+	}, signer25519(priv))
 	if err != nil {
 		t.Fatalf("Create() failed: %v", err)
 	}
 
-	b := a.NewUpdater(func(update *AUM) error {
-		update.sign25519(priv)
-		return nil
-	})
+	b := a.NewUpdater(signer25519(priv))
 	if err := b.RemoveKey(key2.ID()); err != nil {
 		t.Fatalf("RemoveKey(%v) failed: %v", key2, err)
 	}
@@ -90,15 +92,12 @@ func TestAuthorityBuilderSetKeyVote(t *testing.T) {
 	a, _, err := Create(&Mem{}, State{
 		Keys:               []Key{key},
 		DisablementSecrets: [][]byte{disablementKDF([]byte{1, 2, 3})},
-	}, priv)
+	}, signer25519(priv))
 	if err != nil {
 		t.Fatalf("Create() failed: %v", err)
 	}
 
-	b := a.NewUpdater(func(update *AUM) error {
-		update.sign25519(priv)
-		return nil
-	})
+	b := a.NewUpdater(signer25519(priv))
 	if err := b.SetKeyVote(key.ID(), 5); err != nil {
 		t.Fatalf("SetKeyVote(%v) failed: %v", key.ID(), err)
 	}
@@ -128,15 +127,12 @@ func TestAuthorityBuilderSetKeyMeta(t *testing.T) {
 	a, _, err := Create(&Mem{}, State{
 		Keys:               []Key{key},
 		DisablementSecrets: [][]byte{disablementKDF([]byte{1, 2, 3})},
-	}, priv)
+	}, signer25519(priv))
 	if err != nil {
 		t.Fatalf("Create() failed: %v", err)
 	}
 
-	b := a.NewUpdater(func(update *AUM) error {
-		update.sign25519(priv)
-		return nil
-	})
+	b := a.NewUpdater(signer25519(priv))
 	if err := b.SetKeyMeta(key.ID(), map[string]string{"b": "c"}); err != nil {
 		t.Fatalf("SetKeyMeta(%v) failed: %v", key, err)
 	}
@@ -166,7 +162,7 @@ func TestAuthorityBuilderMultiple(t *testing.T) {
 	a, _, err := Create(&Mem{}, State{
 		Keys:               []Key{key},
 		DisablementSecrets: [][]byte{disablementKDF([]byte{1, 2, 3})},
-	}, priv)
+	}, signer25519(priv))
 	if err != nil {
 		t.Fatalf("Create() failed: %v", err)
 	}
@@ -174,10 +170,7 @@ func TestAuthorityBuilderMultiple(t *testing.T) {
 	pub2, _ := testingKey25519(t, 2)
 	key2 := Key{Kind: Key25519, Public: pub2, Votes: 1}
 
-	b := a.NewUpdater(func(update *AUM) error {
-		update.sign25519(priv)
-		return nil
-	})
+	b := a.NewUpdater(signer25519(priv))
 	if err := b.AddKey(key2); err != nil {
 		t.Fatalf("AddKey(%v) failed: %v", key2, err)
 	}

tka/key.go

@@ -53,9 +53,8 @@ type Key struct {
 
 // Clone makes an independent copy of Key.
 //
-// NOTE: There is a difference between a nil slice and an empty
-//       slice for encoding purposes, so an implementation of Clone()
-//       must take care to preserve this.
+// NOTE: There is a difference between a nil slice and an empty slice for encoding purposes,
+// so an implementation of Clone() must take care to preserve this.
 func (k Key) Clone() Key {
 	out := k
 

tka/tka.go

@@ -7,7 +7,6 @@ package tka
 
 import (
 	"bytes"
-	"crypto/ed25519"
 	"errors"
 	"fmt"
 	"os"
@@ -99,6 +98,7 @@ func computeChainCandidates(storage Chonk, lastKnownOldest *AUMHash, maxIter int
 // AUM in the chain, possibly applying fork resolution logic.
 //
 // In other words: given an AUM with 3 children like this:
+//
 //	  / - 1
 //	P   - 2
 //	  \ - 3
@@ -473,7 +473,7 @@ func Open(storage Chonk) (*Authority, error) {
 //
 // Do not use this to initialize a TKA that already exists, use Open()
 // or Bootstrap() instead.
-func Create(storage Chonk, state State, signer ed25519.PrivateKey) (*Authority, AUM, error) {
+func Create(storage Chonk, state State, signer Signer) (*Authority, AUM, error) {
 	// Generate & sign a checkpoint, our genesis update.
 	genesis := AUM{
 		MessageKind: AUMCheckpoint,
@@ -483,7 +483,9 @@ func Create(storage Chonk, state State, signer ed25519.PrivateKey) (*Authority,
 		// This serves as an easy way to validate the given state.
 		return nil, AUM{}, fmt.Errorf("invalid state: %v", err)
 	}
-	genesis.sign25519(signer)
+	if err := signer.SignAUM(&genesis); err != nil {
+		return nil, AUM{}, fmt.Errorf("signing failed: %v", err)
+	}
 
 	a, err := Bootstrap(storage, genesis)
 	return a, genesis, err

tka/tka_test.go

@@ -301,7 +301,7 @@ func TestCreateBootstrapAuthority(t *testing.T) {
 	a1, genesisAUM, err := Create(&Mem{}, State{
 		Keys:               []Key{key},
 		DisablementSecrets: [][]byte{disablementKDF([]byte{1, 2, 3})},
-	}, priv)
+	}, signer25519(priv))
 	if err != nil {
 		t.Fatalf("Create() failed: %v", err)
 	}

types/key/nl.go

@@ -0,0 +1,74 @@
+// Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package key
+
+import (
+	"crypto/ed25519"
+
+	"go4.org/mem"
+	"tailscale.com/tka"
+	"tailscale.com/types/structs"
+)
+
+const (
+	// nlPrivateHexPrefix is the prefix used to identify a
+	// hex-encoded network-lock key.
+	nlPrivateHexPrefix = "nlpriv:"
+)
+
+// NLPrivate is a node-managed network-lock key, used for signing
+// node-key signatures and authority update messages.
+type NLPrivate struct {
+	_ structs.Incomparable // because == isn't constant-time
+	k [ed25519.PrivateKeySize]byte
+}
+
+// NewNLPrivate creates and returns a new network-lock key.
+func NewNLPrivate() NLPrivate {
+	// ed25519.GenerateKey 'clamps' the key, not that it
+	// matters given we don't do Diffie-Hellman.
+	_, priv, err := ed25519.GenerateKey(nil) // nil == crypto/rand
+	if err != nil {
+		panic(err)
+	}
+
+	var out NLPrivate
+	copy(out.k[:], priv)
+	return out
+}
+
+// MarshalText implements encoding.TextUnmarshaler.
+func (k *NLPrivate) UnmarshalText(b []byte) error {
+	return parseHex(k.k[:], mem.B(b), mem.S(nlPrivateHexPrefix))
+}
+
+// MarshalText implements encoding.TextMarshaler.
+func (k NLPrivate) MarshalText() ([]byte, error) {
+	return toHex(k.k[:], nlPrivateHexPrefix), nil
+}
+
+// Public returns the public component of this key.
+func (k NLPrivate) Public() ed25519.PublicKey {
+	return ed25519.PrivateKey(k.k[:]).Public().(ed25519.PublicKey)
+}
+
+// KeyID returns an identifier for this key.
+func (k NLPrivate) KeyID() tka.KeyID {
+	return tka.Key{
+		Kind:   tka.Key25519,
+		Public: k.Public(),
+	}.ID()
+}
+
+// SignAUM implements tka.UpdateSigner.
+func (k NLPrivate) SignAUM(a *tka.AUM) error {
+	sigHash := a.SigHash()
+
+	a.Signatures = append(a.Signatures, tka.Signature{
+		KeyID:     k.KeyID(),
+		Signature: ed25519.Sign(k.k[:], sigHash[:]),
+	})
+	return nil
+}

types/key/nl_test.go

@@ -0,0 +1,47 @@
+// Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package key
+
+import (
+	"bytes"
+	"testing"
+
+	"tailscale.com/tka"
+)
+
+func TestNLPrivate(t *testing.T) {
+	p := NewNLPrivate()
+
+	encoded, err := p.MarshalText()
+	if err != nil {
+		t.Fatal(err)
+	}
+	var decoded NLPrivate
+	if err := decoded.UnmarshalText(encoded); err != nil {
+		t.Fatal(err)
+	}
+	if !bytes.Equal(decoded.k[:], p.k[:]) {
+		t.Error("decoded and generated NLPrivate bytes differ")
+	}
+
+	// Test that NLPrivate implements tka.Signer by making a new
+	// authority.
+	k := tka.Key{Kind: tka.Key25519, Public: p.Public(), Votes: 1}
+	_, aum, err := tka.Create(&tka.Mem{}, tka.State{
+		Keys:               []tka.Key{k},
+		DisablementSecrets: [][]byte{bytes.Repeat([]byte{1}, 32)},
+	}, p)
+	if err != nil {
+		t.Fatalf("tka.Create() failed: %v", err)
+	}
+
+	// Make sure the generated genesis AUM was signed.
+	if got, want := len(aum.Signatures), 1; got != want {
+		t.Fatalf("len(signatures) = %d, want %d", got, want)
+	}
+	if err := aum.Signatures[0].Verify(aum.SigHash(), k); err != nil {
+		t.Errorf("signature did not verify: %v", err)
+	}
+}