clientupdate/distsign: add new library for package signing/verification (#8943)

This library is intended for use during release to sign packages which
are then served from pkgs.tailscale.com.
The library is also then used by clients downloading packages for
`tailscale update` where OS package managers / app stores aren't used.

Updates https://github.com/tailscale/tailscale/issues/8760
Updates https://github.com/tailscale/tailscale/issues/6995

Signed-off-by: Andrew Lytvynov <awly@tailscale.com>

clientupdate/distsign/distsign_test.go

@@ -0,0 +1,347 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package distsign
+
+import (
+	"bytes"
+	"crypto/ed25519"
+	"net/http"
+	"net/http/httptest"
+	"net/url"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"golang.org/x/crypto/blake2s"
+)
+
+func TestDownload(t *testing.T) {
+	srv := newTestServer(t)
+	c := srv.client(t)
+
+	tests := []struct {
+		desc    string
+		before  func(*testing.T)
+		src     string
+		want    []byte
+		wantErr bool
+	}{
+		{
+			desc:    "missing file",
+			before:  func(*testing.T) {},
+			src:     "hello",
+			wantErr: true,
+		},
+		{
+			desc: "success",
+			before: func(*testing.T) {
+				srv.addSigned("hello", []byte("world"))
+			},
+			src:  "hello",
+			want: []byte("world"),
+		},
+		{
+			desc: "no signature",
+			before: func(*testing.T) {
+				srv.add("hello", []byte("world"))
+			},
+			src:     "hello",
+			wantErr: true,
+		},
+		{
+			desc: "bad signature",
+			before: func(*testing.T) {
+				srv.add("hello", []byte("world"))
+				srv.add("hello.sig", []byte("potato"))
+			},
+			src:     "hello",
+			wantErr: true,
+		},
+		{
+			desc: "signed with untrusted key",
+			before: func(t *testing.T) {
+				srv.add("hello", []byte("world"))
+				srv.add("hello.sig", newSigningKeyPair(t).sign([]byte("world")))
+			},
+			src:     "hello",
+			wantErr: true,
+		},
+		{
+			desc: "signed with root key",
+			before: func(t *testing.T) {
+				srv.add("hello", []byte("world"))
+				srv.add("hello.sig", srv.roots[0].sign([]byte("world")))
+			},
+			src:     "hello",
+			wantErr: true,
+		},
+		{
+			desc: "bad signing key signature",
+			before: func(t *testing.T) {
+				srv.add("distsign.pub.sig", []byte("potato"))
+				srv.addSigned("hello", []byte("world"))
+			},
+			src:     "hello",
+			wantErr: true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.desc, func(t *testing.T) {
+			srv.reset()
+			tt.before(t)
+
+			dst := filepath.Join(t.TempDir(), tt.src)
+			t.Cleanup(func() {
+				os.Remove(dst)
+			})
+			err := c.Download(tt.src, dst)
+			if err != nil {
+				if tt.wantErr {
+					return
+				}
+				t.Fatalf("unexpected error from Download(%q): %v", tt.src, err)
+			}
+			if tt.wantErr {
+				t.Fatalf("Download(%q) succeeded, expected an error", tt.src)
+			}
+			got, err := os.ReadFile(dst)
+			if err != nil {
+				t.Fatal(err)
+			}
+			if !bytes.Equal(tt.want, got) {
+				t.Errorf("Download(%q): got %q, want %q", tt.src, got, tt.want)
+			}
+		})
+	}
+}
+
+func TestRotateRoot(t *testing.T) {
+	srv := newTestServer(t)
+	c1 := srv.client(t)
+
+	srv.addSigned("hello", []byte("world"))
+	if err := c1.Download("hello", filepath.Join(t.TempDir(), "hello")); err != nil {
+		t.Fatalf("Download failed on a fresh server: %v", err)
+	}
+
+	// Remove first root and replace it with a new key.
+	srv.roots = append(srv.roots[1:], newRootKeyPair(t))
+
+	// Old client can still download files because it still trusts the old
+	// root key.
+	if err := c1.Download("hello", filepath.Join(t.TempDir(), "hello")); err != nil {
+		t.Fatalf("Download failed after root rotation on old client: %v", err)
+	}
+	// New client should fail download because current signing key is signed by
+	// the revoked root that new client doesn't trust.
+	c2 := srv.client(t)
+	if err := c2.Download("hello", filepath.Join(t.TempDir(), "hello")); err == nil {
+		t.Fatalf("Download succeeded on new client, but signing key is signed with revoked root key")
+	}
+	// Re-sign signing key with another valid root that client still trusts.
+	srv.resignSigningKeys()
+	// Both old and new clients should now be able to download.
+	//
+	// Note: we don't need to re-sign the "hello" file because signing key
+	// didn't change (only signing key's signature).
+	if err := c1.Download("hello", filepath.Join(t.TempDir(), "hello")); err != nil {
+		t.Fatalf("Download failed after root rotation on old client with re-signed signing key: %v", err)
+	}
+	if err := c2.Download("hello", filepath.Join(t.TempDir(), "hello")); err != nil {
+		t.Fatalf("Download failed after root rotation on new client with re-signed signing key: %v", err)
+	}
+}
+
+func TestRotateSigning(t *testing.T) {
+	srv := newTestServer(t)
+	c := srv.client(t)
+
+	srv.addSigned("hello", []byte("world"))
+	if err := c.Download("hello", filepath.Join(t.TempDir(), "hello")); err != nil {
+		t.Fatalf("Download failed on a fresh server: %v", err)
+	}
+
+	// Replace signing key but don't publish it yet.
+	srv.sign = append(srv.sign, newSigningKeyPair(t))
+	if err := c.Download("hello", filepath.Join(t.TempDir(), "hello")); err != nil {
+		t.Fatalf("Download failed after new signing key added but before publishing it: %v", err)
+	}
+
+	// Publish new signing key bundle with both keys.
+	srv.resignSigningKeys()
+	if err := c.Download("hello", filepath.Join(t.TempDir(), "hello")); err != nil {
+		t.Fatalf("Download failed after new signing key was published: %v", err)
+	}
+
+	// Re-sign the "hello" file with new signing key.
+	srv.add("hello.sig", srv.sign[1].sign([]byte("world")))
+	if err := c.Download("hello", filepath.Join(t.TempDir(), "hello")); err != nil {
+		t.Fatalf("Download failed after re-signing with new signing key: %v", err)
+	}
+
+	// Drop the old signing key.
+	srv.sign = srv.sign[1:]
+	srv.resignSigningKeys()
+	if err := c.Download("hello", filepath.Join(t.TempDir(), "hello")); err != nil {
+		t.Fatalf("Download failed after removing old signing key: %v", err)
+	}
+
+	// Add another key and re-sign the file with it *before* publishing.
+	srv.sign = append(srv.sign, newSigningKeyPair(t))
+	srv.add("hello.sig", srv.sign[1].sign([]byte("world")))
+	if err := c.Download("hello", filepath.Join(t.TempDir(), "hello")); err == nil {
+		t.Fatalf("Download succeeded when signed with a not-yet-published signing key")
+	}
+	// Fix this by publishing the new key.
+	srv.resignSigningKeys()
+	if err := c.Download("hello", filepath.Join(t.TempDir(), "hello")); err != nil {
+		t.Fatalf("Download failed after publishing new signing key: %v", err)
+	}
+}
+
+type testServer struct {
+	roots []rootKeyPair
+	sign  []signingKeyPair
+	files map[string][]byte
+	srv   *httptest.Server
+}
+
+func newTestServer(t *testing.T) *testServer {
+	var roots []rootKeyPair
+	for i := 0; i < 3; i++ {
+		roots = append(roots, newRootKeyPair(t))
+	}
+
+	ts := &testServer{
+		roots: roots,
+		sign:  []signingKeyPair{newSigningKeyPair(t)},
+	}
+	ts.reset()
+	ts.srv = httptest.NewServer(ts)
+	t.Cleanup(ts.srv.Close)
+	return ts
+}
+
+func (s *testServer) client(t *testing.T) *Client {
+	roots := make([]ed25519.PublicKey, 0, len(s.roots))
+	for _, r := range s.roots {
+		pub, err := parseSinglePublicKey(r.pubRaw)
+		if err != nil {
+			t.Fatalf("parsePublicKey: %v", err)
+		}
+		roots = append(roots, pub)
+	}
+	u, err := url.Parse(s.srv.URL)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return &Client{
+		roots:    roots,
+		pkgsAddr: u,
+	}
+}
+
+func (s *testServer) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	path := strings.TrimPrefix(r.URL.Path, "/")
+	data, ok := s.files[path]
+	if !ok {
+		http.NotFound(w, r)
+		return
+	}
+	w.Write(data)
+}
+
+func (s *testServer) addSigned(name string, data []byte) {
+	s.files[name] = data
+	s.files[name+".sig"] = s.sign[0].sign(data)
+}
+
+func (s *testServer) add(name string, data []byte) {
+	s.files[name] = data
+}
+
+func (s *testServer) reset() {
+	s.files = make(map[string][]byte)
+	s.resignSigningKeys()
+}
+
+func (s *testServer) resignSigningKeys() {
+	var pubs [][]byte
+	for _, k := range s.sign {
+		pubs = append(pubs, k.pubRaw)
+	}
+	bundle := bytes.Join(pubs, []byte("\n"))
+	sig := s.roots[0].sign(bundle)
+	s.files["distsign.pub"] = bundle
+	s.files["distsign.pub.sig"] = sig
+}
+
+type rootKeyPair struct {
+	*RootKey
+	keyPair
+}
+
+func newRootKeyPair(t *testing.T) rootKeyPair {
+	kp := newKeyPair(t)
+	priv, err := parsePrivateKey(kp.privRaw)
+	if err != nil {
+		t.Fatalf("parsePrivateKey: %v", err)
+	}
+	return rootKeyPair{
+		RootKey: &RootKey{Signer: priv},
+		keyPair: kp,
+	}
+}
+
+func (s rootKeyPair) sign(bundle []byte) []byte {
+	sig, err := s.SignSigningKeys(bundle)
+	if err != nil {
+		panic(err)
+	}
+	return sig
+}
+
+type signingKeyPair struct {
+	*SigningKey
+	keyPair
+}
+
+func newSigningKeyPair(t *testing.T) signingKeyPair {
+	kp := newKeyPair(t)
+	priv, err := parsePrivateKey(kp.privRaw)
+	if err != nil {
+		t.Fatalf("parsePrivateKey: %v", err)
+	}
+	return signingKeyPair{
+		SigningKey: &SigningKey{Signer: priv},
+		keyPair:    kp,
+	}
+}
+
+func (s signingKeyPair) sign(blob []byte) []byte {
+	hash := blake2s.Sum256(blob)
+	sig, err := s.SignPackageHash(hash[:], int64(len(blob)))
+	if err != nil {
+		panic(err)
+	}
+	return sig
+}
+
+type keyPair struct {
+	privRaw []byte
+	pubRaw  []byte
+}
+
+func newKeyPair(t *testing.T) keyPair {
+	privRaw, pubRaw, err := GenerateKey()
+	if err != nil {
+		t.Fatalf("GenerateKey: %v", err)
+	}
+	return keyPair{
+		privRaw: privRaw,
+		pubRaw:  pubRaw,
+	}
+}