Believe a hand-over only signed by the mesh, and give it through no link (hq issue 356, review)

Two holes the review proved. The ask's subject proved nothing — a responder can reach it through a reply — so the engine now verifies the mesh's signature over a fixed context with the key it verifies declarations with, before reading anything, and refuses an ask for another node, expired, good too long, without a nonce, or replayed. And the apply followed a link the hand-over had refused: a component swapped for a link after the record carried root's chmod and chown to its target. A handed-over directory must now still resolve to itself, and is changed only through one descriptor opened from / with no link followed (fchmod, fchown). Tests for the reply guard, the run loop's hook, the fallback renewal on proof and a non-regular fallback; 27 checks each removed in turn, each failing a test.

cmd/mesh-host/found_test.go

@@ -143,24 +143,25 @@ func TestAHandOverIsRefusedForEveryPathThatIsNotTheFoundDirectory(t *testing.T)
 	}}); err != nil {
 		t.Fatal(err)
 	}
-	refused := map[string]string{
-		"a relative path":               "found",
-		"a path with ..":                dir + "/x/../found",
-		"a doubled separator":           dir + "//found",
-		"a trailing separator":          found + "/",
-		"a directory not used as found": filepath.Join(dir, "other"),
-		"a link at the path":            linked,
-		"a link in the path":            through,
-		"a file":                        file,
-		"a directory that is gone":      gone,
+	// Each refused by its own check, which the reason names.
+	refused := map[string][2]string{
+		"a relative path":               {"found", "not one"},
+		"a path with ..":                {dir + "/x/../found", "as the engine states it"},
+		"a doubled separator":           {dir + "//found", "as the engine states it"},
+		"a trailing separator":          {found + "/", "as the engine states it"},
+		"a directory not used as found": {filepath.Join(dir, "other"), "not a directory this machine uses as found"},
+		"a link at the path":            {linked, "is a link"},
+		"a link in the path":            {through, "reached through a link"},
+		"a file":                        {file, "is not a directory on this machine"},
+		"a directory that is gone":      {gone, "cannot be read on this machine"},
 	}
-	for name, path := range refused {
-		said, err := handOverFound(statePath, path, "jo")
+	for name, c := range refused {
+		said, err := handOverFound(statePath, c[0], "jo")
 		if err == nil || said != "" {
-			t.Fatalf("%s (%s) was handed over: %q", name, path, said)
+			t.Fatalf("%s (%s) was handed over: %q", name, c[0], said)
 		}
-		if !strings.Contains(err.Error(), "nothing was handed over") {
-			t.Fatalf("%s's refusal does not say nothing was handed over: %v", name, err)
+		if !strings.Contains(err.Error(), "nothing was handed over") || !strings.Contains(err.Error(), c[1]) {
+			t.Fatalf("%s's refusal does not say %q and that nothing was handed over: %v", name, c[1], err)
 		}
 	}
 	if all, err := store.LoadHandedOver(statePath); err != nil || len(all) != 0 {

cmd/mesh-host/witnessing_test.go

@@ -5,6 +5,7 @@ import (
 	"os/exec"
 	"path/filepath"
 	"strconv"
+	"strings"
 	"testing"
 	"time"
 
@@ -17,6 +18,12 @@ import (
 func TestOnlyAnAccountUnderThisBuildProvesIt(t *testing.T) {
 	state := filepath.Join(t.TempDir(), "state.json")
 	t.Setenv("MESH_HOST_LIBEXEC", t.TempDir())
+	// Never the machine's own engine placed by hand: proving renews it (issue 356).
+	fallback := filepath.Join(t.TempDir(), "nox-mesh-host")
+	if err := os.WriteFile(fallback, []byte("a stale engine placed by hand"), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	t.Setenv(upgrade.FallbackVar, fallback)
 	var said []string
 	p := &proving{statePath: state, version: "2.0", say: func(s string) { said = append(said, s) }}
 
@@ -29,6 +36,9 @@ func TestOnlyAnAccountUnderThisBuildProvesIt(t *testing.T) {
 		if !unproved(state, "2.0") {
 			t.Fatalf("%+v proved this build", r)
 		}
+		if got, _ := os.ReadFile(fallback); string(got) != "a stale engine placed by hand" {
+			t.Fatalf("%+v renewed the engine placed by hand", r)
+		}
 	}
 	p.heard(link.Report{Host: "2.0", Declared: "abc"})
 	if unproved(state, "2.0") {
@@ -37,6 +47,23 @@ func TestOnlyAnAccountUnderThisBuildProvesIt(t *testing.T) {
 	if got, _ := upgrade.ReadKnownGood(upgrade.KnownGoodPath(state)); got != "2.0" {
 		t.Fatalf("known-good is %q", got)
 	}
+	// **Proved, the engine placed by hand becomes this build** (issue 356): the command on the machine's PATH is
+	// the engine that runs it, and that is said.
+	self, err := os.Executable()
+	if err != nil {
+		t.Fatal(err)
+	}
+	want, _ := os.ReadFile(self)
+	if got, _ := os.ReadFile(fallback); len(want) == 0 || string(got) != string(want) {
+		t.Fatalf("the engine placed by hand was not renewed to this build (%d bytes, want %d)", len(got), len(want))
+	}
+	renewed := false
+	for _, s := range said {
+		renewed = renewed || strings.Contains(s, "renewed the engine placed by hand at "+fallback)
+	}
+	if !renewed {
+		t.Fatalf("the renewal was not said: %q", said)
+	}
 }
 
 // Every verdict that stands is said, and a reconcile that has one says it unasked.

internal/apply/apply.go

@@ -1047,10 +1047,10 @@ func applyDirectory(r *declaration.Directory, previous store.Applied, wasBefore
 	}
 	if existed && wasBefore {
 		ours := previous.Target != "" && filepath.Clean(previous.Target) == filepath.Clean(r.Path) && !previous.AsFound
-		// Or handed to the mesh by the operator (`nox node hand-over`, or `hand-over` at the machine): then it is given the
-		// declared owner and mode below, as any directory of the mesh's.
+		// Or handed to the mesh by the operator (`nox node hand-over`, or `hand-over` at the machine): then it is
+		// given the declared owner and mode — through a descriptor that followed no link (giveHandedOver).
 		if !ours && HandedOver()[handOverKey(r.ID, r.Path)] {
-			ours = true
+			return applyHandedOver(r, out, mode)
 		}
 		if !ours {
 			owned, err := ownedBy(r.Path, r.Owner)
@@ -1122,6 +1122,45 @@ func applyDirectory(r *declaration.Directory, previous store.Applied, wasBefore
 	return out, nil
 }
 
+// applyHandedOver gives a directory the operator handed to the mesh its declared mode and owner (novox/hq issue
+// 356). **No link, checked again at the moment of use**: the hand-over refused a link when it was recorded, and a
+// component of the path may have become one since — a parent swapped for a link to /etc would carry root's chmod
+// and chown there. So the path must still resolve to itself, and the change is made through one descriptor opened
+// from / with no link followed on any component (giveHandedOver); a link anywhere is refused and nothing changes.
+func applyHandedOver(r *declaration.Directory, out Outcome, mode os.FileMode) (Outcome, error) {
+	clean := filepath.Clean(r.Path)
+	resolved, err := filepath.EvalSymlinks(clean)
+	if err != nil {
+		return out, err
+	}
+	if resolved != clean {
+		return out, &PlacementRefusedError{Path: r.Path, Why: fmt.Sprintf("it was handed to the mesh, and is now "+
+			"reached through a link (it is %s): the node-engine follows no link to a directory it gives an owner", resolved)}
+	}
+	uid, gid, chown := 0, 0, r.Owner != ""
+	if chown {
+		if uid, gid, err = idsOf(r.Owner); err != nil {
+			return out, fmt.Errorf("%s should belong to %q: %w", r.Path, r.Owner, err)
+		}
+	}
+	before, beforeUID, beforeGID, err := giveHandedOver(clean, mode, uid, gid, chown)
+	if err != nil {
+		var link *LinkUnderHomeError
+		if errors.As(err, &link) {
+			return out, &PlacementRefusedError{Path: r.Path, Why: fmt.Sprintf("it was handed to the mesh, and %s is "+
+				"now a link: the node-engine follows no link to a directory it gives an owner", link.Link)}
+		}
+		return out, err
+	}
+	out.Action = "unchanged"
+	if before.Perm() != mode.Perm() || (chown && (beforeUID != uid || beforeGID != gid)) {
+		out.Action = "updated"
+		out.Detail = fmt.Sprintf("handed to the mesh: owner %d:%d, mode %o, given what was declared", beforeUID, beforeGID,
+			before.Perm())
+	}
+	return out, nil
+}
+
 // applyAccess confirms an operator-owned path is present, and owns nothing about it.
 //
 // **The mirror image of applyDirectory** (novox/hq ADR 0051). A directory the host makes, chmods,

internal/apply/found_is_said_test.go

@@ -136,3 +136,101 @@ func TestAHandOverIsForTheResourceThatAskedForIt(t *testing.T) {
 		t.Fatalf("a hand-over for notes.data gave other.data the directory: mode %o", info.Mode().Perm())
 	}
 }
+
+// **A handed-over directory is changed through no link, at the moment of use** (review of issue 356). The
+// hand-over refused a link when it was recorded; between then and the apply its parent is swapped for a link to
+// a victim tree. The apply refuses, and the victim's directory keeps its mode — and so does a directory whose own
+// name became a link, and one whose grandparent did.
+func TestAHandedOverDirectoryIsChangedThroughNoLink(t *testing.T) {
+	for _, swap := range []string{"parent", "grandparent", "itself"} {
+		t.Run(swap, func(t *testing.T) {
+			dir, err := filepath.EvalSymlinks(t.TempDir())
+			if err != nil {
+				t.Fatal(err)
+			}
+			found := filepath.Join(dir, "top", "parent", "data")
+			if err := os.MkdirAll(found, 0o700); err != nil {
+				t.Fatal(err)
+			}
+			decl := `{"id":"notes.data","type":"directory","path":"` + found + `","mode":"0755"}`
+			_, state, err := Apply(context.Background(), archHost(t), declare(t, decl), store.State{}, store.OriginDeclared,
+				noServices, nil, nil)
+			if err != nil {
+				t.Fatal(err)
+			}
+			if a, _ := state.Find("notes.data"); !a.AsFound {
+				t.Fatalf("not used as found: %+v", a)
+			}
+			before := HandedOver
+			HandedOver = func() map[string]bool { return map[string]bool{handOverKey("notes.data", found): true} }
+			t.Cleanup(func() { HandedOver = before })
+
+			// The victim: a tree of the same shape somewhere else, its data directory mode 0700.
+			victim := filepath.Join(dir, "victim", "top", "parent", "data")
+			if err := os.MkdirAll(victim, 0o700); err != nil {
+				t.Fatal(err)
+			}
+			var swapped, target string
+			switch swap {
+			case "parent":
+				swapped, target = filepath.Join(dir, "top", "parent"), filepath.Join(dir, "victim", "top", "parent")
+			case "grandparent":
+				swapped, target = filepath.Join(dir, "top"), filepath.Join(dir, "victim", "top")
+			case "itself":
+				swapped, target = found, victim
+			}
+			if err := os.Rename(swapped, swapped+".real"); err != nil {
+				t.Fatal(err)
+			}
+			if err := os.Symlink(target, swapped); err != nil {
+				t.Fatal(err)
+			}
+			_, _, err = Apply(context.Background(), archHost(t), declare(t, decl), state, store.OriginDeclared,
+				noServices, nil, nil)
+			if err == nil {
+				t.Fatal("a handed-over directory reached through a link was applied")
+			}
+			// Refused by the path's re-check before any descriptor is opened (a link at the name itself is refused
+			// earlier still, by the directory applier's own look at it).
+			want := "reached through a link"
+			if swap == "itself" {
+				want = "symbolic link"
+			}
+			if !strings.Contains(err.Error(), want) {
+				t.Fatalf("refused as %v; want %q", err, want)
+			}
+			if info, _ := os.Stat(victim); info.Mode().Perm() != 0o700 {
+				t.Fatalf("the victim's directory was given the declared mode through the link: %o", info.Mode().Perm())
+			}
+		})
+	}
+}
+
+// The check at the moment of use, without the path's re-check: a component swapped for a link is refused by the
+// descriptor walk itself, whatever was resolved a moment before.
+func TestGiveHandedOverFollowsNoLink(t *testing.T) {
+	dir, err := filepath.EvalSymlinks(t.TempDir())
+	if err != nil {
+		t.Fatal(err)
+	}
+	victim := filepath.Join(dir, "victim", "data")
+	if err := os.MkdirAll(victim, 0o700); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.Symlink(filepath.Join(dir, "victim"), filepath.Join(dir, "parent")); err != nil {
+		t.Fatal(err)
+	}
+	if _, _, _, err := giveHandedOver(filepath.Join(dir, "parent", "data"), 0o755, 0, 0, false); err == nil {
+		t.Fatal("a directory reached through a link was changed")
+	}
+	if info, _ := os.Stat(victim); info.Mode().Perm() != 0o700 {
+		t.Fatalf("the victim was changed: %o", info.Mode().Perm())
+	}
+	mode, _, _, err := giveHandedOver(victim, 0o750, 0, 0, false)
+	if err != nil || mode.Perm() != 0o700 {
+		t.Fatalf("the directory itself: before %o, %v", mode, err)
+	}
+	if info, _ := os.Stat(victim); info.Mode().Perm() != 0o750 {
+		t.Fatalf("the directory itself was not given its mode: %o", info.Mode().Perm())
+	}
+}

internal/apply/homes_linux.go

@@ -239,3 +239,31 @@ func writeUnder(home, path string, content []byte, mode os.FileMode) error {
 	}
 	return werr
 }
+
+// giveHandedOver gives a directory the operator handed to the mesh its declared mode and owner (novox/hq issue
+// 356, issue 339) through one descriptor opened from / with no link followed on any component (openUnderAs with /
+// as the home), judged a directory by fstat on that descriptor, and changed with fchmod and fchown on it. The
+// hand-over refused a link when it was recorded; this refuses one at the moment of use, so a component swapped for
+// a link between the record and the apply cannot carry root's chmod and chown to wherever it points. Returns the
+// mode and owner the directory had before.
+func giveHandedOver(path string, mode os.FileMode, uid, gid int, chown bool) (before os.FileMode, beforeUID, beforeGID int, err error) {
+	fd, err := openUnderAs("/", path, kindDir)
+	if err != nil {
+		return 0, 0, 0, err
+	}
+	defer unix.Close(fd)
+	var st unix.Stat_t
+	if err := unix.Fstat(fd, &st); err != nil {
+		return 0, 0, 0, &os.PathError{Op: "fstat", Path: path, Err: err}
+	}
+	before, beforeUID, beforeGID = os.FileMode(st.Mode&0o777), int(st.Uid), int(st.Gid)
+	if err := unix.Fchmod(fd, uint32(mode.Perm())); err != nil {
+		return 0, 0, 0, &os.PathError{Op: "chmod", Path: path, Err: err}
+	}
+	if chown && (beforeUID != uid || beforeGID != gid) {
+		if err := unix.Fchown(fd, uid, gid); err != nil {
+			return 0, 0, 0, &os.PathError{Op: "chown", Path: path, Err: err}
+		}
+	}
+	return before, beforeUID, beforeGID, nil
+}

internal/apply/homes_other.go

@@ -57,3 +57,10 @@ func writeUnder(home, path string, content []byte, mode os.FileMode) error {
 	}
 	return writeAtomicallyByPath(path, content, mode)
 }
+
+// giveHandedOver is refused where there is no openat with O_NOFOLLOW: a handed-over directory is never changed by
+// path (novox/hq issue 356). The node-engine runs on Linux.
+func giveHandedOver(path string, _ os.FileMode, _, _ int, _ bool) (os.FileMode, int, int, error) {
+	return 0, 0, 0, &PlacementRefusedError{Path: path, Why: "a handed-over directory is changed only through a " +
+		"descriptor that followed no link, which this system cannot open"}
+}

internal/link/handover.go

@@ -1,32 +1,63 @@
 package link
 
 import (
+	"crypto/ed25519"
+	"encoding/hex"
 	"encoding/json"
 	"fmt"
 	"strings"
+	"sync"
+	"time"
 )
 
 // A hand-over asked over the bus (novox/hq issue 356, issue 339).
 //
 // A directory used as found is handed to the mesh by its operator, at the controller's terminal:
 // `nox node hand-over <node> <path>` on the control-node (novox/hq ADR 0272). The controller asks this
-// node's engine on AskHandOverSubject — a request on core NATS, its own machine's subject, which only the
-// controller may publish — and the engine answers once, on the reply the request carried. The engine
-// judges every value it is sent and records the hand-over, or refuses and records nothing; the next
-// apply gives the directory its declared owner and mode. The controller holds the same two shapes in its
-// own link code, with a test on each side holding the field names.
+// node's engine on AskHandOverSubject, a request on core NATS, and the engine answers once, on the reply
+// the request carried.
+//
+// **The subject proves nothing; the signature does** (review of issue 356). Only the controller is granted
+// a publish on the subject, but the bus lets any principal allowed to answer reply to a message it received,
+// on whatever reply subject that message named — a tool server asked on its own subject with this reply
+// would deliver a body of its choosing here. So the controller signs the ask with the mesh's key, over
+// HandOverContext and the ask's bytes, and the engine verifies it with the key it verifies declarations with
+// before it reads anything out of it; then it holds the ask to this node, to its expiry, and to a nonce it
+// takes once. Then it judges every value and records the hand-over, or refuses and records nothing; the next
+// apply gives the directory its declared owner and mode. The controller holds the same shapes in its own link
+// code, with a test on each side holding the field names.
 
 // AskHandOverSubject is where the controller asks this node's engine to take a directory used as found as
 // the mesh's.
 func AskHandOverSubject(node string) string { return "mesh.node." + node + ".ask.hand-over" }
 
-// HandOverAsk is one hand-over asked: the directory's absolute path as the engine states it, and who asked,
-// in the controller's words.
+// HandOverAsk is one hand-over asked: the node it is for, the directory's absolute path as the engine states
+// it, who asked in the controller's words, when it stops being good, and a nonce taken once.
 type HandOverAsk struct {
-	Path string `json:"path"`
-	By   string `json:"by"`
+	Node    string    `json:"node"`
+	Path    string    `json:"path"`
+	By      string    `json:"by"`
+	Expires time.Time `json:"expires"`
+	Nonce   string    `json:"nonce"`
 }
 
+// SignedHandOver is the ask as it travels: its bytes exactly as signed, and the mesh's signature.
+type SignedHandOver struct {
+	Ask       []byte `json:"ask"`
+	Signature []byte `json:"signature"`
+}
+
+// HandOverContext is prefixed to an ask's bytes before signing: a hand-over's signature is never a
+// declaration's, which the same key makes over a declaration's bytes alone.
+const HandOverContext = "novox-mesh hand-over v1\n"
+
+// HandOverGood is how long the controller makes an ask good for; HandOverSkew is how far the machines' clocks
+// may differ. An ask whose expiry is further ahead than both together is refused: it would be good too long.
+const (
+	HandOverGood = time.Minute
+	HandOverSkew = time.Minute
+)
+
 // HandOverAnswer is the engine's answer: what it recorded, or why it refused — never both, never neither.
 type HandOverAnswer struct {
 	Said    string `json:"said,omitempty"`
@@ -36,34 +67,101 @@ type HandOverAnswer struct {
 // HandOverFunc judges and records one hand-over: the words to answer with, or the refusal.
 type HandOverFunc func(path, by string) (string, error)
 
-// MaxHandOverAsk bounds a request: a path and a name are hundreds of bytes.
+// MaxHandOverAsk bounds a request: a path, a name, a time, a nonce and a signature are hundreds of bytes.
 const MaxHandOverAsk = 8 << 10
 
 // MaxHandOverBy bounds who asked.
 const MaxHandOverBy = 200
 
-// AnswerHandOver is the engine's judgement of one ask's bytes: every value is checked before anything is
-// recorded, and a refusal records nothing. The handler is given only what passed.
-func AnswerHandOver(handle HandOverFunc, body []byte) HandOverAnswer {
+// HandOverCheck is what an ask is held to: the mesh's key, this node's name, the clock, and the nonces taken.
+type HandOverCheck struct {
+	Signer ed25519.PublicKey
+	Node   string
+	Now    func() time.Time
+	Nonces *Nonces
+}
+
+// Nonces are the nonces of asks taken, each kept until its ask expires: an ask is taken once. Kept in memory —
+// an ask outlives no restart by more than its minute, and taking one again records the same hand-over again.
+type Nonces struct {
+	mu   sync.Mutex
+	seen map[string]time.Time
+}
+
+// take records a nonce until its expiry, and says false when it was already taken.
+func (n *Nonces) take(nonce string, expires, now time.Time) bool {
+	n.mu.Lock()
+	defer n.mu.Unlock()
+	if n.seen == nil {
+		n.seen = map[string]time.Time{}
+	}
+	for k, until := range n.seen {
+		if now.After(until) {
+			delete(n.seen, k)
+		}
+	}
+	if _, taken := n.seen[nonce]; taken {
+		return false
+	}
+	n.seen[nonce] = expires
+	return true
+}
+
+// HandOverNonces are this process's: every link it opens takes from the same set, so a reconnect is no replay.
+var HandOverNonces = &Nonces{}
+
+func refusedAsk(why string) HandOverAnswer {
+	return HandOverAnswer{Refused: why + "; nothing was handed over"}
+}
+
+// AnswerHandOver is the engine's judgement of one ask's bytes: the signature first, then this node, the expiry
+// and the nonce, then every value — and only then the handler, given only what passed. A refusal records nothing.
+func AnswerHandOver(check HandOverCheck, handle HandOverFunc, body []byte) HandOverAnswer {
 	if handle == nil {
-		return HandOverAnswer{Refused: "this engine takes no hand-over"}
+		return refusedAsk("this engine takes no hand-over")
 	}
 	if len(body) > MaxHandOverAsk {
-		return HandOverAnswer{Refused: fmt.Sprintf("the ask is %d bytes, more than a path and a name; nothing was handed over",
-			len(body))}
+		return refusedAsk(fmt.Sprintf("the ask is %d bytes, more than a hand-over", len(body)))
+	}
+	var signed SignedHandOver
+	if err := json.Unmarshal(body, &signed); err != nil {
+		return refusedAsk("the ask is not a signed hand-over: " + err.Error())
+	}
+	if len(check.Signer) != ed25519.PublicKeySize ||
+		!ed25519.Verify(check.Signer, append([]byte(HandOverContext), signed.Ask...), signed.Signature) {
+		return refusedAsk("the ask is not signed by the mesh")
 	}
 	var ask HandOverAsk
-	if err := json.Unmarshal(body, &ask); err != nil {
-		return HandOverAnswer{Refused: "the ask is not a hand-over: " + err.Error() + "; nothing was handed over"}
+	if err := json.Unmarshal(signed.Ask, &ask); err != nil {
+		return refusedAsk("the signed ask is not a hand-over: " + err.Error())
+	}
+	now := time.Now()
+	if check.Now != nil {
+		now = check.Now()
+	}
+	nonces := check.Nonces
+	if nonces == nil {
+		nonces = HandOverNonces
 	}
 	by := strings.TrimSpace(ask.By)
+	nonce, nonceErr := hex.DecodeString(ask.Nonce)
 	switch {
+	case check.Node == "" || ask.Node != check.Node:
+		return refusedAsk(fmt.Sprintf("the ask is for %q, and this is %q", ask.Node, check.Node))
+	case ask.Expires.IsZero() || !now.Before(ask.Expires):
+		return refusedAsk("the ask has expired")
+	case ask.Expires.After(now.Add(HandOverGood + HandOverSkew)):
+		return refusedAsk("the ask is good for longer than an ask may be")
+	case nonceErr != nil || len(nonce) < 16:
+		return refusedAsk("the ask carries no nonce of its own")
 	case by == "":
-		return HandOverAnswer{Refused: "the ask names nobody as asking; nothing was handed over"}
+		return refusedAsk("the ask names nobody as asking")
 	case len(by) > MaxHandOverBy || strings.ContainsAny(by, "\r\n\x00"):
-		return HandOverAnswer{Refused: "who asked is not one short line; nothing was handed over"}
+		return refusedAsk("who asked is not one short line")
 	case ask.Path == "":
-		return HandOverAnswer{Refused: "the ask names no directory; nothing was handed over"}
+		return refusedAsk("the ask names no directory")
+	case !nonces.take(ask.Nonce, ask.Expires, now):
+		return refusedAsk("the ask was taken already")
 	}
 	said, err := handle(ask.Path, by)
 	if err != nil {
@@ -72,6 +170,19 @@ func AnswerHandOver(handle HandOverFunc, body []byte) HandOverAnswer {
 	return HandOverAnswer{Said: said}
 }
 
+// HandOverReply is what a hand-over request is answered with, and whether to answer: a request with no reply
+// subject is nobody's question, so nothing is judged, recorded or said.
+func HandOverReply(check HandOverCheck, handle HandOverFunc, reply string, body []byte) ([]byte, bool) {
+	if reply == "" {
+		return nil, false
+	}
+	answer, err := json.Marshal(AnswerHandOver(check, handle, body))
+	if err != nil {
+		return nil, false
+	}
+	return answer, true
+}
+
 // HandOvers is a link that hears the controller asking this node to hand a directory over. Optional, as
 // Asked is.
 type HandOvers interface {

internal/link/handover_test.go

@@ -1,20 +1,27 @@
 package link
 
 import (
+	"context"
+	"crypto/ed25519"
 	"encoding/json"
 	"errors"
 	"sort"
 	"strings"
 	"testing"
+	"time"
 )
 
 // The hand-over's ask and answer hold these field names; the controller's side holds the same list
 // (mesh-controller internal/link, TestTheHandOverAskAndAnswerKeepTheirFieldNames).
 func TestTheHandOverAskAndAnswerKeepTheirFieldNames(t *testing.T) {
-	body, _ := json.Marshal(HandOverAsk{Path: "/srv/notes", By: "jo through mesh-cli on anchor"})
-	if got := keysOf(t, body); got != "by path" {
+	body, _ := json.Marshal(HandOverAsk{Node: "laptop", Path: "/srv/notes", By: "jo", Expires: time.Now(), Nonce: "n"})
+	if got := keysOf(t, body); got != "by expires node nonce path" {
 		t.Fatalf("the ask's fields are %q", got)
 	}
+	body, _ = json.Marshal(SignedHandOver{Ask: []byte("{}"), Signature: []byte("s")})
+	if got := keysOf(t, body); got != "ask signature" {
+		t.Fatalf("the signed ask's fields are %q", got)
+	}
 	body, _ = json.Marshal(HandOverAnswer{Said: "s", Refused: "r"})
 	if got := keysOf(t, body); got != "refused said" {
 		t.Fatalf("the answer's fields are %q", got)
@@ -22,6 +29,9 @@ func TestTheHandOverAskAndAnswerKeepTheirFieldNames(t *testing.T) {
 	if AskHandOverSubject("laptop") != "mesh.node.laptop.ask.hand-over" {
 		t.Fatalf("the subject is %q", AskHandOverSubject("laptop"))
 	}
+	if HandOverContext != "novox-mesh hand-over v1\n" {
+		t.Fatalf("the signing context is %q; the controller holds the same words", HandOverContext)
+	}
 }
 
 func keysOf(t *testing.T, body []byte) string {
@@ -38,52 +48,193 @@ func keysOf(t *testing.T, body []byte) string {
 	return strings.Join(keys, " ")
 }
 
-// Every value of an ask is judged before the handler sees it, and a refusal hands nothing to the handler:
-// an ask too large, not a hand-over, naming nobody, naming somebody over more than a line, or naming no
-// directory. What passes reaches the handler with its path as given and who asked trimmed.
-func TestAHandOverAskIsJudgedBeforeAnythingIsRecorded(t *testing.T) {
+const aNonce = "00112233445566778899aabbccddeeff"
+
+// signedAsk is an ask as the controller makes it, signed with key over the context and its bytes.
+func signedAsk(t *testing.T, key ed25519.PrivateKey, ask HandOverAsk) []byte {
+	t.Helper()
+	raw, err := json.Marshal(ask)
+	if err != nil {
+		t.Fatal(err)
+	}
+	body, err := json.Marshal(SignedHandOver{Ask: raw, Signature: ed25519.Sign(key, append([]byte(HandOverContext), raw...))})
+	if err != nil {
+		t.Fatal(err)
+	}
+	return body
+}
+
+// anAsk is a good ask at now.
+func anAsk(now time.Time) HandOverAsk {
+	return HandOverAsk{Node: "n1", Path: "/srv/notes", By: " jo through mesh-cli on anchor ", Expires: now.Add(HandOverGood),
+		Nonce: aNonce}
+}
+
+// **An ask is believed only signed by the mesh, for this node, unexpired, once** (review of issue 356): a
+// responder can reach the subject through a reply, so the subject proves nothing. Each refusal below hands
+// the handler nothing; the good ask reaches it with its path and who asked, and the same ask again is refused.
+func TestAHandOverAskIsBelievedOnlySignedForThisNodeUnexpiredAndOnce(t *testing.T) {
+	m, key := aMember(t)
+	_, stranger, _ := ed25519.GenerateKey(nil)
+	now := time.Date(2026, 10, 9, 18, 0, 0, 0, time.UTC)
 	var given []string
 	handle := func(path, by string) (string, error) {
 		given = append(given, path+" by "+by)
 		if path == "/refused" {
-			return "", errors.New("not a directory this machine uses as found")
+			return "", errors.New("not a directory this machine uses as found; nothing was handed over")
 		}
 		return path + " is handed to the mesh", nil
 	}
+	check := HandOverCheck{Signer: m.Signer, Node: m.Node, Now: func() time.Time { return now }, Nonces: &Nonces{}}
+	with := func(change func(*HandOverAsk)) HandOverAsk {
+		a := anAsk(now)
+		change(&a)
+		return a
+	}
+	good, _ := json.Marshal(anAsk(now))
+	unsignedBody, _ := json.Marshal(SignedHandOver{Ask: good})
+	// Signed as a declaration is, over the bytes alone: not a hand-over's signature.
+	asDeclaration, _ := json.Marshal(SignedHandOver{Ask: good, Signature: ed25519.Sign(key, good)})
 	refusals := map[string][]byte{
-		"too large":         []byte(`{"path":"/` + strings.Repeat("a", MaxHandOverAsk) + `","by":"jo"}`),
-		"not a hand-over":   []byte(`not json`),
-		"nobody":            []byte(`{"path":"/srv/notes","by":"  "}`),
-		"two lines":         []byte(`{"path":"/srv/notes","by":"jo\nroot"}`),
-		"a name too long":   []byte(`{"path":"/srv/notes","by":"` + strings.Repeat("x", MaxHandOverBy+1) + `"}`),
-		"no directory":      []byte(`{"path":"","by":"jo"}`),
-		"a NUL in the name": []byte(`{"path":"/srv/notes","by":"jo\u0000"}`),
+		"too large":               signedAsk(t, key, with(func(a *HandOverAsk) { a.Path = "/" + strings.Repeat("a", MaxHandOverAsk) })),
+		"not a signed hand-over":  []byte(`not json`),
+		"unsigned":                unsignedBody,
+		"signed by another key":   signedAsk(t, stranger, anAsk(now)),
+		"signed as a declaration": asDeclaration,
+		"the bare ask":            good,
+		"for another node":        signedAsk(t, key, with(func(a *HandOverAsk) { a.Node = "n2" })),
+		"expired":                 signedAsk(t, key, with(func(a *HandOverAsk) { a.Expires = now })),
+		"no expiry":               signedAsk(t, key, with(func(a *HandOverAsk) { a.Expires = time.Time{} })),
+		"good for too long":       signedAsk(t, key, with(func(a *HandOverAsk) { a.Expires = now.Add(HandOverGood + HandOverSkew + time.Second) })),
+		"no nonce":                signedAsk(t, key, with(func(a *HandOverAsk) { a.Nonce = "" })),
+		"a short nonce":           signedAsk(t, key, with(func(a *HandOverAsk) { a.Nonce = "0011" })),
+		"nobody":                  signedAsk(t, key, with(func(a *HandOverAsk) { a.By = "  " })),
+		"two lines":               signedAsk(t, key, with(func(a *HandOverAsk) { a.By = "jo\nroot" })),
+		"a name too long":         signedAsk(t, key, with(func(a *HandOverAsk) { a.By = strings.Repeat("x", MaxHandOverBy+1) })),
+		"a NUL in the name":       signedAsk(t, key, with(func(a *HandOverAsk) { a.By = "jo\x00" })),
+		"no directory":            signedAsk(t, key, with(func(a *HandOverAsk) { a.Path = "" })),
 	}
 	for name, body := range refusals {
-		a := AnswerHandOver(handle, body)
-		if a.Refused == "" || a.Said != "" {
-			t.Fatalf("%s was answered %+v; want a refusal", name, a)
-		}
-		if !strings.Contains(a.Refused, "nothing was handed over") {
-			t.Fatalf("%s's refusal does not say nothing was handed over: %q", name, a.Refused)
+		a := AnswerHandOver(check, handle, body)
+		if a.Refused == "" || a.Said != "" || !strings.Contains(a.Refused, "nothing was handed over") {
+			t.Fatalf("%s was answered %+v; want a refusal saying nothing was handed over", name, a)
 		}
 	}
 	if len(given) != 0 {
 		t.Fatalf("the handler was given %v although every ask was refused", given)
 	}
-	if a := AnswerHandOver(nil, []byte(`{"path":"/srv/notes","by":"jo"}`)); a.Refused == "" {
+	if a := AnswerHandOver(HandOverCheck{Node: "n1", Nonces: &Nonces{}}, handle, signedAsk(t, key, anAsk(time.Now()))); a.Refused == "" {
+		t.Fatal("an engine with no key believed an ask")
+	}
+	if a := AnswerHandOver(check, nil, signedAsk(t, key, anAsk(now))); a.Refused == "" {
 		t.Fatal("an engine with no handler took a hand-over")
 	}
 
-	a := AnswerHandOver(handle, []byte(`{"path":"/refused","by":"jo"}`))
-	if a.Refused != "not a directory this machine uses as found" || a.Said != "" {
-		t.Fatalf("the handler's refusal was answered as %+v", a)
-	}
-	a = AnswerHandOver(handle, []byte(`{"path":"/srv/notes","by":" jo through mesh-cli on anchor "}`))
+	body := signedAsk(t, key, anAsk(now))
+	a := AnswerHandOver(check, handle, body)
 	if a.Said != "/srv/notes is handed to the mesh" || a.Refused != "" {
-		t.Fatalf("answered %+v", a)
+		t.Fatalf("the good ask was answered %+v", a)
 	}
-	if strings.Join(given, "; ") != "/refused by jo; /srv/notes by jo through mesh-cli on anchor" {
+	if again := AnswerHandOver(check, handle, body); !strings.Contains(again.Refused, "taken already") {
+		t.Fatalf("the same ask again was answered %+v", again)
+	}
+	refused := signedAsk(t, key, with(func(a *HandOverAsk) { a.Path = "/refused"; a.Nonce = strings.Repeat("ab", 16) }))
+	if a := AnswerHandOver(check, handle, refused); !strings.HasPrefix(a.Refused, "not a directory") {
+		t.Fatalf("the handler's refusal was answered %+v", a)
+	}
+	if strings.Join(given, "; ") != "/srv/notes by jo through mesh-cli on anchor; /refused by jo through mesh-cli on anchor" {
 		t.Fatalf("the handler was given %v", given)
 	}
+	// A nonce is forgotten once its ask has expired, so the set does not grow for ever.
+	now = now.Add(HandOverGood + time.Second)
+	check.Nonces.take("ff", now.Add(time.Second), now)
+	if len(check.Nonces.seen) != 1 {
+		t.Fatalf("expired nonces are kept: %v", check.Nonces.seen)
+	}
+}
+
+// **A request with no reply is nobody's question**: nothing is judged, recorded or said. One with a reply is
+// answered with the judgement.
+func TestAHandOverIsAnsweredOnlyWhenItCarriesAReply(t *testing.T) {
+	m, key := aMember(t)
+	now := time.Now()
+	called := 0
+	handle := func(string, string) (string, error) { called++; return "handed", nil }
+	check := HandOverCheck{Signer: m.Signer, Node: m.Node, Nonces: &Nonces{}}
+	if body, answer := HandOverReply(check, handle, "", signedAsk(t, key, anAsk(now))); answer || body != nil || called != 0 {
+		t.Fatalf("a request with no reply was answered %q %v, handled %d", body, answer, called)
+	}
+	body, answer := HandOverReply(check, handle, "_INBOX.x", signedAsk(t, key, anAsk(now)))
+	var said HandOverAnswer
+	if !answer || json.Unmarshal(body, &said) != nil || said.Said != "handed" || called != 1 {
+		t.Fatalf("answered %q %v, handled %d", body, answer, called)
+	}
+}
+
+// handingLink is a quiet link that hears hand-overs, keeping the handler it was given.
+type handingLink struct {
+	*quietLink
+	given chan HandOverFunc
+}
+
+func (l handingLink) HearHandOvers(h HandOverFunc) { l.given <- h }
+
+// **The link is given the queue's hand-over** when it is served: what the controller asks reaches the engine's
+// judgement, and nothing else.
+func TestALinkThatHearsHandOversIsGivenTheQueues(t *testing.T) {
+	m, _ := aMember(t)
+	l := handingLink{quietLink: newQuietLink(), given: make(chan HandOverFunc, 1)}
+	q := aQueue(m, (&applies{}).apply, &keptInMemory{})
+	q.HandOver = func(path, by string) (string, error) { return "the queue's: " + path + " by " + by, nil }
+	ctx, stop := context.WithCancel(context.Background())
+	served := make(chan error, 1)
+	go func() { served <- serve(ctx, l, m, q, nil, time.Second) }()
+	select {
+	case h := <-l.given:
+		if said, err := h("/srv/notes", "jo"); err != nil || said != "the queue's: /srv/notes by jo" {
+			t.Fatalf("the link was given another handler: %q %v", said, err)
+		}
+	case <-time.After(5 * time.Second):
+		t.Fatal("the link was never given the queue's hand-over")
+	}
+	stop()
+	<-served
+}
+
+// **Against a real server**: a signed request on the node's own subject is answered; one on another node's is
+// not heard; a message with no reply is answered by nobody.
+func TestNatsAHandOverIsAskedOfTheNode(t *testing.T) {
+	conn, js := aBus(t)
+	m, key := aMember(t)
+	l := &natsLink{conn: conn, js: js, node: m.Node, signer: m.Signer}
+	handled := make(chan string, 4)
+	l.HearHandOvers(func(path, by string) (string, error) { handled <- path; return "handed " + path, nil })
+	defer l.Close()
+	if err := conn.Flush(); err != nil {
+		t.Fatal(err)
+	}
+	ask := anAsk(time.Now())
+	ask.Nonce = strings.Repeat("cd", 16)
+	reply, err := conn.Request(AskHandOverSubject(m.Node), signedAsk(t, key, ask), 5*time.Second)
+	if err != nil {
+		t.Fatal(err)
+	}
+	var said HandOverAnswer
+	if err := json.Unmarshal(reply.Data, &said); err != nil || said.Said != "handed /srv/notes" {
+		t.Fatalf("answered %s, %v", reply.Data, err)
+	}
+	ask.Nonce = strings.Repeat("ef", 16)
+	if err := conn.Publish(AskHandOverSubject(m.Node), signedAsk(t, key, ask)); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := conn.Request(AskHandOverSubject("another"), signedAsk(t, key, ask), 300*time.Millisecond); err == nil {
+		t.Fatal("another node's ask was answered here")
+	}
+	if err := conn.Flush(); err != nil {
+		t.Fatal(err)
+	}
+	time.Sleep(100 * time.Millisecond)
+	if len(handled) != 1 {
+		t.Fatalf("handled %d asks; want only the one with a reply", len(handled))
+	}
 }

internal/link/hearing_nats.go

@@ -2,7 +2,7 @@ package link
 
 import (
 	"context"
-	"encoding/json"
+	"crypto/ed25519"
 	"errors"
 	"fmt"
 	"strings"
@@ -63,6 +63,8 @@ type natsLink struct {
 	asked   chan struct{}
 	handing *nats.Subscription
 	node    string
+	// signer is the mesh's key, which a hand-over ask is verified with as a declaration is (issue 356).
+	signer  ed25519.PublicKey
 	arrived chan Declaration
 	lost    chan error
 }
@@ -112,7 +114,7 @@ func dialNats(ctx context.Context, m Membership, timeout time.Duration) (Link, e
 	}
 
 	l := &natsLink{
-		conn: conn, js: js, node: m.Node,
+		conn: conn, js: js, node: m.Node, signer: m.Signer,
 		arrived: make(chan Declaration, drainDepth),
 		lost:    make(chan error, 1),
 		asked:   make(chan struct{}, 1),
@@ -195,24 +197,20 @@ func (l *natsLink) hearAsks() {
 }
 
 // HearHandOvers listens for the controller asking this node to hand a directory used as found to the mesh
-// (novox/hq issue 356): a request on this machine's own subject, answered once on the reply it carried.
-// The engine judges the ask (AnswerHandOver) and runs nothing it is sent. A bus whose user list is older
-// than the grant refuses the subscription asynchronously, said by the client; the operator's line then
-// says that nothing on this node answers.
+// (novox/hq issue 356): a request on this machine's own subject, answered once on the reply it carried. The
+// ask is verified with the mesh's key before anything is read out of it (AnswerHandOver), because a responder
+// can reach this subject through a reply; the engine runs nothing it is sent. A bus whose user list is older
+// than the grant refuses the subscription asynchronously, said by the client; the operator's line then says
+// that nothing on this node answers.
 func (l *natsLink) HearHandOvers(handle HandOverFunc) {
 	if handle == nil {
 		return
 	}
+	check := HandOverCheck{Signer: l.signer, Node: l.node}
 	handing, err := l.conn.Subscribe(AskHandOverSubject(l.node), func(msg *nats.Msg) {
-		if msg.Reply == "" {
-			// Nobody to answer: nothing is handed over on a word nobody waits for.
-			return
+		if body, answer := HandOverReply(check, handle, msg.Reply, msg.Data); answer {
+			_ = msg.Respond(body)
 		}
-		body, err := json.Marshal(AnswerHandOver(handle, msg.Data))
-		if err != nil {
-			return
-		}
-		_ = msg.Respond(body)
 	})
 	if err == nil {
 		l.handing = handing

internal/upgrade/fallback.go

@@ -41,8 +41,8 @@ func FallbackPath() string {
 
 // RenewFallback makes the file at fallback a copy of the executable at self, when it is not one already.
 //
-// Nothing is written when the two are the same file (an engine running from the fallback itself, while
-// nothing is delivered) or hold the same bytes. A fallback that is a link is refused: the mesh writes
+// Nothing is written when the two hold the same bytes — which an engine running from the fallback itself, while
+// nothing is delivered, always does. A fallback that is a link is refused: the mesh writes
 // through no link, and a link there would be somebody else's arrangement. Reports whether it was renewed.
 func RenewFallback(self, fallback string) (bool, error) {
 	if self == "" || fallback == "" {
@@ -66,8 +66,6 @@ func RenewFallback(self, fallback string) (bool, error) {
 			"it is left as it is", fallback)
 	case !placed.Mode().IsRegular():
 		return false, fmt.Errorf("%s is not a regular file: it is left as it is", fallback)
-	case os.SameFile(mine, placed):
-		return false, nil
 	default:
 		same, err := sameBytes(self, fallback)
 		if err != nil {

internal/upgrade/fallback_test.go

@@ -152,3 +152,35 @@ func TestTheFallbackPathIsTheLaunchers(t *testing.T) {
 		t.Fatalf("the override is %q", FallbackPath())
 	}
 }
+
+// A fallback that is not a regular file — a directory where the engine should be — is refused and left: the mesh
+// replaces a file, never something else at its name.
+func TestAFallbackThatIsNotARegularFileIsRefused(t *testing.T) {
+	dir := t.TempDir()
+	self := filepath.Join(dir, "v1", BinaryName)
+	if err := os.MkdirAll(filepath.Dir(self), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(self, []byte("proved"), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	fallback := filepath.Join(dir, BinaryName)
+	if err := os.Mkdir(fallback, 0o755); err != nil {
+		t.Fatal(err)
+	}
+	renewed, err := RenewFallback(self, fallback)
+	if err == nil || renewed || !strings.Contains(err.Error(), "not a regular file") {
+		t.Fatalf("a directory at the fallback was taken: %v, %v", renewed, err)
+	}
+	if info, err := os.Lstat(fallback); err != nil || !info.IsDir() {
+		t.Fatalf("the directory was replaced: %v %v", info, err)
+	}
+	// And a running engine that is not a regular file is refused before anything is written.
+	if _, err := RenewFallback(dir, filepath.Join(dir, "elsewhere")); err == nil || !strings.Contains(err.Error(),
+		"not a regular file") {
+		t.Fatalf("a directory was taken as the running engine: %v", err)
+	}
+	if _, err := os.Lstat(filepath.Join(dir, "elsewhere")); err == nil {
+		t.Fatal("something was placed from a directory")
+	}
+}