An enrolling node signs its request with the identity it just generated, so the mesh can tell it from anyone who knows its public key (novox/hq issue 083)

cmd/mesh-host/main.go

@@ -493,8 +493,12 @@ func enrol(ctx context.Context, opts options) error {
 		_ = json.Unmarshal(raw, &reported)
 	}
 
+	// Signed with the identity just generated, so the mesh can tell this machine from anyone else
+	// who knows its public key (novox/hq issue 083).
+	proof := mine.Sign(link.EnrolProof(token.Secret, mine.Public, mine.Overlay.Public,
+		sealing.Public, serving.Public))
 	reply, err := link.Enrol(ctx, token.Broker, token.Fingerprint, *name, token.Secret,
-		mine.Public, mine.Overlay.Public, sealing.Public, serving.Public, reported, opts.timeout)
+		mine.Public, mine.Overlay.Public, sealing.Public, serving.Public, reported, proof, opts.timeout)
 	if err != nil {
 		return err
 	}

internal/link/enrol.go

@@ -2,6 +2,7 @@ package link
 
 import (
 	"context"
+	"encoding/base64"
 	"encoding/json"
 	"errors"
 	"fmt"
@@ -46,6 +47,11 @@ type EnrolRequest struct {
 	ServingKey string `json:"serving_key,omitempty"`
 
 	Profile map[string]any `json:"profile,omitempty"`
+
+	// Proof is this node's identity key signing EnrolProof over this request: that the presenter
+	// holds the private half of PublicKey. The mesh asks for it before letting an enrolment finish
+	// on a token this key already spent (novox/hq issue 083).
+	Proof []byte `json:"proof,omitempty"`
 }
 
 // EnrolReply is what the mesh says back.
@@ -71,6 +77,13 @@ type EnrolReply struct {
 	Refusal string `json:"refusal,omitempty"`
 }
 
+// EnrolProof is what a node signs with its identity key when it enrols: the token and every key it
+// presents, so a proof cannot be moved to another request. The mesh builds the same bytes.
+func EnrolProof(secret string, public []byte, overlay, sealing, serving string) []byte {
+	return []byte("novox-mesh-enrol\x00" + secret + "\x00" + base64.StdEncoding.EncodeToString(public) +
+		"\x00" + overlay + "\x00" + sealing + "\x00" + serving)
+}
+
 // ErrRefused is what a node gets when the mesh will not have it.
 var ErrRefused = errors.New("the mesh refused this enrolment")
 
@@ -111,7 +124,7 @@ func answered(reply EnrolReply, asking time.Duration) (again bool, err error) {
 // says once it is in, and the secret travels again because the control plane must not have to ask
 // the broker who connected.
 func Enrol(ctx context.Context, address, pin, node, secret string, public []byte,
-	overlayKey, sealingKey, servingKey string, profile map[string]any,
+	overlayKey, sealingKey, servingKey string, profile map[string]any, proof []byte,
 	timeout time.Duration) (EnrolReply, error) {
 
 	config, err := PinnedConfig(pin)
@@ -158,7 +171,8 @@ func Enrol(ctx context.Context, address, pin, node, secret string, public []byte
 	}
 
 	request := EnrolRequest{Node: node, Secret: secret, PublicKey: public,
-		OverlayKey: overlayKey, SealingKey: sealingKey, ServingKey: servingKey, Profile: profile}
+		OverlayKey: overlayKey, SealingKey: sealingKey, ServingKey: servingKey, Profile: profile,
+		Proof: proof}
 	body, err := json.Marshal(request)
 	if err != nil {
 		return EnrolReply{}, err

internal/link/enrol_proof_test.go

@@ -0,0 +1,13 @@
+package link
+
+import "testing"
+
+// The bytes a node signs when it enrols. The mesh builds the same bytes to check the signature, in
+// another repository; this known answer is repeated in its test, so the two cannot drift apart
+// without one of them failing (novox/hq issue 083).
+func TestWhatAnEnrollingNodeSignsIsFixed(t *testing.T) {
+	got := string(EnrolProof("s", []byte{1, 2, 3}, "o", "e", "v"))
+	if want := "novox-mesh-enrol\x00s\x00AQID\x00o\x00e\x00v"; got != want {
+		t.Fatalf("the enrolment proof's message changed: %q, want %q", got, want)
+	}
+}