Review of 083: finishing an enrolment whose token was spent takes proof of the key's private half, a live lease and a first delivery — a public key alone cannot replay a spent token; shutdown leaves held messages for the broker; identical builds supersede; what is held leaves room in the prefetch

internal/link/protocol.go

@@ -6,6 +6,8 @@
 // traffic between them, each receiving half of what it expects. That has happened here before.
 package link
 
+import "encoding/base64"
+
 // Exchange is where nodes publish everything they have to say.
 const Exchange = "mesh"
 
@@ -58,6 +60,18 @@ type EnrolRequest struct {
 	// Profile is what this machine can be asked to do. The control plane cannot decide what a
 	// node should run without it, so it arrives with enrolment rather than being asked for after.
 	Profile map[string]any `json:"profile,omitempty"`
+
+	// Proof is the node's identity key signing EnrolProof over this request: that the presenter
+	// holds the private half of PublicKey, not only knows the public one. Required to finish an
+	// enrolment whose token this key already spent — the case of an answer lost after the spend —
+	// because a public key is no secret, and without it anyone holding a leaked token and a
+	// node's public key could replay the spent token (novox/hq issue 083, on review).
+	Proof []byte `json:"proof,omitempty"`
+
+	// Redelivered is set by the control plane, never sent: the broker handed this request over a
+	// second time. Such a request does not finish an enrolment already spent — the first time may
+	// have answered, and the node holds what it was told.
+	Redelivered bool `json:"-"`
 }
 
 // Signed is a declaration and the signature over it.
@@ -137,3 +151,10 @@ type EnrolReply struct {
 	// token can fail — unknown, spent, expired — so that guessing learns nothing.
 	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.
+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)
+}