feat: add root-only rendezvous derivation

README.md

@@ -142,6 +142,14 @@ Create a `TreeRoot` from a bech32 nsec string or raw 32-byte key. An intermediat
 
 Derive a child `Identity` from a `TreeRoot`. Returns `{ nsec, npub, privateKey, publicKey, purpose, index }`. The index defaults to `0`.
 
+### `deriveRendezvous(root, index?)`
+
+Derive the Vennel `rendezvous` root child for a Link or quiet-drop pairing.
+It is deliberately derived from the `TreeRoot`, never an account identity or
+persona. Rotate it by incrementing `index`; provision the private half only by
+an authenticated private device flow, then call `zeroise()` on the temporary
+identity. This helper does not create a pairing link or export a private key.
+
 ### `deriveFromIdentity(identity, purpose, index?)`
 
 Derive a child `Identity` from any existing `Identity`, enabling arbitrary-depth

src/core.ts

@@ -1,5 +1,6 @@
 export { fromNsec } from './root-nsec.js'
 export { derive, zeroise } from './derive.js'
+export { deriveRendezvous, RENDEZVOUS_PURPOSE } from './rendezvous.js'
 export { recover } from './recover.js'
 export type { TreeRoot, Identity } from './types.js'
 export { NsecTreeError, DEFAULT_SCAN_RANGE, MAX_SCAN_RANGE, MAX_INDEX } from './types.js'

src/index.ts

@@ -1,6 +1,7 @@
 export { fromNsec } from './root-nsec.js'
 export { fromMnemonic } from './root-mnemonic.js'
 export { derive, zeroise } from './derive.js'
+export { deriveRendezvous, RENDEZVOUS_PURPOSE } from './rendezvous.js'
 export { recover } from './recover.js'
 export { createBlindProof, createFullProof, verifyProof } from './proof.js'
 export type { TreeRoot, Identity, LinkageProof } from './types.js'

src/rendezvous.ts

@@ -0,0 +1,16 @@
+import { derive } from './derive.js'
+import type { Identity, TreeRoot } from './types.js'
+
+/** The sole root-child purpose used for a person's Vennel rendezvous key. */
+export const RENDEZVOUS_PURPOSE = 'rendezvous'
+
+/**
+ * Derive one rotated rendezvous identity directly from the person's tree root.
+ *
+ * This deliberately accepts a `TreeRoot`, not a persona or ordinary identity:
+ * Link tags and quiet drops must not use an account identity key. The returned
+ * private key is caller-owned and must be zeroised after private provisioning.
+ */
+export function deriveRendezvous(root: TreeRoot, index = 0): Identity {
+  return derive(root, RENDEZVOUS_PURPOSE, index)
+}

test/exports.test.ts

@@ -6,6 +6,8 @@ describe('subpath exports', () => {
     expect(mod.fromNsec).toBeDefined()
     expect(mod.fromMnemonic).toBeDefined()
     expect(mod.derive).toBeDefined()
+    expect(mod.deriveRendezvous).toBeDefined()
+    expect(mod.RENDEZVOUS_PURPOSE).toBe('rendezvous')
     expect(mod.recover).toBeDefined()
     expect(mod.zeroise).toBeDefined()
     expect(mod.createBlindProof).toBeDefined()
@@ -18,6 +20,8 @@ describe('subpath exports', () => {
     const mod = await import('../src/core.js')
     expect(mod.fromNsec).toBeDefined()
     expect(mod.derive).toBeDefined()
+    expect(mod.deriveRendezvous).toBeDefined()
+    expect(mod.RENDEZVOUS_PURPOSE).toBe('rendezvous')
     expect(mod.recover).toBeDefined()
     expect(mod.zeroise).toBeDefined()
     expect((mod as any).fromMnemonic).toBeUndefined()

test/rendezvous.test.ts

@@ -0,0 +1,41 @@
+import { describe, expect, it } from 'vitest'
+import { derive, zeroise } from '../src/derive.js'
+import { fromNsec } from '../src/root-nsec.js'
+import { deriveRendezvous, RENDEZVOUS_PURPOSE } from '../src/rendezvous.js'
+
+describe('deriveRendezvous', () => {
+  it('uses the sole Vennel root-child purpose and preserves the requested rotation index', () => {
+    const root = fromNsec(new Uint8Array(32).fill(0x61))
+    const rendezvous = deriveRendezvous(root, 3)
+    const generic = derive(root, 'rendezvous', 3)
+
+    expect(RENDEZVOUS_PURPOSE).toBe('rendezvous')
+    expect(rendezvous.purpose).toBe(RENDEZVOUS_PURPOSE)
+    expect(rendezvous.index).toBe(3)
+    expect(rendezvous.publicKey).toEqual(generic.publicKey)
+    expect(rendezvous.privateKey).toEqual(generic.privateKey)
+
+    zeroise(rendezvous)
+    zeroise(generic)
+    root.destroy()
+  })
+
+  it('rotates by index without changing the person root', () => {
+    const root = fromNsec(new Uint8Array(32).fill(0x62))
+    const current = deriveRendezvous(root, 0)
+    const rotated = deriveRendezvous(root, 1)
+
+    expect(rotated.publicKey).not.toEqual(current.publicKey)
+    expect(rotated.npub).not.toBe(current.npub)
+
+    zeroise(current)
+    zeroise(rotated)
+    root.destroy()
+  })
+
+  it('cannot derive after the root is destroyed', () => {
+    const root = fromNsec(new Uint8Array(32).fill(0x63))
+    root.destroy()
+    expect(() => deriveRendezvous(root)).toThrow('destroyed')
+  })
+})