diff --git a/apps/api/src/notifications/unsubscribe-token.test.ts b/apps/api/src/notifications/unsubscribe-token.test.ts
new file mode 100644
index 0000000..56ebf14
--- /dev/null
+++ b/apps/api/src/notifications/unsubscribe-token.test.ts
@@ -0,0 +1,72 @@
+import { createHmac } from 'node:crypto';
+
+import { deriveTokenKey } from '@dorfteich/shared/token-crypto';
+import { describe, expect, it } from 'vitest';
+
+import {
+  LEGACY_VERIFY_UNTIL,
+  signUnsubscribeToken,
+  verifyUnsubscribeToken,
+} from './unsubscribe-token';
+
+const secret = 'test-secret-at-least-16-chars-long';
+const TTL_MS = 90 * 24 * 60 * 60 * 1000;
+
+/** A token as the pre-#188 implementation minted it: root secret + purpose prefix. */
+function legacyToken(userId: string, now = Date.now()): string {
+  const body = Buffer.from(
+    JSON.stringify({ userId, exp: Math.floor(now / 1000) + 90 * 24 * 60 * 60 }),
+    'utf8',
+  ).toString('base64url');
+  const sig = createHmac('sha256', secret).update(`digest-unsubscribe.${body}`).digest('base64url');
+  return `${body}.${sig}`;
+}
+
+describe('unsubscribe token', () => {
+  it('round-trips a user id', () => {
+    expect(verifyUnsubscribeToken(signUnsubscribeToken('u1', secret), secret)).toBe('u1');
+  });
+
+  it('rejects an expired token', () => {
+    const token = signUnsubscribeToken('u1', secret);
+    expect(verifyUnsubscribeToken(token, secret, Date.now() + TTL_MS + 1000)).toBeNull();
+  });
+
+  it('rejects garbage and a wrong secret', () => {
+    expect(verifyUnsubscribeToken('nonsense', secret)).toBeNull();
+    expect(verifyUnsubscribeToken('a.b', secret)).toBeNull();
+    const token = signUnsubscribeToken('u1', secret);
+    expect(verifyUnsubscribeToken(token, 'another-secret-16-chars')).toBeNull();
+  });
+
+  it("rejects a token HMACed with a different purpose's subkey (ADR 0020)", () => {
+    const body = Buffer.from(
+      JSON.stringify({ userId: 'u1', exp: Math.floor(Date.now() / 1000) + 60 }),
+      'utf8',
+    ).toString('base64url');
+    const sig = createHmac('sha256', deriveTokenKey(secret, 'collab'))
+      .update(body)
+      .digest('base64url');
+    expect(verifyUnsubscribeToken(`${body}.${sig}`, secret)).toBeNull();
+  });
+
+  it('accepts a pre-separation legacy token inside the dual-verify window', () => {
+    const inWindow = LEGACY_VERIFY_UNTIL - 24 * 60 * 60 * 1000;
+    expect(verifyUnsubscribeToken(legacyToken('u1', inWindow - TTL_MS / 2), secret, inWindow)).toBe(
+      'u1',
+    );
+  });
+
+  it('rejects a legacy token once the dual-verify window has closed', () => {
+    const afterWindow = LEGACY_VERIFY_UNTIL + 1000;
+    // Unexpired on its own terms — rejected purely because the window closed.
+    const token = legacyToken('u1', afterWindow - 1000);
+    expect(verifyUnsubscribeToken(token, secret, afterWindow)).toBeNull();
+  });
+
+  it('verifies freshly minted tokens via the subkey, independent of the window', () => {
+    const afterWindow = LEGACY_VERIFY_UNTIL + 24 * 60 * 60 * 1000;
+    const token = signUnsubscribeToken('u1', secret, afterWindow);
+    expect(verifyUnsubscribeToken(token, secret, afterWindow + 1000)).toBe('u1');
+  });
+});
diff --git a/apps/api/src/notifications/unsubscribe-token.ts b/apps/api/src/notifications/unsubscribe-token.ts
index adaa4e7..87d18ea 100644
--- a/apps/api/src/notifications/unsubscribe-token.ts
+++ b/apps/api/src/notifications/unsubscribe-token.ts
@@ -1,31 +1,54 @@
 import { createHmac, timingSafeEqual } from 'node:crypto';
 
+import { deriveTokenKey } from '@dorfteich/shared/token-crypto';
+
 /**
  * Single-purpose unsubscribe tokens for the digest mails (issue #95): a
  * signed `{userId, exp}` blob whose only power is flipping that user's
  * digest setting to `off` — it never creates a session (ADR 0007's "signed
- * tokens" pattern, purpose-bound so it cannot be replayed anywhere else).
+ * tokens" pattern). Purpose-binding is structural since #188 (ADR 0020):
+ * the HMAC key is the HKDF subkey for `unsubscribe`, so a token signed for
+ * any other purpose cannot verify here regardless of its content.
  */
 
-const PURPOSE = 'digest-unsubscribe';
 const TTL_SECONDS = 90 * 24 * 60 * 60;
 
-function signature(body: string, secret: string): Buffer {
-  return createHmac('sha256', secret).update(`${PURPOSE}.${body}`).digest();
+/**
+ * Dual-verify window (#188, ADR 0020): before the key separation, tokens
+ * were HMACed with the root secret over a `digest-unsubscribe.` prefix.
+ * Those links live in digest mails that are already sent and stay valid
+ * for their full 90-day TTL, so verification accepts the legacy derivation
+ * until every pre-separation token has expired. Tokens are only ever
+ * SIGNED with the new subkey; the legacy path is verify-only and goes dead
+ * automatically on the date below (last possible legacy expiry, rounded up).
+ */
+export const LEGACY_VERIFY_UNTIL = Date.parse('2026-11-01T00:00:00Z');
+const LEGACY_PURPOSE = 'digest-unsubscribe';
+
+function signature(body: string, rootSecret: string): Buffer {
+  return createHmac('sha256', deriveTokenKey(rootSecret, 'unsubscribe')).update(body).digest();
 }
 
-export function signUnsubscribeToken(userId: string, secret: string, now = Date.now()): string {
+function legacySignature(body: string, rootSecret: string): Buffer {
+  return createHmac('sha256', rootSecret).update(`${LEGACY_PURPOSE}.${body}`).digest();
+}
+
+export function signUnsubscribeToken(userId: string, rootSecret: string, now = Date.now()): string {
   const body = Buffer.from(
     JSON.stringify({ userId, exp: Math.floor(now / 1000) + TTL_SECONDS }),
     'utf8',
   ).toString('base64url');
-  return `${body}.${signature(body, secret).toString('base64url')}`;
+  return `${body}.${signature(body, rootSecret).toString('base64url')}`;
+}
+
+function matches(provided: Buffer, expected: Buffer): boolean {
+  return provided.length === expected.length && timingSafeEqual(provided, expected);
 }
 
 /** The user id, or null for anything invalid or expired. Never throws. */
 export function verifyUnsubscribeToken(
   token: string,
-  secret: string,
+  rootSecret: string,
   now = Date.now(),
 ): string | null {
   const [body, sig] = token.split('.');
@@ -36,8 +59,10 @@ export function verifyUnsubscribeToken(
   } catch {
     return null;
   }
-  const expected = signature(body, secret);
-  if (provided.length !== expected.length || !timingSafeEqual(provided, expected)) return null;
+  const current = matches(provided, signature(body, rootSecret));
+  const legacy =
+    !current && now < LEGACY_VERIFY_UNTIL && matches(provided, legacySignature(body, rootSecret));
+  if (!current && !legacy) return null;
   try {
     const payload = JSON.parse(Buffer.from(body, 'base64url').toString('utf8')) as {
       userId?: string;
diff --git a/apps/api/src/pages/collab-token.e2e.db.test.ts b/apps/api/src/pages/collab-token.e2e.db.test.ts
index 1612955..6a282bf 100644
--- a/apps/api/src/pages/collab-token.e2e.db.test.ts
+++ b/apps/api/src/pages/collab-token.e2e.db.test.ts
@@ -104,7 +104,7 @@ describe.skipIf(!hasTestDb)('collab token (e2e, issue #34)', () => {
     expect(res.body.mode).toBe('rw');
     expect(res.body.expiresInSeconds).toBe(60);
 
-    const verified = verifyCollabToken(res.body.token, secret);
+    const verified = await verifyCollabToken(res.body.token, secret);
     expect(verified.valid).toBe(true);
     if (verified.valid) {
       expect(collabTokenClaimsSchema.parse(verified.claims)).toEqual({
diff --git a/apps/api/src/pages/pages.service.ts b/apps/api/src/pages/pages.service.ts
index 85f3865..ce8b06a 100644
--- a/apps/api/src/pages/pages.service.ts
+++ b/apps/api/src/pages/pages.service.ts
@@ -378,7 +378,7 @@ export class PagesService {
     const canWrite = await this.permissions.canAccessPage(user, page, 'write');
     const mode = canWrite ? 'rw' : 'ro';
     const userId = user?.id ?? null;
-    const token = signCollabToken(
+    const token = await signCollabToken(
       { userId, pageId: page.id, mode },
       this.config.env.COLLAB_TOKEN_SECRET,
       COLLAB_TOKEN_TTL_SECONDS,
diff --git a/apps/collab/src/access-listener.db.test.ts b/apps/collab/src/access-listener.db.test.ts
index b32aeb4..1d4f69e 100644
--- a/apps/collab/src/access-listener.db.test.ts
+++ b/apps/collab/src/access-listener.db.test.ts
@@ -108,11 +108,8 @@ describe.skipIf(!url)('access listener (DB-backed)', () => {
     // A string reuses one token; a function is re-invoked on every (re)connect,
     // exactly as the web client re-fetches from the api — which is how a
     // downgrade to `ro` takes effect on reconnect (issue #53).
-    token: string | (() => string) = signCollabToken(
-      { userId, pageId: name, mode: 'rw' },
-      secret,
-      60,
-    ),
+    token: string | (() => Promise<string>) = () =>
+      signCollabToken({ userId, pageId: name, mode: 'rw' }, secret, 60),
   ): {
     provider: HocuspocusProvider;
     doc: Y.Doc;
diff --git a/apps/collab/src/auth.test.ts b/apps/collab/src/auth.test.ts
index 9e047f0..457e5fc 100644
--- a/apps/collab/src/auth.test.ts
+++ b/apps/collab/src/auth.test.ts
@@ -83,7 +83,11 @@ describe('collab authentication', () => {
   }
 
   it('accepts a valid token whose page matches the document', async () => {
-    const token = signCollabToken({ userId: 'u1', pageId: 'page-ok', mode: 'rw' }, secret, 60);
+    const token = await signCollabToken(
+      { userId: 'u1', pageId: 'page-ok', mode: 'rw' },
+      secret,
+      60,
+    );
     const client = connect('page-ok', token);
     await waitFor(client.authenticated);
     expect(client.authenticated()).toBe(true);
@@ -93,7 +97,7 @@ describe('collab authentication', () => {
   });
 
   const claims: CollabTokenClaims = { userId: 'u1', pageId: 'page-x', mode: 'rw' };
-  const rejectionCases: { label: string; name: string; token: () => string }[] = [
+  const rejectionCases: { label: string; name: string; token: () => Promise<string> }[] = [
     {
       label: 'an expired token',
       name: 'page-x',
@@ -102,8 +106,8 @@ describe('collab authentication', () => {
     {
       label: 'a tampered token',
       name: 'page-x',
-      token: () => {
-        const [h, p, s = ''] = signCollabToken(claims, secret, 60).split('.');
+      token: async () => {
+        const [h, p, s = ''] = (await signCollabToken(claims, secret, 60)).split('.');
         // Flip the last character of the signature.
         const flipped = s.slice(0, -1) + (s.endsWith('A') ? 'B' : 'A');
         return `${h}.${p}.${flipped}`;
@@ -122,7 +126,7 @@ describe('collab authentication', () => {
   ];
 
   it.each(rejectionCases)('rejects $label', async ({ name, token }) => {
-    const client = connect(name, token());
+    const client = connect(name, await token());
     await waitFor(client.failed);
     expect(client.failed()).toBe(true);
     expect(client.authenticated()).toBe(false);
@@ -134,11 +138,11 @@ describe('collab authentication', () => {
     const page = 'page-shared';
     const rw = connect(
       page,
-      signCollabToken({ userId: 'w', pageId: page, mode: 'rw' }, secret, 60),
+      await signCollabToken({ userId: 'w', pageId: page, mode: 'rw' }, secret, 60),
     );
     const ro = connect(
       page,
-      signCollabToken({ userId: 'r', pageId: page, mode: 'ro' }, secret, 60),
+      await signCollabToken({ userId: 'r', pageId: page, mode: 'ro' }, secret, 60),
     );
     await waitFor(() => rw.synced() && ro.synced());
 
diff --git a/apps/collab/src/persistence.test.ts b/apps/collab/src/persistence.test.ts
index 7a01f38..b0d4d16 100644
--- a/apps/collab/src/persistence.test.ts
+++ b/apps/collab/src/persistence.test.ts
@@ -45,7 +45,8 @@ describe('collab persistence hooks', () => {
 
   function connect(url: string, pageId: string): { provider: HocuspocusProvider; doc: Y.Doc } {
     const doc = new Y.Doc();
-    const token = signCollabToken({ userId: `u-${pageId}`, pageId, mode: 'rw' }, secret, 60);
+    const token = (): Promise<string> =>
+      signCollabToken({ userId: `u-${pageId}`, pageId, mode: 'rw' }, secret, 60);
     const provider = new HocuspocusProvider({ url, name: pageId, document: doc, token });
     cleanups.push(() => provider.destroy());
     return { provider, doc };
diff --git a/apps/collab/src/restore-listener.db.test.ts b/apps/collab/src/restore-listener.db.test.ts
index ed2b9a7..ca682b8 100644
--- a/apps/collab/src/restore-listener.db.test.ts
+++ b/apps/collab/src/restore-listener.db.test.ts
@@ -120,7 +120,7 @@ describe.skipIf(!url)('restore listener (DB-backed, issue #42)', () => {
       url: wsUrl,
       name: pageId,
       document: doc,
-      token: signCollabToken({ userId, pageId, mode: 'rw' }, secret, 60),
+      token: await signCollabToken({ userId, pageId, mode: 'rw' }, secret, 60),
     });
     await waitFor(() => provider.isSynced);
 
diff --git a/apps/collab/src/server.ts b/apps/collab/src/server.ts
index 8f70237..4af8b18 100644
--- a/apps/collab/src/server.ts
+++ b/apps/collab/src/server.ts
@@ -113,7 +113,7 @@ export function createCollabServer(deps: CollabServerDeps): Server {
         );
         throw new Error('maintenance');
       }
-      const result = verifyCollabToken(token, tokenSecret);
+      const result = await verifyCollabToken(token, tokenSecret);
       if (!result.valid) {
         logger.info(
           { event: 'auth.rejected', documentName, reason: result.reason },
diff --git a/deploy/compose/.env.example b/deploy/compose/.env.example
index c244da7..d72e3d6 100644
--- a/deploy/compose/.env.example
+++ b/deploy/compose/.env.example
@@ -5,9 +5,12 @@
 # PostgreSQL password for the `dorfteich` database user.
 POSTGRES_PASSWORD=change-me
 
-# Secret that signs/verifies the short-lived collaboration tokens (issue #34).
-# The api and collab services share this one value; use a long random string
-# (e.g. `openssl rand -base64 32`). Min length 16.
+# ROOT key of the token key hierarchy (ADR 0020, issue #188): every token
+# purpose (collaboration tokens, digest unsubscribe links) derives its own
+# HKDF subkey from this value — nothing signs with it directly. The api and
+# collab services share this one value; use a long random string
+# (e.g. `openssl rand -base64 32`). Min length 16. Rotating it rotates all
+# derived keys at once and invalidates outstanding tokens.
 COLLAB_TOKEN_SECRET=change-me-to-a-long-random-string
 
 # --- images -----------------------------------------------------------------
diff --git a/docs/architecture/security.md b/docs/architecture/security.md
index 6f80528..fe32def 100644
--- a/docs/architecture/security.md
+++ b/docs/architecture/security.md
@@ -66,13 +66,27 @@ or sloppy plugin authors, compromised dependencies.
 
 ## Secrets & configuration
 
-- Secrets (DB password, collab signing key, SMTP credentials) live only in
+- Secrets (DB password, token root key, SMTP credentials) live only in
   the stage `.env` (mode 600, never in git) and container env — not in the
   database (`instance_settings` stores non-secret config; the SMTP password
   entered in the setup wizard is written to the env-backed secret store,
   not to a DB row).
-- Key rotation: collab signing key and session pepper rotate via env change
-  plus rolling restart; procedure documented in `operations.md` runbooks.
+- Token key hierarchy (ADR 0020, issue #188): `COLLAB_TOKEN_SECRET` is a
+  ROOT key. Each token purpose uses its own HKDF-SHA-256 subkey
+  (`deriveTokenKey` in `packages/shared/src/token-crypto.ts`): `collab`
+  for the collaboration JWTs (signed and verified by `jose`, HS256 as an
+  explicit allowlist), `unsubscribe` for the digest unsubscribe links. No
+  code path signs with the root key directly, so a compromise of one
+  purpose's tokens is not transferable to the other.
+- Dual-verify window: unsubscribe links minted before the key separation
+  live in already-sent mail (90-day TTL). Verification accepts the legacy
+  derivation (root key + purpose prefix) until **2026-11-01**
+  (`LEGACY_VERIFY_UNTIL` in `apps/api/src/notifications/unsubscribe-token.ts`),
+  after which the legacy path goes dead automatically. New tokens are only
+  ever signed with the subkey.
+- Key rotation: rotating the root key rotates every derived subkey at once
+  (desired: one secret to rotate) via env change plus rolling restart;
+  procedure documented in `operations.md` runbooks.
 - Dependencies: lockfile-pinned; monthly update batch; images pinned to
   digests in Prod.
 
diff --git a/docs/vs-nfd/20-massnahmenplan.md b/docs/vs-nfd/20-massnahmenplan.md
index 2f8134c..3c08fcc 100644
--- a/docs/vs-nfd/20-massnahmenplan.md
+++ b/docs/vs-nfd/20-massnahmenplan.md
@@ -84,9 +84,9 @@ _Meilensteine: `M24 — VS-NfD: security quick wins`; die nachgezogenen
 Punkte (#199, #200, #201, #202) in `M25 — VS-NfD: hardening & supply
 chain`_
 
-- [ ] **Schlüsseltrennung `COLLAB_TOKEN_SECRET`** per HKDF (zweckgebundene
+- [x] **Schlüsseltrennung `COLLAB_TOKEN_SECRET`** per HKDF (zweckgebundene
       Subkeys) — echter Fund, vor allen Features · 1–2 AT · #188
-- [ ] **Eigenbau-HMAC-JWT durch `jose` ersetzen** · +2–3 AT · #188 ⟵ neu aus Roadmap
+- [x] **Eigenbau-HMAC-JWT durch `jose` ersetzen** · +2–3 AT · #188 ⟵ neu aus Roadmap
       _Gebündelt mit der Zeile darüber, weil dieselbe Datei
       (`packages/shared/src/token-crypto.ts`). Einzeln wären es 5–6 AT._
       Achtung: Unsubscribe-Tokens leben lang in versandten Mails →
diff --git a/packages/shared/package.json b/packages/shared/package.json
index 134b3a5..1f12113 100644
--- a/packages/shared/package.json
+++ b/packages/shared/package.json
@@ -46,6 +46,7 @@
     "test": "vitest run --passWithNoTests"
   },
   "dependencies": {
+    "jose": "^6.2.4",
     "markdown-it": "^14.3.0",
     "prosemirror-markdown": "^1.13.4",
     "prosemirror-model": "^1.25.9",
diff --git a/packages/shared/src/collab-token.test.ts b/packages/shared/src/collab-token.test.ts
index 5d6889c..1bf297c 100644
--- a/packages/shared/src/collab-token.test.ts
+++ b/packages/shared/src/collab-token.test.ts
@@ -1,64 +1,122 @@
+import { SignJWT } from 'jose';
 import { describe, expect, it } from 'vitest';
 
 import type { CollabTokenClaims } from './collab-token';
-import { signCollabToken, verifyCollabToken } from './token-crypto';
+import { deriveTokenKey, signCollabToken, verifyCollabToken } from './token-crypto';
 
 const secret = 'test-secret-at-least-16-chars-long';
 const claims: CollabTokenClaims = { userId: 'user-1', pageId: 'page-1', mode: 'rw' };
 
+/** Sign the claims with an arbitrary key — for cross-purpose/root-key forgeries. */
+async function signWithKey(key: Uint8Array): Promise<string> {
+  const now = Math.floor(Date.now() / 1000);
+  return await new SignJWT({ ...claims })
+    .setProtectedHeader({ alg: 'HS256', typ: 'JWT' })
+    .setIssuedAt(now)
+    .setExpirationTime(now + 60)
+    .sign(key);
+}
+
 describe('collab token', () => {
-  it('round-trips valid claims', () => {
-    const token = signCollabToken(claims, secret, 60);
-    const result = verifyCollabToken(token, secret);
+  it('round-trips valid claims', async () => {
+    const token = await signCollabToken(claims, secret, 60);
+    const result = await verifyCollabToken(token, secret);
     expect(result).toEqual({ valid: true, claims });
   });
 
-  it('rejects a token signed with a different secret', () => {
-    const token = signCollabToken(claims, secret, 60);
-    expect(verifyCollabToken(token, 'another-secret-16-chars')).toEqual({
+  it('rejects a token signed with a different secret', async () => {
+    const token = await signCollabToken(claims, secret, 60);
+    expect(await verifyCollabToken(token, 'another-secret-16-chars')).toEqual({
       valid: false,
       reason: 'bad_signature',
     });
   });
 
-  it('rejects a tampered payload', () => {
-    const token = signCollabToken(claims, secret, 60);
+  it("rejects a token signed with a different purpose's subkey (ADR 0020)", async () => {
+    const forged = await signWithKey(deriveTokenKey(secret, 'unsubscribe'));
+    expect(await verifyCollabToken(forged, secret)).toEqual({
+      valid: false,
+      reason: 'bad_signature',
+    });
+  });
+
+  it('rejects a token signed with the raw root secret — no code path signs with it', async () => {
+    const forged = await signWithKey(new TextEncoder().encode(secret));
+    expect(await verifyCollabToken(forged, secret)).toEqual({
+      valid: false,
+      reason: 'bad_signature',
+    });
+  });
+
+  it('rejects a tampered payload', async () => {
+    const token = await signCollabToken(claims, secret, 60);
     const [header, , signature] = token.split('.');
     const forgedPayload = Buffer.from(
       JSON.stringify({ ...claims, mode: 'rw', iat: 0, exp: 9999999999 }),
       'utf8',
     ).toString('base64url');
     const forged = `${header}.${forgedPayload}.${signature}`;
-    expect(verifyCollabToken(forged, secret).valid).toBe(false);
+    expect((await verifyCollabToken(forged, secret)).valid).toBe(false);
   });
 
-  it('rejects an expired token', () => {
-    const token = signCollabToken(claims, secret, 60);
+  it('rejects an expired token', async () => {
+    const token = await signCollabToken(claims, secret, 60);
     // 61 seconds later the 60 s token is past its expiry.
     const later = Date.now() + 61_000;
-    expect(verifyCollabToken(token, secret, later)).toEqual({ valid: false, reason: 'expired' });
+    expect(await verifyCollabToken(token, secret, later)).toEqual({
+      valid: false,
+      reason: 'expired',
+    });
   });
 
-  it('rejects a malformed token', () => {
-    expect(verifyCollabToken('not-a-jwt', secret)).toEqual({ valid: false, reason: 'malformed' });
-    expect(verifyCollabToken('a.b', secret).valid).toBe(false);
+  it('rejects a malformed token', async () => {
+    expect(await verifyCollabToken('not-a-jwt', secret)).toEqual({
+      valid: false,
+      reason: 'malformed',
+    });
+    expect((await verifyCollabToken('a.b', secret)).valid).toBe(false);
   });
 
-  it('rejects an alg:none token even if the signature check is bypassed', () => {
-    // Craft a header advertising `none` and an empty signature; the empty
-    // signature cannot match a real HMAC, so this is caught as bad_signature,
-    // but the algorithm guard is the intended second line of defense.
+  it('rejects alg:none — HS256 is an explicit allowlist', async () => {
     const header = Buffer.from(JSON.stringify({ alg: 'none', typ: 'JWT' })).toString('base64url');
     const payload = Buffer.from(JSON.stringify({ ...claims, iat: 0, exp: 9999999999 })).toString(
       'base64url',
     );
-    const result = verifyCollabToken(`${header}.${payload}.`, secret);
+    const result = await verifyCollabToken(`${header}.${payload}.`, secret);
     expect(result.valid).toBe(false);
   });
 
-  it('preserves the read-only mode claim', () => {
-    const roToken = signCollabToken({ ...claims, mode: 'ro' }, secret, 60);
-    const result = verifyCollabToken(roToken, secret);
+  it('rejects an RS256 header before any signature check', async () => {
+    const header = Buffer.from(JSON.stringify({ alg: 'RS256', typ: 'JWT' })).toString('base64url');
+    const payload = Buffer.from(JSON.stringify({ ...claims, iat: 0, exp: 9999999999 })).toString(
+      'base64url',
+    );
+    const result = await verifyCollabToken(`${header}.${payload}.AAAA`, secret);
+    expect(result).toEqual({ valid: false, reason: 'bad_algorithm' });
+  });
+
+  it('rejects a signed token without an expiry claim', async () => {
+    const token = await new SignJWT({ ...claims })
+      .setProtectedHeader({ alg: 'HS256', typ: 'JWT' })
+      .setIssuedAt()
+      .sign(deriveTokenKey(secret, 'collab'));
+    expect(await verifyCollabToken(token, secret)).toEqual({
+      valid: false,
+      reason: 'invalid_claims',
+    });
+  });
+
+  it('preserves the read-only mode claim', async () => {
+    const roToken = await signCollabToken({ ...claims, mode: 'ro' }, secret, 60);
+    const result = await verifyCollabToken(roToken, secret);
     expect(result).toEqual({ valid: true, claims: { ...claims, mode: 'ro' } });
   });
+
+  it('derives distinct, stable subkeys per purpose', () => {
+    const collab = deriveTokenKey(secret, 'collab');
+    const unsubscribe = deriveTokenKey(secret, 'unsubscribe');
+    expect(collab).toHaveLength(32);
+    expect(Buffer.from(collab).equals(Buffer.from(unsubscribe))).toBe(false);
+    expect(Buffer.from(collab).equals(Buffer.from(deriveTokenKey(secret, 'collab')))).toBe(true);
+  });
 });
diff --git a/packages/shared/src/collab-token.ts b/packages/shared/src/collab-token.ts
index 2cf0578..c6b6d53 100644
--- a/packages/shared/src/collab-token.ts
+++ b/packages/shared/src/collab-token.ts
@@ -4,8 +4,8 @@ import { z } from 'zod';
  * Types and schemas for the short-lived collaboration tokens (issue #34,
  * ADR 0003/0007). These are browser-safe (no Node built-ins) so the web app
  * can import them from the package barrel. The signing/verifying helpers live
- * in `./token-crypto` (Node `crypto`) and are imported only by the api and the
- * collab server.
+ * in `./token-crypto` (HKDF subkeys + `jose`, ADR 0020) and are imported only
+ * by the api and the collab server.
  */
 
 export const collabTokenModeSchema = z.enum(['rw', 'ro']);
diff --git a/packages/shared/src/token-crypto.crossruntime.test.ts b/packages/shared/src/token-crypto.crossruntime.test.ts
new file mode 100644
index 0000000..f1533b7
--- /dev/null
+++ b/packages/shared/src/token-crypto.crossruntime.test.ts
@@ -0,0 +1,67 @@
+import { execFileSync } from 'node:child_process';
+import { existsSync } from 'node:fs';
+import path from 'node:path';
+import { fileURLToPath, pathToFileURL } from 'node:url';
+
+import { describe, expect, it } from 'vitest';
+
+/**
+ * The reason the homegrown JWT existed was the guarantee that the identical
+ * code runs in the CommonJS api and the ESM collab server (ADR 0020). With
+ * `jose` that property must be PROVEN, not assumed — jose v6 is ESM-only
+ * and reaches CJS via Node's `require(esm)`. This test loads the actually
+ * built dist artefacts (what api and collab load at runtime) in child
+ * processes of both module systems and round-trips a token in each
+ * direction. Requires a current `pnpm --filter @dorfteich/shared build`
+ * (CI builds before testing).
+ */
+
+const here = path.dirname(fileURLToPath(import.meta.url));
+const distCjs = path.resolve(here, '../dist/token-crypto.js');
+const distEsm = path.resolve(here, '../dist/token-crypto.mjs');
+
+const secret = 'cross-runtime-secret-32-characters';
+const claims = { userId: 'user-x', pageId: 'page-x', mode: 'rw' } as const;
+
+function run(args: string[], extraEnv: Record<string, string>): string {
+  return execFileSync(process.execPath, args, {
+    cwd: path.resolve(here, '..'),
+    env: { ...process.env, SECRET: secret, ...extraEnv },
+    encoding: 'utf8',
+  });
+}
+
+const signCjs = `require(process.env.DIST).signCollabToken(JSON.parse(process.env.CLAIMS), process.env.SECRET, 60).then((t) => process.stdout.write(t));`;
+const verifyCjs = `require(process.env.DIST).verifyCollabToken(process.env.TOKEN, process.env.SECRET).then((r) => process.stdout.write(JSON.stringify(r)));`;
+const signEsm = `const m = await import(process.env.DIST_URL); process.stdout.write(await m.signCollabToken(JSON.parse(process.env.CLAIMS), process.env.SECRET, 60));`;
+const verifyEsm = `const m = await import(process.env.DIST_URL); process.stdout.write(JSON.stringify(await m.verifyCollabToken(process.env.TOKEN, process.env.SECRET)));`;
+
+describe('token crypto across module systems (built dist)', () => {
+  it('has a built dist to test against', () => {
+    for (const file of [distCjs, distEsm]) {
+      if (!existsSync(file)) {
+        throw new Error(`${file} missing — run \`pnpm --filter @dorfteich/shared build\` first`);
+      }
+    }
+  });
+
+  it('verifies a CommonJS-signed token in an ESM runtime', () => {
+    const token = run(['-e', signCjs], { DIST: distCjs, CLAIMS: JSON.stringify(claims) });
+    expect(token.split('.')).toHaveLength(3);
+    const result = run(['--input-type=module', '-e', verifyEsm], {
+      DIST_URL: pathToFileURL(distEsm).href,
+      TOKEN: token,
+    });
+    expect(JSON.parse(result)).toEqual({ valid: true, claims });
+  });
+
+  it('verifies an ESM-signed token in a CommonJS runtime', () => {
+    const token = run(['--input-type=module', '-e', signEsm], {
+      DIST_URL: pathToFileURL(distEsm).href,
+      CLAIMS: JSON.stringify(claims),
+    });
+    expect(token.split('.')).toHaveLength(3);
+    const result = run(['-e', verifyCjs], { DIST: distCjs, TOKEN: token });
+    expect(JSON.parse(result)).toEqual({ valid: true, claims });
+  });
+});
diff --git a/packages/shared/src/token-crypto.ts b/packages/shared/src/token-crypto.ts
index cab3aa9..4df5add 100644
--- a/packages/shared/src/token-crypto.ts
+++ b/packages/shared/src/token-crypto.ts
@@ -1,5 +1,6 @@
-import { createHmac, timingSafeEqual } from 'node:crypto';
+import { hkdfSync } from 'node:crypto';
 
+import { errors as joseErrors, jwtVerify, SignJWT } from 'jose';
 import { z } from 'zod';
 
 import {
@@ -9,91 +10,87 @@ import {
 } from './collab-token';
 
 /**
- * Sign and verify collaboration tokens (issue #34, ADR 0007). These are the
- * ONLY JWTs in the system. Implemented with `node:crypto` rather than a library
- * so the exact same code runs in the CommonJS api and the ESM collab server
- * without module-interop or dependency-version drift. This module pulls in a
- * Node built-in, so it lives outside the browser-safe package barrel and is
- * imported via `@dorfteich/shared/token-crypto`.
+ * Purpose-bound token keys and collaboration-token signing (issue #34,
+ * ADR 0007; key separation and `jose` per ADR 0020, issue #188).
  *
- * The token is a standard compact HS256 JWT. Only HS256 is ever produced or
- * accepted; the signature is checked in constant time before any untrusted
- * field is read.
+ * The configured `COLLAB_TOKEN_SECRET` is a ROOT key: every purpose derives
+ * its own subkey via HKDF-SHA-256, and no code path signs with the root key
+ * directly. A token signed for one purpose cannot verify under another
+ * because the keys differ — the separation is structural, not a string
+ * prefix in the signed payload.
+ *
+ * Signing and verification go through `jose` (vetted, dependency-free)
+ * with HS256 as an explicit algorithm allowlist. The same module must work
+ * in the CommonJS api and the ESM collab server — proven by
+ * `token-crypto.crossruntime.test.ts` rather than assumed (jose v6 is
+ * ESM-only and reaches CJS via Node's `require(esm)`, available in the
+ * pinned Node 22 images). This module pulls in Node built-ins, so it lives
+ * outside the browser-safe package barrel and is imported via
+ * `@dorfteich/shared/token-crypto`.
  */
 
-const HEADER = { alg: 'HS256', typ: 'JWT' } as const;
+/** Every purpose a subkey is derived for. Add here — never reuse a key. */
+export type TokenPurpose = 'collab' | 'unsubscribe';
+
+/** Fixed HKDF salt: domain-separates this application's key hierarchy. */
+const HKDF_SALT = 'dorfteich-token-keys';
+
+/**
+ * Derive the 32-byte subkey for `purpose` from the configured root secret.
+ * Deterministic: rotation of the root secret rotates every subkey at once,
+ * which is the intended behaviour (ADR 0020).
+ */
+export function deriveTokenKey(rootSecret: string, purpose: TokenPurpose): Uint8Array {
+  return new Uint8Array(hkdfSync('sha256', rootSecret, HKDF_SALT, `dorfteich/${purpose}/v1`, 32));
+}
 
 /** Full JWT payload: the claims plus standard `iat`/`exp` (seconds since epoch). */
 const payloadSchema = collabTokenClaimsSchema.extend({
   iat: z.number().int().nonnegative(),
-  exp: z.number().int().nonnegative(),
+  exp: z.number().int(),
 });
 
-function b64url(value: string): string {
-  return Buffer.from(value, 'utf8').toString('base64url');
-}
-
-function hmac(signingInput: string, secret: string): string {
-  return createHmac('sha256', secret).update(signingInput).digest('base64url');
-}
-
 /** Sign a collaboration token that expires `ttlSeconds` from now. */
-export function signCollabToken(
+export async function signCollabToken(
   claims: CollabTokenClaims,
-  secret: string,
+  rootSecret: string,
   ttlSeconds: number,
-): string {
+): Promise<string> {
   const now = Math.floor(Date.now() / 1000);
-  const payload = { ...collabTokenClaimsSchema.parse(claims), iat: now, exp: now + ttlSeconds };
-  const signingInput = `${b64url(JSON.stringify(HEADER))}.${b64url(JSON.stringify(payload))}`;
-  return `${signingInput}.${hmac(signingInput, secret)}`;
+  return await new SignJWT({ ...collabTokenClaimsSchema.parse(claims) })
+    .setProtectedHeader({ alg: 'HS256', typ: 'JWT' })
+    .setIssuedAt(now)
+    .setExpirationTime(now + ttlSeconds)
+    .sign(deriveTokenKey(rootSecret, 'collab'));
+}
+
+/** Map a `jose` verification error onto the stable rejection reasons. */
+function reasonOf(error: unknown): Exclude<CollabTokenVerification, { valid: true }>['reason'] {
+  if (error instanceof joseErrors.JWTExpired) return 'expired';
+  if (error instanceof joseErrors.JOSEAlgNotAllowed) return 'bad_algorithm';
+  if (error instanceof joseErrors.JWSSignatureVerificationFailed) return 'bad_signature';
+  if (error instanceof joseErrors.JWTClaimValidationFailed) return 'invalid_claims';
+  return 'malformed';
 }
 
 /** Verify signature, algorithm, claims, and expiry. Never throws. */
-export function verifyCollabToken(
+export async function verifyCollabToken(
   token: string,
-  secret: string,
+  rootSecret: string,
   now: number = Date.now(),
-): CollabTokenVerification {
-  const parts = token.split('.');
-  const [headerPart, payloadPart, signaturePart] = parts;
-  if (
-    parts.length !== 3 ||
-    headerPart === undefined ||
-    payloadPart === undefined ||
-    signaturePart === undefined
-  ) {
-    return { valid: false, reason: 'malformed' };
-  }
-  const signingInput = `${headerPart}.${payloadPart}`;
-
-  // Verify the signature (recomputed with HS256, ignoring the header's claimed
-  // algorithm) before reading any field — defeats alg-confusion and forgery.
-  const expected = Buffer.from(hmac(signingInput, secret), 'utf8');
-  const provided = Buffer.from(signaturePart, 'utf8');
-  if (expected.length !== provided.length || !timingSafeEqual(expected, provided)) {
-    return { valid: false, reason: 'bad_signature' };
-  }
-
-  let header: unknown;
+): Promise<CollabTokenVerification> {
   let payload: unknown;
   try {
-    header = JSON.parse(Buffer.from(headerPart, 'base64url').toString('utf8'));
-    payload = JSON.parse(Buffer.from(payloadPart, 'base64url').toString('utf8'));
-  } catch {
-    return { valid: false, reason: 'malformed' };
+    ({ payload } = await jwtVerify(token, deriveTokenKey(rootSecret, 'collab'), {
+      algorithms: ['HS256'],
+      currentDate: new Date(now),
+      clockTolerance: 0,
+    }));
+  } catch (error) {
+    return { valid: false, reason: reasonOf(error) };
   }
-
-  // Defense in depth: even though the signature already pinned HS256, reject a
-  // token whose header advertises anything else (e.g. `none`).
-  if (
-    typeof header !== 'object' ||
-    header === null ||
-    (header as { alg?: unknown }).alg !== 'HS256'
-  ) {
-    return { valid: false, reason: 'bad_algorithm' };
-  }
-
+  // `jose` treats `exp` as optional; the schema makes it mandatory so a
+  // token without an expiry can never verify.
   const parsed = payloadSchema.safeParse(payload);
   if (!parsed.success) {
     return { valid: false, reason: 'invalid_claims' };
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index 51a08d4..db78446 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -559,6 +559,9 @@ importers:
 
   packages/shared:
     dependencies:
+      jose:
+        specifier: ^6.2.4
+        version: 6.2.4
       markdown-it:
         specifier: ^14.3.0
         version: 14.3.0
@@ -4951,6 +4954,9 @@ packages:
   jose@6.2.3:
     resolution: {integrity: sha512-YYVDInQKFJfR/xa3ojUTl8c2KoTwiL1R5Wg9YCydwH0x0B9grbzlg5HC7mMjCtUJjbQ/YnGEZIhI5tCgfTb4Hw==}
 
+  jose@6.2.4:
+    resolution: {integrity: sha512-N8acGzVsQy6M/fjFcxtysNc4Q379TcM5dM/qKkNtsHFji88yANnXTr7BLeP75iPnFwBfQzM/jg2BZ9+HZrHCZA==}
+
   jotai-scope@0.7.2:
     resolution: {integrity: sha512-Gwed97f3dDObrO43++2lRcgOqw4O2sdr4JCjP/7eHK1oPACDJ7xKHGScpJX9XaflU+KBHXF+VhwECnzcaQiShg==}
     peerDependencies:
@@ -11482,6 +11488,8 @@ snapshots:
 
   jose@6.2.3: {}
 
+  jose@6.2.4: {}
+
   jotai-scope@0.7.2(jotai@2.11.0(@types/react@19.2.17)(react@19.2.7))(react@19.2.7):
     dependencies:
       jotai: 2.11.0(@types/react@19.2.17)(react@19.2.7)