fix(relay-auth): close 5 pre-production security findings (F1–F5)

F1 (HIGH): bind step-up freshness to the required method (stepUpMethod) so a
  login-time or weaker (TOTP) factor can no longer satisfy a passkey requirement.
F2 (HIGH): make the step-up gate host-driven and fail-closed on connect+reattach
  (deny when policy.required and the principal is missing/stale/wrong-method).
F3 (MED): bind the WebAuthn assertion to the stored credential (credentialId
  cross-check + forward publicKey/credentialId to the verifier).
F4 (LOW): make verifyDpopProof totally fail-safe (no unhandled throw), wrap the
  authz boundary into a clean audited 401, and validate cnfJkt format at issue.
F5 (LOW): return newSignCount from finishAuthentication so the caller can persist
  the advanced counter (WebAuthn clone detection).

All fixes ship regression tests. relay-auth: 122 tests green, tsc clean.

relay-auth/test/stepup.test.ts

@@ -1,10 +1,10 @@
 import { describe, it, expect } from 'vitest'
-import { needsStepUp, recordStepUp, policyForHost } from '../src/human/stepup/stepup.js'
+import { needsStepUp, recordStepUp, policyForHost, NO_STEPUP_POLICY } from '../src/human/stepup/stepup.js'
 import type { StepUpPolicy } from '../src/types.js'
 import { principal, makeHost, uuid } from './_helpers.js'
 
 const NOW = 1_700_000_000
-const POLICY: StepUpPolicy = { maxAgeSeconds: 300, requiredMethod: 'passkey' }
+const POLICY: StepUpPolicy = { required: true, maxAgeSeconds: 300, requiredMethod: 'passkey' }
 
 describe('step-up before session open (T8, Finding-3)', () => {
   it('fresh login but stepUpAt=null → needsStepUp true (login ≠ step-up)', () => {
@@ -13,34 +13,61 @@ describe('step-up before session open (T8, Finding-3)', () => {
   })
 
   it('stale step-up (older than maxAgeSeconds) → true', () => {
-    const p = principal('acct-A', { stepUpAt: NOW - 301, amr: ['passkey', 'stepup'] })
+    const p = principal('acct-A', { stepUpAt: NOW - 301, amr: ['passkey', 'stepup'], stepUpMethod: 'passkey' })
     expect(needsStepUp(p, POLICY, NOW)).toBe(true)
   })
 
-  it('required method absent from amr → true', () => {
-    const p = principal('acct-A', { stepUpAt: NOW, amr: ['totp'] })
+  it('required method not the LAST step-up method → true', () => {
+    const p = principal('acct-A', { stepUpAt: NOW, amr: ['totp'], stepUpMethod: 'totp' })
     expect(needsStepUp(p, POLICY, NOW)).toBe(true)
   })
 
-  it('fresh passkey step-up → false', () => {
-    const p = principal('acct-A', { stepUpAt: NOW, amr: ['passkey'] })
+  it('fresh passkey step-up satisfies passkey policy → false (F1)', () => {
+    const p = principal('acct-A', { stepUpAt: NOW, amr: ['passkey'], stepUpMethod: 'passkey' })
     expect(needsStepUp(p, POLICY, NOW)).toBe(false)
   })
 
+  it('fresh TOTP step-up does NOT satisfy passkey policy → true (F1 factor-downgrade regression)', () => {
+    // A weaker (TOTP) step-up performed just now must not satisfy a passkey requirement, even though
+    // it is fresh — the required method itself must be the LAST step-up.
+    const p = principal('acct-A', { stepUpAt: NOW, amr: ['passkey', 'totp'], stepUpMethod: 'totp' })
+    expect(needsStepUp(p, POLICY, NOW)).toBe(true)
+  })
+
+  it('login-time passkey in amr but no step-up (stepUpMethod null) → true (login ≠ step-up)', () => {
+    const p = principal('acct-A', { stepUpAt: NOW, amr: ['passkey'], stepUpMethod: null })
+    expect(needsStepUp(p, POLICY, NOW)).toBe(true)
+  })
+
   it('recordStepUp returns a NEW principal (immutable), original untouched', () => {
-    const p = principal('acct-A', { stepUpAt: null, amr: ['totp'] })
+    const p = principal('acct-A', { stepUpAt: null, amr: ['totp'], stepUpMethod: 'totp' })
     const stepped = recordStepUp(p, 'passkey', NOW)
     expect(stepped).not.toBe(p)
     expect(p.stepUpAt).toBeNull()
     expect(p.amr).toEqual(['totp'])
+    expect(p.stepUpMethod).toBe('totp')
     expect(stepped.stepUpAt).toBe(NOW)
     expect(stepped.amr).toContain('passkey')
     expect(needsStepUp(stepped, POLICY, NOW)).toBe(false)
   })
 
-  it('policyForHost yields a passkey freshness requirement', () => {
+  it('recordStepUp stamps stepUpMethod with the method used (F1)', () => {
+    const p = principal('acct-A', { stepUpAt: null, amr: ['passkey'], stepUpMethod: null })
+    const stepped = recordStepUp(p, 'passkey', NOW)
+    expect(stepped.stepUpMethod).toBe('passkey')
+    expect(stepped.stepUpAt).toBe(NOW)
+  })
+
+  it('policyForHost yields a REQUIRED passkey freshness requirement', () => {
     const policy = policyForHost(makeHost('acct-A', uuid()))
+    expect(policy.required).toBe(true)
     expect(policy.requiredMethod).toBe('passkey')
     expect(policy.maxAgeSeconds).toBeGreaterThan(0)
   })
+
+  it('NO_STEPUP_POLICY is the v0.9 never-required default', () => {
+    expect(NO_STEPUP_POLICY.required).toBe(false)
+    expect(NO_STEPUP_POLICY.requiredMethod).toBe('passkey')
+    expect(NO_STEPUP_POLICY.maxAgeSeconds).toBeGreaterThan(0)
+  })
 })