fix(api): 登录验证码与 passkey 挑战不再被他人的 start 作废，冷却内重复 start 照常 202，登录挑战按来源限量

internal/api/api.go

@@ -962,7 +962,8 @@ func (c *cooldownLimiter) record(name string) {
 }
 
 // reserve atomically checks name's cooldown AND, if the window is open, records it
-// in the same critical section, returning the reservation time and true. Unlike
+// in the same critical section, returning the reservation time and true; inside the
+// window it returns the standing reservation's time and false. Unlike
 // allowed→record there is no gap between the check and the commit, so a burst of
 // truly concurrent callers yields exactly one winner. Use it where the throttle is
 // the SOLE defense and each admitted call has a non-idempotent side effect (an OTP
@@ -979,7 +980,7 @@ func (c *cooldownLimiter) reserve(name string, window time.Duration) (time.Time,
 	t := c.now()
 	c.noteWindow(window, t)
 	if last, ok := c.last[name]; ok && t.Sub(last) < window {
-		return time.Time{}, false
+		return last, false
 	}
 	c.last[name] = t
 	return t, true

internal/api/api_test.go

@@ -83,6 +83,9 @@ type fakeRepo struct {
 	// player email OTPs (spec §B2). Keyed by row id; the verify path scans for the
 	// newest live (user, purpose) just as the PG query does.
 	otps map[string]*fakeEmailOTP
+	// otpSeq orders codes by insertion (the PG created_at): a frozen test clock mints
+	// several codes at one instant, so time cannot tell the oldest apart.
+	otpSeq int
 	// otpBudget mirrors otp_failure_windows, keyed user|purpose.
 	otpBudget map[string]*fakeOTPBudget
 	// op-login requests (spec §B op-login). opLogins mirrors op_login_requests keyed
@@ -140,6 +143,9 @@ type fakePasskeyChallenge struct {
 	expiresAt   time.Time
 	consumed    bool
 	createdAt   time.Time
+	// challenge and source are set on email-first login rows only (migration 0029).
+	challenge string
+	source    string
 }
 
 // fakeDiscoverableChallenge mirrors a webauthn_discoverable_challenges row (task #40): no user
@@ -149,6 +155,7 @@ type fakeDiscoverableChallenge struct {
 	sessionData []byte
 	expiresAt   time.Time
 	consumed    bool
+	source      string
 }
 
 // fakeDataHold mirrors a player_data_holds row at the granularity the verifiable
@@ -177,10 +184,13 @@ func (f *fakeRepo) OTPLockedUntil(_ context.Context, userID, purpose string, now
 	return otpLockEnd(b.windowStart, b.failures, now), nil
 }
 
-// chargeOTP mirrors chargeOTPMismatch: one wrong guess on the code and the budget.
-func (f *fakeRepo) chargeOTP(live *fakeEmailOTP, now time.Time) error {
-	live.attempts++
-	key := live.userID + "|" + live.purpose
+// chargeOTP mirrors chargeOTPMismatch: one wrong guess on each open code and one on
+// the (user, purpose) budget.
+func (f *fakeRepo) chargeOTP(open []*fakeEmailOTP, userID, purpose string, now time.Time) error {
+	for _, o := range open {
+		o.attempts++
+	}
+	key := userID + "|" + purpose
 	b := f.otpBudget[key]
 	if b == nil || !b.windowStart.Add(otpFailureWindow).After(now) {
 		b = &fakeOTPBudget{windowStart: now}
@@ -206,6 +216,7 @@ type fakeEmailOTP struct {
 	expiresAt time.Time
 	consumed  bool
 	createdAt time.Time
+	seq       int
 }
 
 // fakeSession mirrors a sessions row: its owner, its expiry, and whether it has
@@ -403,12 +414,42 @@ func (f *fakeRepo) CreateEmailOTP(_ context.Context, id, userID, email, codeHash
 			delete(f.otps, k)
 		}
 	}
+	f.otpSeq++
 	f.otps[id] = &fakeEmailOTP{
 		id: id, userID: userID, email: email, codeHash: codeHash, purpose: purpose,
 		expiresAt: expiresAt, createdAt: expiresAt, // createdAt proxy: constant TTL ⇒ later expiry == later creation
+		seq: f.otpSeq,
 	}
 	return nil
 }
+
+// AddLoginEmailOTP mirrors PGRepo.AddLoginEmailOTP: the live codes of (user, purpose)
+// that expired at now go, then all but the newest otpLiveLoginCodes-1, and the new
+// code joins the rest.
+func (f *fakeRepo) AddLoginEmailOTP(_ context.Context, id, userID, email, codeHash, purpose string, now, expiresAt time.Time) error {
+	var live []*fakeEmailOTP
+	for k, o := range f.otps {
+		if o.userID != userID || o.purpose != purpose || o.consumed {
+			continue
+		}
+		if !o.expiresAt.After(now) {
+			delete(f.otps, k)
+			continue
+		}
+		live = append(live, o)
+	}
+	sort.Slice(live, func(i, j int) bool { return live[i].seq > live[j].seq })
+	for _, o := range live[min(len(live), otpLiveLoginCodes-1):] {
+		delete(f.otps, o.id)
+	}
+	f.otpSeq++
+	f.otps[id] = &fakeEmailOTP{
+		id: id, userID: userID, email: email, codeHash: codeHash, purpose: purpose,
+		expiresAt: expiresAt, createdAt: now, seq: f.otpSeq,
+	}
+	return nil
+}
+
 func (f *fakeRepo) VerifyEmailOTP(_ context.Context, userID, purpose, codeHash string, now time.Time) (string, error) {
 	var live *fakeEmailOTP
 	for _, o := range f.otps { // newest live (user, purpose)
@@ -432,7 +473,7 @@ func (f *fakeRepo) VerifyEmailOTP(_ context.Context, userID, purpose, codeHash s
 		return "", ErrOTPLocked
 	}
 	if live.codeHash != codeHash {
-		return "", f.chargeOTP(live, now) // a typo costs an attempt but does not consume the code
+		return "", f.chargeOTP([]*fakeEmailOTP{live}, userID, purpose, now) // a typo costs an attempt but does not consume the code
 	}
 	// A DIFFERENT verified holder of the same address → ErrEmailTaken, code left
 	// live — mirrors PGRepo's guard + the users_verified_email_unique index.
@@ -478,6 +519,44 @@ func (f *fakeRepo) CreatePasskeyChallenge(_ context.Context, id, userID, purpose
 	}
 	return nil
 }
+
+// AddPasskeyLoginChallenge / ConsumePasskeyLoginChallenge mirror PGRepo's email-first
+// login pair: begin reaps the account's spent login rows, refuses once source holds
+// maxLiveChallengesPerSource live login rows, and stores the challenge beside the
+// others; consume redeems the live row whose challenge the browser signed.
+func (f *fakeRepo) AddPasskeyLoginChallenge(_ context.Context, id, userID, purpose, source, challenge string, sessionData []byte, now, expiresAt time.Time) error {
+	fromSource := 0
+	for k, c := range f.passkeyChallenges {
+		if c.userID == userID && c.purpose == purpose && (c.consumed || !c.expiresAt.After(now)) {
+			delete(f.passkeyChallenges, k)
+			continue
+		}
+		if c.source == source && !c.consumed && c.expiresAt.After(now) {
+			fromSource++
+		}
+	}
+	if fromSource >= maxLiveChallengesPerSource {
+		return ErrTooManyPasskeyChallenges
+	}
+	f.passkeyChallenges[id] = &fakePasskeyChallenge{
+		id: id, userID: userID, purpose: purpose, sessionData: sessionData,
+		expiresAt: expiresAt, createdAt: now, challenge: challenge, source: source,
+	}
+	return nil
+}
+func (f *fakeRepo) ConsumePasskeyLoginChallenge(_ context.Context, userID, purpose, challenge string, now time.Time) ([]byte, error) {
+	for _, c := range f.passkeyChallenges {
+		if c.userID != userID || c.purpose != purpose || c.challenge != challenge || c.consumed {
+			continue
+		}
+		if !c.expiresAt.After(now) {
+			return nil, ErrPasskeyChallengeInvalid
+		}
+		c.consumed = true
+		return c.sessionData, nil
+	}
+	return nil, ErrPasskeyChallengeInvalid
+}
 func (f *fakeRepo) ConsumePasskeyChallengeByUser(_ context.Context, userID, purpose string, now time.Time) ([]byte, error) {
 	var live *fakePasskeyChallenge
 	for _, c := range f.passkeyChallenges { // newest live (user, purpose)
@@ -496,19 +575,28 @@ func (f *fakeRepo) ConsumePasskeyChallengeByUser(_ context.Context, userID, purp
 }
 
 // CreateDiscoverableChallenge / ConsumeDiscoverableChallenge mirror PGRepo's non-user-keyed
-// contract (task #40): begin reaps expired/consumed rows then stashes under the opaque handle,
-// and consume redeems by handle, single-use, expiry checked. discoverableFull forces the capped
-// path so the begin 429 branch is reachable without inserting thousands of rows.
-func (f *fakeRepo) CreateDiscoverableChallenge(_ context.Context, id string, sessionData []byte, now, expiresAt time.Time) error {
+// contract (task #40): begin reaps expired/consumed rows, refuses once source holds
+// maxLiveChallengesPerSource live rows, then stashes under the opaque handle; consume redeems
+// by handle, single-use, expiry checked. discoverableFull forces the global cap so the begin
+// 429 branch is reachable without inserting thousands of rows.
+func (f *fakeRepo) CreateDiscoverableChallenge(_ context.Context, id, source string, sessionData []byte, now, expiresAt time.Time) error {
 	if f.discoverableFull {
-		return ErrTooManyDiscoverableChallenges
+		return ErrTooManyPasskeyChallenges
 	}
+	fromSource := 0
 	for k, c := range f.discoverableChallenges { // reap (DELETE ... expires_at<=now OR consumed_at NOT NULL)
 		if c.consumed || !c.expiresAt.After(now) {
 			delete(f.discoverableChallenges, k)
+			continue
+		}
+		if c.source == source {
+			fromSource++
 		}
 	}
-	f.discoverableChallenges[id] = &fakeDiscoverableChallenge{sessionData: sessionData, expiresAt: expiresAt}
+	if fromSource >= maxLiveChallengesPerSource {
+		return ErrTooManyPasskeyChallenges
+	}
+	f.discoverableChallenges[id] = &fakeDiscoverableChallenge{sessionData: sessionData, expiresAt: expiresAt, source: source}
 	return nil
 }
 func (f *fakeRepo) ConsumeDiscoverableChallenge(_ context.Context, id string, now time.Time) ([]byte, error) {
@@ -624,6 +712,9 @@ type fakePasskeyVerifier struct {
 	// stashed SessionData round-trips and the existing credentials reach the verifier.
 	lastUser    PasskeyUser
 	lastSession []byte
+	// loginSession, when set, is the SessionData BeginLogin hands out in place of the
+	// per-user marker, so a test can tell two live login ceremonies apart at finish.
+	loginSession []byte
 	// discoverableUserHandle is the userHandle the fake feeds to FinishDiscoverableLogin's
 	// resolver, so a handler test drives the userHandle → UserByID → session-mint wiring for a
 	// chosen account (or an unknown handle, to exercise the resolve-fails branch).
@@ -657,6 +748,9 @@ func (v *fakePasskeyVerifier) BeginLogin(user PasskeyUser) (json.RawMessage, []b
 	if opts == nil {
 		opts = json.RawMessage(`{"publicKey":{"challenge":"YXNzZXJ0"}}`)
 	}
+	if v.loginSession != nil {
+		return opts, v.loginSession, nil
+	}
 	return opts, []byte("login-session:" + user.ID), nil
 }
 
@@ -1473,36 +1567,40 @@ func (f *fakeRepo) UserByEmail(_ context.Context, email string) (*StaffUser, err
 	return nil, ErrNotFound
 }
 
-// ConsumeLoginEmailOTP mirrors PGRepo.ConsumeLoginEmailOTP: it redeems the newest
-// live code for (user, purpose) WITHOUT the identity side-effect (login already
-// resolved the userID via UserByEmail, so the address is settled). It charges an
-// attempt on a hash mismatch (exactly like VerifyEmailOTP) but never writes
-// users.email or runs the verified-email guard. A missing/expired/consumed code →
-// ErrOTPInvalid; a mismatch → ErrOTPInvalid too (and costs an attempt without
-// consuming); a locked code → ErrOTPLocked; a match → consumed, nil.
+// ConsumeLoginEmailOTP mirrors PGRepo.ConsumeLoginEmailOTP: every live (unconsumed,
+// unexpired) code of (user, purpose) is a candidate. Nothing live → ErrOTPInvalid;
+// the account lock next; every live code out of attempts → ErrOTPLocked; no match →
+// one attempt on each open code plus one budget failure, nothing consumed; a match
+// spends every live code. No identity side-effect (login already resolved the
+// userID via UserByEmail, so the address is settled).
 func (f *fakeRepo) ConsumeLoginEmailOTP(_ context.Context, userID, purpose, codeHash string, now time.Time) error {
-	var live *fakeEmailOTP
-	for _, o := range f.otps { // newest live (user, purpose), mirroring VerifyEmailOTP
-		if o.userID != userID || o.purpose != purpose || o.consumed {
+	var live, open []*fakeEmailOTP
+	matched := false
+	for _, o := range f.otps {
+		if o.userID != userID || o.purpose != purpose || o.consumed || !o.expiresAt.After(now) {
 			continue
 		}
-		if live == nil || o.createdAt.After(live.createdAt) {
-			live = o
+		live = append(live, o)
+		if o.attempts < otpMaxAttempts {
+			open = append(open, o)
+			matched = matched || o.codeHash == codeHash
 		}
 	}
-	if live == nil || !live.expiresAt.After(now) {
+	if len(live) == 0 {
 		return ErrOTPInvalid
 	}
 	if until, _ := f.OTPLockedUntil(context.Background(), userID, purpose, now); !until.IsZero() {
 		return &OTPAccountLockedError{Until: until}
 	}
-	if live.attempts >= otpMaxAttempts {
+	if len(open) == 0 {
 		return ErrOTPLocked
 	}
-	if live.codeHash != codeHash {
-		return f.chargeOTP(live, now) // a typo costs an attempt but does not consume the code
+	if !matched {
+		return f.chargeOTP(open, userID, purpose, now) // a typo costs an attempt but consumes nothing
+	}
+	for _, o := range live {
+		o.consumed = true
 	}
-	live.consumed = true
 	return nil
 }
 

internal/api/errors.go

@@ -87,14 +87,13 @@ var (
 	// guard and gets a 409 instead of a raw unique-violation 500. Distinct from
 	// ErrConflict so the message can name the cause (the email is spoken for).
 	ErrEmailTaken = errors.New("email already verified on another account")
-	// ErrTooManyDiscoverableChallenges means the non-user-keyed discoverable ("usernameless")
-	// login challenge store is at its hard cap of live rows (task #40, migration 0013).
-	// Unlike the user-keyed enrollment/login challenges — which self-bound via a per-user
-	// supersede — a from-zero begin has no principal to key a fair per-caller limit on, so the
-	// table is capped globally and a begin over the cap is refused. Distinct from the other
-	// sentinels so the handler answers 429 (a transient "too busy, retry" — the cap self-clears
-	// as challenges expire), never a 400 that invites an immediate retry.
-	ErrTooManyDiscoverableChallenges = errors.New("too many discoverable login challenges in flight")
+	// ErrTooManyPasskeyChallenges means a passkey login begin was refused because too many
+	// login challenges are live: the caller's source already holds its allowance
+	// (maxLiveChallengesPerSource), or the discoverable store is at its global cap
+	// (maxLiveDiscoverableChallenges). Distinct from the other sentinels so the handler
+	// answers 429 (a transient "too busy, retry" — both bounds clear as challenges expire),
+	// never a 400 that invites an immediate retry.
+	ErrTooManyPasskeyChallenges = errors.New("too many passkey login challenges in flight")
 	// ErrNotStopped means a world-volume operation was refused because the server is
 	// not fully stopped: desiredState is not Stopped, or its pod is still shutting
 	// down (phase Stopping) and holds the volume while it saves.

internal/api/handlers_auth_email.go

@@ -60,6 +60,11 @@ type loginEmailStartRequest struct {
 // no code minted: the response never distinguishes the two, and the reservation is
 // kept on that path too so repeated probing of one address is throttled identically
 // to repeated sends.
+//
+// A start never cancels the codes already mailed (AddLoginEmailOTP keeps the newest
+// otpLiveLoginCodes live), and a start inside the cooldown answers the same 202 without
+// minting: the code mailed moments ago is still good. So anyone who knows an address
+// can only add codes to its owner's inbox, never keep the owner from signing in.
 func (a *API) handleLoginEmailStart(w http.ResponseWriter, r *http.Request) {
 	if !localAuthEnabled(r.Context(), a.Repo) {
 		writeError(w, r, newError(http.StatusForbidden, "local_auth_disabled",
@@ -98,8 +103,11 @@ func (a *API) handleLoginEmailStart(w http.ResponseWriter, r *http.Request) {
 	lim := a.otpLimiter()
 	emailAt, ok := lim.reserve(emailKey, otpResendCooldown)
 	if !ok {
-		writeError(w, r, newError(http.StatusTooManyRequests, "otp_resend_cooldown",
-			"a code was sent recently; wait a moment before requesting another"))
+		// A start for this address went through less than a cooldown ago, and the code
+		// it mailed (if the address has an account) is still live. Answer as that start
+		// did, expiry included, and mail nothing: the owner — or whoever typed the
+		// address — lands on the code screen and the code already in the inbox works.
+		writeJSON(w, http.StatusAccepted, map[string]any{"sent": true, "expires_at": emailAt.Add(otpTTL).UTC()})
 		return
 	}
 	committed := false
@@ -109,16 +117,17 @@ func (a *API) handleLoginEmailStart(w http.ResponseWriter, r *http.Request) {
 		}
 	}()
 
-	// Compute the expiry once so the neutral (no-account) branch and the real-send
-	// branch return byte-identical bodies.
-	expiresAt := a.now().Add(otpTTL)
+	// Compute the expiry once, from the reservation, so the neutral (no-account)
+	// branch, the real-send branch and a start inside the window all return
+	// byte-identical bodies.
+	expiresAt := emailAt.Add(otpTTL)
 
 	u, err := a.Repo.UserByEmail(r.Context(), email)
 	switch {
 	case errors.Is(err, ErrNotFound):
 		// No verified account for this address. Return the same 202 as a real send
-		// (no code minted) and KEEP the reservation, so probing an unknown address is
-		// throttled exactly like resending to a known one — the throttle reveals
+		// (no code minted) and KEEP the reservation, so a probe of an unknown address
+		// holds the window exactly like a send to a known one — the window reveals
 		// nothing, and the accepted /auth/options oracle is where existence is learnt.
 		committed = true
 		writeJSON(w, http.StatusAccepted, map[string]any{"sent": true, "expires_at": expiresAt.UTC()})
@@ -158,7 +167,7 @@ func (a *API) handleLoginEmailStart(w http.ResponseWriter, r *http.Request) {
 	// record. The login redeem (ConsumeLoginEmailOTP) never reads or writes this
 	// address, so the stored casing is authoritative and the row's email snapshot is
 	// purely for the audit trail.
-	if err := a.Repo.CreateEmailOTP(r.Context(), id, u.ID, u.Email, otpCodeHash(code), otpPurposeLogin, expiresAt); err != nil {
+	if err := a.Repo.AddLoginEmailOTP(r.Context(), id, u.ID, u.Email, otpCodeHash(code), otpPurposeLogin, a.now(), expiresAt); err != nil {
 		writeError(w, r, err)
 		return
 	}

internal/api/handlers_auth_email_test.go

@@ -152,8 +152,7 @@ func TestLoginEmailVertical(t *testing.T) {
 // TestLoginEmailStartNeutralOnUnknownAddress pins the start-side anti-enumeration
 // contract: an address with no verified account yields a 202 BYTE-IDENTICAL to a
 // real send (frozen clock ⇒ same expires_at), mints and mails nothing, audits
-// nothing — and still burns the cooldown window, so probing is throttled exactly
-// like sending.
+// nothing — and a re-probe inside the cooldown answers the same 202 a resend does.
 func TestLoginEmailStartNeutralOnUnknownAddress(t *testing.T) {
 	// A real send for comparison.
 	apiK, _, _ := seedLoginEmailAPI(t)
@@ -183,12 +182,14 @@ func TestLoginEmailStartNeutralOnUnknownAddress(t *testing.T) {
 		t.Errorf("neutral path must mint/mail/audit nothing, got otps=%d mails=%d audits=%d",
 			len(repoU.otps), mailerU.calls, len(repoU.audits))
 	}
-	// The reservation is KEPT on the neutral path: re-probing the same unknown
-	// address inside the window is throttled identically to a resend.
-	if w := do(ehU, "POST", "/api/v1/auth/email/start", `{"email":"[email protected]"}`, jsonHeader); w.Code != http.StatusTooManyRequests || decodeErr(t, w) != "otp_resend_cooldown" {
-		t.Fatalf("re-probe of unknown address: code = %d body %s, want 429 otp_resend_cooldown",
+	// Re-probing inside the window answers exactly as the first probe did.
+	if w := do(ehU, "POST", "/api/v1/auth/email/start", `{"email":"[email protected]"}`, jsonHeader); w.Code != http.StatusAccepted || w.Body.String() != wK.Body.String() {
+		t.Fatalf("re-probe of unknown address: code = %d body %s, want the same 202 as a real send",
 			w.Code, w.Body.String())
 	}
+	if len(repoU.otps) != 0 || mailerU.calls != 0 {
+		t.Errorf("re-probe must mint/mail nothing, got otps=%d mails=%d", len(repoU.otps), mailerU.calls)
+	}
 
 	// An UNVERIFIED account is indistinguishable from no account: UserByEmail only
 	// resolves proven addresses, so the door never mails one nobody controls.
@@ -278,13 +279,14 @@ func TestLoginEmailGates(t *testing.T) {
 // TestLoginEmailStartRateLimited closes the unauthenticated email-bomb vector on the
 // public door: one send per recipient per window, keyed case-insensitively, and
 // namespaced apart from the authenticated onboarding throttle so neither door can
-// starve the other.
+// starve the other. A start inside the window is answered like the one that sent,
+// so whoever asks lands on the code screen with the mail already in the inbox.
 func TestLoginEmailStartRateLimited(t *testing.T) {
 	start := func(eh http.Handler, email string) *httptest.ResponseRecorder {
 		return do(eh, "POST", "/api/v1/auth/email/start", `{"email":"`+email+`"}`, jsonHeader)
 	}
 
-	t.Run("same recipient is throttled, then recovers after the cooldown", func(t *testing.T) {
+	t.Run("a start inside the window mails nothing and keeps the first code", func(t *testing.T) {
 		api, repo, mailer := seedLoginEmailAPI(t)
 		clock := time.Unix(1_700_000_000, 0)
 		api.Now = func() time.Time { return clock }
@@ -293,28 +295,41 @@ func TestLoginEmailStartRateLimited(t *testing.T) {
 		if w := start(eh, "[email protected]"); w.Code != http.StatusAccepted {
 			t.Fatalf("first send: code = %d, want 202 (%s)", w.Code, w.Body.String())
 		}
-		if w := start(eh, "[email protected]"); w.Code != http.StatusTooManyRequests || decodeErr(t, w) != "otp_resend_cooldown" {
-			t.Fatalf("immediate resend: code = %d body %s, want 429 otp_resend_cooldown", w.Code, w.Body.String())
+		code := mailer.code
+		clock = clock.Add(30 * time.Second)
+		w := start(eh, "[email protected]")
+		if w.Code != http.StatusAccepted {
+			t.Fatalf("resend inside the window: code = %d, want 202 (%s)", w.Code, w.Body.String())
+		}
+		// The expiry is the first code's, the one the inbox holds.
+		if b := acctBody(t, w); b["sent"] != true || b["expires_at"] != "2023-11-14T22:23:20Z" {
+			t.Errorf("resend body = %v, want sent:true expires_at:2023-11-14T22:23:20Z", b)
 		}
 		if mailer.calls != 1 || len(repo.otps) != 1 {
-			t.Errorf("throttled resend must not mint or mail: mails=%d otps=%d, want 1/1",
+			t.Errorf("resend inside the window must not mint or mail: mails=%d otps=%d, want 1/1",
 				mailer.calls, len(repo.otps))
 		}
-		clock = clock.Add(otpResendCooldown + time.Second)
-		if w := start(eh, "[email protected]"); w.Code != http.StatusAccepted {
-			t.Fatalf("post-cooldown send: code = %d, want 202 (%s)", w.Code, w.Body.String())
+		if w := do(eh, "POST", "/api/v1/auth/email/verify",
+			`{"email":"[email protected]","code":"`+code+`"}`, jsonHeader); w.Code != http.StatusOK {
+			t.Fatalf("first code after a resend: code = %d, want 200 (%s)", w.Code, w.Body.String())
+		}
+		clock = clock.Add(otpResendCooldown)
+		if w := start(eh, "[email protected]"); w.Code != http.StatusAccepted || mailer.calls != 2 {
+			t.Fatalf("post-cooldown send: code = %d mails = %d, want 202 and a second mail (%s)",
+				w.Code, mailer.calls, w.Body.String())
 		}
 	})
 
 	t.Run("throttle key is case-insensitive", func(t *testing.T) {
-		api, _, _ := seedLoginEmailAPI(t)
+		api, _, mailer := seedLoginEmailAPI(t)
 		eh := api.ExternalHandler()
 		if w := start(eh, "[email protected]"); w.Code != http.StatusAccepted {
 			t.Fatalf("first send: code = %d, want 202 (%s)", w.Code, w.Body.String())
 		}
 		// A recased retype is the same mailbox: it must hit the same window.
-		if w := start(eh, "[email protected]"); w.Code != http.StatusTooManyRequests {
-			t.Fatalf("recased resend: code = %d, want 429 (key must be lowercased)", w.Code)
+		if w := start(eh, "[email protected]"); w.Code != http.StatusAccepted || mailer.calls != 1 {
+			t.Fatalf("recased resend: code = %d mails = %d, want 202 and no second mail (key must be lowercased)",
+				w.Code, mailer.calls)
 		}
 	})
 
@@ -339,6 +354,95 @@ func TestLoginEmailStartRateLimited(t *testing.T) {
 	})
 }
 
+// TestLoginStartsInsideTheWindowMatchExactly: with a clock that moves on every read,
+// a start inside the cooldown still answers with the very expires_at the first start
+// returned, on both email doors and for known and unknown addresses alike, so a
+// repeat start is indistinguishable from the first down to the nanosecond.
+func TestLoginStartsInsideTheWindowMatchExactly(t *testing.T) {
+	ticking := func(api *API) {
+		clock := time.Unix(1_700_000_000, 0)
+		api.Now = func() time.Time {
+			clock = clock.Add(time.Millisecond)
+			return clock
+		}
+	}
+	expiry := func(t *testing.T, w *httptest.ResponseRecorder) string {
+		t.Helper()
+		if w.Code != http.StatusAccepted {
+			t.Fatalf("start: code = %d, want 202 (%s)", w.Code, w.Body.String())
+		}
+		e, _ := acctBody(t, w)["expires_at"].(string)
+		return e
+	}
+	for _, email := range []string{"[email protected]", "[email protected]"} {
+		api, _, _ := seedLoginEmailAPI(t)
+		ticking(api)
+		eh := api.ExternalHandler()
+		start := func() *httptest.ResponseRecorder {
+			return do(eh, "POST", "/api/v1/auth/email/start", `{"email":"`+email+`"}`, jsonHeader)
+		}
+		first, again := expiry(t, start()), expiry(t, start())
+		if first != "2023-11-14T22:23:20.001Z" || again != first {
+			t.Errorf("email door %s: expires_at %q then %q, want 2023-11-14T22:23:20.001Z twice", email, first, again)
+		}
+	}
+	for _, email := range []string{"[email protected]", "[email protected]"} {
+		api, _, _ := seedOpLoginAPI(t)
+		ticking(api)
+		eh := api.ExternalHandler()
+		first, again := expiry(t, startOp(eh, email)), expiry(t, startOp(eh, email))
+		if first != "2023-11-14T22:23:20.001Z" || again != first {
+			t.Errorf("op door %s: expires_at %q then %q, want 2023-11-14T22:23:20.001Z twice", email, first, again)
+		}
+	}
+}
+
+// TestLoginEmailStartKeepsEarlierCodes is the stranger-keeps-starting case: someone
+// who knows the address starts a login every cooldown. Each start adds a code to the
+// owner's inbox and never cancels one, so the owner's own code keeps working until
+// otpLiveLoginCodes newer ones exist; signing in spends every code still out.
+func TestLoginEmailStartKeepsEarlierCodes(t *testing.T) {
+	api, repo, mailer := seedLoginEmailAPI(t)
+	clock := time.Unix(1_700_000_000, 0)
+	api.Now = func() time.Time { return clock }
+	eh := api.ExternalHandler()
+	start := func() string {
+		t.Helper()
+		if w := do(eh, "POST", "/api/v1/auth/email/start", `{"email":"[email protected]"}`, jsonHeader); w.Code != http.StatusAccepted {
+			t.Fatalf("start: code = %d, want 202 (%s)", w.Code, w.Body.String())
+		}
+		code := mailer.code
+		clock = clock.Add(otpResendCooldown)
+		return code
+	}
+	verify := func(code string) *httptest.ResponseRecorder {
+		return do(eh, "POST", "/api/v1/auth/email/verify",
+			`{"email":"[email protected]","code":"`+code+`"}`, jsonHeader)
+	}
+
+	owner := start()
+	second := start()
+	third := start()
+	if mailer.calls != 3 || len(repo.otps) != 3 {
+		t.Fatalf("mails=%d otps=%d, want 3/3 (a start must not cancel earlier codes)", mailer.calls, len(repo.otps))
+	}
+	fourth := start()
+	if len(repo.otps) != 3 {
+		t.Fatalf("otps = %d after a fourth start, want 3 (the oldest goes)", len(repo.otps))
+	}
+	if w := verify(owner); w.Code != http.StatusBadRequest || decodeErr(t, w) != "invalid_code" {
+		t.Fatalf("code with three newer ones: code = %d body %s, want 400 invalid_code", w.Code, w.Body.String())
+	}
+	if w := verify(second); w.Code != http.StatusOK {
+		t.Fatalf("second code while two newer are live: code = %d, want 200 (%s)", w.Code, w.Body.String())
+	}
+	for name, code := range map[string]string{"third": third, "fourth": fourth} {
+		if w := verify(code); w.Code != http.StatusBadRequest || decodeErr(t, w) != "invalid_code" {
+			t.Errorf("%s code after the sign-in: code = %d body %s, want 400 invalid_code", name, w.Code, w.Body.String())
+		}
+	}
+}
+
 // TestLoginEmailVerifyRejections is the redeem-side failure matrix. The anchor case
 // is uniformity: an unknown address and a wrong code for a known address answer with
 // the same (code, message) envelope, so the verify half never doubles as an

internal/api/handlers_email_otp.go

@@ -57,6 +57,15 @@ const (
 	// use a passkey.
 	otpFailureBudget = 10
 	otpFailureWindow = 24 * time.Hour
+	// otpLiveLoginCodes is how many codes a pre-session login door (email login,
+	// op-login) keeps redeemable per (account, purpose). Anyone who knows an address
+	// can start a login for it, so a start never cancels the codes already mailed:
+	// with one mail per otpResendCooldown, every code stays good for at least
+	// otpLiveLoginCodes cooldowns (or its TTL), and a stranger's starts only add
+	// codes to the owner's inbox. A wrong guess is compared with every live code, so
+	// the daily blind-hit chance rises to otpFailureBudget*otpLiveLoginCodes/1e6
+	// (3e-5). Enforced in the Repo so the fake and PG agree.
+	otpLiveLoginCodes = 3
 )
 
 // OTPMailer delivers a one-time code to an email address. It is a seam, not a

internal/api/handlers_op_login.go

@@ -65,6 +65,12 @@ type opLoginStartRequest struct {
 // unknown address yields the SAME 202 with a random, non-persisted handle and no mail,
 // so this never doubles as a staff-enumeration oracle (op.console's own Zero-Trust is
 // the edge gate; this app-layer neutrality covers the hostname-agnostic route).
+//
+// Anyone who knows a staff address can start a login for it, so a start never cancels
+// the codes already mailed (AddLoginEmailOTP), and a start inside the per-recipient
+// cooldown still gets a request of its own but mails nothing: the code mailed moments
+// ago is live and finishes it. A stranger's starts therefore only add codes to the
+// staff inbox and never keep its owner from signing in.
 func (a *API) handleOpLoginStart(w http.ResponseWriter, r *http.Request) {
 	if !localAuthEnabled(r.Context(), a.Repo) {
 		writeError(w, r, newError(http.StatusForbidden, "local_auth_disabled",
@@ -94,33 +100,32 @@ func (a *API) handleOpLoginStart(w http.ResponseWriter, r *http.Request) {
 	}
 
 	// Per-recipient cooldown reserved BEFORE any work, identical to the console email
-	// door: one winner per window, and the neutral (non-staff) branch keeps the
-	// reservation too so probing an address is throttled exactly like a real send. The
-	// key is namespaced apart from the console door's "login:email:" so the two
+	// door: one mail per window, and the neutral (non-staff) branch keeps the
+	// reservation too so a probe holds the window exactly like a real send. The key is
+	// namespaced apart from the console door's "login:email:" so the two
 	// unauthenticated doors never perturb each other's throttle.
 	emailKey := "oplogin:email:" + strings.ToLower(email)
 	lim := a.otpLimiter()
-	emailAt, ok := lim.reserve(emailKey, otpResendCooldown)
-	if !ok {
-		writeError(w, r, newError(http.StatusTooManyRequests, "otp_resend_cooldown",
-			"a code was sent recently; wait a moment before requesting another"))
-		return
-	}
+	emailAt, fresh := lim.reserve(emailKey, otpResendCooldown)
 	committed := false
-	defer func() {
-		if !committed {
-			lim.release(emailKey, emailAt)
-		}
-	}()
+	if fresh {
+		defer func() {
+			if !committed {
+				lim.release(emailKey, emailAt)
+			}
+		}()
+	}
 
-	// Compute expiry once so the neutral and real branches return identical-shaped
-	// bodies and (real branch) the request row and its OTP are coterminous.
-	expiresAt := a.now().Add(otpTTL)
+	// Compute expiry once, from the reservation, so the neutral and real branches
+	// return identical bodies and the request row and its OTP are coterminous. Inside
+	// the cooldown the live code is the one the standing reservation mailed, so the
+	// request ends with it.
+	expiresAt := emailAt.Add(otpTTL)
 
 	// neutral returns the indistinguishable no-op success: a plausible but non-persisted
 	// handle that status(id) reads approved:false forever (no row, never approvable). It
-	// mints nothing and mails nothing, and KEEPS the reservation so probing is throttled
-	// exactly like a real send.
+	// mints nothing and mails nothing, and KEEPS the reservation so a probe holds the
+	// window exactly like a real send.
 	neutral := func() {
 		fakeID, err := newOTPID()
 		if err != nil {
@@ -172,6 +177,15 @@ func (a *API) handleOpLoginStart(w http.ResponseWriter, r *http.Request) {
 		writeError(w, r, err)
 		return
 	}
+	if !fresh {
+		// Inside the cooldown: the code mailed with the standing reservation finishes
+		// this request too, and the mailbox still sees one code per window.
+		a.auditAccount(r, u, "auth.op_login.requested", "")
+		writeJSON(w, http.StatusAccepted, map[string]any{
+			"request_id": id, "expires_at": expiresAt.UTC(),
+		})
+		return
+	}
 	code, err := newEmailOTP()
 	if err != nil {
 		writeError(w, r, err)
@@ -184,7 +198,7 @@ func (a *API) handleOpLoginStart(w http.ResponseWriter, r *http.Request) {
 	}
 	// Mint+mail against the STORED staff address (UserByEmail matched case-insensitively);
 	// the request row snapshots the same address for its audit trail.
-	if err := a.Repo.CreateEmailOTP(r.Context(), otpID, u.ID, u.Email, otpCodeHash(code), otpPurposeOpLogin, expiresAt); err != nil {
+	if err := a.Repo.AddLoginEmailOTP(r.Context(), otpID, u.ID, u.Email, otpCodeHash(code), otpPurposeOpLogin, a.now(), expiresAt); err != nil {
 		writeError(w, r, err)
 		return
 	}

internal/api/handlers_op_login_test.go

@@ -212,8 +212,8 @@ func TestOpLoginOwnerAdmitted(t *testing.T) {
 
 // TestOpLoginStartNeutral pins the start-side anti-enumeration contract: op.console is
 // the STAFF door, so a non-admin account AND an unknown address both get a 202 carrying
-// a request_id + expires_at, mint/mail nothing, and still burn the per-recipient
-// cooldown — so neither the response nor the throttle tells a caller who is staff.
+// a request_id + expires_at and mint/mail nothing, and a re-probe inside the cooldown
+// does the same — so neither the response nor the throttle tells a caller who is staff.
 func TestOpLoginStartNeutral(t *testing.T) {
 	check := func(t *testing.T, seed func(*fakeRepo), email, wantReason, wantUser string) {
 		t.Helper()
@@ -248,9 +248,14 @@ func TestOpLoginStartNeutral(t *testing.T) {
 			repo.audits[0].ActorUserID != wantUser {
 			t.Errorf("neutral start audits = %+v, want one auth.op_login.failed %s by %q", repo.audits, wantReason, wantUser)
 		}
-		// The reservation is KEPT: re-probing the same address is throttled like a resend.
-		if w := startOp(eh, email); w.Code != http.StatusTooManyRequests || decodeErr(t, w) != "otp_resend_cooldown" {
-			t.Fatalf("re-probe: code = %d body %s, want 429 otp_resend_cooldown", w.Code, w.Body.String())
+		// Re-probing inside the window: the same 202 shape, still nothing behind it.
+		w = startOp(eh, email)
+		if b := acctBody(t, w); w.Code != http.StatusAccepted || b["request_id"] == "" || b["expires_at"] != "2023-11-14T22:23:20Z" {
+			t.Fatalf("re-probe: code = %d body %s, want 202 with a request_id and the first expiry", w.Code, w.Body.String())
+		}
+		if len(repo.opLogins) != 0 || len(repo.otps) != 0 || mailer.calls != 0 {
+			t.Errorf("re-probe must mint/mail nothing: reqs=%d otps=%d mails=%d",
+				len(repo.opLogins), len(repo.otps), mailer.calls)
 		}
 	}
 
@@ -264,6 +269,64 @@ func TestOpLoginStartNeutral(t *testing.T) {
 	})
 }
 
+// TestOpLoginStartByAStranger is the case of someone who knows a staff address and
+// keeps starting logins for it. Their start mails the code to the staff inbox; the
+// staff member's own start inside the cooldown still gets a request of its own
+// (nothing new mailed, same expiry as the live code), and that inbox code finishes
+// it. A start after the cooldown mails a second code without cancelling the first.
+func TestOpLoginStartByAStranger(t *testing.T) {
+	api, repo, mailer := seedOpLoginAPI(t)
+	clock := time.Unix(1_700_000_000, 0)
+	api.Now = func() time.Time { return clock }
+	eh := api.ExternalHandler()
+	ih := api.InternalHandler()
+	requestID := func(w *httptest.ResponseRecorder) string {
+		t.Helper()
+		if w.Code != http.StatusAccepted {
+			t.Fatalf("start: code = %d, want 202 (%s)", w.Code, w.Body.String())
+		}
+		id, _ := acctBody(t, w)["request_id"].(string)
+		return id
+	}
+
+	strangers := requestID(startOp(eh, "[email protected]"))
+	inboxCode := mailer.code
+	clock = clock.Add(30 * time.Second)
+	w := startOp(eh, "[email protected]")
+	own := requestID(w)
+	if own == strangers || repo.opLogins[own] == nil {
+		t.Fatalf("own request %q must be a new, stored request beside the stranger's %q", own, strangers)
+	}
+	if b := acctBody(t, w); b["expires_at"] != "2023-11-14T22:23:20Z" {
+		t.Errorf("own start expires_at = %v, want 2023-11-14T22:23:20Z (the live code's)", b["expires_at"])
+	}
+	if mailer.calls != 1 || len(repo.otps) != 1 {
+		t.Fatalf("start inside the cooldown: mails=%d otps=%d, want 1/1", mailer.calls, len(repo.otps))
+	}
+	if w := approveOp(ih, own, opUUID); w.Code != http.StatusOK {
+		t.Fatalf("approve: code = %d (%s)", w.Code, w.Body.String())
+	}
+	if w := finishOp(eh, own, inboxCode); w.Code != http.StatusOK {
+		t.Fatalf("finish own request with the inbox code: code = %d, want 200 (%s)", w.Code, w.Body.String())
+	}
+
+	// After the cooldown a start mails a second code; the first one still works.
+	clock = clock.Add(otpResendCooldown)
+	first := requestID(startOp(eh, "[email protected]"))
+	firstCode := mailer.code
+	clock = clock.Add(otpResendCooldown)
+	requestID(startOp(eh, "[email protected]"))
+	if mailer.calls != 3 {
+		t.Fatalf("mails = %d, want 3", mailer.calls)
+	}
+	if w := approveOp(ih, first, opUUID); w.Code != http.StatusOK {
+		t.Fatalf("approve: code = %d (%s)", w.Code, w.Body.String())
+	}
+	if w := finishOp(eh, first, firstCode); w.Code != http.StatusOK {
+		t.Fatalf("finish with the earlier code after a newer start: code = %d, want 200 (%s)", w.Code, w.Body.String())
+	}
+}
+
 // TestOpLoginStatusNeutral proves status is never an enumeration oracle: it returns
 // approved:true ONLY for a genuinely approved, live, unconsumed request, and
 // approved:false (never 404) for an unknown, expired, denied, or consumed handle — all

internal/api/handlers_passkey.go

@@ -4,9 +4,11 @@ import (
 	"bytes"
 	"context"
 	"crypto/rand"
+	"encoding/base64"
 	"encoding/hex"
 	"encoding/json"
 	"errors"
+	"fmt"
 	"io"
 	"net/http"
 	"strings"
@@ -241,6 +243,61 @@ func unwrapPublicKey(options json.RawMessage) json.RawMessage {
 	return env.PublicKey
 }
 
+// optionsChallenge returns the challenge in go-webauthn's request options
+// ({"publicKey": {"challenge": ...}}) in canonical form: the value the browser will
+// sign into the assertion's clientDataJSON.
+func optionsChallenge(options json.RawMessage) (string, error) {
+	var env struct {
+		PublicKey struct {
+			Challenge string `json:"challenge"`
+		} `json:"publicKey"`
+	}
+	if err := json.Unmarshal(options, &env); err != nil {
+		return "", err
+	}
+	return canonicalChallenge(env.PublicKey.Challenge)
+}
+
+// assertionChallenge returns, in canonical form, the challenge a
+// navigator.credentials.get() result signed: response.clientDataJSON is base64url
+// JSON whose challenge member is that value. It only picks the ceremony to verify
+// against; the verifier checks the signature over the same bytes.
+func assertionChallenge(assertion json.RawMessage) (string, error) {
+	var body struct {
+		Response struct {
+			ClientDataJSON string `json:"clientDataJSON"`
+		} `json:"response"`
+	}
+	if err := json.Unmarshal(assertion, &body); err != nil {
+		return "", err
+	}
+	raw, err := base64.RawURLEncoding.DecodeString(strings.TrimRight(body.Response.ClientDataJSON, "="))
+	if err != nil {
+		return "", err
+	}
+	var clientData struct {
+		Challenge string `json:"challenge"`
+	}
+	if err := json.Unmarshal(raw, &clientData); err != nil {
+		return "", err
+	}
+	return canonicalChallenge(clientData.Challenge)
+}
+
+// canonicalChallenge decodes a base64url challenge, padded or not (go-webauthn
+// accepts both), and re-encodes it unpadded, so the stored and the signed forms
+// compare equal. An empty or undecodable challenge is an error.
+func canonicalChallenge(s string) (string, error) {
+	b, err := base64.RawURLEncoding.DecodeString(strings.TrimRight(s, "="))
+	if err != nil {
+		return "", err
+	}
+	if len(b) == 0 {
+		return "", errors.New("empty challenge")
+	}
+	return base64.RawURLEncoding.EncodeToString(b), nil
+}
+
 // handlePasskeyRegisterBegin mints a credential-creation challenge for the caller
 // (spec §14, external app face). It loads the passkeys the caller has already bound so
 // the ceremony excludes them (one authenticator binds once), asks the verifier for the
@@ -461,12 +518,16 @@ type passkeyLoginBeginRequest struct {
 // (Public, pre-session). It resolves the typed email to an account, loads the
 // passkeys that account has bound, and asks the verifier for the assertion options
 // + opaque SessionData the browser needs for navigator.credentials.get(). The
-// SessionData is stashed under a short TTL, keyed to the user so the finish step
-// can consume it. Requires local sessions to be enabled (like the other pre-session
-// doors). A user with no bound passkey, an unknown email, and a real account with
-// passkeys are distinguished by status code (400 vs 200) — this is an accepted
-// enumeration trade-off (the /auth/options oracle is the sanctioned place to learn
-// existence), but the per-recipient cooldown below makes probing impractical.
+// SessionData is stashed under a short TTL beside the account's other live login
+// ceremonies, tagged with the challenge the browser will sign, so finish consumes
+// exactly the ceremony it answers: anyone who knows the address can begin a login for
+// it, and a begin never cancels the owner's. The store holds each network to
+// maxLiveChallengesPerSource live login challenges (429 too_many_challenges past it).
+// Requires local sessions to be enabled (like the other pre-session doors). A user
+// with no bound passkey, an unknown email, and a real account with passkeys are
+// distinguished by status code (400 vs 200) — this is an accepted enumeration
+// trade-off (the /auth/options oracle is the sanctioned place to learn existence);
+// volume per caller is bounded by the auth-door bucket.
 func (a *API) handlePasskeyLoginBegin(w http.ResponseWriter, r *http.Request) {
 	if !localAuthEnabled(r.Context(), a.Repo) {
 		writeError(w, r, newError(http.StatusForbidden, "local_auth_disabled",
@@ -492,29 +553,9 @@ func (a *API) handlePasskeyLoginBegin(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	// Per-recipient cooldown reserved BEFORE any work, identical to the email-OTP and
-	// op-login doors: one winner per window, so a burst of probes is throttled. The
-	// key is namespaced apart from the other pre-session doors so they never perturb
-	// each other's throttle.
-	emailKey := "passkey:login:" + strings.ToLower(email)
-	lim := a.otpLimiter()
-	emailAt, ok := lim.reserve(emailKey, otpResendCooldown)
-	if !ok {
-		writeError(w, r, newError(http.StatusTooManyRequests, "otp_resend_cooldown",
-			"a passkey login was started recently; wait a moment before requesting another"))
-		return
-	}
-	committed := false
-	defer func() {
-		if !committed {
-			lim.release(emailKey, emailAt)
-		}
-	}()
-
 	u, err := a.Repo.UserByEmail(r.Context(), email)
 	if err != nil {
 		if errors.Is(err, ErrNotFound) {
-			committed = true // keep the reservation so probing is throttled
 			writeError(w, r, newError(http.StatusBadRequest, "no_passkey",
 				"no passkey enrolled for this account; use email or operator login"))
 			return
@@ -529,7 +570,6 @@ func (a *API) handlePasskeyLoginBegin(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 	if len(creds) == 0 {
-		committed = true
 		writeError(w, r, newError(http.StatusBadRequest, "no_passkey",
 			"no passkey enrolled for this account; use email or operator login"))
 		return
@@ -547,17 +587,26 @@ func (a *API) handlePasskeyLoginBegin(w http.ResponseWriter, r *http.Request) {
 			"could not start passkey login"))
 		return
 	}
+	challenge, err := optionsChallenge(options)
+	if err != nil {
+		writeError(w, r, fmt.Errorf("passkey login options carry no challenge: %w", err))
+		return
+	}
 	id, err := newPasskeyID()
 	if err != nil {
 		writeError(w, r, err)
 		return
 	}
-	expiresAt := a.now().Add(passkeyChallengeTTL)
-	if err := a.Repo.CreatePasskeyChallenge(r.Context(), id, u.ID, passkeyPurposeLogin, sessionData, expiresAt); err != nil {
+	now := a.now()
+	if err := a.Repo.AddPasskeyLoginChallenge(r.Context(), id, u.ID, passkeyPurposeLogin,
+		challengeSource(a.clientIP(r)), challenge, sessionData, now, now.Add(passkeyChallengeTTL)); err != nil {
+		if errors.Is(err, ErrTooManyPasskeyChallenges) {
+			writeError(w, r, errTooManyChallenges)
+			return
+		}
 		writeError(w, r, err)
 		return
 	}
-	committed = true
 	// go-webauthn wraps the assertion options as {"publicKey": {...}}; the panel's
 	// username-login flow reads them flat (options.challenge, options.allowCredentials),
 	// so strip the envelope. (Discoverable login keeps the envelope — see its handler.)
@@ -575,7 +624,8 @@ type passkeyLoginFinishRequest struct {
 
 // handlePasskeyLoginFinish verifies a passkey assertion and mints a session (Public,
 // pre-session). It resolves the email to the account, atomically consumes the
-// stashed login challenge (a missing or expired one → 400), verifies the assertion
+// stashed login challenge the assertion signed (clientDataJSON names it; a missing,
+// expired, or unnamed one → 400), verifies the assertion
 // against the SessionData, and mints a felis_session. Both players and staff may
 // log in this way — the passkey is a two-factor authenticator (possession +
 // biometric/PIN), strong enough to stand alone without the in-game approval the
@@ -623,7 +673,14 @@ func (a *API) handlePasskeyLoginFinish(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	sessionData, err := a.Repo.ConsumePasskeyChallengeByUser(r.Context(), u.ID, passkeyPurposeLogin, a.now())
+	challenge, err := assertionChallenge(req.Assertion)
+	if err != nil {
+		a.authFailure(r, "passkey", "bad_assertion", u)
+		writeError(w, r, newError(http.StatusBadRequest, "passkey_login_invalid",
+			"passkey login could not be completed; begin again"))
+		return
+	}
+	sessionData, err := a.Repo.ConsumePasskeyLoginChallenge(r.Context(), u.ID, passkeyPurposeLogin, challenge, a.now())
 	if err != nil {
 		if errors.Is(err, ErrPasskeyChallengeInvalid) {
 			a.authFailure(r, "passkey", "challenge_invalid", u)

internal/api/handlers_passkey_discoverable.go

@@ -19,12 +19,17 @@ import (
 // and username-first passkey remain the fallbacks, so an authenticator that stored no resident
 // key is never locked out — only its from-zero convenience is unavailable.
 //
-// Anti-abuse divergence from the email-first door: that door reserves a per-recipient cooldown
-// (a.otpLimiter) keyed on the typed email. A usernameless begin has no recipient OR principal to
-// key a fair per-caller limit on, so one client is bounded by the per-address token bucket every
-// public auth door sits behind (throttleAuthDoor), and the table by a hard global cap on live
-// challenges enforced atomically in CreateDiscoverableChallenge (ErrTooManyDiscoverableChallenges
-// → 429).
+// Anti-abuse: a usernameless begin has no recipient OR principal to key a limit on, so one client
+// is bounded by the per-address token bucket every public auth door sits behind
+// (throttleAuthDoor), each network (IPv4 host or IPv6 /48, challengeSource) by
+// maxLiveChallengesPerSource live challenges, and the table by a global cap on live challenges;
+// CreateDiscoverableChallenge enforces both bounds atomically (ErrTooManyPasskeyChallenges →
+// 429). A flood from one network fills its own allowance and leaves every other network its
+// sign-ins.
+
+// errTooManyChallenges answers a passkey login begin over a challenge bound.
+var errTooManyChallenges = newError(http.StatusTooManyRequests, "too_many_challenges",
+	"too many passkey logins in progress from this network; try again in a few minutes")
 
 // handlePasskeyLoginDiscoverableBegin starts a usernameless assertion ceremony (Public,
 // pre-session). It has no request body — the whole point is that the caller supplies no
@@ -59,10 +64,9 @@ func (a *API) handlePasskeyLoginDiscoverableBegin(w http.ResponseWriter, r *http
 		return
 	}
 	now := a.now()
-	if err := a.Repo.CreateDiscoverableChallenge(r.Context(), id, sessionData, now, now.Add(passkeyChallengeTTL)); err != nil {
-		if errors.Is(err, ErrTooManyDiscoverableChallenges) {
-			writeError(w, r, newError(http.StatusTooManyRequests, "too_many_challenges",
-				"too many passkey logins in progress; try again shortly"))
+	if err := a.Repo.CreateDiscoverableChallenge(r.Context(), id, challengeSource(a.clientIP(r)), sessionData, now, now.Add(passkeyChallengeTTL)); err != nil {
+		if errors.Is(err, ErrTooManyPasskeyChallenges) {
+			writeError(w, r, errTooManyChallenges)
 			return
 		}
 		writeError(w, r, err)

internal/api/handlers_passkey_discoverable_test.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"encoding/json"
 	"errors"
+	"fmt"
 	"net/http"
 	"net/http/httptest"
 	"testing"
@@ -136,11 +137,10 @@ func TestPasskeyDiscoverableLoginVertical(t *testing.T) {
 	}
 }
 
-// TestPasskeyDiscoverableLoginBeginCapped pins the server-side volumetric bound: with the store
-// at its hard cap, begin answers 429 too_many_challenges and stashes nothing. This is the only
-// per-server brake on the usernameless begin (there is no recipient/principal to key a per-caller
-// cooldown on, so volumetric per-source limiting is delegated to the edge) — a reap alone cannot
-// bound a burst, since freshly-inserted rows are not yet expired.
+// TestPasskeyDiscoverableLoginBeginCapped pins the store-wide bound: with the store at its hard
+// cap, begin answers 429 too_many_challenges and stashes nothing. A reap alone cannot bound a
+// burst, since freshly-inserted rows are not yet expired; the per-source bound below keeps one
+// network from reaching this cap.
 func TestPasskeyDiscoverableLoginBeginCapped(t *testing.T) {
 	api, repo, _ := seedDiscoverableLoginAPI(t)
 	repo.discoverableFull = true
@@ -155,6 +155,35 @@ func TestPasskeyDiscoverableLoginBeginCapped(t *testing.T) {
 	}
 }
 
+// TestPasskeyDiscoverableLoginBeginPerSourceCap: the usernameless begin has no account to key
+// on, so the store holds each network (IPv4 address, IPv6 /48) to 32 live challenges. The
+// begins come from 33 different /64s inside one /48, so the per-/64 auth-door bucket never
+// trips and only the /48 bound refuses the last; another network still begins.
+func TestPasskeyDiscoverableLoginBeginPerSourceCap(t *testing.T) {
+	api, repo, _ := seedDiscoverableLoginAPI(t)
+	api.ClientIPHeader = "CF-Connecting-IP"
+	eh := api.ExternalHandler()
+	from := func(ip string) *httptest.ResponseRecorder {
+		return do(eh, "POST", "/api/v1/auth/passkey/login/discoverable/begin", `{}`,
+			map[string]string{"Content-Type": "application/json", "CF-Connecting-IP": ip})
+	}
+	for i := 1; i <= 32; i++ {
+		if w := from(fmt.Sprintf("2001:db8:7:%x::1", i)); w.Code != http.StatusOK {
+			t.Fatalf("begin %d: code = %d, want 200 (%s)", i, w.Code, w.Body.String())
+		}
+	}
+	w := from("2001:db8:7:ff::1")
+	if w.Code != http.StatusTooManyRequests || decodeErr(t, w) != "too_many_challenges" {
+		t.Fatalf("33rd begin from the /48: code = %d body %s, want 429 too_many_challenges", w.Code, w.Body.String())
+	}
+	if len(repo.discoverableChallenges) != 32 {
+		t.Errorf("stashed = %d, want 32", len(repo.discoverableChallenges))
+	}
+	if w := from("2001:db8:8::1"); w.Code != http.StatusOK {
+		t.Fatalf("begin from another /48: code = %d, want 200 (%s)", w.Code, w.Body.String())
+	}
+}
+
 // TestPasskeyDiscoverableLoginBeginVerifierError pins the verifier-fault path: a
 // BeginDiscoverableLogin failure is a server-side fault, answered passkey_login_failed, and
 // stashes no challenge (there is nothing to stash — the ceremony never started).

internal/api/handlers_passkey_login_test.go

@@ -2,8 +2,10 @@ package api
 
 import (
 	"bytes"
+	"encoding/base64"
 	"encoding/json"
 	"errors"
+	"fmt"
 	"net/http"
 	"net/http/httptest"
 	"testing"
@@ -23,8 +25,9 @@ import (
 //   - Anti-enumeration on finish: unknown email, no live challenge, expired challenge and
 //     a bad assertion all collapse to ONE passkey_login_invalid envelope, so the finish
 //     half is never an existence/state oracle.
-//   - Cooldown seals the accepted begin-side trade-off: has-passkey (200) vs no_passkey
-//     (400) is a status oracle, but one begin per recipient per window throttles probing.
+//   - Ceremonies coexist: finish picks the live challenge the browser signed, so a
+//     begin by someone else who knows the address never cancels the owner's, and each
+//     network holds a bounded number of live login challenges.
 //   - Staff admitted: unlike the email door's staff_account refusal, a passkey stands
 //     alone (possession + user-verification), so role=admin mints a session here.
 
@@ -57,14 +60,30 @@ func seedLoginPasskeyAPI(t *testing.T) (*API, *fakeRepo, *fakePasskeyVerifier) {
 // plantLoginChallenge seeds a stashed LOGIN-purpose challenge for u1 directly, so the
 // finish-side branches (expired, verification failure) are reachable under the frozen
 // clock without running begin first. Mirrors plantPasskeyChallenge, but scoped to
-// passkeyPurposeLogin so it is only ever consumed by the login door.
+// passkeyPurposeLogin so it is only ever consumed by the login door. Its challenge is
+// the one the fake verifier hands out by default, so loginAssertion("cred-1",
+// "YXNzZXJ0") answers it.
 func plantLoginChallenge(repo *fakeRepo, id string, expiresAt time.Time) {
 	repo.passkeyChallenges[id] = &fakePasskeyChallenge{
-		id: id, userID: "u1", purpose: passkeyPurposeLogin,
+		id: id, userID: "u1", purpose: passkeyPurposeLogin, challenge: "YXNzZXJ0", source: "192.0.2.1",
 		sessionData: []byte("login-session:u1"), expiresAt: expiresAt, createdAt: expiresAt,
 	}
 }
 
+// loginAssertion is the JSON a browser posts back from navigator.credentials.get(): the
+// credential id plus response.clientDataJSON, the base64url JSON that carries the signed
+// challenge.
+func loginAssertion(credID, challenge string) string {
+	clientData := base64.RawURLEncoding.EncodeToString(
+		[]byte(`{"type":"webauthn.get","challenge":"` + challenge + `","origin":"https://console.example.net"}`))
+	return `{"id":"` + credID + `","type":"public-key","response":{"clientDataJSON":"` + clientData + `"}}`
+}
+
+// finishBody is a username-first finish body answering challenge with cred-1.
+func finishBody(email, challenge string) string {
+	return `{"email":"` + email + `","assertion":` + loginAssertion("cred-1", challenge) + `}`
+}
+
 // TestPasskeyLoginVertical walks the whole returning-player slice across the external
 // face: begin resolves the mixed-case account from a lowercase-typed email, hands its
 // bound credential to the verifier, returns the assertion options verbatim and stashes
@@ -105,8 +124,7 @@ func TestPasskeyLoginVertical(t *testing.T) {
 	// 2) finish — typed in yet another casing, proving the finish-side resolver is
 	// case-insensitive too — verifies the assertion and mints the session. The body
 	// carries NO challenge, only email+assertion.
-	w = do(eh, "POST", "/api/v1/auth/passkey/login/finish",
-		`{"email":"[email protected]","assertion":{"id":"cred-1","type":"public-key"}}`, jsonHeader)
+	w = do(eh, "POST", "/api/v1/auth/passkey/login/finish", finishBody("[email protected]", "YXNzZXJ0"), jsonHeader)
 	if w.Code != http.StatusOK {
 		t.Fatalf("finish: code = %d, want 200 (%s)", w.Code, w.Body.String())
 	}
@@ -144,8 +162,7 @@ func TestPasskeyLoginVertical(t *testing.T) {
 	}
 
 	// 3) single-use: the consumed challenge buys nothing a second time.
-	if w := do(eh, "POST", "/api/v1/auth/passkey/login/finish",
-		`{"email":"[email protected]","assertion":{"id":"cred-1","type":"public-key"}}`, jsonHeader); w.Code != http.StatusBadRequest || decodeErr(t, w) != "passkey_login_invalid" {
+	if w := do(eh, "POST", "/api/v1/auth/passkey/login/finish", finishBody("[email protected]", "YXNzZXJ0"), jsonHeader); w.Code != http.StatusBadRequest || decodeErr(t, w) != "passkey_login_invalid" {
 		t.Fatalf("replay of consumed challenge: code = %d body %s, want 400 passkey_login_invalid", w.Code, w.Body.String())
 	}
 }
@@ -153,8 +170,8 @@ func TestPasskeyLoginVertical(t *testing.T) {
 // TestPasskeyLoginBeginNoPasskey pins the begin-side anti-enumeration floor: an unknown
 // email and a KNOWN verified account that has enrolled no passkey answer the SAME
 // no_passkey envelope, so the two are indistinguishable. (The remaining has-passkey-vs-not
-// status split is the documented, accepted trade-off; the cooldown below makes probing
-// it impractical.) Neither path stashes a challenge, and both KEEP the reservation.
+// status split is the documented, accepted trade-off; the auth-door bucket bounds how
+// fast one address can probe it.) Neither path stashes a challenge.
 func TestPasskeyLoginBeginNoPasskey(t *testing.T) {
 	begin := func(eh http.Handler, email string) *httptest.ResponseRecorder {
 		return do(eh, "POST", "/api/v1/auth/passkey/login/begin", `{"email":"`+email+`"}`, jsonHeader)
@@ -184,26 +201,11 @@ func TestPasskeyLoginBeginNoPasskey(t *testing.T) {
 				len(repoU.passkeyChallenges), len(repoN.passkeyChallenges))
 		}
 	})
-
-	t.Run("the no_passkey path KEEPS the reservation so probing is throttled", func(t *testing.T) {
-		api, _, _ := seedLoginPasskeyAPI(t)
-		eh := api.ExternalHandler()
-		if w := begin(eh, "[email protected]"); w.Code != http.StatusBadRequest || decodeErr(t, w) != "no_passkey" {
-			t.Fatalf("first probe: code = %d body %s, want 400 no_passkey", w.Code, w.Body.String())
-		}
-		// Re-probing the same unknown address inside the window is throttled identically to
-		// a real begin — the response is not the only channel; the throttle is sealed too.
-		if w := begin(eh, "[email protected]"); w.Code != http.StatusTooManyRequests || decodeErr(t, w) != "otp_resend_cooldown" {
-			t.Fatalf("re-probe: code = %d body %s, want 429 otp_resend_cooldown", w.Code, w.Body.String())
-		}
-	})
 }
 
-// TestPasskeyLoginBeginVerifierError pins the reserve→rollback path: a BeginLogin failure
-// is a server-side fault, not a probe signal, so it answers passkey_login_failed AND
-// RELEASES the reservation — the immediate retry is admitted, not 429'd. Distinguishing
-// 400-not-429 on the retry is what proves the release: a kept reservation would 429 before
-// ever reaching BeginLogin.
+// TestPasskeyLoginBeginVerifierError pins the verifier-fault path: a BeginLogin failure is
+// a server-side fault, answered passkey_login_failed, stashing nothing, and the immediate
+// retry reaches the verifier again.
 func TestPasskeyLoginBeginVerifierError(t *testing.T) {
 	api, repo, v := seedLoginPasskeyAPI(t)
 	v.beginLoginErr = errors.New("no assertable credential")
@@ -216,7 +218,7 @@ func TestPasskeyLoginBeginVerifierError(t *testing.T) {
 		t.Errorf("a failed begin must stash no challenge, got %d", len(repo.passkeyChallenges))
 	}
 	if w := do(eh, "POST", "/api/v1/auth/passkey/login/begin", `{"email":"[email protected]"}`, jsonHeader); w.Code != http.StatusBadRequest || decodeErr(t, w) != "passkey_login_failed" {
-		t.Fatalf("retry after verifier error: code = %d body %s, want 400 passkey_login_failed (reservation must be released, not 429)", w.Code, w.Body.String())
+		t.Fatalf("retry after verifier error: code = %d body %s, want 400 passkey_login_failed", w.Code, w.Body.String())
 	}
 }
 
@@ -317,32 +319,41 @@ func TestPasskeyLoginGates(t *testing.T) {
 // directly: the frozen clock makes that the only deterministic route to that branch.
 func TestPasskeyLoginFinishRejections(t *testing.T) {
 	const finishPath = "/api/v1/auth/passkey/login/finish"
-	finish := func(eh http.Handler, email string) *httptest.ResponseRecorder {
-		return do(eh, "POST", finishPath,
-			`{"email":"`+email+`","assertion":{"id":"cred-1","type":"public-key"}}`, jsonHeader)
+	finish := func(eh http.Handler, email, assertion string) *httptest.ResponseRecorder {
+		if assertion == "" {
+			assertion = loginAssertion("cred-1", "YXNzZXJ0")
+		}
+		return do(eh, "POST", finishPath, `{"email":"`+email+`","assertion":`+assertion+`}`, jsonHeader)
 	}
 
 	cases := []struct {
-		name  string
-		email string
-		setup func(repo *fakeRepo, v *fakePasskeyVerifier)
+		name      string
+		email     string
+		assertion string // empty: cred-1 over the planted challenge
+		setup     func(repo *fakeRepo, v *fakePasskeyVerifier)
 	}{
-		{"unknown email", "[email protected]", func(repo *fakeRepo, v *fakePasskeyVerifier) {}},
-		{"known account, no live challenge", "[email protected]", func(repo *fakeRepo, v *fakePasskeyVerifier) {}},
-		{"expired challenge", "[email protected]", func(repo *fakeRepo, v *fakePasskeyVerifier) {
+		{"unknown email", "[email protected]", "", func(repo *fakeRepo, v *fakePasskeyVerifier) {}},
+		{"known account, no live challenge", "[email protected]", "", func(repo *fakeRepo, v *fakePasskeyVerifier) {}},
+		{"expired challenge", "[email protected]", "", func(repo *fakeRepo, v *fakePasskeyVerifier) {
 			plantLoginChallenge(repo, "ex", frozenNow.Add(-time.Second))
 		}},
-		{"assertion fails verification", "[email protected]", func(repo *fakeRepo, v *fakePasskeyVerifier) {
+		{"assertion fails verification", "[email protected]", "", func(repo *fakeRepo, v *fakePasskeyVerifier) {
 			plantLoginChallenge(repo, "live", frozenNow.Add(passkeyChallengeTTL))
 			v.failErr = errors.New("bad assertion")
 		}},
+		{"assertion signs a challenge nobody issued", "[email protected]", loginAssertion("cred-1", "bm9ib2R5"), func(repo *fakeRepo, v *fakePasskeyVerifier) {
+			plantLoginChallenge(repo, "live", frozenNow.Add(passkeyChallengeTTL))
+		}},
+		{"assertion without clientDataJSON", "[email protected]", `{"id":"cred-1","type":"public-key"}`, func(repo *fakeRepo, v *fakePasskeyVerifier) {
+			plantLoginChallenge(repo, "live", frozenNow.Add(passkeyChallengeTTL))
+		}},
 	}
 
 	var envelopes [][2]string
 	for _, c := range cases {
 		api, repo, v := seedLoginPasskeyAPI(t)
 		c.setup(repo, v)
-		w := finish(api.ExternalHandler(), c.email)
+		w := finish(api.ExternalHandler(), c.email, c.assertion)
 		if w.Code != http.StatusBadRequest {
 			t.Fatalf("%s: code = %d, want 400 (%s)", c.name, w.Code, w.Body.String())
 		}
@@ -368,43 +379,106 @@ func TestPasskeyLoginFinishRejections(t *testing.T) {
 	}
 }
 
-// TestPasskeyLoginBeginRateLimited closes the unauthenticated probing/DoS vector on the
-// public door: one begin per recipient per window, keyed case-insensitively (a recased
-// retype is the same mailbox), recovering after the window elapses. This is what makes the
-// accepted has-passkey-vs-not status oracle impractical to farm.
-func TestPasskeyLoginBeginRateLimited(t *testing.T) {
-	begin := func(eh http.Handler, email string) *httptest.ResponseRecorder {
-		return do(eh, "POST", "/api/v1/auth/passkey/login/begin", `{"email":"`+email+`"}`, jsonHeader)
+// TestPasskeyLoginCeremoniesCoexist is the case of someone who knows the address and
+// begins logins for it while its owner signs in. Every begin stashes a ceremony of its
+// own; finish verifies against the one whose challenge the browser signed, so the
+// owner's earlier ceremony survives any number of later begins, and a stranger's
+// assertion over a challenge nobody issued consumes nothing.
+func TestPasskeyLoginCeremoniesCoexist(t *testing.T) {
+	api, repo, v := seedLoginPasskeyAPI(t)
+	eh := api.ExternalHandler()
+	begin := func(challenge, session string) {
+		t.Helper()
+		v.options = json.RawMessage(`{"publicKey":{"challenge":"` + challenge + `"}}`)
+		v.loginSession = []byte(session)
+		if w := do(eh, "POST", "/api/v1/auth/passkey/login/begin", `{"email":"[email protected]"}`, jsonHeader); w.Code != http.StatusOK {
+			t.Fatalf("begin %s: code = %d, want 200 (%s)", challenge, w.Code, w.Body.String())
+		}
+	}
+	finish := func(challenge string) *httptest.ResponseRecorder {
+		return do(eh, "POST", "/api/v1/auth/passkey/login/finish", finishBody("[email protected]", challenge), jsonHeader)
 	}
 
-	t.Run("same recipient is throttled, then recovers after the cooldown", func(t *testing.T) {
-		api, _, _ := seedLoginPasskeyAPI(t)
-		clock := frozenNow
-		api.Now = func() time.Time { return clock }
-		eh := api.ExternalHandler()
+	begin("b3duZXI", "owner-session")   // the owner's ceremony
+	begin("c3RyYW5nZXI", "later-begin") // someone else's begin right after
+	if len(repo.passkeyChallenges) != 2 {
+		t.Fatalf("live login challenges = %d, want 2 (a begin must not cancel another)", len(repo.passkeyChallenges))
+	}
 
-		if w := begin(eh, "[email protected]"); w.Code != http.StatusOK {
-			t.Fatalf("first begin: code = %d, want 200 (%s)", w.Code, w.Body.String())
-		}
-		if w := begin(eh, "[email protected]"); w.Code != http.StatusTooManyRequests || decodeErr(t, w) != "otp_resend_cooldown" {
-			t.Fatalf("immediate re-begin: code = %d body %s, want 429 otp_resend_cooldown", w.Code, w.Body.String())
-		}
-		clock = clock.Add(otpResendCooldown + time.Second)
-		if w := begin(eh, "[email protected]"); w.Code != http.StatusOK {
-			t.Fatalf("post-cooldown begin: code = %d, want 200 (%s)", w.Code, w.Body.String())
-		}
-	})
+	if w := finish("bm9ib2R5"); w.Code != http.StatusBadRequest || decodeErr(t, w) != "passkey_login_invalid" {
+		t.Fatalf("finish over an unissued challenge: code = %d body %s, want 400 passkey_login_invalid", w.Code, w.Body.String())
+	}
+	if w := finish("b3duZXI"); w.Code != http.StatusOK {
+		t.Fatalf("owner's finish after a later begin: code = %d, want 200 (%s)", w.Code, w.Body.String())
+	}
+	if string(v.lastSession) != "owner-session" {
+		t.Errorf("owner's finish verified against %q, want owner-session", v.lastSession)
+	}
+	if w := finish("c3RyYW5nZXI"); w.Code != http.StatusOK || string(v.lastSession) != "later-begin" {
+		t.Fatalf("later ceremony: code = %d session %q, want 200 later-begin", w.Code, v.lastSession)
+	}
+	if w := finish("b3duZXI"); w.Code != http.StatusBadRequest {
+		t.Fatalf("replay of the owner's challenge: code = %d, want 400", w.Code)
+	}
+}
 
-	t.Run("throttle key is case-insensitive", func(t *testing.T) {
-		api, _, _ := seedLoginPasskeyAPI(t)
-		eh := api.ExternalHandler()
-		if w := begin(eh, "[email protected]"); w.Code != http.StatusOK {
-			t.Fatalf("first begin: code = %d, want 200 (%s)", w.Code, w.Body.String())
+// TestPasskeyLoginBeginPerSourceCap bounds the challenge store by network: 32 live login
+// challenges begun from one IPv6 /48 fill that network's allowance (429
+// too_many_challenges, nothing stashed), while a begin from another network still gets
+// its ceremony. The begins come from 33 different /64s inside the /48, so the per-/64
+// auth-door bucket never trips and only the /48 bound can refuse the last one.
+func TestPasskeyLoginBeginPerSourceCap(t *testing.T) {
+	api, repo, _ := seedLoginPasskeyAPI(t)
+	api.ClientIPHeader = "CF-Connecting-IP"
+	eh := api.ExternalHandler()
+	from := func(ip string) *httptest.ResponseRecorder {
+		return do(eh, "POST", "/api/v1/auth/passkey/login/begin", `{"email":"[email protected]"}`,
+			map[string]string{"Content-Type": "application/json", "CF-Connecting-IP": ip})
+	}
+	for i := 1; i <= 32; i++ {
+		if w := from(fmt.Sprintf("2001:db8:7:%x::1", i)); w.Code != http.StatusOK {
+			t.Fatalf("begin %d: code = %d, want 200 (%s)", i, w.Code, w.Body.String())
 		}
-		if w := begin(eh, "[email protected]"); w.Code != http.StatusTooManyRequests {
-			t.Fatalf("recased re-begin: code = %d, want 429 (key must be lowercased)", w.Code)
+	}
+	w := from("2001:db8:7:ff::1")
+	if w.Code != http.StatusTooManyRequests || decodeErr(t, w) != "too_many_challenges" {
+		t.Fatalf("33rd begin from the /48: code = %d body %s, want 429 too_many_challenges", w.Code, w.Body.String())
+	}
+	if len(repo.passkeyChallenges) != 32 {
+		t.Errorf("stashed = %d, want 32", len(repo.passkeyChallenges))
+	}
+	if w := from("2001:db8:8::1"); w.Code != http.StatusOK {
+		t.Fatalf("begin from another /48: code = %d, want 200 (%s)", w.Code, w.Body.String())
+	}
+	if w := from("203.0.113.9"); w.Code != http.StatusOK {
+		t.Fatalf("begin from an IPv4 address: code = %d, want 200 (%s)", w.Code, w.Body.String())
+	}
+}
+
+// TestPasskeyChallengeCanonicalForm pins that the stored and the signed challenge compare
+// equal whatever padding each side used: go-webauthn accepts padded and unpadded
+// base64url, and a browser's clientDataJSON may arrive either way.
+func TestPasskeyChallengeCanonicalForm(t *testing.T) {
+	if got, err := optionsChallenge(json.RawMessage(`{"publicKey":{"challenge":"ZmFrZQ=="}}`)); err != nil || got != "ZmFrZQ" {
+		t.Errorf("optionsChallenge(padded) = %q, %v; want ZmFrZQ", got, err)
+	}
+	padded := base64.URLEncoding.EncodeToString([]byte(`{"challenge":"ZmFrZQ"}`)) // 22 bytes: ends in "=="
+	unpadded := base64.RawURLEncoding.EncodeToString([]byte(`{"challenge":"ZmFrZQ=="}`))
+	for name, cd := range map[string]string{"padded clientDataJSON": padded, "padded challenge": unpadded} {
+		got, err := assertionChallenge(json.RawMessage(`{"response":{"clientDataJSON":"` + cd + `"}}`))
+		if err != nil || got != "ZmFrZQ" {
+			t.Errorf("%s: assertionChallenge = %q, %v; want ZmFrZQ", name, got, err)
 		}
-	})
+	}
+	for name, a := range map[string]string{
+		"no response":         `{"id":"cred-1"}`,
+		"clientDataJSON junk": `{"response":{"clientDataJSON":"!!"}}`,
+		"empty challenge":     `{"response":{"clientDataJSON":"` + base64.RawURLEncoding.EncodeToString([]byte(`{"challenge":""}`)) + `"}}`,
+	} {
+		if got, err := assertionChallenge(json.RawMessage(a)); err == nil {
+			t.Errorf("%s: assertionChallenge = %q, want an error", name, got)
+		}
+	}
 }
 
 // TestPasskeyLoginAllowsStaff pins the deliberate contrast with the email door: that door
@@ -434,7 +508,7 @@ func TestPasskeyLoginAllowsStaff(t *testing.T) {
 		t.Fatalf("begin for staff: code = %d, want 200 (%s)", w.Code, w.Body.String())
 	}
 	w := do(eh, "POST", "/api/v1/auth/passkey/login/finish",
-		`{"email":"[email protected]","assertion":{"id":"cred-a1","type":"public-key"}}`, jsonHeader)
+		`{"email":"[email protected]","assertion":`+loginAssertion("cred-a1", "YXNzZXJ0")+`}`, jsonHeader)
 	if w.Code != http.StatusOK {
 		t.Fatalf("finish for staff: code = %d, want 200 (passkey admits staff) (%s)", w.Code, w.Body.String())
 	}
@@ -472,8 +546,7 @@ func TestPasskeyLoginFinishCloneRejected(t *testing.T) {
 	plantLoginChallenge(repo, "live", frozenNow.Add(passkeyChallengeTTL))
 	eh := api.ExternalHandler()
 
-	w := do(eh, "POST", "/api/v1/auth/passkey/login/finish",
-		`{"email":"[email protected]","assertion":{"id":"cred-1","type":"public-key"}}`, jsonHeader)
+	w := do(eh, "POST", "/api/v1/auth/passkey/login/finish", finishBody("[email protected]", "YXNzZXJ0"), jsonHeader)
 
 	if w.Code != http.StatusBadRequest || decodeErr(t, w) != "passkey_login_invalid" {
 		t.Fatalf("clone finish: code = %d body %s, want 400 passkey_login_invalid (opaque refusal)", w.Code, w.Body.String())

internal/api/pgrepo.go

@@ -823,6 +823,38 @@ func (p *PGRepo) CreateEmailOTP(ctx context.Context, id, userID, email, codeHash
 	return tx.Commit()
 }
 
+// AddLoginEmailOTP stores a code for a pre-session login door beside the codes
+// already mailed for (user, purpose), keeping the newest otpLiveLoginCodes live:
+// it drops every unconsumed code outside the newest otpLiveLoginCodes-1 unexpired
+// ones (so expired codes go too), then inserts. It never cancels a code just because another start came in, so knowing
+// an address is not enough to keep its owner from holding a working code
+// (ConsumeLoginEmailOTP accepts any live one).
+func (p *PGRepo) AddLoginEmailOTP(ctx context.Context, id, userID, email, codeHash, purpose string, now, expiresAt time.Time) error {
+	tx, err := p.db.BeginTx(ctx, nil)
+	if err != nil {
+		return err
+	}
+	defer tx.Rollback() //nolint:errcheck // no-op after commit
+
+	if _, err := tx.ExecContext(ctx,
+		`DELETE FROM email_otps
+		 WHERE user_id = $1 AND purpose = $2 AND consumed_at IS NULL
+		   AND id NOT IN (
+		       SELECT id FROM email_otps
+		       WHERE user_id = $1 AND purpose = $2 AND consumed_at IS NULL AND expires_at > $3
+		       ORDER BY created_at DESC, id DESC LIMIT $4)`,
+		userID, purpose, now, otpLiveLoginCodes-1); err != nil {
+		return fmt.Errorf("trim live login codes: %w", err)
+	}
+	if _, err := tx.ExecContext(ctx,
+		`INSERT INTO email_otps (id, user_id, email, code_hash, purpose, expires_at)
+		 VALUES ($1, $2, $3, $4, $5, $6)`,
+		id, userID, email, codeHash, purpose, expiresAt); err != nil {
+		return fmt.Errorf("insert otp: %w", err)
+	}
+	return tx.Commit()
+}
+
 // VerifyEmailOTP redeems the newest live code for (user, purpose) in one
 // transaction (spec §B2). The row is taken FOR UPDATE so a concurrent verify of the
 // same code cannot double-spend it. The branch order is deliberate: expiry and the
@@ -870,7 +902,7 @@ func (p *PGRepo) VerifyEmailOTP(ctx context.Context, userID, purpose, codeHash s
 		return "", ErrOTPLocked
 	}
 	if storedHash != codeHash {
-		return "", chargeOTPMismatch(ctx, tx, id, userID, purpose, now)
+		return "", chargeOTPMismatch(ctx, tx, userID, purpose, now)
 	}
 
 	// A DIFFERENT account may not also prove this address: the pre-session login
@@ -1280,25 +1312,109 @@ func (p *PGRepo) ConsumePasskeyChallengeByUser(ctx context.Context, userID, purp
 	return sessionData, nil
 }
 
+// maxLiveChallengesPerSource bounds the live login challenges one source (an IPv4
+// address or IPv6 /48, see challengeSource) holds in each login store. A ceremony
+// takes seconds and a challenge lives passkeyChallengeTTL, so a network with a few
+// dozen people signing in at once stays well inside it, while a flood of begins
+// fills its own allowance and leaves the other networks their sign-ins. The count
+// and the insert are not serialised, so a burst of truly concurrent begins can pass
+// it together; the per-source token bucket in front of the door caps that burst.
+const maxLiveChallengesPerSource = 32
+
+// AddPasskeyLoginChallenge stores an email-first login ceremony beside the ones
+// already live for (user, purpose); finish finds it by the challenge the browser
+// signed (ConsumePasskeyLoginChallenge), so a begin by anyone who knows the address
+// never cancels its owner's ceremony. It reaps the account's spent login rows,
+// refuses with ErrTooManyPasskeyChallenges once source holds
+// maxLiveChallengesPerSource live login challenges, then inserts.
+func (p *PGRepo) AddPasskeyLoginChallenge(ctx context.Context, id, userID, purpose, source, challenge string, sessionData []byte, now, expiresAt time.Time) error {
+	tx, err := p.db.BeginTx(ctx, nil)
+	if err != nil {
+		return err
+	}
+	defer tx.Rollback() //nolint:errcheck // no-op after commit
+
+	if _, err := tx.ExecContext(ctx,
+		`DELETE FROM webauthn_challenges
+		 WHERE user_id = $1 AND purpose = $2 AND (expires_at <= $3 OR consumed_at IS NOT NULL)`,
+		userID, purpose, now); err != nil {
+		return fmt.Errorf("reap login challenges: %w", err)
+	}
+	var fromSource int
+	if err := tx.QueryRowContext(ctx,
+		`SELECT count(*) FROM webauthn_challenges
+		 WHERE source = $1 AND consumed_at IS NULL AND expires_at > $2`,
+		source, now).Scan(&fromSource); err != nil {
+		return fmt.Errorf("count login challenges: %w", err)
+	}
+	if fromSource >= maxLiveChallengesPerSource {
+		return ErrTooManyPasskeyChallenges
+	}
+	if _, err := tx.ExecContext(ctx,
+		`INSERT INTO webauthn_challenges (id, user_id, purpose, session_data, expires_at, challenge, source)
+		 VALUES ($1, $2, $3, $4, $5, $6, $7)`,
+		id, userID, purpose, sessionData, expiresAt, challenge, source); err != nil {
+		return fmt.Errorf("insert login challenge: %w", err)
+	}
+	return tx.Commit()
+}
+
+// ConsumePasskeyLoginChallenge redeems the live login challenge of (user, purpose)
+// whose challenge is the one the browser signed, single-use: the row is taken FOR
+// UPDATE, expiry is checked against now, consumed_at is stamped, and the stashed
+// SessionData is returned. No such live row → ErrPasskeyChallengeInvalid.
+func (p *PGRepo) ConsumePasskeyLoginChallenge(ctx context.Context, userID, purpose, challenge string, now time.Time) ([]byte, error) {
+	tx, err := p.db.BeginTx(ctx, nil)
+	if err != nil {
+		return nil, err
+	}
+	defer tx.Rollback() //nolint:errcheck // no-op after commit
+
+	var (
+		id          string
+		sessionData []byte
+		expiresAt   time.Time
+	)
+	switch err := tx.QueryRowContext(ctx,
+		`SELECT id, session_data, expires_at FROM webauthn_challenges
+		 WHERE user_id = $1 AND purpose = $2 AND challenge = $3 AND consumed_at IS NULL
+		 FOR UPDATE`,
+		userID, purpose, challenge).Scan(&id, &sessionData, &expiresAt); {
+	case errors.Is(err, sql.ErrNoRows):
+		return nil, ErrPasskeyChallengeInvalid
+	case err != nil:
+		return nil, err
+	}
+	if !expiresAt.After(now) {
+		return nil, ErrPasskeyChallengeInvalid
+	}
+	if _, err := tx.ExecContext(ctx,
+		`UPDATE webauthn_challenges SET consumed_at = $2 WHERE id = $1`, id, now); err != nil {
+		return nil, fmt.Errorf("consume login challenge: %w", err)
+	}
+	if err := tx.Commit(); err != nil {
+		return nil, err
+	}
+	return sessionData, nil
+}
+
 // ---- discoverable ("usernameless") passkey login (task #40, migration 0013) ----
 
 // maxLiveDiscoverableChallenges hard-bounds the non-user-keyed discoverable-login challenge
-// store. webauthn_challenges self-bounds via a per-(user,purpose) supersede; a from-zero begin
-// has no such key, so the table is capped: once this many LIVE (unexpired, unconsumed) rows
-// exist, a new begin is refused (ErrTooManyDiscoverableChallenges → 429). The cap is generous —
-// a login challenge lives only passkeyChallengeTTL (5 min) and each row is ~1 KB — so real
-// concurrency never approaches it, while an abusive begin-flood is bounded to a few MB instead
-// of growing without limit. One client's volume is bounded before it gets here, by the
-// per-address token bucket in front of every public auth door (ratelimit.go).
-const maxLiveDiscoverableChallenges = 4096
+// store: once this many LIVE (unexpired, unconsumed) rows exist, a new begin is refused
+// (ErrTooManyPasskeyChallenges → 429). Each source is held to maxLiveChallengesPerSource
+// first, so filling the store takes this many / 32 distinct networks; a row is a few hundred
+// bytes, so the ceiling is a few MB.
+const maxLiveDiscoverableChallenges = 16384
 
 // CreateDiscoverableChallenge stashes a discoverable-login ceremony under an opaque handle,
 // bounding the table in one transaction (see the Repo interface for the full contract). It
-// reaps expired/consumed rows first — the non-user-keyed analog of CreatePasskeyChallenge's
-// supersede — then refuses over the cap rather than inserting. Because the reap ran first, the
-// COUNT is exactly the live-row count, so the cap bounds an adversarial begin-flood (which a
-// reap alone cannot: a burst inside the TTL leaves every fresh row live).
-func (p *PGRepo) CreateDiscoverableChallenge(ctx context.Context, id string, sessionData []byte, now, expiresAt time.Time) error {
+// reaps expired/consumed rows first, then refuses when source already holds
+// maxLiveChallengesPerSource live rows or the table holds maxLiveDiscoverableChallenges.
+// Because the reap ran first, the counts are exactly the live rows, so the bounds hold under
+// an adversarial begin-flood (which a reap alone cannot: a burst inside the TTL leaves every
+// fresh row live).
+func (p *PGRepo) CreateDiscoverableChallenge(ctx context.Context, id, source string, sessionData []byte, now, expiresAt time.Time) error {
 	tx, err := p.db.BeginTx(ctx, nil)
 	if err != nil {
 		return err
@@ -1310,18 +1426,19 @@ func (p *PGRepo) CreateDiscoverableChallenge(ctx context.Context, id string, ses
 		now); err != nil {
 		return fmt.Errorf("reap discoverable challenges: %w", err)
 	}
-	var live int
+	var live, fromSource int
 	if err := tx.QueryRowContext(ctx,
-		`SELECT count(*) FROM webauthn_discoverable_challenges`).Scan(&live); err != nil {
+		`SELECT count(*), count(*) FILTER (WHERE source = $1) FROM webauthn_discoverable_challenges`,
+		source).Scan(&live, &fromSource); err != nil {
 		return fmt.Errorf("count discoverable challenges: %w", err)
 	}
-	if live >= maxLiveDiscoverableChallenges {
-		return ErrTooManyDiscoverableChallenges
+	if fromSource >= maxLiveChallengesPerSource || live >= maxLiveDiscoverableChallenges {
+		return ErrTooManyPasskeyChallenges
 	}
 	if _, err := tx.ExecContext(ctx,
-		`INSERT INTO webauthn_discoverable_challenges (id, session_data, expires_at)
-		 VALUES ($1, $2, $3)`,
-		id, sessionData, expiresAt); err != nil {
+		`INSERT INTO webauthn_discoverable_challenges (id, session_data, expires_at, source)
+		 VALUES ($1, $2, $3, $4)`,
+		id, sessionData, expiresAt, source); err != nil {
 		return fmt.Errorf("insert discoverable challenge: %w", err)
 	}
 	return tx.Commit()
@@ -2230,14 +2347,19 @@ func (p *PGRepo) UserByEmail(ctx context.Context, email string) (*StaffUser, err
 	return &u, nil
 }
 
-// ConsumeLoginEmailOTP redeems the live code for the PRE-SESSION email login door
-// with the SAME lifecycle as VerifyEmailOTP (FOR UPDATE, expiry + attempt cap
-// before the hash compare, a mismatch charges one attempt without consuming) but
-// with NO identity side-effects: it neither writes users.email nor runs the
-// verified-email uniqueness guard — login already resolved the userID via
-// UserByEmail, which requires email_verified, so the address is settled. Errors
-// are exactly ErrOTPInvalid / ErrOTPLocked (ErrEmailTaken is structurally
-// impossible here).
+// ConsumeLoginEmailOTP redeems a live code for the PRE-SESSION login doors (and
+// the step-up doors, which keep one code live) in one transaction. Every live,
+// unexpired code for (user, purpose) is taken FOR UPDATE, since a login door keeps
+// several (AddLoginEmailOTP). The branch order matches VerifyEmailOTP: nothing live
+// is ErrOTPInvalid; the account lock comes next; when every live code has used up
+// its attempts the answer is ErrOTPLocked; a code matching none charges one attempt
+// to each code still open and one failure to the account, and consumes nothing. A
+// match consumes that code and every other live one for (user, purpose): the
+// sign-in they were mailed for has happened. There are no identity side-effects:
+// it neither writes users.email nor runs the verified-email uniqueness guard —
+// login already resolved the userID via UserByEmail, which requires
+// email_verified, so the address is settled. Errors are exactly ErrOTPInvalid /
+// ErrOTPLocked / *OTPAccountLockedError.
 func (p *PGRepo) ConsumeLoginEmailOTP(ctx context.Context, userID, purpose, codeHash string, now time.Time) error {
 	tx, err := p.db.BeginTx(ctx, nil)
 	if err != nil {
@@ -2245,25 +2367,37 @@ func (p *PGRepo) ConsumeLoginEmailOTP(ctx context.Context, userID, purpose, code
 	}
 	defer tx.Rollback() //nolint:errcheck // no-op after commit
 
-	var (
-		id         string
-		storedHash string
-		attempts   int
-		expiresAt  time.Time
-	)
-	switch err := tx.QueryRowContext(ctx,
-		`SELECT id, code_hash, attempts, expires_at FROM email_otps
-		 WHERE user_id = $1 AND purpose = $2 AND consumed_at IS NULL
-		 ORDER BY created_at DESC LIMIT 1 FOR UPDATE`,
-		userID, purpose).Scan(&id, &storedHash, &attempts, &expiresAt); {
-	case errors.Is(err, sql.ErrNoRows):
-		// Nothing live: never minted, already consumed, or superseded.
-		return ErrOTPInvalid
-	case err != nil:
+	rows, err := tx.QueryContext(ctx,
+		`SELECT code_hash, attempts FROM email_otps
+		 WHERE user_id = $1 AND purpose = $2 AND consumed_at IS NULL AND expires_at > $3
+		 FOR UPDATE`,
+		userID, purpose, now)
+	if err != nil {
 		return err
 	}
-
-	if !expiresAt.After(now) {
+	var live, open int
+	matched := false
+	for rows.Next() {
+		var (
+			storedHash string
+			attempts   int
+		)
+		if err := rows.Scan(&storedHash, &attempts); err != nil {
+			rows.Close()
+			return err
+		}
+		live++
+		if attempts < otpMaxAttempts {
+			open++
+			matched = matched || storedHash == codeHash
+		}
+	}
+	rows.Close()
+	if err := rows.Err(); err != nil {
+		return err
+	}
+	if live == 0 {
+		// Nothing live: never minted, already consumed, trimmed, or expired.
 		return ErrOTPInvalid
 	}
 	if until, err := otpLockedUntil(ctx, tx, userID, purpose, now, true); err != nil {
@@ -2271,15 +2405,17 @@ func (p *PGRepo) ConsumeLoginEmailOTP(ctx context.Context, userID, purpose, code
 	} else if !until.IsZero() {
 		return &OTPAccountLockedError{Until: until}
 	}
-	if attempts >= otpMaxAttempts {
+	if open == 0 {
 		return ErrOTPLocked
 	}
-	if storedHash != codeHash {
-		return chargeOTPMismatch(ctx, tx, id, userID, purpose, now)
+	if !matched {
+		return chargeOTPMismatch(ctx, tx, userID, purpose, now)
 	}
 
 	if _, err := tx.ExecContext(ctx,
-		`UPDATE email_otps SET consumed_at = $2 WHERE id = $1`, id, now); err != nil {
+		`UPDATE email_otps SET consumed_at = $3
+		 WHERE user_id = $1 AND purpose = $2 AND consumed_at IS NULL`,
+		userID, purpose, now); err != nil {
 		return fmt.Errorf("consume otp: %w", err)
 	}
 	return tx.Commit()
@@ -2325,12 +2461,17 @@ func otpLockEnd(windowStart time.Time, failures int, now time.Time) time.Time {
 	return end
 }
 
-// chargeOTPMismatch records one wrong guess against the code and the account
-// budget, commits, and returns what the caller should answer: ErrOTPInvalid, or
-// the lock this guess just tripped. A window that has ended starts over at 1.
-func chargeOTPMismatch(ctx context.Context, tx *sql.Tx, codeID, userID, purpose string, now time.Time) error {
+// chargeOTPMismatch records one wrong guess against every live code of (user,
+// purpose) that still has attempts left and against the account budget, commits,
+// and returns what the caller should answer: ErrOTPInvalid, or the lock this guess
+// just tripped. The caller holds those codes FOR UPDATE, so the charged set is the
+// set it compared. A window that has ended starts over at 1.
+func chargeOTPMismatch(ctx context.Context, tx *sql.Tx, userID, purpose string, now time.Time) error {
 	if _, err := tx.ExecContext(ctx,
-		`UPDATE email_otps SET attempts = attempts + 1 WHERE id = $1`, codeID); err != nil {
+		`UPDATE email_otps SET attempts = attempts + 1
+		 WHERE user_id = $1 AND purpose = $2 AND consumed_at IS NULL
+		   AND expires_at > $3 AND attempts < $4`,
+		userID, purpose, now, otpMaxAttempts); err != nil {
 		return fmt.Errorf("record otp attempt: %w", err)
 	}
 	var (

internal/api/ratelimit.go

@@ -264,3 +264,20 @@ func sourceKey(ip netip.Addr) string {
 		return p.String()
 	}
 }
+
+// challengeSource is the network a passkey login challenge is counted against
+// (maxLiveChallengesPerSource): IPv4 per host, IPv6 per /48. A /48 is what one site
+// is handed, so its holder cannot spread a flood of begins over the 65,536 /64s the
+// auth-door bucket would see as separate callers. An unparseable address shares one
+// key.
+func challengeSource(ip netip.Addr) string {
+	switch {
+	case !ip.IsValid():
+		return "unknown"
+	case ip.Is4():
+		return ip.String()
+	default:
+		p, _ := ip.Prefix(48)
+		return p.String()
+	}
+}

internal/api/ratelimit_test.go

@@ -127,6 +127,26 @@ func TestSourceKeyGroupsIPv6By64(t *testing.T) {
 	}
 }
 
+// TestChallengeSource pins how passkey login challenges are grouped by network: an
+// IPv4 address stands alone, an IPv6 address counts toward its /48 (one site's
+// allocation, so hopping /64s inside it stays one source).
+func TestChallengeSource(t *testing.T) {
+	cases := []struct{ in, want string }{
+		{"203.0.113.9", "203.0.113.9"},
+		{"2001:db8:7:1::1", "2001:db8:7::/48"},
+		{"2001:db8:7:ffff:1:2:3:4", "2001:db8:7::/48"},
+		{"2001:db8:8::1", "2001:db8:8::/48"},
+	}
+	for _, c := range cases {
+		if got := challengeSource(netip.MustParseAddr(c.in)); got != c.want {
+			t.Errorf("challengeSource(%s) = %q, want %q", c.in, got, c.want)
+		}
+	}
+	if got := challengeSource(netip.Addr{}); got != "unknown" {
+		t.Errorf("challengeSource(invalid) = %q, want unknown", got)
+	}
+}
+
 func TestAuthDoorsThrottlePerClientAddress(t *testing.T) {
 	api, _, _ := seedLoginEmailAPI(t)
 	api.AuthDoorLimit = RateLimit{Burst: 3, PerMinute: 3}

internal/api/repo.go

@@ -375,6 +375,13 @@ type Repo interface {
 	// outstanding code per purpose — a re-request invalidates the earlier mail.
 	// expiresAt is the API clock + TTL so expiry is driven by one authoritative clock.
 	CreateEmailOTP(ctx context.Context, id, userID, email, codeHash, purpose string, expiresAt time.Time) error
+	// AddLoginEmailOTP persists a code for a PRE-SESSION login door (email login,
+	// op-login) beside the codes already mailed for (userID, purpose): it keeps the
+	// newest otpLiveLoginCodes live, dropping live codes that expired at now and the
+	// oldest beyond the allowance. Unlike CreateEmailOTP it never cancels a code
+	// because another start came in — anyone who knows an address can start a login
+	// for it, and ConsumeLoginEmailOTP accepts any live code.
+	AddLoginEmailOTP(ctx context.Context, id, userID, email, codeHash, purpose string, now, expiresAt time.Time) error
 	// VerifyEmailOTP redeems the newest live code for (userID, purpose) against
 	// codeHash, atomically (spec §B2). No live code, an expired one, or a consumed
 	// one → ErrOTPInvalid; an exhausted attempt budget → ErrOTPLocked; a spent
@@ -398,10 +405,13 @@ type Repo interface {
 	// address is stored unverified for a later Settings/SMTP flow to verify. An unknown
 	// userID returns ErrNotFound.
 	SetUserEmail(ctx context.Context, userID, email string) error
-	// ConsumeLoginEmailOTP redeems the newest live code for (userID, purpose) against
-	// codeHash for the PRE-SESSION email LOGIN door, with the SAME code lifecycle as
-	// VerifyEmailOTP (FOR UPDATE, expiry+lockout before hash compare, mismatch charges
-	// one attempt without consuming) but with NO identity side-effects: it neither
+	// ConsumeLoginEmailOTP redeems a code for (userID, purpose) against codeHash for
+	// the PRE-SESSION login doors and the step-up doors. Every live (unconsumed,
+	// unexpired at now) code is a candidate, since a login door keeps several. The
+	// lifecycle matches VerifyEmailOTP (FOR UPDATE, lockout before hash compare; all
+	// live codes out of attempts → ErrOTPLocked; a mismatch charges one attempt to each
+	// code still open plus one account failure, consuming nothing), and a match spends
+	// every live code of (userID, purpose). It has NO identity side-effects: it neither
 	// writes users.email nor runs the verified-email uniqueness guard. Login resolved
 	// userID via UserByEmail, which already requires email_verified, so the address is
 	// settled — re-proving control of a code this session must not re-touch the row.
@@ -464,19 +474,32 @@ type Repo interface {
 	// returns the stashed SessionData so finish can validate the attestation against
 	// it. No live challenge → ErrPasskeyChallengeInvalid. Single-use: a second finish
 	// for the same ceremony finds nothing live and fails. now is the API clock so
-	// expiry is testable. Bound to user_id — enrollment and username-first login both
-	// know the principal at begin; the usernameless from-zero door instead uses the
-	// non-user-keyed pair below.
+	// expiry is testable. Bound to user_id — enrollment and step-up know the principal
+	// at begin and only its holder can begin one; the public login doors use the
+	// challenge-matched and non-user-keyed pairs below.
 	ConsumePasskeyChallengeByUser(ctx context.Context, userID, purpose string, now time.Time) (sessionData []byte, err error)
+	// AddPasskeyLoginChallenge persists an email-first passkey LOGIN ceremony for
+	// (userID, purpose) beside the ones already live, recording challenge (the
+	// canonical base64url challenge handed to the browser) and source (the caller's
+	// network, see challengeSource). Anyone who knows an address can begin a login
+	// for it, so a begin never cancels another; it reaps the account's expired or
+	// consumed login rows and refuses with ErrTooManyPasskeyChallenges once source
+	// holds maxLiveChallengesPerSource live login challenges.
+	AddPasskeyLoginChallenge(ctx context.Context, id, userID, purpose, source, challenge string, sessionData []byte, now, expiresAt time.Time) error
+	// ConsumePasskeyLoginChallenge redeems the live login challenge of (userID,
+	// purpose) whose challenge is the one the browser signed, atomically and
+	// single-use, returning its SessionData. No such live row →
+	// ErrPasskeyChallengeInvalid.
+	ConsumePasskeyLoginChallenge(ctx context.Context, userID, purpose, challenge string, now time.Time) (sessionData []byte, err error)
 	// CreateDiscoverableChallenge persists a DISCOVERABLE ("usernameless") login ceremony
 	// (task #40, migration 0013), keyed by an opaque server-minted handle id — NOT a user,
-	// since a from-zero begin has no principal. In one transaction it reaps expired/consumed
-	// rows (the non-user-keyed analog of CreatePasskeyChallenge's supersede) and then, if the
-	// live count is at the hard cap, refuses with ErrTooManyDiscoverableChallenges rather than
-	// inserting — the cap, not the reap, bounds an adversarial begin-flood, since a burst
-	// inside the TTL leaves every fresh row live. now and expiresAt are both the API clock
-	// (now drives the reap; expiresAt = now + TTL drives liveness).
-	CreateDiscoverableChallenge(ctx context.Context, id string, sessionData []byte, now, expiresAt time.Time) error
+	// since a from-zero begin has no principal — and tagged with source (the caller's
+	// network, see challengeSource). In one transaction it reaps expired/consumed rows and
+	// then refuses with ErrTooManyPasskeyChallenges, rather than inserting, when source
+	// already holds maxLiveChallengesPerSource live rows or the table holds
+	// maxLiveDiscoverableChallenges. now and expiresAt are both the API clock (now drives
+	// the reap; expiresAt = now + TTL drives liveness).
+	CreateDiscoverableChallenge(ctx context.Context, id, source string, sessionData []byte, now, expiresAt time.Time) error
 	// ConsumeDiscoverableChallenge redeems the discoverable challenge under handle id,
 	// atomically and single-use (mirrors ConsumePasskeyChallengeByUser without the user key):
 	// it takes the row FOR UPDATE, checks expiry against now, stamps consumed_at, and returns

internal/pgint/challenges_test.go

@@ -0,0 +1,287 @@
+//go:build pgint
+
+package pgint
+
+import (
+	"context"
+	"errors"
+	"testing"
+	"time"
+
+	"felis.lolicon.best/internal/api"
+)
+
+// ---- login codes and ceremonies that coexist (migration 0029) --------------------
+
+func liveLoginCodes(t *testing.T, userID, purpose string, now time.Time) int {
+	t.Helper()
+	var n int
+	if err := db.QueryRow(`SELECT count(*) FROM email_otps
+		WHERE user_id = $1 AND purpose = $2 AND consumed_at IS NULL AND expires_at > $3`,
+		userID, purpose, now).Scan(&n); err != nil {
+		t.Fatalf("count live codes: %v", err)
+	}
+	return n
+}
+
+// A login start keeps the three newest live codes: a fourth drops the oldest, and
+// redeeming any live one spends its siblings too.
+func TestAddLoginEmailOTPKeepsNewestThree(t *testing.T) {
+	ctx := context.Background()
+	u := newUser(t, "user", "otp-keep")
+	purpose := "login_email"
+	addr := "keep-" + suffix(t) + "@example.net"
+	t0 := mustNow().Truncate(time.Second)
+	add := func(hash string, at time.Time) {
+		t.Helper()
+		if err := repo.AddLoginEmailOTP(ctx, "keep-"+suffix(t), u.ID, addr, hash, purpose, at, at.Add(10*time.Minute)); err != nil {
+			t.Fatalf("AddLoginEmailOTP(%s): %v", hash, err)
+		}
+	}
+	add("h1", t0)
+	add("h2", t0.Add(time.Minute))
+	add("h3", t0.Add(2*time.Minute))
+	if n := liveLoginCodes(t, u.ID, purpose, t0.Add(2*time.Minute)); n != 3 {
+		t.Fatalf("live codes after three starts = %d, want 3", n)
+	}
+	add("h4", t0.Add(3*time.Minute))
+	at := t0.Add(3 * time.Minute)
+	if n := liveLoginCodes(t, u.ID, purpose, at); n != 3 {
+		t.Fatalf("live codes after four starts = %d, want 3", n)
+	}
+	if err := repo.ConsumeLoginEmailOTP(ctx, u.ID, purpose, "h1", at); !errors.Is(err, api.ErrOTPInvalid) {
+		t.Fatalf("oldest code after a fourth start = %v, want ErrOTPInvalid", err)
+	}
+	if err := repo.ConsumeLoginEmailOTP(ctx, u.ID, purpose, "h2", at); err != nil {
+		t.Fatalf("second code = %v, want nil", err)
+	}
+	for _, h := range []string{"h3", "h4"} {
+		if err := repo.ConsumeLoginEmailOTP(ctx, u.ID, purpose, h, at); !errors.Is(err, api.ErrOTPInvalid) {
+			t.Fatalf("sibling %s after a sign-in = %v, want ErrOTPInvalid", h, err)
+		}
+	}
+
+	// Expired codes leave the allowance: a start after they lapse is the only live one.
+	add("h5", t0.Add(4*time.Minute))
+	add("h6", t0.Add(5*time.Minute))
+	later := t0.Add(20 * time.Minute)
+	add("h7", later)
+	if n := liveLoginCodes(t, u.ID, purpose, later); n != 1 {
+		t.Fatalf("live codes after the others expired = %d, want 1", n)
+	}
+	var unconsumed int
+	if err := db.QueryRow(`SELECT count(*) FROM email_otps WHERE user_id = $1 AND purpose = $2 AND consumed_at IS NULL`,
+		u.ID, purpose).Scan(&unconsumed); err != nil {
+		t.Fatalf("count unconsumed: %v", err)
+	}
+	if unconsumed != 1 {
+		t.Fatalf("unconsumed rows = %d, want 1 (expired codes are deleted, not kept)", unconsumed)
+	}
+}
+
+// A wrong guess with several live codes costs each open code one attempt and the
+// account one failure; when every live code is spent the door answers ErrOTPLocked,
+// and a newer code still signs in.
+func TestConsumeLoginEmailOTPAcrossLiveCodes(t *testing.T) {
+	ctx := context.Background()
+	u := newUser(t, "user", "otp-multi")
+	purpose := "op_login"
+	addr := "multi-" + suffix(t) + "@example.net"
+	t0 := mustNow().Truncate(time.Second)
+	add := func(hash string, at time.Time) {
+		t.Helper()
+		if err := repo.AddLoginEmailOTP(ctx, "multi-"+suffix(t), u.ID, addr, hash, purpose, at, at.Add(10*time.Minute)); err != nil {
+			t.Fatalf("AddLoginEmailOTP(%s): %v", hash, err)
+		}
+	}
+	add("h-a", t0)
+	add("h-b", t0.Add(time.Minute))
+	at := t0.Add(time.Minute)
+	attempts := func() map[string]int {
+		t.Helper()
+		rows, err := db.Query(`SELECT code_hash, attempts FROM email_otps WHERE user_id = $1 AND purpose = $2`, u.ID, purpose)
+		if err != nil {
+			t.Fatalf("read attempts: %v", err)
+		}
+		defer rows.Close()
+		got := map[string]int{}
+		for rows.Next() {
+			var h string
+			var n int
+			if err := rows.Scan(&h, &n); err != nil {
+				t.Fatalf("scan: %v", err)
+			}
+			got[h] = n
+		}
+		return got
+	}
+
+	if err := repo.ConsumeLoginEmailOTP(ctx, u.ID, purpose, "h-wrong", at); !errors.Is(err, api.ErrOTPInvalid) {
+		t.Fatalf("wrong guess = %v, want ErrOTPInvalid", err)
+	}
+	if got := attempts(); got["h-a"] != 1 || got["h-b"] != 1 {
+		t.Fatalf("attempts after one wrong guess = %v, want h-a:1 h-b:1", got)
+	}
+	for i := 2; i <= 5; i++ {
+		if err := repo.ConsumeLoginEmailOTP(ctx, u.ID, purpose, "h-wrong", at); !errors.Is(err, api.ErrOTPInvalid) {
+			t.Fatalf("wrong guess %d = %v, want ErrOTPInvalid", i, err)
+		}
+	}
+	if err := repo.ConsumeLoginEmailOTP(ctx, u.ID, purpose, "h-a", at); !errors.Is(err, api.ErrOTPLocked) {
+		t.Fatalf("right code once every live code is spent = %v, want ErrOTPLocked", err)
+	}
+	var failures int
+	if err := db.QueryRow(`SELECT failures FROM otp_failure_windows WHERE user_id = $1 AND purpose = $2`,
+		u.ID, purpose).Scan(&failures); err != nil {
+		t.Fatalf("read budget row: %v", err)
+	}
+	if failures != 5 {
+		t.Fatalf("account failures = %d, want 5 (one per wrong guess, not one per code)", failures)
+	}
+
+	add("h-c", t0.Add(2*time.Minute))
+	at = t0.Add(2 * time.Minute)
+	if err := repo.ConsumeLoginEmailOTP(ctx, u.ID, purpose, "h-b", at); !errors.Is(err, api.ErrOTPInvalid) {
+		t.Fatalf("spent code beside a fresh one = %v, want ErrOTPInvalid", err)
+	}
+	if got := attempts(); got["h-c"] != 1 || got["h-a"] != 5 || got["h-b"] != 5 {
+		t.Fatalf("attempts after a guess of a spent code = %v, want h-c:1 and the spent codes left at 5", got)
+	}
+	if err := repo.ConsumeLoginEmailOTP(ctx, u.ID, purpose, "h-c", at); err != nil {
+		t.Fatalf("fresh code = %v, want nil", err)
+	}
+	if n := liveLoginCodes(t, u.ID, purpose, at); n != 0 {
+		t.Fatalf("live codes after a sign-in = %d, want 0", n)
+	}
+}
+
+// Email-first passkey ceremonies of one account coexist and are redeemed by the
+// challenge the browser signed; one network holds at most 32 live ones.
+func TestPasskeyLoginChallengesCoexist(t *testing.T) {
+	ctx := context.Background()
+	u := newUser(t, "user", "pk-login")
+	purpose := "passkey_login"
+	source := "203.0.113." + suffix(t)
+	now := mustNow()
+	add := func(id, source, challenge, session string, expiresAt time.Time) error {
+		return repo.AddPasskeyLoginChallenge(ctx, id+"-"+suffix(t), u.ID, purpose, source, challenge, []byte(session), now, expiresAt)
+	}
+	if err := add("c1", source, "Y2hhbGxlbmdlMQ", "s1", now.Add(5*time.Minute)); err != nil {
+		t.Fatalf("add c1: %v", err)
+	}
+	if err := add("c2", source, "Y2hhbGxlbmdlMg", "s2", now.Add(5*time.Minute)); err != nil {
+		t.Fatalf("add c2: %v", err)
+	}
+	if _, err := repo.ConsumePasskeyLoginChallenge(ctx, u.ID, purpose, "bm9ib2R5", now); !errors.Is(err, api.ErrPasskeyChallengeInvalid) {
+		t.Fatalf("unissued challenge = %v, want ErrPasskeyChallengeInvalid", err)
+	}
+	if sd, err := repo.ConsumePasskeyLoginChallenge(ctx, u.ID, purpose, "Y2hhbGxlbmdlMQ", now); err != nil || string(sd) != "s1" {
+		t.Fatalf("earlier ceremony = %q, %v; want s1 (a later begin must not cancel it)", sd, err)
+	}
+	if _, err := repo.ConsumePasskeyLoginChallenge(ctx, u.ID, purpose, "Y2hhbGxlbmdlMQ", now); !errors.Is(err, api.ErrPasskeyChallengeInvalid) {
+		t.Fatalf("replay = %v, want ErrPasskeyChallengeInvalid", err)
+	}
+	if sd, err := repo.ConsumePasskeyLoginChallenge(ctx, u.ID, purpose, "Y2hhbGxlbmdlMg", now); err != nil || string(sd) != "s2" {
+		t.Fatalf("later ceremony = %q, %v; want s2", sd, err)
+	}
+	// The same challenge under another purpose is a different door.
+	if err := add("c3", source, "Y2hhbGxlbmdlMw", "s3", now.Add(5*time.Minute)); err != nil {
+		t.Fatalf("add c3: %v", err)
+	}
+	if _, err := repo.ConsumePasskeyLoginChallenge(ctx, u.ID, "passkey_register", "Y2hhbGxlbmdlMw", now); !errors.Is(err, api.ErrPasskeyChallengeInvalid) {
+		t.Fatalf("other purpose = %v, want ErrPasskeyChallengeInvalid", err)
+	}
+	if _, err := repo.ConsumePasskeyLoginChallenge(ctx, u.ID, purpose, "Y2hhbGxlbmdlMw", now.Add(6*time.Minute)); !errors.Is(err, api.ErrPasskeyChallengeInvalid) {
+		t.Fatalf("expired ceremony = %v, want ErrPasskeyChallengeInvalid", err)
+	}
+
+	// Other accounts' spent rows from the same source hold no slot: one expired, one
+	// redeemed. Two accounts, since an account's own begin reaps its spent rows.
+	other := newUser(t, "user", "pk-login-other")
+	if err := repo.AddPasskeyLoginChallenge(ctx, "ox-"+suffix(t), other.ID, purpose, source, "ZXhwaXJlZA", []byte("s"), now, now.Add(-time.Second)); err != nil {
+		t.Fatalf("other account's expired begin: %v", err)
+	}
+	third := newUser(t, "user", "pk-login-third")
+	if err := repo.AddPasskeyLoginChallenge(ctx, "oc-"+suffix(t), third.ID, purpose, source, "cmVkZWVtZWQ", []byte("s"), now, now.Add(5*time.Minute)); err != nil {
+		t.Fatalf("third account's begin: %v", err)
+	}
+	if _, err := repo.ConsumePasskeyLoginChallenge(ctx, third.ID, purpose, "cmVkZWVtZWQ", now); err != nil {
+		t.Fatalf("third account's finish: %v", err)
+	}
+
+	// Per-source bound: c3 is still live at now, so 31 more fill the allowance.
+	for i := 0; i < 31; i++ {
+		if err := add("cap", source, "Y2Fw", "s", now.Add(5*time.Minute)); err != nil {
+			t.Fatalf("add %d from the source: %v", i+2, err)
+		}
+	}
+	if err := add("over", source, "b3Zlcg", "s", now.Add(5*time.Minute)); !errors.Is(err, api.ErrTooManyPasskeyChallenges) {
+		t.Fatalf("33rd live challenge from one source = %v, want ErrTooManyPasskeyChallenges", err)
+	}
+	if err := repo.AddPasskeyLoginChallenge(ctx, "x-"+suffix(t), other.ID, purpose, source, "eA", []byte("s"), now, now.Add(5*time.Minute)); !errors.Is(err, api.ErrTooManyPasskeyChallenges) {
+		t.Fatalf("another account's begin from the full source = %v, want ErrTooManyPasskeyChallenges", err)
+	}
+	if err := add("elsewhere", "198.51.100."+suffix(t), "ZWxzZXdoZXJl", "s", now.Add(5*time.Minute)); err != nil {
+		t.Fatalf("begin from another source: %v", err)
+	}
+	// A redeemed ceremony frees its slot.
+	if _, err := repo.ConsumePasskeyLoginChallenge(ctx, u.ID, purpose, "b3Zlcg", now); !errors.Is(err, api.ErrPasskeyChallengeInvalid) {
+		t.Fatalf("the refused begin left a row: %v", err)
+	}
+	var capID string
+	if err := db.QueryRow(`SELECT challenge FROM webauthn_challenges WHERE user_id = $1 AND source = $2 AND consumed_at IS NULL LIMIT 1`,
+		u.ID, source).Scan(&capID); err != nil {
+		t.Fatalf("pick a live challenge: %v", err)
+	}
+	if _, err := repo.ConsumePasskeyLoginChallenge(ctx, u.ID, purpose, capID, now); err != nil {
+		t.Fatalf("redeem one from the full source: %v", err)
+	}
+	if err := add("after", source, "YWZ0ZXI", "s", now.Add(5*time.Minute)); err != nil {
+		t.Fatalf("begin after one was redeemed = %v, want nil", err)
+	}
+}
+
+// The usernameless store holds each source to 32 live challenges and the whole
+// table to 16384.
+func TestDiscoverableChallengeBounds(t *testing.T) {
+	ctx := context.Background()
+	now := mustNow()
+	source := "2001:db8:" + suffix(t)[:4] + "::/48"
+	t.Cleanup(func() {
+		db.Exec(`DELETE FROM webauthn_discoverable_challenges WHERE source LIKE 'pgint-bulk-%' OR source = $1`, source) //nolint:errcheck
+	})
+	var ids []string
+	for i := 0; i < 32; i++ {
+		id := "disc-" + suffix(t)
+		if err := repo.CreateDiscoverableChallenge(ctx, id, source, []byte("s"), now, now.Add(5*time.Minute)); err != nil {
+			t.Fatalf("create %d: %v", i+1, err)
+		}
+		ids = append(ids, id)
+	}
+	if err := repo.CreateDiscoverableChallenge(ctx, "disc-over-"+suffix(t), source, []byte("s"), now, now.Add(5*time.Minute)); !errors.Is(err, api.ErrTooManyPasskeyChallenges) {
+		t.Fatalf("33rd from one source = %v, want ErrTooManyPasskeyChallenges", err)
+	}
+	if err := repo.CreateDiscoverableChallenge(ctx, "disc-else-"+suffix(t), "pgint-bulk-else", []byte("s"), now, now.Add(5*time.Minute)); err != nil {
+		t.Fatalf("another source: %v", err)
+	}
+	if _, err := repo.ConsumeDiscoverableChallenge(ctx, ids[0], now); err != nil {
+		t.Fatalf("consume: %v", err)
+	}
+	if err := repo.CreateDiscoverableChallenge(ctx, "disc-after-"+suffix(t), source, []byte("s"), now, now.Add(5*time.Minute)); err != nil {
+		t.Fatalf("after a redeem freed a slot = %v, want nil", err)
+	}
+
+	// Fill the table to its store-wide bound from many sources, none of them full.
+	var live int
+	if err := db.QueryRow(`SELECT count(*) FROM webauthn_discoverable_challenges WHERE consumed_at IS NULL AND expires_at > $1`, now).Scan(&live); err != nil {
+		t.Fatalf("count: %v", err)
+	}
+	if _, err := db.Exec(`INSERT INTO webauthn_discoverable_challenges (id, session_data, expires_at, source)
+		SELECT 'bulk-' || $1 || '-' || i, '\x00'::bytea, $2, 'pgint-bulk-' || (i % 1000)
+		FROM generate_series(1, $3) AS i`, suffix(t), now.Add(5*time.Minute), 16384-live); err != nil {
+		t.Fatalf("bulk insert: %v", err)
+	}
+	if err := repo.CreateDiscoverableChallenge(ctx, "disc-full-"+suffix(t), "pgint-bulk-fresh", []byte("s"), now, now.Add(5*time.Minute)); !errors.Is(err, api.ErrTooManyPasskeyChallenges) {
+		t.Fatalf("begin with the table full = %v, want ErrTooManyPasskeyChallenges", err)
+	}
+}

internal/store/migrations/0029_challenge_source.sql

@@ -0,0 +1,20 @@
+-- Passkey login challenges stop being one-per-account and are bounded per source.
+--
+-- An email-first passkey login used to keep one challenge per account: every begin
+-- deleted the previous one, so anyone who knew an address could cancel its owner's
+-- ceremony by starting another. A login challenge now lives beside the others and
+-- finish picks the one whose challenge the browser signed (clientDataJSON carries
+-- it), so a stranger's begin never touches the owner's.
+--
+-- Both login stores are then bounded by where the begins come from: source is the
+-- caller's IPv4 address or IPv6 /48, and a source holds only so many live login
+-- challenges at once (maxLiveChallengesPerSource). One network flooding begins
+-- fills its own allowance and leaves every other network able to sign in.
+ALTER TABLE webauthn_challenges ADD COLUMN challenge text;
+ALTER TABLE webauthn_challenges ADD COLUMN source text;
+ALTER TABLE webauthn_discoverable_challenges ADD COLUMN source text;
+
+CREATE INDEX webauthn_challenges_source_idx
+    ON webauthn_challenges (source) WHERE source IS NOT NULL;
+CREATE INDEX webauthn_discoverable_challenges_source_idx
+    ON webauthn_discoverable_challenges (source);