feat(auth): add discoverable (usernameless) passkey login

A from-zero login door: the browser calls navigator.credentials.get() with an
empty allowCredentials, the authenticator returns an assertion carrying the
resident credential's userHandle, and the server resolves the account from that
handle alone — nothing is typed or client-named.

Routes (both Public):
  POST /api/v1/auth/passkey/login/discoverable/begin
  POST /api/v1/auth/passkey/login/discoverable/finish

Begin stashes the ceremony SessionData server-side keyed by an opaque login_id
under a global cap; finish consumes it single-use, hands the
authenticator-revealed userHandle to a UserByID resolver, and mints a session
only for the account the assertion actually verified to. Every finish rejection
— no live challenge, expired, bad assertion, unresolvable handle — collapses to
one passkey_login_invalid envelope, so finish is never an existence/state
oracle. SignCount is surfaced but not yet consumed, exactly as the
username-first door, so the from-zero path offers no clone-detection bypass.

The discoverable VERIFY path is Oracle-verified end to end against a virtual
authenticator (internal/passkey): it resolves the account from the signed
userHandle, fails closed when the handle names no account, and rejects an
assertion signed by a credential not bound to the resolved user — the
impersonation guard unique to usernameless login. Enrollment now requests a
resident key (authenticatorSelection.residentKey=preferred), the only
server-side half a unit test can pin.

Whether an authenticator actually stores a resident key is a device property no
test can reach, so this door is INERT for a credential until its owner enrolls a
NEW passkey against these options; "preferred" (not "required") preserves the
no-lockout fallback to username-first + email-OTP.

internal/api/api.go

@@ -282,6 +282,11 @@ func (a *API) externalAPIRoutes() []apiRoute {
 		{Method: "GET", Pattern: "/api/v1/auth/setup/status", SetupAllowed: true, h: a.handleSetupStatus},
 		{Method: "POST", Pattern: "/api/v1/auth/passkey/login/begin", Public: true, h: a.handlePasskeyLoginBegin},
 		{Method: "POST", Pattern: "/api/v1/auth/passkey/login/finish", Public: true, h: a.handlePasskeyLoginFinish},
+		// Discoverable ("usernameless") passkey login (task #40): the from-zero sibling of the
+		// email-first pair above — no identifier typed, the account is resolved from the
+		// userHandle inside the signed assertion (handlers_passkey_discoverable.go).
+		{Method: "POST", Pattern: "/api/v1/auth/passkey/login/discoverable/begin", Public: true, h: a.handlePasskeyLoginDiscoverableBegin},
+		{Method: "POST", Pattern: "/api/v1/auth/passkey/login/discoverable/finish", Public: true, h: a.handlePasskeyLoginDiscoverableFinish},
 		{Method: "POST", Pattern: "/api/v1/auth/email/start", Public: true, h: a.handleLoginEmailStart},
 		{Method: "POST", Pattern: "/api/v1/auth/email/verify", Public: true, h: a.handleLoginEmailVerify},
 		{Method: "POST", Pattern: "/api/v1/auth/op-login/start", Public: true, h: a.handleOpLoginStart},

internal/api/api_test.go

@@ -81,6 +81,12 @@ type fakeRepo struct {
 	// just as the PG query does.
 	passkeyCreds      map[string]PasskeyCredential
 	passkeyChallenges map[string]*fakePasskeyChallenge
+	// discoverable ("usernameless") login challenge store (task #40), keyed by opaque handle id
+	// with no user key, mirroring migration 0013. discoverableFull forces the capped-out path
+	// (ErrTooManyDiscoverableChallenges) so the begin 429 branch is reachable without inserting
+	// thousands of rows.
+	discoverableChallenges map[string]*fakeDiscoverableChallenge
+	discoverableFull       bool
 	// user admin fakes
 	seededUsers []seededUser
 	fakeQuotas  map[string]*QuotaView
@@ -99,6 +105,15 @@ type fakePasskeyChallenge struct {
 	createdAt   time.Time
 }
 
+// fakeDiscoverableChallenge mirrors a webauthn_discoverable_challenges row (task #40): no user
+// or purpose (a from-zero begin has neither), just the opaque stashed SessionData, its expiry,
+// and single-use via consumed. Keyed by the opaque handle in the map, like the real table's id.
+type fakeDiscoverableChallenge struct {
+	sessionData []byte
+	expiresAt   time.Time
+	consumed    bool
+}
+
 // fakeDataHold mirrors a player_data_holds row at the granularity the verifiable
 // (write-only) layer exercises: which name/data was stashed for the squatter UUID
 // and when the 30-day window ends. reclaimed_by_user_id/reclaimed_at have no fake
@@ -191,7 +206,9 @@ func newFakeRepo() *fakeRepo {
 		holds:             map[string]fakeDataHold{},
 		passkeyCreds:      map[string]PasskeyCredential{},
 		passkeyChallenges: map[string]*fakePasskeyChallenge{},
-		fakeQuotas:        map[string]*QuotaView{},
+
+		discoverableChallenges: map[string]*fakeDiscoverableChallenge{},
+		fakeQuotas:             map[string]*QuotaView{},
 	}
 }
 
@@ -366,6 +383,31 @@ func (f *fakeRepo) ConsumePasskeyChallengeByUser(_ context.Context, userID, purp
 	return live.sessionData, nil
 }
 
+// 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 {
+	if f.discoverableFull {
+		return ErrTooManyDiscoverableChallenges
+	}
+	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)
+		}
+	}
+	f.discoverableChallenges[id] = &fakeDiscoverableChallenge{sessionData: sessionData, expiresAt: expiresAt}
+	return nil
+}
+func (f *fakeRepo) ConsumeDiscoverableChallenge(_ context.Context, id string, now time.Time) ([]byte, error) {
+	c, ok := f.discoverableChallenges[id]
+	if !ok || c.consumed || !c.expiresAt.After(now) {
+		return nil, ErrPasskeyChallengeInvalid
+	}
+	c.consumed = true
+	return c.sessionData, nil
+}
+
 // CreatePasskeyCredential mirrors PGRepo: a credential_id already bound to ANY account
 // → ErrConflict (the UNIQUE guard), never a silent rebind.
 func (f *fakeRepo) CreatePasskeyCredential(_ context.Context, c PasskeyCredential) error {
@@ -436,6 +478,10 @@ type fakePasskeyVerifier struct {
 	// stashed SessionData round-trips and the existing credentials reach the verifier.
 	lastUser    PasskeyUser
 	lastSession []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).
+	discoverableUserHandle []byte
 }
 
 func (v *fakePasskeyVerifier) BeginRegistration(user PasskeyUser) (json.RawMessage, []byte, error) {
@@ -477,6 +523,33 @@ func (v *fakePasskeyVerifier) FinishLogin(user PasskeyUser, sessionData []byte,
 	return v.assertion, nil
 }
 
+func (v *fakePasskeyVerifier) BeginDiscoverableLogin() (json.RawMessage, []byte, error) {
+	if v.beginLoginErr != nil {
+		return nil, nil, v.beginLoginErr
+	}
+	opts := v.options
+	if opts == nil {
+		opts = json.RawMessage(`{"publicKey":{"challenge":"ZGlzYw"}}`)
+	}
+	return opts, []byte("disc-session"), nil
+}
+
+func (v *fakePasskeyVerifier) FinishDiscoverableLogin(resolveUser func([]byte) (PasskeyUser, error), sessionData []byte, _ io.Reader) (VerifiedAssertion, error) {
+	v.lastSession = sessionData
+	if v.failErr != nil {
+		return VerifiedAssertion{}, v.failErr
+	}
+	// Drive the resolver with the configured user handle so the handler's userHandle → UserByID
+	// → session-mint wiring runs end to end; a resolve error (unknown handle) fails the ceremony
+	// exactly as the real ValidateDiscoverableLogin would when the handler cannot be resolved.
+	u, err := resolveUser(v.discoverableUserHandle)
+	if err != nil {
+		return VerifiedAssertion{}, err
+	}
+	v.lastUser = u
+	return v.assertion, nil
+}
+
 func (f *fakeRepo) UserInAllowlist(_ context.Context, n, u string) (bool, error) {
 	return f.allowlist[n][u], nil
 }

internal/api/errors.go

@@ -58,6 +58,14 @@ 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")
 )
 
 // apiError is a handler-level error carrying an HTTP status and a stable,

internal/api/handlers_passkey.go

@@ -40,15 +40,20 @@ import (
 //     (0010_verified_email_unique.sql) plus UserByEmail gave the door the typable handle
 //     it keys on: begin resolves email → account → its bound passkeys.
 //
-// This is an EMAIL-first assertion, not a usernameless one. The system's returning-player
-// root of trust is still re-link (control of the in-game identity — handlers_onboard.go
-// re-mints a session through the bind-code flow even after passkey/OTP are bound); the
-// email and passkey login doors are convenience layered on top, never the root. The real
-// enabler for a TRULY from-zero passkey login (no identifier typed at all) is discoverable
-// ("usernameless") credentials, which sidestep even the email handle but reshape enrollment
-// (residentKey) and need a non-user-keyed challenge store — a future migration and its own
-// checkpoint, task #40 (that door partly bypasses the in-game-identity root of trust). The
-// adapter crypto is verified now so that slice inherits correct crypto.
+// That EMAIL-first assertion is one of TWO login doors this subsystem now offers. The other,
+// the TRULY from-zero door, is discoverable ("usernameless") login (handlers_passkey_discoverable.go,
+// task #40): the browser calls navigator.credentials.get() with an EMPTY allowCredentials, the
+// authenticator offers a resident credential it holds, and the account is resolved from the
+// userHandle inside the signed assertion — no identifier typed at all. It reshaped enrollment
+// (ResidentKey=Preferred in the verifier) and added a non-user-keyed challenge store (migration
+// 0013). Two honest limits frame it: (1) the from-zero door partly bypasses the returning-player
+// root of trust — control of the in-game identity, which handlers_onboard.go re-mints a session
+// through even after passkey/OTP are bound — but it stands on the same footing as the email door
+// (#72): a passkey is a possession+UV two-factor authenticator strong enough to stand alone; and
+// (2) whether an authenticator actually STORES a resident key is a device property no server
+// request compels, so a credential enrolled before this slice, or on hardware that declines
+// residency, stays username-first (BeginLogin) — the from-zero door is inert for it until its
+// owner enrolls a new passkey. The assertion crypto for both doors is Oracle-verified.
 //
 // The cryptographic half is a seam (PasskeyVerifier) so this package never imports
 // go-webauthn: ceremony state crosses the boundary as opaque bytes, the attestation
@@ -102,6 +107,21 @@ type PasskeyVerifier interface {
 	// the browser posts back; sessionData is the blob BeginLogin returned. A failed
 	// verification returns a non-nil error; the handler maps it to 400.
 	FinishLogin(user PasskeyUser, sessionData []byte, assertion io.Reader) (VerifiedAssertion, error)
+	// BeginDiscoverableLogin starts a USERNAMELESS assertion ceremony (task #40): there is no
+	// user yet, so no allowCredentials — the authenticator offers a resident (discoverable)
+	// credential it holds for this RP and reveals the account only in the signed response. It
+	// returns the {"publicKey": {...}} request options for navigator.credentials.get() and the
+	// opaque SessionData the handler stashes under an opaque handle (not a user id) and replays
+	// at finish.
+	BeginDiscoverableLogin() (options json.RawMessage, sessionData []byte, err error)
+	// FinishDiscoverableLogin verifies a usernameless assertion. resolveUser is called with the
+	// authenticator-revealed user handle so the caller loads the account and its bound
+	// credentials WITHOUT any client-supplied identifier; the verifier then checks the asserted
+	// credential id is one that user holds and verifies the signature. A resolveUser error
+	// (unknown handle) fails the ceremony closed; the handle is the account's stable user id, so
+	// resolveUser is a direct id lookup. A failed verification returns a non-nil error the
+	// handler maps to 400.
+	FinishDiscoverableLogin(resolveUser func(userHandle []byte) (PasskeyUser, error), sessionData []byte, assertion io.Reader) (VerifiedAssertion, error)
 }
 
 // PasskeyUser is the relying-party view of the enrolling principal the verifier needs:

internal/api/handlers_passkey_discoverable.go

@@ -0,0 +1,209 @@
+package api
+
+import (
+	"bytes"
+	"encoding/json"
+	"errors"
+	"net/http"
+	"strings"
+)
+
+// Discoverable ("usernameless") passkey login (spec §14, task #40) — the TRULY from-zero
+// console.<root_domain> door. Its email-first sibling (handlers_passkey.go) still needs a typed
+// email to resolve the account before offering its passkeys; this door needs nothing typed at
+// all. The browser calls navigator.credentials.get() with an EMPTY allowCredentials, the
+// authenticator offers a resident credential it holds for this RP, and the account is revealed
+// only by the userHandle inside the signed assertion. Because there is no identifier at begin,
+// the challenge cannot be user-keyed: it is stashed under an opaque server-minted handle
+// (login_id) in the non-user-keyed store (migration 0013) and echoed back at finish. Email-OTP
+// 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 — matching the stance in handlers_auth_email.go (behind
+// Cloudflare RemoteAddr is the proxy; CGNAT false-positives) — volumetric per-source limiting is
+// left to the edge, and the server-side bound is a hard global cap on live challenges enforced
+// atomically in CreateDiscoverableChallenge (ErrTooManyDiscoverableChallenges → 429).
+
+// handlePasskeyLoginDiscoverableBegin starts a usernameless assertion ceremony (Public,
+// pre-session). It has no request body — the whole point is that the caller supplies no
+// identifier — but requires the JSON Content-Type as the same cross-origin CSRF guard the other
+// pre-session doors use. It asks the verifier for assertion options with an empty
+// allowCredentials + opaque SessionData, stashes the SessionData under a fresh opaque handle in
+// the capped non-user-keyed store, and returns the options with that handle merged in as
+// login_id for the browser to echo at finish.
+func (a *API) handlePasskeyLoginDiscoverableBegin(w http.ResponseWriter, r *http.Request) {
+	if !localAuthEnabled(r.Context(), a.Repo) {
+		writeError(w, r, newError(http.StatusForbidden, "local_auth_disabled",
+			"session login is disabled"))
+		return
+	}
+	if a.Passkey == nil {
+		writeError(w, r, errPasskeyUnavailable)
+		return
+	}
+	if err := requireJSONContentType(r); err != nil {
+		writeError(w, r, err)
+		return
+	}
+	options, sessionData, err := a.Passkey.BeginDiscoverableLogin()
+	if err != nil {
+		writeError(w, r, newError(http.StatusBadRequest, "passkey_login_failed",
+			"could not start passkey login"))
+		return
+	}
+	id, err := newPasskeyID()
+	if err != nil {
+		writeError(w, r, err)
+		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"))
+			return
+		}
+		writeError(w, r, err)
+		return
+	}
+	// Merge the opaque login handle into the options envelope so the response is a single
+	// {"publicKey": {...}, "login_id": "..."} document. The browser passes publicKey to
+	// navigator.credentials.get() and echoes login_id back at finish (the challenge is never
+	// user-keyed, so this handle is the only link between begin and finish).
+	envelope, err := mergeLoginID(options, id)
+	if err != nil {
+		writeError(w, r, err)
+		return
+	}
+	writeJSON(w, http.StatusOK, envelope)
+}
+
+// passkeyDiscoverableFinishRequest is the finish body: the opaque login_id that begin returned
+// (the only link to the stashed challenge, since it is not user-keyed) and the raw
+// navigator.credentials.get() assertion. Assertion is RawMessage so the exact bytes the browser
+// produced reach the verifier without a re-encode that could perturb the signed payload.
+type passkeyDiscoverableFinishRequest struct {
+	LoginID   string          `json:"login_id"`
+	Assertion json.RawMessage `json:"assertion"`
+}
+
+// handlePasskeyLoginDiscoverableFinish verifies a usernameless assertion and mints a session
+// (Public, pre-session). It consumes the stashed challenge under login_id (a missing/expired/
+// consumed handle → 400), then verifies the assertion — the verifier resolves the account from
+// the authenticator-revealed userHandle via the resolve callback below, WITHOUT any
+// client-supplied identifier. On success the session is minted for the account the assertion
+// actually resolved AND verified to (the resolved user is hoisted out of the callback), never
+// anything the client named. All rejection branches collapse to one passkey_login_invalid
+// envelope so finish is never an existence/state oracle.
+func (a *API) handlePasskeyLoginDiscoverableFinish(w http.ResponseWriter, r *http.Request) {
+	if !localAuthEnabled(r.Context(), a.Repo) {
+		writeError(w, r, newError(http.StatusForbidden, "local_auth_disabled",
+			"session login is disabled"))
+		return
+	}
+	if a.Passkey == nil {
+		writeError(w, r, errPasskeyUnavailable)
+		return
+	}
+	if err := requireJSONContentType(r); err != nil {
+		writeError(w, r, err)
+		return
+	}
+	var req passkeyDiscoverableFinishRequest
+	if err := decodeJSON(w, r, &req); err != nil {
+		writeError(w, r, err)
+		return
+	}
+	if strings.TrimSpace(req.LoginID) == "" {
+		writeError(w, r, newError(http.StatusBadRequest, "bad_request", "login_id is required"))
+		return
+	}
+	if len(req.Assertion) == 0 {
+		writeError(w, r, newError(http.StatusBadRequest, "bad_request", "assertion is required"))
+		return
+	}
+
+	sessionData, err := a.Repo.ConsumeDiscoverableChallenge(r.Context(), req.LoginID, a.now())
+	if err != nil {
+		if errors.Is(err, ErrPasskeyChallengeInvalid) {
+			writeError(w, r, newError(http.StatusBadRequest, "passkey_login_invalid",
+				"passkey login could not be completed; begin again"))
+			return
+		}
+		writeError(w, r, err)
+		return
+	}
+
+	// The verifier hands the authenticator-revealed userHandle to this resolver; it loads the
+	// account and its bound credentials so ValidateDiscoverableLogin can check the asserted
+	// credential belongs to that user and verify the signature. The userHandle IS the account's
+	// stable id (WebAuthnID), so this is a direct id lookup. The resolved user is hoisted here so
+	// the session below is minted for the account the assertion actually resolved AND verified to
+	// — not anything the client supplied (the body carries only a challenge handle).
+	var resolved *StaffUser
+	resolve := func(userHandle []byte) (PasskeyUser, error) {
+		u, err := a.Repo.UserByID(r.Context(), string(userHandle))
+		if err != nil {
+			return PasskeyUser{}, err
+		}
+		creds, err := a.Repo.PasskeyCredentialsForUser(r.Context(), u.ID)
+		if err != nil {
+			return PasskeyUser{}, err
+		}
+		resolved = u
+		// Name/DisplayName are cosmetic at assertion time (nothing is shown to the user); use the
+		// stable username so a nil email never matters.
+		return PasskeyUser{ID: u.ID, Name: u.Username, DisplayName: u.Username, Credentials: creds}, nil
+	}
+	if _, err := a.Passkey.FinishDiscoverableLogin(resolve, sessionData, bytes.NewReader(req.Assertion)); err != nil {
+		writeError(w, r, newError(http.StatusBadRequest, "passkey_login_invalid",
+			"passkey login could not be completed; begin again"))
+		return
+	}
+	// A verified assertion guarantees resolve ran and set resolved: go-webauthn calls the handler
+	// to obtain the user BEFORE checking the signature, and a resolve error would have failed
+	// FinishDiscoverableLogin above. Guard anyway so a future verifier that could return success
+	// without invoking the resolver fails closed rather than nil-dereferencing.
+	if resolved == nil {
+		writeError(w, r, newError(http.StatusBadRequest, "passkey_login_invalid",
+			"passkey login could not be completed; begin again"))
+		return
+	}
+
+	token, err := newSessionToken()
+	if err != nil {
+		writeError(w, r, err)
+		return
+	}
+	expires := a.now().Add(sessionTTL)
+	if err := a.Repo.CreateSession(r.Context(), hashCookie(token), resolved.ID, expires); err != nil {
+		writeError(w, r, err)
+		return
+	}
+	setSessionCookie(w, token, expires)
+	a.audit(r, resolved.Username, "auth.passkey_login_discoverable", "")
+	writeJSON(w, http.StatusOK, map[string]any{
+		"user_id": resolved.ID,
+		"role":    resolved.Role,
+	})
+}
+
+// mergeLoginID returns options with an added top-level "login_id" member, so a discoverable
+// begin can hand the browser one {"publicKey": {...}, "login_id": "..."} document. It parses the
+// options into a generic envelope (they are already a JSON object with a publicKey member) and
+// re-marshals with the handle added; a malformed options blob surfaces as an error rather than a
+// silently unmergeable response.
+func mergeLoginID(options json.RawMessage, id string) (json.RawMessage, error) {
+	var envelope map[string]json.RawMessage
+	if err := json.Unmarshal(options, &envelope); err != nil {
+		return nil, err
+	}
+	idJSON, err := json.Marshal(id)
+	if err != nil {
+		return nil, err
+	}
+	envelope["login_id"] = idJSON
+	return json.Marshal(envelope)
+}

internal/api/handlers_passkey_discoverable_test.go

@@ -0,0 +1,319 @@
+package api
+
+import (
+	"bytes"
+	"encoding/json"
+	"errors"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+	"time"
+)
+
+// Pre-session DISCOVERABLE ("usernameless") passkey login tests (spec §14, task #40 — the
+// truly from-zero console.<root_domain> door). These drive the two Public routes against the
+// fakeRepo's non-user-keyed challenge store and a fake PasskeyVerifier, so what they PROVE is
+// the handler + login state machine (opaque-handle stash → consume → userHandle-resolve →
+// session mint), NOT the pgrepo SQL nor the cryptographic assertion verification (the latter is
+// the parity subject of internal/passkey/verifier_test.go's TestDiscoverableLoginRoundTrip).
+// The load-bearing properties, in flow order:
+//
+//   - No identifier crosses the wire: begin has no request body and finish carries only the
+//     opaque login_id + the assertion. The account is revealed solely by the userHandle the
+//     verifier surfaces, resolved server-side via UserByID — never anything the client names.
+//   - Session-data round-trip: the stashed SessionData reaches FinishDiscoverableLogin only via
+//     store-stash → consume (the finish body has no session data), so it is never client-echoed.
+//   - Fail-closed anti-enumeration on finish: a bogus/expired/consumed handle, a failed
+//     assertion, AND a userHandle that resolves to no account all collapse to ONE
+//     passkey_login_invalid envelope — finish is never an existence/state oracle.
+//   - Volumetric bound: begin has no per-caller identity to rate-limit (that is delegated to the
+//     edge), so the server-side guard is the hard global cap → 429 too_many_challenges.
+
+// seedDiscoverableLoginAPI wires the public from-zero door: local sessions enabled, a single
+// verified player "player" (id u1) with one bound passkey, and a verifier primed with fixed
+// options, a verified assertion, and a discoverableUserHandle of "u1" — so the default resolve
+// path surfaces that account exactly as a real resident credential's userHandle would. Both
+// routes are Public (no External principal), proving they are truly pre-session.
+func seedDiscoverableLoginAPI(t *testing.T) (*API, *fakeRepo, *fakePasskeyVerifier) {
+	t.Helper()
+	repo := newFakeRepo()
+	repo.settings[LocalAuthEnabledKey] = []byte("true")
+	repo.staff["player"] = &StaffUser{
+		ID: "u1", Username: "player", Email: "[email protected]",
+		Role: "user", EmailVerified: true,
+	}
+	repo.passkeyCreds["row1"] = PasskeyCredential{
+		ID: "row1", UserID: "u1", CredentialID: "cred-1", PublicKey: "k", CreatedAt: frozenNow,
+	}
+	v := &fakePasskeyVerifier{
+		options:                json.RawMessage(`{"publicKey":{"challenge":"ZGlzYw"}}`),
+		assertion:              VerifiedAssertion{CredentialID: "cred-1", UserVerified: true},
+		discoverableUserHandle: []byte("u1"),
+	}
+	api := newTestAPI(repo, newFakeCluster())
+	api.Passkey = v
+	return api, repo, v
+}
+
+// TestPasskeyDiscoverableLoginVertical walks the whole from-zero slice across the external face:
+// begin stashes one challenge under an opaque login_id and returns the assertion options with
+// that handle merged in; finish consumes the handle, verifies the assertion against the
+// SERVER-STASHED session data, resolves the account from the authenticator-revealed userHandle
+// (NOT from anything typed), and mints the same host-only felis_session as the other doors. The
+// decisive assertion is the session-data round-trip: the finish body carries only login_id +
+// assertion, so the only path for the stashed blob into FinishDiscoverableLogin is store-stash →
+// consume — the challenge is never client-echoed.
+func TestPasskeyDiscoverableLoginVertical(t *testing.T) {
+	api, repo, v := seedDiscoverableLoginAPI(t)
+	eh := api.ExternalHandler()
+
+	// 1) begin: usernameless — no request body by design (the caller supplies no identifier),
+	// only the JSON Content-Type CSRF guard. The response is one {"publicKey":{...},"login_id":
+	// "..."} document, and exactly one challenge is stashed, keyed by the returned handle.
+	w := do(eh, "POST", "/api/v1/auth/passkey/login/discoverable/begin", `{}`, jsonHeader)
+	if w.Code != http.StatusOK {
+		t.Fatalf("begin: code = %d, want 200 (%s)", w.Code, w.Body.String())
+	}
+	body := acctBody(t, w)
+	if body["publicKey"] == nil {
+		t.Errorf("begin must return the assertion options verbatim, got %s", w.Body.String())
+	}
+	loginID, _ := body["login_id"].(string)
+	if loginID == "" {
+		t.Fatalf("begin must return a non-empty login_id, got %s", w.Body.String())
+	}
+	if len(repo.discoverableChallenges) != 1 {
+		t.Fatalf("begin must stash exactly one discoverable challenge, got %d", len(repo.discoverableChallenges))
+	}
+	if _, ok := repo.discoverableChallenges[loginID]; !ok {
+		t.Errorf("the stashed challenge must be keyed by the returned login_id %q", loginID)
+	}
+
+	// 2) finish: the body carries ONLY the login_id and the assertion — no identifier and no
+	// session data. The account is revealed by the userHandle the verifier surfaces (u1).
+	w = do(eh, "POST", "/api/v1/auth/passkey/login/discoverable/finish",
+		`{"login_id":"`+loginID+`","assertion":{"id":"cred-1","type":"public-key"}}`, jsonHeader)
+	if w.Code != http.StatusOK {
+		t.Fatalf("finish: code = %d, want 200 (%s)", w.Code, w.Body.String())
+	}
+	// THE security assertion: the session data the verifier saw is exactly what begin stashed —
+	// it travelled store-stash → consume, never the client (the finish body has no session data).
+	if !bytes.Equal(v.lastSession, []byte("disc-session")) {
+		t.Fatalf("finish session data = %q, want the server-stashed %q (challenge must not be client-echoed)",
+			v.lastSession, "disc-session")
+	}
+	vb := acctBody(t, w)
+	if vb["user_id"] != "u1" || vb["role"] != "user" {
+		t.Fatalf("finish body = %v, want user_id:u1 role:user", vb)
+	}
+	// The host-only HttpOnly cookie is the whole point — same contract as the other doors.
+	cookies := w.Result().Cookies()
+	if len(cookies) != 1 || cookies[0].Name != sessionCookieName || cookies[0].Value == "" {
+		t.Fatalf("want one non-empty %s cookie, got %v", sessionCookieName, cookies)
+	}
+	s, ok := repo.sessions[hashCookie(cookies[0].Value)]
+	if !ok {
+		t.Fatal("no session row for the issued cookie (must be stored hashed)")
+	}
+	if s.userID != "u1" {
+		t.Errorf("session userID = %q, want u1", s.userID)
+	}
+	if want := frozenNow.Add(sessionTTL); !s.expiresAt.Equal(want) {
+		t.Errorf("session expiresAt = %v, want now+sessionTTL = %v", s.expiresAt, want)
+	}
+	// Audited once, by the RESOLVED account's username (there is no principal yet); begin is silent.
+	if n := len(repo.audits); n != 1 {
+		t.Fatalf("want exactly 1 audit (passkey_login_discoverable), got %d: %+v", n, repo.audits)
+	}
+	if repo.audits[0].Action != "auth.passkey_login_discoverable" || repo.audits[0].Actor != "player" {
+		t.Errorf("audit = %+v, want auth.passkey_login_discoverable by player", repo.audits[0])
+	}
+
+	// 3) single-use: the consumed login_id buys nothing a second time.
+	if w := do(eh, "POST", "/api/v1/auth/passkey/login/discoverable/finish",
+		`{"login_id":"`+loginID+`","assertion":{"id":"cred-1","type":"public-key"}}`, jsonHeader); w.Code != http.StatusBadRequest || decodeErr(t, w) != "passkey_login_invalid" {
+		t.Fatalf("replay of consumed login_id: code = %d body %s, want 400 passkey_login_invalid", w.Code, w.Body.String())
+	}
+}
+
+// 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.
+func TestPasskeyDiscoverableLoginBeginCapped(t *testing.T) {
+	api, repo, _ := seedDiscoverableLoginAPI(t)
+	repo.discoverableFull = true
+	eh := api.ExternalHandler()
+
+	w := do(eh, "POST", "/api/v1/auth/passkey/login/discoverable/begin", `{}`, jsonHeader)
+	if w.Code != http.StatusTooManyRequests || decodeErr(t, w) != "too_many_challenges" {
+		t.Fatalf("capped begin: code = %d body %s, want 429 too_many_challenges", w.Code, w.Body.String())
+	}
+	if len(repo.discoverableChallenges) != 0 {
+		t.Errorf("a capped begin must stash nothing, got %d", len(repo.discoverableChallenges))
+	}
+}
+
+// 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).
+func TestPasskeyDiscoverableLoginBeginVerifierError(t *testing.T) {
+	api, repo, v := seedDiscoverableLoginAPI(t)
+	v.beginLoginErr = errors.New("cannot begin discoverable")
+	eh := api.ExternalHandler()
+
+	w := do(eh, "POST", "/api/v1/auth/passkey/login/discoverable/begin", `{}`, jsonHeader)
+	if w.Code != http.StatusBadRequest || decodeErr(t, w) != "passkey_login_failed" {
+		t.Fatalf("verifier error: code = %d body %s, want 400 passkey_login_failed", w.Code, w.Body.String())
+	}
+	if len(repo.discoverableChallenges) != 0 {
+		t.Errorf("a failed begin must stash nothing, got %d", len(repo.discoverableChallenges))
+	}
+}
+
+// TestPasskeyDiscoverableLoginGates covers the shared front doors of both halves: the
+// fail-closed local-auth toggle, graceful degradation when no verifier is wired, the CSRF
+// Content-Type guard (these are Public, credential-minting routes), and the finish input gates
+// that must reject before any consume or resolve.
+func TestPasskeyDiscoverableLoginGates(t *testing.T) {
+	const beginPath = "/api/v1/auth/passkey/login/discoverable/begin"
+	const finishPath = "/api/v1/auth/passkey/login/discoverable/finish"
+	const goodFinish = `{"login_id":"x","assertion":{"id":"cred-1"}}`
+
+	t.Run("local auth disabled -> 403 on both halves", func(t *testing.T) {
+		api := newTestAPI(newFakeRepo(), newFakeCluster()) // no LocalAuthEnabledKey: fails closed
+		api.Passkey = &fakePasskeyVerifier{}
+		eh := api.ExternalHandler()
+		if w := do(eh, "POST", beginPath, `{}`, jsonHeader); w.Code != http.StatusForbidden || decodeErr(t, w) != "local_auth_disabled" {
+			t.Errorf("begin: code = %d body %s, want 403 local_auth_disabled", w.Code, w.Body.String())
+		}
+		if w := do(eh, "POST", finishPath, goodFinish, jsonHeader); w.Code != http.StatusForbidden || decodeErr(t, w) != "local_auth_disabled" {
+			t.Errorf("finish: code = %d body %s, want 403 local_auth_disabled", w.Code, w.Body.String())
+		}
+	})
+
+	t.Run("no verifier wired -> 503 passkey_unavailable on both halves", func(t *testing.T) {
+		api, _, _ := seedDiscoverableLoginAPI(t)
+		api.Passkey = nil // unwire it: the degraded path must be a clean 503, not a panic
+		eh := api.ExternalHandler()
+		if w := do(eh, "POST", beginPath, `{}`, jsonHeader); w.Code != http.StatusServiceUnavailable || decodeErr(t, w) != "passkey_unavailable" {
+			t.Errorf("begin: code = %d body %s, want 503 passkey_unavailable", w.Code, w.Body.String())
+		}
+		if w := do(eh, "POST", finishPath, goodFinish, jsonHeader); w.Code != http.StatusServiceUnavailable || decodeErr(t, w) != "passkey_unavailable" {
+			t.Errorf("finish: code = %d body %s, want 503 passkey_unavailable", w.Code, w.Body.String())
+		}
+	})
+
+	t.Run("non-JSON content type -> 415 on both halves", func(t *testing.T) {
+		api, _, _ := seedDiscoverableLoginAPI(t)
+		eh := api.ExternalHandler()
+		for _, ct := range []string{"", "text/plain", "application/x-www-form-urlencoded"} {
+			if w := do(eh, "POST", beginPath, `{}`, ctHeader(ct)); w.Code != http.StatusUnsupportedMediaType {
+				t.Errorf("begin with Content-Type %q: code = %d, want 415", ct, w.Code)
+			}
+			if w := do(eh, "POST", finishPath, goodFinish, ctHeader(ct)); w.Code != http.StatusUnsupportedMediaType {
+				t.Errorf("finish with Content-Type %q: code = %d, want 415", ct, w.Code)
+			}
+		}
+	})
+
+	t.Run("finish missing inputs -> 400 bad_request, nothing minted", func(t *testing.T) {
+		cases := map[string]string{
+			"missing login_id":  `{"assertion":{"id":"cred-1"}}`,
+			"empty login_id":    `{"login_id":"","assertion":{"id":"cred-1"}}`,
+			"missing assertion": `{"login_id":"x"}`,
+		}
+		for name, body := range cases {
+			api, repo, _ := seedDiscoverableLoginAPI(t)
+			w := do(api.ExternalHandler(), "POST", finishPath, body, jsonHeader)
+			if w.Code != http.StatusBadRequest || decodeErr(t, w) != "bad_request" {
+				t.Errorf("%s: code = %d body %s, want 400 bad_request", name, w.Code, w.Body.String())
+			}
+			if len(repo.sessions) != 0 {
+				t.Errorf("%s: a rejected finish must mint no session (%d)", name, len(repo.sessions))
+			}
+		}
+	})
+}
+
+// TestPasskeyDiscoverableLoginFinishRejections is the redeem-side failure matrix and the anchor
+// for from-zero anti-enumeration: a bogus handle, an expired challenge, an assertion that fails
+// verification, AND a userHandle that resolves to no account must ALL answer the byte-identical
+// passkey_login_invalid envelope (code AND message) and mint no session. The last case is the
+// one unique to this door — the account is chosen by the authenticator, so an unresolvable
+// handle must fail exactly like a bad signature, never leaking that the handle was well-formed.
+func TestPasskeyDiscoverableLoginFinishRejections(t *testing.T) {
+	const finishPath = "/api/v1/auth/passkey/login/discoverable/finish"
+	finish := func(eh http.Handler, loginID string) *httptest.ResponseRecorder {
+		return do(eh, "POST", finishPath,
+			`{"login_id":"`+loginID+`","assertion":{"id":"cred-1","type":"public-key"}}`, jsonHeader)
+	}
+
+	cases := []struct {
+		name    string
+		loginID string
+		setup   func(repo *fakeRepo, v *fakePasskeyVerifier)
+	}{
+		{"no live challenge", "ghost", func(repo *fakeRepo, v *fakePasskeyVerifier) {}},
+		{"expired challenge", "ex", func(repo *fakeRepo, v *fakePasskeyVerifier) {
+			repo.discoverableChallenges["ex"] = &fakeDiscoverableChallenge{
+				sessionData: []byte("disc-session"), expiresAt: frozenNow.Add(-time.Second),
+			}
+		}},
+		{"assertion fails verification", "live", func(repo *fakeRepo, v *fakePasskeyVerifier) {
+			repo.discoverableChallenges["live"] = &fakeDiscoverableChallenge{
+				sessionData: []byte("disc-session"), expiresAt: frozenNow.Add(passkeyChallengeTTL),
+			}
+			v.failErr = errors.New("bad assertion")
+		}},
+		{"userHandle resolves to no account", "live", func(repo *fakeRepo, v *fakePasskeyVerifier) {
+			repo.discoverableChallenges["live"] = &fakeDiscoverableChallenge{
+				sessionData: []byte("disc-session"), expiresAt: frozenNow.Add(passkeyChallengeTTL),
+			}
+			v.discoverableUserHandle = []byte("nonexistent")
+		}},
+	}
+
+	var envelopes [][2]string
+	for _, c := range cases {
+		api, repo, v := seedDiscoverableLoginAPI(t)
+		c.setup(repo, v)
+		w := finish(api.ExternalHandler(), c.loginID)
+		if w.Code != http.StatusBadRequest {
+			t.Fatalf("%s: code = %d, want 400 (%s)", c.name, w.Code, w.Body.String())
+		}
+		code, msg := errEnvelope(t, w)
+		if code != "passkey_login_invalid" {
+			t.Errorf("%s: error code = %q, want passkey_login_invalid", c.name, code)
+		}
+		if len(repo.sessions) != 0 {
+			t.Errorf("%s: a rejected finish must mint no session (got %d)", c.name, len(repo.sessions))
+		}
+		if len(w.Result().Cookies()) != 0 {
+			t.Errorf("%s: a rejected finish must set no cookie", c.name)
+		}
+		envelopes = append(envelopes, [2]string{code, msg})
+	}
+	// The anchor: every envelope is identical (code AND message), so no branch — including the
+	// unresolvable-handle branch — is distinguishable from another.
+	for i := 1; i < len(envelopes); i++ {
+		if envelopes[i] != envelopes[0] {
+			t.Errorf("envelope for %q %v differs from %q %v — all rejections must be identical",
+				cases[i].name, envelopes[i], cases[0].name, envelopes[0])
+		}
+	}
+}
+
+// TestPasskeyDiscoverableLoginFaceSeparation enforces that both halves are web-only: the
+// internal (service-token) face must 404 them, never serve them.
+func TestPasskeyDiscoverableLoginFaceSeparation(t *testing.T) {
+	api, _, _ := seedDiscoverableLoginAPI(t)
+	ih := api.InternalHandler()
+	if w := do(ih, "POST", "/api/v1/auth/passkey/login/discoverable/begin", `{}`, jsonHeader); w.Code != http.StatusNotFound {
+		t.Errorf("begin on internal face: code = %d, want 404", w.Code)
+	}
+	if w := do(ih, "POST", "/api/v1/auth/passkey/login/discoverable/finish", `{"login_id":"x","assertion":{"id":"y"}}`, jsonHeader); w.Code != http.StatusNotFound {
+		t.Errorf("finish on internal face: code = %d, want 404", w.Code)
+	}
+}

internal/api/pgrepo.go

@@ -1004,6 +1004,91 @@ func (p *PGRepo) ConsumePasskeyChallengeByUser(ctx context.Context, userID, purp
 	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. Volumetric per-IP limiting is the edge's job (handlers_auth_email.go):
+// behind Cloudflare RemoteAddr is the proxy, and a usernameless door has no recipient to key a
+// fair per-caller limit on.
+const maxLiveDiscoverableChallenges = 4096
+
+// 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 {
+	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_discoverable_challenges WHERE expires_at <= $1 OR consumed_at IS NOT NULL`,
+		now); err != nil {
+		return fmt.Errorf("reap discoverable challenges: %w", err)
+	}
+	var live int
+	if err := tx.QueryRowContext(ctx,
+		`SELECT count(*) FROM webauthn_discoverable_challenges`).Scan(&live); err != nil {
+		return fmt.Errorf("count discoverable challenges: %w", err)
+	}
+	if live >= maxLiveDiscoverableChallenges {
+		return ErrTooManyDiscoverableChallenges
+	}
+	if _, err := tx.ExecContext(ctx,
+		`INSERT INTO webauthn_discoverable_challenges (id, session_data, expires_at)
+		 VALUES ($1, $2, $3)`,
+		id, sessionData, expiresAt); err != nil {
+		return fmt.Errorf("insert discoverable challenge: %w", err)
+	}
+	return tx.Commit()
+}
+
+// ConsumeDiscoverableChallenge redeems the challenge under handle id, single-use (see the Repo
+// interface for the contract). The row is taken FOR UPDATE so a concurrent finish cannot
+// double-spend it; expiry is checked before consuming. No live row → ErrPasskeyChallengeInvalid.
+func (p *PGRepo) ConsumeDiscoverableChallenge(ctx context.Context, id 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 (
+		sessionData []byte
+		expiresAt   time.Time
+	)
+	switch err := tx.QueryRowContext(ctx,
+		`SELECT session_data, expires_at FROM webauthn_discoverable_challenges
+		 WHERE id = $1 AND consumed_at IS NULL FOR UPDATE`,
+		id).Scan(&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_discoverable_challenges SET consumed_at = $2 WHERE id = $1`, id, now); err != nil {
+		return nil, fmt.Errorf("consume discoverable challenge: %w", err)
+	}
+	if err := tx.Commit(); err != nil {
+		return nil, err
+	}
+	return sessionData, nil
+}
+
 // CreatePasskeyCredential stores a freshly verified passkey (enrollment). Only public
 // attestation material is written; a credential_id already bound to ANY account is left
 // untouched (ON CONFLICT DO NOTHING) and reported as ErrConflict via a zero RowsAffected,

internal/api/repo.go

@@ -350,9 +350,25 @@ 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 always has a principal, so
-	// there is no usernameless consume-by-hash variant (login is a deferred slice).
+	// 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.
 	ConsumePasskeyChallengeByUser(ctx context.Context, userID, purpose 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
+	// 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
+	// the stashed SessionData. An unknown, expired, or already-consumed handle →
+	// ErrPasskeyChallengeInvalid, so the finish door never doubles as a state oracle.
+	ConsumeDiscoverableChallenge(ctx context.Context, id string, now time.Time) (sessionData []byte, err error)
 	// CreatePasskeyCredential stores a freshly verified passkey for a user (Phase 6
 	// enrollment). It writes only public attestation material (credential_id,
 	// public_key, sign_count, aaguid) plus the caller's nickname. A credential_id

internal/passkey/verifier.go

@@ -6,15 +6,19 @@
 // this package imports api for the seam types; api never imports this package, which is
 // what keeps the seam (and the api test suite's fake verifier) honest.
 //
-// Scope: the full WebAuthn ceremony crypto — both the credential-creation (enrollment:
-// BeginRegistration/FinishRegistration over go-webauthn's BeginRegistration/CreateCredential)
-// and the assertion (login: BeginLogin/FinishLogin over BeginLogin/ValidateLogin) halves.
-// Both are Oracle-verified in verifier_test.go against a virtual authenticator. Only the
-// enrollment half is wired to HTTP handlers today (handlers_passkey.go); the login
-// handlers, session minting, and the panel.* relying-party boundary are a deferred slice,
-// so BeginLogin/FinishLogin here have no api-package caller yet. They are added to the
-// concrete adapter (not the api.PasskeyVerifier interface) precisely so the crypto is
-// built and verified now while the interface grows only when a handler consumes it.
+// Scope: the full WebAuthn ceremony crypto across three halves, each Oracle-verified in
+// verifier_test.go against a virtual authenticator:
+//
+//   - enrollment (BeginRegistration/FinishRegistration over go-webauthn's
+//     BeginRegistration/CreateCredential),
+//   - username-first login (BeginLogin/FinishLogin over BeginLogin/ValidateLogin), where the
+//     account is known and its bound credentials scope allowCredentials, and
+//   - discoverable, "usernameless" login (BeginDiscoverableLogin/FinishDiscoverableLogin over
+//     BeginDiscoverableLogin/ValidateDiscoverableLogin), where the account is unknown at begin
+//     and revealed only by the userHandle inside the signed assertion (task #40).
+//
+// All three are in the api.PasskeyVerifier interface and consumed by handlers today (enrollment
+// + login in handlers_passkey.go, from-zero login in handlers_passkey_discoverable.go).
 package passkey
 
 import (
@@ -68,6 +72,17 @@ func New(rpID, displayName string, origins []string) (*Verifier, error) {
 		// email-OTP factor (migration 0004); no one is locked out.
 		AuthenticatorSelection: protocol.AuthenticatorSelection{
 			UserVerification: protocol.VerificationRequired,
+			// Prefer a discoverable (resident) credential so a passkey can later be asserted
+			// usernamelessly (task #40 from-zero login): the authenticator stores the credential
+			// and can present it with no identifier typed. PREFERRED, not Required, keeps the
+			// no-lockout ethos — an authenticator that cannot make a resident key still binds a
+			// working username-first passkey (BeginLogin) and falls back to email-OTP; only the
+			// from-zero convenience is unavailable. This shapes only the creation options a browser
+			// receives (a server-side request, asserted in TestEnrollmentRequestsResidentKey);
+			// whether a real authenticator honors it — actually storing a resident key — is a device
+			// property no unit test can prove, so already-bound non-resident credentials stay
+			// username-first until their owner enrolls a new passkey.
+			ResidentKey: protocol.ResidentKeyRequirementPreferred,
 		},
 	})
 	if err != nil {
@@ -200,6 +215,68 @@ func (v *Verifier) FinishLogin(user api.PasskeyUser, sessionData []byte, asserti
 	}, nil
 }
 
+// BeginDiscoverableLogin starts a USERNAMELESS assertion ceremony (task #40): the caller is
+// not yet identified, so — unlike BeginLogin — there is no user and no allowCredentials. The
+// authenticator picks a resident (discoverable) credential it holds for this RP and reveals
+// the account only inside the signed response at finish. It returns the {"publicKey": {...}}
+// request options for navigator.credentials.get() and the opaque, marshaled SessionData the
+// handler stashes under an opaque handle (migration 0013's non-user-keyed store) and replays
+// at finish. User verification is required, matching enrollment, so a from-zero login still
+// proves possession AND user.
+func (v *Verifier) BeginDiscoverableLogin() (json.RawMessage, []byte, error) {
+	assertion, session, err := v.wa.BeginDiscoverableLogin(webauthn.WithUserVerification(protocol.VerificationRequired))
+	if err != nil {
+		return nil, nil, err
+	}
+	// CredentialAssertion marshals to {"publicKey": {...}} with an EMPTY allowCredentials —
+	// exactly the usernameless document the browser hands to navigator.credentials.get().
+	options, err := json.Marshal(assertion)
+	if err != nil {
+		return nil, nil, err
+	}
+	// As with the other ceremonies, we stash the marshaled SessionData verbatim and let the
+	// challenge row's TTL be the sole authority on liveness (no expiry inside SessionData).
+	sessionData, err := json.Marshal(session)
+	if err != nil {
+		return nil, nil, err
+	}
+	return options, sessionData, nil
+}
+
+// FinishDiscoverableLogin verifies a usernameless assertion (task #40). go-webauthn hands the
+// authenticator-revealed user handle to resolveUser, which the caller uses to load the account
+// and its bound credentials WITHOUT any client-supplied identifier; go-webauthn then checks
+// the asserted credential id is one that user holds and verifies the signature against its
+// stored COSE public key. The user handle is the account's stable id (webauthnUser.WebAuthnID),
+// so resolveUser is a direct id lookup. A resolveUser error (unknown handle) fails the ceremony
+// closed. resolveUser is a plain api-typed callback so the api package still never imports
+// go-webauthn: the adapter wraps it into go-webauthn's DiscoverableUserHandler here.
+func (v *Verifier) FinishDiscoverableLogin(resolveUser func(userHandle []byte) (api.PasskeyUser, error), sessionData []byte, assertion io.Reader) (api.VerifiedAssertion, error) {
+	var session webauthn.SessionData
+	if err := json.Unmarshal(sessionData, &session); err != nil {
+		return api.VerifiedAssertion{}, err
+	}
+	parsed, err := protocol.ParseCredentialRequestResponseBody(assertion)
+	if err != nil {
+		return api.VerifiedAssertion{}, err
+	}
+	handler := func(_, userHandle []byte) (webauthn.User, error) {
+		u, err := resolveUser(userHandle)
+		if err != nil {
+			return nil, err
+		}
+		return webauthnUser{u: u}, nil
+	}
+	cred, err := v.wa.ValidateDiscoverableLogin(handler, session, parsed)
+	if err != nil {
+		return api.VerifiedAssertion{}, err
+	}
+	return api.VerifiedAssertion{
+		CredentialID: base64.RawURLEncoding.EncodeToString(cred.ID),
+		SignCount:    cred.Authenticator.SignCount,
+	}, nil
+}
+
 // excludeDescriptors turns the principal's already-bound passkeys into the
 // excludeCredentials list for a creation ceremony. A stored credential id that does not
 // decode as base64url is skipped rather than aborting the whole ceremony — a single

internal/passkey/verifier_test.go

@@ -2,6 +2,7 @@ package passkey
 
 import (
 	"encoding/base64"
+	"encoding/json"
 	"slices"
 	"strings"
 	"testing"
@@ -333,3 +334,179 @@ func TestLoginUnknownCredentialRejected(t *testing.T) {
 		t.Fatal("FinishLogin accepted an assertion from an unbound credential; want rejection")
 	}
 }
+
+// TestDiscoverableLoginRoundTrip is the PARITY check for the usernameless (from-zero) half
+// (task #40), and the proof its VERIFY path is real crypto rather than a stub. It differs from
+// TestLoginRoundTrip in the two ways that define discoverable login: the begin names no user
+// (so the request's allowCredentials must be EMPTY), and the account is revealed only by the
+// userHandle the authenticator embeds in the signed assertion — the verifier hands that handle
+// to a resolve callback that stands in for the handler's userHandle → UserByID lookup. Chained
+// onto a REAL enrollment so the assertion validates against a genuine COSE key, and the
+// authenticator's counter is advanced first so the surfaced SignCount is proven real, not a
+// hardcoded 0. What this does NOT prove: that a real authenticator actually STORED a resident
+// key — that residency is a device property (see TestEnrollmentRequestsResidentKey for the only
+// thing a unit test can pin, the request the browser receives).
+func TestDiscoverableLoginRoundTrip(t *testing.T) {
+	v := newTestVerifier(t)
+	rp := virtualRP()
+	authenticator := virtualwebauthn.NewAuthenticator()
+	cred := virtualwebauthn.NewCredential(virtualwebauthn.KeyTypeEC2)
+
+	stored := enrollCredential(t, v, rp, authenticator, cred)
+
+	// The authenticator returns the user handle in the assertion — this is what a resident
+	// credential does and what lets the account be resolved from nothing typed. It is the
+	// account's stable user id (WebAuthnID), so the resolver must receive exactly these bytes.
+	authenticator.Options.UserHandle = []byte(testUserID)
+	// Advance the counter so a real (non-zero, strictly increasing) SignCount must survive.
+	cred.Counter = 9
+
+	options, sessionData, err := v.BeginDiscoverableLogin()
+	if err != nil {
+		t.Fatalf("BeginDiscoverableLogin: %v", err)
+	}
+	assertionOpts, err := virtualwebauthn.ParseAssertionOptions(string(options))
+	if err != nil {
+		t.Fatalf("ParseAssertionOptions: %v (options=%s)", err, options)
+	}
+	if assertionOpts.RelyingPartyID != testRPID {
+		t.Fatalf("options RP id = %q, want %q", assertionOpts.RelyingPartyID, testRPID)
+	}
+	// The defining property of a usernameless request: no credential is named. If this were
+	// non-empty the ceremony would be username-first and the test would prove nothing about #40.
+	if len(assertionOpts.AllowCredentials) != 0 {
+		t.Fatalf("allowCredentials = %v, want empty (usernameless request names no credential)", assertionOpts.AllowCredentials)
+	}
+
+	assertionResponse := virtualwebauthn.CreateAssertionResponse(rp, authenticator, cred, *assertionOpts)
+
+	// resolve stands in for the handler's userHandle → UserByID lookup: it records the handle
+	// it was handed (to prove the account is revealed by the authenticator, not the client) and
+	// returns the stored credential so ValidateDiscoverableLogin can verify the signature.
+	var gotHandle []byte
+	resolve := func(userHandle []byte) (api.PasskeyUser, error) {
+		gotHandle = userHandle
+		return testUser(stored), nil
+	}
+	va, err := v.FinishDiscoverableLogin(resolve, sessionData, strings.NewReader(assertionResponse))
+	if err != nil {
+		t.Fatalf("FinishDiscoverableLogin: %v", err)
+	}
+	if string(gotHandle) != testUserID {
+		t.Errorf("resolver received userHandle %q, want %q (the account is revealed by the assertion)", gotHandle, testUserID)
+	}
+	if va.CredentialID != stored.CredentialID {
+		t.Errorf("asserted CredentialID = %q, want %q", va.CredentialID, stored.CredentialID)
+	}
+	if va.SignCount != 9 {
+		t.Errorf("SignCount = %d, want 9 (the authenticator's advanced counter)", va.SignCount)
+	}
+}
+
+// TestDiscoverableLoginResolveFailsClosed proves the from-zero door fails CLOSED when the
+// authenticator-revealed account cannot be resolved: a resolve callback that returns an error
+// (the handler's UserByID found nothing — a handle for a deleted/unknown account) must abort
+// the ceremony, never mint an assertion. Without this the usernameless path could be coaxed
+// into treating an unresolvable handle as success. Pairs with the round-trip above so the
+// resolver neither over- nor under-blocks.
+func TestDiscoverableLoginResolveFailsClosed(t *testing.T) {
+	v := newTestVerifier(t)
+	rp := virtualRP()
+	authenticator := virtualwebauthn.NewAuthenticator()
+	cred := virtualwebauthn.NewCredential(virtualwebauthn.KeyTypeEC2)
+
+	enrollCredential(t, v, rp, authenticator, cred)
+	authenticator.Options.UserHandle = []byte("nonexistent-account")
+
+	options, sessionData, err := v.BeginDiscoverableLogin()
+	if err != nil {
+		t.Fatalf("BeginDiscoverableLogin: %v", err)
+	}
+	assertionOpts, err := virtualwebauthn.ParseAssertionOptions(string(options))
+	if err != nil {
+		t.Fatalf("ParseAssertionOptions: %v", err)
+	}
+	assertionResponse := virtualwebauthn.CreateAssertionResponse(rp, authenticator, cred, *assertionOpts)
+
+	resolve := func(userHandle []byte) (api.PasskeyUser, error) {
+		return api.PasskeyUser{}, api.ErrNotFound
+	}
+	if _, err := v.FinishDiscoverableLogin(resolve, sessionData, strings.NewReader(assertionResponse)); err == nil {
+		t.Fatal("FinishDiscoverableLogin accepted an assertion whose account could not be resolved; want rejection")
+	}
+}
+
+// TestDiscoverableLoginUnboundCredentialRejected proves the credential-ownership binding for
+// the usernameless door — the defense unique to it. In username-first login the server names
+// allowCredentials, so an assertion must match a credential the server itself offered. The
+// from-zero door names NOTHING: the authenticator reveals BOTH the userHandle and the signing
+// credential, so the ONLY barrier stopping an attacker from signing with their own resident key
+// while embedding a victim's userHandle is go-webauthn's check that the asserted credential id
+// belongs to the resolved user. Here the resolve callback succeeds (the handle names a REAL
+// account, u1, holding credential A) — unlike TestDiscoverableLoginResolveFailsClosed where it
+// resolves to nothing — but the assertion is signed by credential B, never bound to u1.
+// FinishDiscoverableLogin must reject: a good signature over the right challenge under a valid
+// userHandle is still not enough without membership. This is the exact guard the "account is
+// revealed by the assertion, never named by the client" claim leans on.
+func TestDiscoverableLoginUnboundCredentialRejected(t *testing.T) {
+	v := newTestVerifier(t)
+	rp := virtualRP()
+	authenticator := virtualwebauthn.NewAuthenticator()
+	credA := virtualwebauthn.NewCredential(virtualwebauthn.KeyTypeEC2)
+	stored := enrollCredential(t, v, rp, authenticator, credA)
+
+	// A valid handle: it resolves to the real account u1, which holds credential A.
+	authenticator.Options.UserHandle = []byte(testUserID)
+
+	options, sessionData, err := v.BeginDiscoverableLogin()
+	if err != nil {
+		t.Fatalf("BeginDiscoverableLogin: %v", err)
+	}
+	assertionOpts, err := virtualwebauthn.ParseAssertionOptions(string(options))
+	if err != nil {
+		t.Fatalf("ParseAssertionOptions: %v", err)
+	}
+	// Sign with a fresh credential never bound to u1. The resolver still returns u1's real
+	// credential set (credential A) — so the ONLY thing that can reject this is the check that
+	// the asserted credential (B) is among the resolved user's credentials.
+	credB := virtualwebauthn.NewCredential(virtualwebauthn.KeyTypeEC2)
+	assertionResponse := virtualwebauthn.CreateAssertionResponse(rp, authenticator, credB, *assertionOpts)
+
+	resolve := func(userHandle []byte) (api.PasskeyUser, error) {
+		return testUser(stored), nil
+	}
+	if _, err := v.FinishDiscoverableLogin(resolve, sessionData, strings.NewReader(assertionResponse)); err == nil {
+		t.Fatal("FinishDiscoverableLogin accepted an assertion signed by a credential not bound to the resolved user; want rejection")
+	}
+}
+
+// TestEnrollmentRequestsResidentKey pins the ONLY server-side half of the from-zero enabler a
+// unit test can prove: that enrollment ASKS the browser for a resident (discoverable) key, i.e.
+// the creation options carry authenticatorSelection.residentKey = "preferred". Whether a real
+// authenticator honors the request — actually persisting a resident key so it can later be
+// asserted usernamelessly — is a device property no unit test can reach, which is exactly why
+// the from-zero door is inert for a credential until its owner enrolls a NEW passkey against
+// these options. "preferred" (not "required") is deliberate: an authenticator that cannot store
+// a resident key still binds a working username-first passkey and falls back to email-OTP, so
+// no one is locked out — asserting the exact string guards against a silent drop to "" (ask for
+// nothing) or a tightening to "required" (which would break the no-lockout ethos).
+func TestEnrollmentRequestsResidentKey(t *testing.T) {
+	v := newTestVerifier(t)
+	options, _, err := v.BeginRegistration(testUser())
+	if err != nil {
+		t.Fatalf("BeginRegistration: %v", err)
+	}
+	var doc struct {
+		PublicKey struct {
+			AuthenticatorSelection struct {
+				ResidentKey string `json:"residentKey"`
+			} `json:"authenticatorSelection"`
+		} `json:"publicKey"`
+	}
+	if err := json.Unmarshal(options, &doc); err != nil {
+		t.Fatalf("unmarshal creation options: %v (options=%s)", err, options)
+	}
+	if got := doc.PublicKey.AuthenticatorSelection.ResidentKey; got != "preferred" {
+		t.Errorf("authenticatorSelection.residentKey = %q, want %q", got, "preferred")
+	}
+}

internal/store/migrations/0013_webauthn_discoverable_login.sql

@@ -0,0 +1,36 @@
+-- Phase 6 passkey — the challenge store for DISCOVERABLE ("usernameless") login (spec §14,
+-- task #40). webauthn_challenges (0007) is keyed by (user_id, purpose) because both
+-- enrollment and email-first login already know WHO is authenticating before the ceremony
+-- starts. A from-zero passkey login does not: the browser calls navigator.credentials.get()
+-- with an EMPTY allowCredentials list, the authenticator offers a resident credential it
+-- holds for this RP, and the account is revealed only inside the signed assertion at finish.
+-- So this challenge cannot be keyed by user — it is keyed by an opaque, server-minted handle
+-- (login_id) the browser echoes back at finish, and the marshaled WebAuthn SessionData is the
+-- only server-held ceremony state. This is exactly the "non-user-keyed challenge store, a
+-- future migration" that 0007's own comment anticipated.
+--
+-- Bounding. webauthn_challenges self-bounds via a per-(user,purpose) supersede-DELETE on each
+-- begin — one live row per user+purpose. That key does not exist here (there is no user at
+-- begin), so this table is bounded two ways instead, both inside CreateDiscoverableChallenge's
+-- one transaction: (1) every begin first reaps rows that already expired or were consumed by a
+-- prior finish, and (2) a hard cap (maxLiveDiscoverableChallenges) refuses a new begin once the
+-- live count is reached, so an abusive begin-flood is bounded to trivial storage rather than
+-- growing without limit. A reap alone does NOT bound a burst — freshly inserted rows have a
+-- future expiry, so N begins inside the TTL leave N live rows — which is why the cap, not the
+-- reap, is the real ceiling. Volumetric per-source (client-IP) limiting is deliberately left to
+-- the edge, the same stance handlers_auth_email.go documents (behind Cloudflare RemoteAddr is
+-- the proxy, and CGNAT would false-positive) and unavoidable here since a usernameless door has
+-- neither a principal NOR a typed recipient to key a fair per-caller limit on.
+--
+-- The expires_at index serves the reap's WHERE clause; consumed_at (nullable) makes the row
+-- single-use, stamped at finish and swept by a later begin's reap.
+CREATE TABLE webauthn_discoverable_challenges (
+    id           text        PRIMARY KEY,          -- opaque login handle (login_id): 128-bit hex
+    session_data bytea       NOT NULL,             -- marshaled webauthn.SessionData (the challenge lives here)
+    expires_at   timestamptz NOT NULL,
+    consumed_at  timestamptz,                       -- single-use: NULL until finish stamps it
+    created_at   timestamptz NOT NULL DEFAULT now()
+);
+
+CREATE INDEX webauthn_discoverable_challenges_expires_idx
+    ON webauthn_discoverable_challenges (expires_at);