Maven/cmd/mavweb/webauthn.go

package main

import (
	"context"
	_ "embed"
	"encoding/json"
	"errors"
	"fmt"
	"log"
	"net/http"
	"time"

	"github.com/kami/maven/internal/auth"
	"github.com/kami/maven/internal/ipc"
	"github.com/kami/maven/internal/webauthn"
)

// assertIPC — satisfies the AssertStepUp caller shape. The only implementation
// is *ipc.Client; in-process CoreAPI adapters return ErrUnknownMethod.
type assertIPC interface {
	AssertStepUp(ctx context.Context) error
}

// keyIPC — satisfies the key-wrap and unlock methods. The only implementation
// is *ipc.Client; in-process CoreAPI adapters do not implement it. When nil,
// StoreEncryptionKey and Unlock are silently skipped.
type keyIPC interface {
	StoreEncryptionKey(ctx context.Context, secret []byte, explicit bool) error
	Unlock(ctx context.Context, secret []byte) error
}

// PasskeyHandle holds the WebAuthn relying party, a local in-memory credential
// store, and the IPC client used to assert step-up and to wrap/unwrap the
// daemon's encryption key. It serves the four WebAuthn HTTP endpoints
// (register/begin, register/finish, assert/begin, assert/finish).
//
// Credentials are kept in-memory only (a single-user daemon restarts
// infrequently, and re-enrolling after restart is acceptable). A future
// version may persist them to disk.
type PasskeyHandle struct {
	rp        *webauthn.RP
	assertFn  assertIPC // *ipc.Client when connected; nil ⇒ no step-up IPC
	encryptFn keyIPC    // *ipc.Client when connected; nil ⇒ key wrap/unlock disabled
	store     *credentialStore
	session   *webauthn.PasskeySession
}

type localCred struct {
	PublicKey []byte
	SignCount int64
}

func newPasskeyHandle(cfg webauthn.Config, core ipc.CoreAPI, storePath string, session *webauthn.PasskeySession) (*PasskeyHandle, error) {
	var af assertIPC
	if c, ok := core.(assertIPC); ok {
		af = c
	}
	var ek keyIPC
	if c, ok := core.(keyIPC); ok {
		ek = c
	}
	store, err := newCredentialStore(storePath)
	if err != nil {
		return nil, fmt.Errorf("credential store: %w", err)
	}
	return &PasskeyHandle{
		rp:        webauthn.NewRP(cfg),
		assertFn:  af,
		encryptFn: ek,
		store:     store,
		session:   session,
	}, nil
}

// Page serves the passkey enrollment + step-up UI. It's the only surface that
// can perform a WebAuthn gesture, so it's the gate the /tools enable depends
// on: assert here (bumps the daemon session to L3 for the assertion TTL), then
// enable a tool on /tools within that window.
func (h *PasskeyHandle) Page(w http.ResponseWriter, r *http.Request) {
	renderPage(w, passkeyTmpl, nil)
}

// passkeyPageHTML — the enrolment page's own markup, wrapped by passkeyTmpl
// with shellTop/shellBottom. It was a Go string constant, which is the one
// place page markup still lived in Go.
//
//go:embed passkey.html
var passkeyPageHTML string

var passkeyTmpl = parsePage("passkey", passkeyPageHTML, nil)

func (h *PasskeyHandle) RegisterBegin(w http.ResponseWriter, r *http.Request) {
	opts, challenge, err := h.rp.CreationOptions([]byte("maven-user"), "maven user")
	if err != nil {
		writeProblem(w, r, http.StatusInternalServerError, problemWebAuthnBegin,
			"passkey registration could not start", fmt.Errorf("webauthn register begin: %w", err))
		return
	}
	w.Header().Set("Content-Type", "application/json")
	json.NewEncoder(w).Encode(map[string]any{"challenge": challenge, "options": opts})
}

func (h *PasskeyHandle) RegisterFinish(w http.ResponseWriter, r *http.Request) {
	if r.Method != http.MethodPost {
		writeProblem(w, r, http.StatusMethodNotAllowed, problemMethodNotAllowed,
			"POST only", nil)
		return
	}
	var body struct {
		Challenge  string         `json:"challenge"`
		Credential map[string]any `json:"credential"`
	}
	if err := json.NewDecoder(r.Body).Decode(&body); err != nil {
		writeProblem(w, r, http.StatusBadRequest, problemInvalidRequest,
			"invalid registration request", fmt.Errorf("decode webauthn registration: %w", err))
		return
	}
	save := func(id string, publicKey []byte, _ []byte, _ string) error {
		return h.store.Save(id, publicKey)
	}
	credID, err := h.rp.FinishRegistration(save, body.Challenge, body.Credential)
	if err != nil {
		writeProblem(w, r, http.StatusBadRequest, problemWebAuthnFinish,
			"passkey registration failed", fmt.Errorf("webauthn register finish: %w", err))
		return
	}
	log.Printf("webauthn: registered credential %s", credID)

	// Note what does NOT happen here: the encryption key is not wrapped at
	// enrolment. Wrapping needs the authenticator's PRF output, and create()
	// does not produce one on most authenticators — it only reports whether
	// the extension is supported. The wrapped key is written on the first
	// assertion instead (see AssertFinish).
	//
	// This used to wrap the key under the credential *public* key, which is
	// written to passkeys.json next to the wrapped blob. See the header of
	// internal/webauthn/keywrap.go.

	json.NewEncoder(w).Encode(map[string]string{"credential_id": credID})
}

func (h *PasskeyHandle) AssertBegin(w http.ResponseWriter, r *http.Request) {
	opts, challenge, err := h.rp.AssertionOptions()
	if err != nil {
		writeProblem(w, r, http.StatusInternalServerError, problemWebAuthnBegin,
			"passkey assertion could not start", fmt.Errorf("webauthn assert begin: %w", err))
		return
	}
	w.Header().Set("Content-Type", "application/json")
	json.NewEncoder(w).Encode(map[string]any{"challenge": challenge, "options": opts})
}

func (h *PasskeyHandle) AssertFinish(w http.ResponseWriter, r *http.Request) {
	if r.Method != http.MethodPost {
		writeProblem(w, r, http.StatusMethodNotAllowed, problemMethodNotAllowed,
			"POST only", nil)
		return
	}
	var body struct {
		Challenge  string         `json:"challenge"`
		Credential map[string]any `json:"credential"`
		// PRF is the base64url WebAuthn PRF output the browser read out of
		// getClientExtensionResults(). Empty when the authenticator has no
		// PRF extension: cold-start unlock is then unavailable and we say so
		// rather than falling back to something weaker.
		//
		// Known property, accepted deliberately: this value is supplied by
		// the client and is NOT covered by the assertion signature. WebAuthn
		// client extension outputs never are, and binding one would need a
		// per-assertion salt, which would make the wrapped blob unopenable on
		// the next boot. Nothing here can tell a real PRF output from 32
		// bytes a compromised page chose. What limits the damage is that the
		// daemon refuses to rewrite an existing blob unless the operator
		// asked for it — see Explicit below and cmd/mavend/keyfile.go.
		PRF string `json:"prf"`
		// Explicit marks the "rewrite cold-start key" button rather than a
		// plain step-up. Only then may the daemon replace a blob that is
		// already on disk.
		Explicit bool `json:"explicit"`
	}
	if err := json.NewDecoder(r.Body).Decode(&body); err != nil {
		writeProblem(w, r, http.StatusBadRequest, problemInvalidRequest,
			"invalid assertion request", fmt.Errorf("decode webauthn assertion: %w", err))
		return
	}

	lookup := func(id string) ([]byte, int64, error) {
		return h.store.Lookup(id)
	}
	update := func(id string, count int64) error {
		return h.store.UpdateSignCount(id, count)
	}

	credID, err := h.rp.FinishAssertion(lookup, update, body.Challenge, body.Credential)
	if err != nil {
		writeProblem(w, r, http.StatusBadRequest, problemWebAuthnFinish,
			"passkey assertion failed", fmt.Errorf("webauthn assert finish: %w", err))
		return
	}

	// Assert step-up on the IPC (mavend) side so subsequent EnableTool calls
	// see L3. Best-effort: if IPC fails (no -core or mavend unreachable), the
	// user still sees success but the enable will fail with AuthStepUp.
	if h.assertFn != nil {
		ctx, cancel := context.WithTimeout(r.Context(), 5*time.Second)
		defer cancel()
		if err := h.assertFn.AssertStepUp(ctx); err != nil {
			writeProblem(w, r, http.StatusBadGateway, problemWebAuthnStepUp,
				"step-up assertion failed", fmt.Errorf("assert step-up in core: %w", err))
			return
		}
	}

	// Cold-start unlock and key wrapping, both keyed on the PRF secret this
	// assertion just produced. The secret is used here and dropped; it is
	// never stored on this side, and it must never be logged — unlike a
	// signature it does not expire, so one copy in a proxy log or a HAR file
	// is permanent access to the wrapped blob.
	//
	// Order matters: unlock first (if the daemon is locked there is nothing to
	// wrap yet), then wrap. Both are best-effort, because the assertion itself
	// is valid either way.
	if h.encryptFn != nil {
		if secret, err := webauthn.DecodePRFResult(body.PRF); err != nil {
			log.Printf("webauthn: no usable PRF secret from credential %s: %v", credID, err)
		} else {
			ctx, cancel := context.WithTimeout(r.Context(), 10*time.Second)
			defer cancel()
			h.coldStart(ctx, credID, secret, body.Explicit)
		}
	}

	// Assert the in-process session so the POST /tools handler sees step-up.
	if h.session != nil {
		h.session.Assert(r.Context(), auth.Scope{})
	}

	log.Printf("webauthn: asserted credential %s", credID)
	json.NewEncoder(w).Encode(map[string]string{"credential_id": credID})
}

// coldStart unlocks a locked daemon with this assertion's PRF output and then
// asks it to wrap the at-rest key. Never fatal: a locked or unreachable daemon
// does not invalidate the step-up.
//
// # The legacy retry
//
// A box enrolled before Vikunja #14 has a v1 blob, wrapped under the
// credential PUBLIC key. The PRF secret cannot open it, and this handler is
// the only caller of Unlock, so without a second attempt that box could never
// cold-start again: it would sit locked while a perfectly good passkey was
// asserted, and the only way back in would be putting MAVEN_DB_KEY into the
// environment — the exact thing cold-start unlock exists to avoid.
//
// So a failed PRF unlock is retried with the public key from the credential
// store. That is not a weaker fallback being offered to new deployments:
// nothing writes v1 any more, and a v2 blob does not open under a public key
// either. It is a one-way door out of the old format, and the operator is told
// to walk through it.
func (h *PasskeyHandle) coldStart(ctx context.Context, credID string, secret []byte, explicit bool) {
	legacy := false
	err := h.encryptFn.Unlock(ctx, secret)
	if err != nil && !errors.Is(err, ipc.ErrUnknownMethod) {
		if pub, _, lerr := h.store.Lookup(credID); lerr == nil && len(pub) > 0 {
			if err2 := h.encryptFn.Unlock(ctx, pub); err2 == nil {
				err, legacy = nil, true
			}
		}
	}
	switch {
	case errors.Is(err, ipc.ErrUnknownMethod):
		// Env-key mode: the daemon was never locked and has no UnlockFn. Not
		// a failure, and the old code logged it as one on every assertion.
	case err != nil:
		log.Printf("webauthn: unlock via credential %s failed: %v", credID, err)
	case legacy:
		log.Printf("SECURITY: webauthn: daemon unlocked from a LEGACY v1 wrapped key using credential %s. That blob is derived from the credential public key, which sits in passkeys.json beside it, so it protects nothing. Press \"rewrite cold-start key\" on this page to replace it with a v2 blob.", credID)
	default:
		log.Printf("webauthn: daemon reports unlocked, credential %s", credID)
	}

	// explicit=false means "write the blob only if there is none". The daemon
	// enforces that; sending the flag is the whole of this side's part in it.
	switch err := h.encryptFn.StoreEncryptionKey(ctx, secret, explicit); {
	case err == nil && explicit:
		log.Printf("webauthn: cold-start key rewritten under credential %s", credID)
	case err == nil:
	case errors.Is(err, ipc.ErrUnknownMethod):
		// No key to wrap: a plaintext dev store, or a daemon still locked.
	default:
		log.Printf("webauthn: wrap encryption key: %v", err)
	}
}