feat(11-04): add flare Web Push with a stdlib VAPID driver

- RFC 8291 aes128gcm encryption from crypto/ecdh, crypto/hkdf and AES-GCM, matching the RFC 8291 Appendix A vector byte for byte - RFC 8292 vapid t=<ES256 JWT>, k=<key> header (aud origin, exp +12h, sub) - Pusher, Subscription, SendOptions, SubscriptionSource, Service and From reading push.* (enabled, keys, subject, ttl, allowed_hosts) - sends only to https endpoints on push.allowed_hosts, checked before dialing, and never follows redirects; 404/410 map to ErrSubscriptionGone - module README and root modules row
2026-09-30 13:43:09 +02:00
parent a8305a015d
commit a9af0d77c7
7 changed files with 1337 additions and 0 deletions
--- a/modules/flare/README.md
+++ b/modules/flare/README.md
@@ -0,0 +1,127 @@
+# flare
+
+Web Push delivery with VAPID (RFC 8292) and aes128gcm payload encryption (RFC 8291) behind a small Pusher interface.
+
+`import "git.golem15.com/golem15/summercms/modules/flare"`
+
+## Overview
+
+flare sends browser push notifications. Push is a separate channel from realtime: `lighthouse` publishes to clients that hold an open connection, while flare hands a message to the browser vendor's push service, which wakes the browser even when no page is open.
+
+The application owns the subscriptions. When a browser subscribes, the frontend posts its `PushSubscription` (the endpoint URL and the `p256dh` and `auth` keys) to the application, which stores it. flare never reads a database. Code that sends a push passes a `flare.Subscription` to a `flare.Pusher`, and operator tooling reads stored subscriptions through a `flare.SubscriptionSource` that the application publishes on the app.
+
+`flare.From` builds the app-scoped `flare.Service` from `push.*` on first use. Its `flare.Service.Pusher` is the VAPID driver, `flare.VAPIDPusher`, which talks to push services directly with the standard library:
+
+- The payload is encrypted for the subscriber with `flare.Encrypt`: an ephemeral P-256 key agreement (`crypto/ecdh`) with the subscription's `p256dh` key, mixed with its `auth` secret through HKDF-SHA-256 (`crypto/hkdf`), then one AES-128-GCM record in the `aes128gcm` content coding. The implementation reproduces the RFC 8291 Appendix A test vector byte for byte.
+- Every request carries `Authorization: vapid t=<JWT>, k=<public key>` from `flare.VAPIDHeader`. The ES256 token's `aud` is the endpoint's origin, `exp` lies `flare.VAPIDTokenLifetime` (12 hours) ahead and `sub` is `push.subject`.
+
+Endpoints come from browsers, so they are untrusted URLs. The driver only sends to `https` endpoints whose host is in `push.allowed_hosts`, checks this before it opens a connection, and never follows a redirect.
+
+## Features
+
+- `flare.Pusher` with one method, `Send(ctx, sub, payload, opts)`. `flare.SendOptions` sets the `TTL` (default `push.ttl`), `Urgency` and `Topic` headers.
+- The VAPID driver POSTs the encrypted body with `TTL`, `Content-Encoding: aes128gcm`, `Content-Type: application/octet-stream`, the optional `Urgency` and `Topic` and the VAPID `Authorization` header. A 2xx answer is success. 404 and 410 return `flare.ErrSubscriptionGone`, so the caller can delete the subscription. Any other status returns a `*flare.StatusError` with the code and without the response body. Requests time out after `flare.DefaultTimeout` (10 s).
+- Nothing is sent while `push.enabled` is false: `Send` returns `flare.ErrPushDisabled`.
+- Endpoint allowlist: `flare.HostAllowed` matches a host against `push.allowed_hosts`, where `*.example.com` matches any subdomain of `example.com` (not `example.com` itself). The defaults, `flare.DefaultAllowedHosts`, are Firebase Cloud Messaging, Mozilla autopush, Apple and Windows push. A refused endpoint returns `flare.ErrEndpointNotAllowed`, and the error names the host, never the endpoint path.
+- Payloads up to `flare.MaxPayloadSize` (3993 bytes, the RFC 8291 limit for a 4096-byte body). A larger one returns `flare.ErrPayloadTooLarge`.
+- VAPID keys: `flare.GenerateVAPIDKeys` returns a P-256 pair as unpadded base64url (`flare.PublicKeyLength`, 87 characters, and `flare.PrivateKeyLength`, 43 characters). `flare.ParseVAPIDKeys` accepts padded or unpadded input and checks that the public key belongs to the private key; a bad pair returns `flare.ErrInvalidVAPIDKeys`. A subject that is not `mailto:` or `https:` returns `flare.ErrInvalidSubject`.
+- The private key never reaches logs or formatted output: `flare.VAPIDKeys` and `flare.Config` redact it in `String`, `GoString` and (for the config) `LogValue`, and no error carries key material.
+
+## Usage
+
+Send a push to one stored subscription:
+
+```go
+svc, err := flare.From(app)
+if err != nil {
+	return err
+}
+payload := []byte(`{"title":"New comment","body":"Someone replied to your post"}`)
+err = svc.Pusher().Send(ctx, flare.Subscription{
+	Endpoint: row.Endpoint,
+	P256dh:   row.P256dh,
+	Auth:     row.Auth,
+}, payload, flare.SendOptions{Urgency: "normal"})
+if errors.Is(err, flare.ErrSubscriptionGone) {
+	// The browser unsubscribed: delete the row.
+}
+```
+
+Publish a subscription source from a plugin's `Boot`, so operator commands can read the stored subscriptions of a user:
+
+```go
+type blogSubscriptions struct{ db *gorm.DB }
+
+func (s blogSubscriptions) Subscriptions(ctx context.Context, userID uint) ([]flare.SubscriptionInfo, error) {
+	var user models.User
+	if err := s.db.WithContext(ctx).First(&user, userID).Error; err != nil {
+		if errors.Is(err, gorm.ErrRecordNotFound) {
+			return nil, flare.ErrUserNotFound
+		}
+		return nil, err
+	}
+	var rows []models.PushSubscription
+	if err := s.db.WithContext(ctx).Where("user_id = ?", userID).Find(&rows).Error; err != nil {
+		return nil, err
+	}
+	out := make([]flare.SubscriptionInfo, 0, len(rows))
+	for _, r := range rows {
+		out = append(out, flare.SubscriptionInfo{
+			Subscription: flare.Subscription{Endpoint: r.Endpoint, P256dh: r.P256dh, Auth: r.Auth},
+			ID:           r.ID,
+			UserAgent:    r.UserAgent,
+			SubscribedAt: &r.CreatedAt,
+		})
+	}
+	return out, nil
+}
+
+// In Boot:
+if err := app.Publish[flare.SubscriptionSource](blogSubscriptions{db: gdb}); err != nil {
+	return err
+}
+```
+
+## API reference
+
+| Identifier | Description |
+|------------|-------------|
+| `flare.From(app)` | The app's `*flare.Service`, built from `push.*` and published on first use. |
+| `flare.Service` | `Config`, `Enabled`, `Pusher`, `SetHTTPClient` (replace the driver's HTTP client, for example in tests) and `Logger`. |
+| `flare.Pusher` | `Send(ctx, sub, payload, opts) error`. |
+| `flare.VAPIDPusher`, `flare.NewVAPIDPusher(cfg, hc)` | The VAPID driver. `hc` may be nil; a given client is copied and never follows redirects. |
+| `flare.Subscription` | `Endpoint`, `P256dh`, `Auth`, as `PushSubscription.toJSON` returns them. |
+| `flare.SendOptions` | `TTL`, `Urgency`, `Topic`. |
+| `flare.SubscriptionSource`, `flare.SubscriptionInfo` | The application's subscription store: `Subscriptions(ctx, userID)` returns the subscriptions with `ID`, `UserAgent`, `SubscribedAt` and `LastUsedAt`. |
+| `flare.Config`, `flare.LoadConfig` | The `push.*` settings with their defaults; `Keys` returns the key pair. |
+| `flare.VAPIDKeys`, `flare.GenerateVAPIDKeys`, `flare.ParseVAPIDKeys` | VAPID key pairs as unpadded base64url. |
+| `flare.VAPIDHeader(endpoint, subject, keys, now)` | The RFC 8292 `Authorization` header value. |
+| `flare.Encrypt(payload, sub)` | The RFC 8291 `aes128gcm` request body. |
+| `flare.HostAllowed(host, allowed)`, `flare.DefaultAllowedHosts` | The endpoint host allowlist and its default. |
+| `flare.ErrPushDisabled`, `flare.ErrEndpointNotAllowed`, `flare.ErrSubscriptionGone`, `flare.ErrUserNotFound`, `flare.ErrPayloadTooLarge`, `flare.ErrInvalidVAPIDKeys`, `flare.ErrInvalidSubject`, `flare.StatusError` | Errors. |
+| `flare.ContentEncoding`, `flare.MaxPayloadSize`, `flare.DefaultTTL`, `flare.DefaultTimeout`, `flare.VAPIDTokenLifetime`, `flare.PublicKeyLength`, `flare.PrivateKeyLength` | Constants. |
+
+## Configuration
+
+| Key | Default | Description |
+|-----|---------|-------------|
+| `push.enabled` | `false` | Nothing is sent while false. |
+| `push.public_key` | `""` | VAPID public key, base64url (87 characters unpadded). |
+| `push.private_key` | `""` | VAPID private key, base64url (43 characters). Keep it out of committed files; set `SUMMER_PUSH__PRIVATE_KEY`. |
+| `push.subject` | `""` | VAPID `sub` claim: a `mailto:` or `https:` contact URI. |
+| `push.ttl` | `2419200` | Default `TTL` header, in seconds or as a duration string. |
+| `push.allowed_hosts` | FCM, Mozilla autopush, `*.push.apple.com`, `*.notify.windows.com` | Push service hosts an endpoint may point at, as a list or a comma-separated string. |
+
+## Dependencies
+
+- `backpack` and `compass` from this repository.
+- `github.com/golang-jwt/jwt/v5` (the ES256 VAPID token).
+- Everything else is the standard library: `crypto/ecdh`, `crypto/ecdsa`, `crypto/hkdf`, `crypto/aes`, `crypto/cipher` and `net/http`. No Web Push library is used.
+
+## Testing
+
+```bash
+go test ./modules/flare/...
+```
+
+`TestRFC8291AppendixA` fixes the RFC's salt and application server key and compares the output with the RFC's published bytes. The send tests run an `httptest.NewTLSServer` push service with `127.0.0.1` in the allowlist; it verifies the VAPID token with the key from `k=` and decrypts the body as a browser would. Pass the test server's client to `flare.NewVAPIDPusher` or `flare.Service.SetHTTPClient` so it trusts the test certificate.
--- a/modules/flare/encrypt.go
+++ b/modules/flare/encrypt.go
@@ -0,0 +1,163 @@
+package flare
+
+import (
+	"crypto/aes"
+	"crypto/cipher"
+	"crypto/ecdh"
+	"crypto/hkdf"
+	"crypto/rand"
+	"crypto/sha256"
+	"encoding/base64"
+	"encoding/binary"
+	"errors"
+	"fmt"
+	"strings"
+)
+
+// ContentEncoding is the only content coding a push message may use
+// (RFC 8291 Section 4).
+const ContentEncoding = "aes128gcm"
+
+// MaxPayloadSize is the largest plaintext Encrypt accepts: a push service
+// need not accept more than 4096 octets of body, and the aes128gcm header
+// (86 octets), the padding delimiter (1) and the GCM tag (16) take the rest.
+const MaxPayloadSize = 3993
+
+// recordSize is the "rs" field of the aes128gcm header. A push message is a
+// single record, so any value above the record length works; 4096 is the
+// value RFC 8291 uses.
+const recordSize = 4096
+
+const (
+	saltSize      = 16
+	authSize      = 16
+	publicKeySize = 65
+	headerSize    = saltSize + 4 + 1 + publicKeySize
+)
+
+// Key derivation labels from RFC 8291 Section 3.4 (the RFC 8188 labels end
+// with a zero octet).
+const (
+	webPushInfo = "WebPush: info\x00"
+	cekInfo     = "Content-Encoding: aes128gcm\x00"
+	nonceInfo   = "Content-Encoding: nonce\x00"
+)
+
+// ErrPayloadTooLarge is returned by Encrypt for a payload above
+// MaxPayloadSize.
+var ErrPayloadTooLarge = errors.New("flare: payload exceeds 3993 bytes")
+
+// Encrypt encrypts payload for the user agent that owns sub, following
+// RFC 8291: an ephemeral P-256 key agreement with sub.P256dh, mixed with the
+// sub.Auth secret, then one aes128gcm record (RFC 8188). The result is the
+// request body: salt || record size || key id length || the ephemeral
+// public key || ciphertext.
+func Encrypt(payload []byte, sub Subscription) ([]byte, error) {
+	asKey, err := ecdh.P256().GenerateKey(rand.Reader)
+	if err != nil {
+		return nil, fmt.Errorf("flare: generate ephemeral key: %w", err)
+	}
+	salt := make([]byte, saltSize)
+	if _, err := rand.Read(salt); err != nil {
+		return nil, fmt.Errorf("flare: generate salt: %w", err)
+	}
+	return encrypt(payload, sub, asKey, salt)
+}
+
+// encrypt is Encrypt with the application server key and the salt given,
+// so the RFC 8291 Appendix A vector can fix both.
+func encrypt(payload []byte, sub Subscription, asKey *ecdh.PrivateKey, salt []byte) ([]byte, error) {
+	if len(payload) > MaxPayloadSize {
+		return nil, ErrPayloadTooLarge
+	}
+	if len(salt) != saltSize {
+		return nil, fmt.Errorf("flare: salt must be %d bytes", saltSize)
+	}
+	uaRaw, err := decodeBase64URL(sub.P256dh)
+	if err != nil {
+		return nil, fmt.Errorf("flare: subscription p256dh is not base64url")
+	}
+	uaPublic, err := ecdh.P256().NewPublicKey(uaRaw)
+	if err != nil {
+		return nil, fmt.Errorf("flare: subscription p256dh is not an uncompressed P-256 point")
+	}
+	authSecret, err := decodeBase64URL(sub.Auth)
+	if err != nil {
+		return nil, fmt.Errorf("flare: subscription auth is not base64url")
+	}
+	if len(authSecret) != authSize {
+		return nil, fmt.Errorf("flare: subscription auth must be %d bytes, got %d", authSize, len(authSecret))
+	}
+
+	ecdhSecret, err := asKey.ECDH(uaPublic)
+	if err != nil {
+		return nil, fmt.Errorf("flare: key agreement: %w", err)
+	}
+	asPublic := asKey.PublicKey().Bytes()
+	cek, nonce, err := deriveKeys(ecdhSecret, authSecret, salt, uaPublic.Bytes(), asPublic)
+	if err != nil {
+		return nil, err
+	}
+
+	block, err := aes.NewCipher(cek)
+	if err != nil {
+		return nil, fmt.Errorf("flare: %w", err)
+	}
+	gcm, err := cipher.NewGCM(block)
+	if err != nil {
+		return nil, fmt.Errorf("flare: %w", err)
+	}
+	// A single, final record: the plaintext followed by the 0x02 padding
+	// delimiter and no padding.
+	plain := make([]byte, 0, len(payload)+1)
+	plain = append(plain, payload...)
+	plain = append(plain, 0x02)
+
+	out := make([]byte, headerSize, headerSize+len(plain)+gcm.Overhead())
+	copy(out, salt)
+	binary.BigEndian.PutUint32(out[saltSize:], recordSize)
+	out[saltSize+4] = byte(len(asPublic))
+	copy(out[saltSize+5:], asPublic)
+	return gcm.Seal(out, nonce, plain, nil), nil
+}
+
+// deriveKeys runs the RFC 8291 Section 3.4 key schedule and returns the
+// content encryption key and the nonce.
+func deriveKeys(ecdhSecret, authSecret, salt, uaPublic, asPublic []byte) (cek, nonce []byte, err error) {
+	prkKey, err := hkdf.Extract(sha256.New, ecdhSecret, authSecret)
+	if err != nil {
+		return nil, nil, fmt.Errorf("flare: %w", err)
+	}
+	keyInfo := make([]byte, 0, len(webPushInfo)+len(uaPublic)+len(asPublic))
+	keyInfo = append(keyInfo, webPushInfo...)
+	keyInfo = append(keyInfo, uaPublic...)
+	keyInfo = append(keyInfo, asPublic...)
+	ikm, err := hkdf.Expand(sha256.New, prkKey, string(keyInfo), 32)
+	if err != nil {
+		return nil, nil, fmt.Errorf("flare: %w", err)
+	}
+	prk, err := hkdf.Extract(sha256.New, ikm, salt)
+	if err != nil {
+		return nil, nil, fmt.Errorf("flare: %w", err)
+	}
+	cek, err = hkdf.Expand(sha256.New, prk, cekInfo, 16)
+	if err != nil {
+		return nil, nil, fmt.Errorf("flare: %w", err)
+	}
+	nonce, err = hkdf.Expand(sha256.New, prk, nonceInfo, 12)
+	if err != nil {
+		return nil, nil, fmt.Errorf("flare: %w", err)
+	}
+	return cek, nonce, nil
+}
+
+// decodeBase64URL decodes base64url with or without padding. Browsers hand
+// out unpadded keys; stored copies sometimes carry padding or use the
+// standard alphabet, which is accepted too.
+func decodeBase64URL(s string) ([]byte, error) {
+	s = strings.TrimRight(strings.TrimSpace(s), "=")
+	if b, err := base64.RawURLEncoding.DecodeString(s); err == nil {
+		return b, nil
+	}
+	return base64.RawStdEncoding.DecodeString(s)
+}
--- a/modules/flare/encrypt_test.go
+++ b/modules/flare/encrypt_test.go
@@ -0,0 +1,112 @@
+package flare
+
+import (
+	"bytes"
+	"crypto/ecdh"
+	"encoding/base64"
+	"strings"
+	"testing"
+)
+
+// Values copied verbatim from RFC 8291 Section 5 and Appendix A
+// (https://www.rfc-editor.org/rfc/rfc8291), with the line-wrapping
+// whitespace removed.
+const (
+	rfcPlaintext  = "V2hlbiBJIGdyb3cgdXAsIEkgd2FudCB0byBiZSBhIHdhdGVybWVsb24"
+	rfcASPublic   = "BP4z9KsN6nGRTbVYI_c7VJSPQTBtkgcy27mlmlMoZIIgDll6e3vCYLocInmYWAmS6TlzAC8wEqKK6PBru3jl7A8"
+	rfcASPrivate  = "yfWPiYE-n46HLnH0KqZOF1fJJU3MYrct3AELtAQ-oRw"
+	rfcUAPublic   = "BCVxsr7N_eNgVRqvHtD0zTZsEc6-VV-JvLexhqUzORcxaOzi6-AYWXvTBHm4bjyPjs7Vd8pZGH6SRpkNtoIAiw4"
+	rfcUAPrivate  = "q1dXpw3UpT5VOmu_cf_v6ih07Aems3njxI-JWgLcM94"
+	rfcSalt       = "DGv6ra1nlYgDCS1FRnbzlw"
+	rfcAuthSecret = "BTBZMqHH6r4Tts7J_aSIgg"
+	rfcCEK        = "oIhVW04MRdy2XN9CiKLxTg"
+	rfcNonce      = "4h_95klXJ5E_qnoN"
+	rfcHeader     = "DGv6ra1nlYgDCS1FRnbzlwAAEABBBP4z9KsN6nGRTbVYI_c7VJSPQTBtkgcy27mlmlMoZIIgDll6e3vCYLocInmYWAmS6TlzAC8wEqKK6PBru3jl7A8"
+	rfcCiphertext = "8pfeW0KbunFT06SuDKoJH9Ql87S1QUrdirN6GcG7sFz1y1sqLgVi1VhjVkHsUoEsbI_0LpXMuGvnzQ"
+	// The request body of Section 5: header || ciphertext.
+	rfcBody = "DGv6ra1nlYgDCS1FRnbzlwAAEABBBP4z9KsN6nGRTbVYI_c7VJSPQTBtkgcy27ml" +
+		"mlMoZIIgDll6e3vCYLocInmYWAmS6TlzAC8wEqKK6PBru3jl7A_yl95bQpu6cVPT" +
+		"pK4Mqgkf1CXztLVBSt2Ks3oZwbuwXPXLWyouBWLVWGNWQexSgSxsj_Qulcy4a-fN"
+)
+
+func b64(t *testing.T, s string) []byte {
+	t.Helper()
+	b, err := base64.RawURLEncoding.DecodeString(s)
+	if err != nil {
+		t.Fatalf("decode %q: %v", s, err)
+	}
+	return b
+}
+
+func TestRFC8291AppendixA(t *testing.T) {
+	plaintext := b64(t, rfcPlaintext)
+	if string(plaintext) != "When I grow up, I want to be a watermelon" {
+		t.Fatalf("plaintext = %q", plaintext)
+	}
+	asKey, err := ecdh.P256().NewPrivateKey(b64(t, rfcASPrivate))
+	if err != nil {
+		t.Fatalf("as_private: %v", err)
+	}
+	if !bytes.Equal(asKey.PublicKey().Bytes(), b64(t, rfcASPublic)) {
+		t.Fatal("as_private does not produce the RFC as_public")
+	}
+	sub := Subscription{Endpoint: "https://push.example.net/push/JzLQ3raZJfFBR0aqvOMsLrt54w4rJUsV", P256dh: rfcUAPublic, Auth: rfcAuthSecret}
+
+	got, err := encrypt(plaintext, sub, asKey, b64(t, rfcSalt))
+	if err != nil {
+		t.Fatalf("encrypt: %v", err)
+	}
+	want := b64(t, rfcBody)
+	if !bytes.Equal(got, want) {
+		t.Fatalf("body mismatch\n got %s\nwant %s", base64.RawURLEncoding.EncodeToString(got), rfcBody)
+	}
+	header := b64(t, rfcHeader)
+	if len(header) != 86 || !bytes.Equal(got[:86], header) {
+		t.Fatalf("header mismatch: %s", base64.RawURLEncoding.EncodeToString(got[:86]))
+	}
+	if !bytes.Equal(got[86:], b64(t, rfcCiphertext)) {
+		t.Fatalf("ciphertext mismatch: %s", base64.RawURLEncoding.EncodeToString(got[86:]))
+	}
+
+	// The intermediate CEK and nonce, derived from the receiver side.
+	uaKey, err := ecdh.P256().NewPrivateKey(b64(t, rfcUAPrivate))
+	if err != nil {
+		t.Fatalf("ua_private: %v", err)
+	}
+	secret, err := uaKey.ECDH(asKey.PublicKey())
+	if err != nil {
+		t.Fatal(err)
+	}
+	cek, nonce, err := deriveKeys(secret, b64(t, rfcAuthSecret), b64(t, rfcSalt), b64(t, rfcUAPublic), b64(t, rfcASPublic))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !bytes.Equal(cek, b64(t, rfcCEK)) || !bytes.Equal(nonce, b64(t, rfcNonce)) {
+		t.Fatalf("CEK/NONCE = %x/%x", cek, nonce)
+	}
+
+	// The receiver decrypts the RFC body back to the plaintext.
+	back, err := decryptForTest(want, rfcUAPrivate, rfcAuthSecret)
+	if err != nil {
+		t.Fatalf("decrypt: %v", err)
+	}
+	if !bytes.Equal(back, plaintext) {
+		t.Fatalf("decrypted %q", back)
+	}
+
+	// Encrypt with fresh randomness also round-trips, and refuses a
+	// payload above 3993 bytes.
+	body, err := Encrypt(plaintext, sub)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if back, err := decryptForTest(body, rfcUAPrivate, rfcAuthSecret); err != nil || !bytes.Equal(back, plaintext) {
+		t.Fatalf("round trip: %q, %v", back, err)
+	}
+	if _, err := Encrypt([]byte(strings.Repeat("x", MaxPayloadSize+1)), sub); err != ErrPayloadTooLarge {
+		t.Fatalf("oversized payload: %v", err)
+	}
+	if _, err := Encrypt([]byte(strings.Repeat("x", MaxPayloadSize)), sub); err != nil {
+		t.Fatalf("3993-byte payload: %v", err)
+	}
+}
--- a/modules/flare/flare.go
+++ b/modules/flare/flare.go
@@ -0,0 +1,423 @@
+// Package flare delivers Web Push notifications: a small Pusher interface
+// and a VAPID driver that encrypts every payload with aes128gcm (RFC 8291)
+// and signs every request with an ES256 VAPID token (RFC 8292). Push is a
+// separate channel from realtime; the application owns the subscription
+// store and hands subscriptions to flare through a SubscriptionSource.
+package flare
+
+import (
+	"bytes"
+	"context"
+	"errors"
+	"fmt"
+	"io"
+	"log/slog"
+	"net/http"
+	"net/url"
+	"strconv"
+	"strings"
+	"sync"
+	"time"
+
+	"git.golem15.com/golem15/summercms/modules/backpack"
+	"git.golem15.com/golem15/summercms/modules/compass"
+)
+
+// DefaultTTL is how long a push service keeps an undelivered message when
+// push.ttl is not set: four weeks.
+const DefaultTTL = 2419200 * time.Second
+
+// DefaultTimeout bounds every request to a push service.
+const DefaultTimeout = 10 * time.Second
+
+// Errors returned by Pusher.Send and SubscriptionSource.
+var (
+	// ErrPushDisabled is returned while push.enabled is false.
+	ErrPushDisabled = errors.New("flare: push is disabled (push.enabled is false)")
+	// ErrEndpointNotAllowed is returned, before any connection is made,
+	// for an endpoint that is not https or whose host is not in
+	// push.allowed_hosts.
+	ErrEndpointNotAllowed = errors.New("flare: push endpoint not allowed")
+	// ErrSubscriptionGone is returned when the push service answers 404 or
+	// 410: the subscription expired and should be deleted.
+	ErrSubscriptionGone = errors.New("flare: push subscription is gone")
+	// ErrUserNotFound is returned by a SubscriptionSource for an unknown
+	// user.
+	ErrUserNotFound = errors.New("flare: user not found")
+)
+
+// StatusError is a push service answer other than 2xx, 404 or 410. It
+// carries the status code, never the response body.
+type StatusError struct {
+	Code int
+}
+
+func (e *StatusError) Error() string {
+	return "flare: push service answered HTTP " + strconv.Itoa(e.Code)
+}
+
+// Subscription is a browser push subscription as PushSubscription.toJSON
+// returns it: the endpoint URL and the user agent's P-256 public key and
+// 16-byte authentication secret, both base64url.
+type Subscription struct {
+	Endpoint string `json:"endpoint"`
+	P256dh   string `json:"p256dh"`
+	Auth     string `json:"auth"`
+}
+
+// SendOptions tune one push. A zero TTL uses push.ttl; Urgency ("very-low",
+// "low", "normal" or "high") and Topic are sent only when set.
+type SendOptions struct {
+	TTL     time.Duration
+	Urgency string
+	Topic   string
+}
+
+// Pusher sends one encrypted push message to one subscription.
+type Pusher interface {
+	Send(ctx context.Context, sub Subscription, payload []byte, opts SendOptions) error
+}
+
+// SubscriptionInfo is a stored subscription with the details the
+// websockets:test-push command reports.
+type SubscriptionInfo struct {
+	Subscription
+	ID           uint
+	UserAgent    string
+	SubscribedAt *time.Time
+	LastUsedAt   *time.Time
+}
+
+// SubscriptionSource is implemented by the application that stores push
+// subscriptions, and published on the app with backpack.App.Publish as a
+// flare.SubscriptionSource. Subscriptions returns ErrUserNotFound for an
+// unknown user and an empty list for a user without subscriptions.
+type SubscriptionSource interface {
+	Subscriptions(ctx context.Context, userID uint) ([]SubscriptionInfo, error)
+}
+
+// Config is the push.* configuration. String and GoString never print
+// PrivateKey.
+type Config struct {
+	// Enabled is push.enabled; nothing is sent while it is false.
+	Enabled bool
+	// PublicKey and PrivateKey are the VAPID key pair, base64url.
+	PublicKey  string
+	PrivateKey string
+	// Subject is the VAPID sub claim, a mailto: or https: URI.
+	Subject string
+	// TTL is the default TTL header.
+	TTL time.Duration
+	// AllowedHosts are the push service hosts an endpoint may point at.
+	// "*.example.com" matches any subdomain of example.com.
+	AllowedHosts []string
+}
+
+// String prints the configuration without the private key.
+func (c Config) String() string {
+	priv := "unset"
+	if c.PrivateKey != "" {
+		priv = "[redacted]"
+	}
+	return fmt.Sprintf("flare.Config{Enabled: %t, PublicKey: %s, PrivateKey: %s, Subject: %s, TTL: %s, AllowedHosts: %v}",
+		c.Enabled, c.PublicKey, priv, c.Subject, c.TTL, c.AllowedHosts)
+}
+
+// GoString is String.
+func (c Config) GoString() string { return c.String() }
+
+// LogValue keeps the private key out of structured logs.
+func (c Config) LogValue() slog.Value { return slog.StringValue(c.String()) }
+
+// Keys returns the configured VAPID key pair.
+func (c Config) Keys() VAPIDKeys {
+	return VAPIDKeys{PublicKey: c.PublicKey, PrivateKey: c.PrivateKey}
+}
+
+// DefaultAllowedHosts are the push services endpoints may point at when
+// push.allowed_hosts is not set: Firebase Cloud Messaging (Chrome, Edge on
+// Android), Mozilla autopush (Firefox), Apple and Windows push.
+func DefaultAllowedHosts() []string {
+	return []string{
+		"fcm.googleapis.com",
+		"updates.push.services.mozilla.com",
+		"*.push.apple.com",
+		"*.notify.windows.com",
+	}
+}
+
+// LoadConfig reads push.* from c, filling the defaults. push.ttl is an
+// integer number of seconds or a duration string; push.allowed_hosts is a
+// list or a comma-separated string.
+func LoadConfig(c *compass.Config) Config {
+	cfg := Config{TTL: DefaultTTL, AllowedHosts: DefaultAllowedHosts()}
+	if c == nil {
+		return cfg
+	}
+	cfg.Enabled = c.Bool("push.enabled")
+	cfg.PublicKey = strings.TrimSpace(c.String("push.public_key"))
+	cfg.PrivateKey = strings.TrimSpace(c.String("push.private_key"))
+	cfg.Subject = strings.TrimSpace(c.String("push.subject"))
+	if d := durationSetting(c, "push.ttl"); d > 0 {
+		cfg.TTL = d
+	}
+	if v, ok := c.Lookup("push.allowed_hosts"); ok {
+		if hosts := stringList(v); len(hosts) > 0 {
+			cfg.AllowedHosts = hosts
+		}
+	}
+	return cfg
+}
+
+func durationSetting(c *compass.Config, key string) time.Duration {
+	raw := strings.TrimSpace(c.String(key))
+	if raw == "" {
+		return 0
+	}
+	if n, err := strconv.ParseInt(raw, 10, 64); err == nil {
+		return time.Duration(n) * time.Second
+	}
+	if d, err := time.ParseDuration(raw); err == nil {
+		return d
+	}
+	return 0
+}
+
+func stringList(v any) []string {
+	var parts []string
+	switch t := v.(type) {
+	case string:
+		parts = strings.Split(t, ",")
+	case []string:
+		parts = t
+	case []any:
+		for _, e := range t {
+			if s, ok := e.(string); ok {
+				parts = append(parts, s)
+			}
+		}
+	}
+	out := make([]string, 0, len(parts))
+	for _, p := range parts {
+		if p = strings.ToLower(strings.TrimSpace(p)); p != "" {
+			out = append(out, p)
+		}
+	}
+	return out
+}
+
+// Service is the app-scoped push service. Get it with From.
+type Service struct {
+	cfg Config
+	log *slog.Logger
+
+	mu     sync.RWMutex
+	pusher *VAPIDPusher
+}
+
+// From returns the app's Service, building it from push.* and publishing it
+// on first use.
+func From(app *backpack.App) (*Service, error) {
+	if app == nil {
+		return nil, fmt.Errorf("flare: app is nil")
+	}
+	if s, ok := app.Lookup[*Service](); ok && s != nil {
+		return s, nil
+	}
+	cfg := LoadConfig(app.Config)
+	svc := &Service{cfg: cfg, log: loggerFromApp(app), pusher: NewVAPIDPusher(cfg, nil)}
+	if err := app.Publish(svc); err != nil {
+		if existing, ok := app.Lookup[*Service](); ok && existing != nil {
+			return existing, nil
+		}
+		return nil, fmt.Errorf("flare: %w", err)
+	}
+	return svc, nil
+}
+
+// Config returns the push configuration the service was built with.
+func (s *Service) Config() Config {
+	if s == nil {
+		return LoadConfig(nil)
+	}
+	return s.cfg
+}
+
+// Enabled reports push.enabled.
+func (s *Service) Enabled() bool { return s != nil && s.cfg.Enabled }
+
+// Pusher returns the VAPID driver.
+func (s *Service) Pusher() Pusher {
+	if s == nil {
+		return NewVAPIDPusher(LoadConfig(nil), nil)
+	}
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+	return s.pusher
+}
+
+// SetHTTPClient replaces the HTTP client of the VAPID driver, for example
+// with an httptest TLS client in tests. Redirects stay refused.
+func (s *Service) SetHTTPClient(hc *http.Client) {
+	if s == nil {
+		return
+	}
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	s.pusher = NewVAPIDPusher(s.cfg, hc)
+}
+
+// Logger returns the app logger.
+func (s *Service) Logger() *slog.Logger {
+	if s == nil || s.log == nil {
+		return slog.Default()
+	}
+	return s.log
+}
+
+// VAPIDPusher is the Pusher that talks to push services directly with
+// VAPID authentication. It is safe for concurrent use.
+type VAPIDPusher struct {
+	cfg Config
+	hc  *http.Client
+	now func() time.Time
+}
+
+var _ Pusher = (*VAPIDPusher)(nil)
+
+// NewVAPIDPusher returns a driver for cfg. hc may be nil for a client with
+// a DefaultTimeout timeout; a given client is copied. Either way the driver
+// never follows redirects, so a push service cannot send it to a host
+// outside push.allowed_hosts.
+func NewVAPIDPusher(cfg Config, hc *http.Client) *VAPIDPusher {
+	var client http.Client
+	if hc != nil {
+		client = *hc
+	}
+	if client.Timeout == 0 {
+		client.Timeout = DefaultTimeout
+	}
+	client.CheckRedirect = func(*http.Request, []*http.Request) error { return http.ErrUseLastResponse }
+	return &VAPIDPusher{cfg: cfg, hc: &client, now: time.Now}
+}
+
+// Send encrypts payload for sub and POSTs it to sub.Endpoint. It returns
+// ErrPushDisabled while push is disabled and ErrEndpointNotAllowed, before
+// dialing, for an endpoint outside the allowlist. A 2xx answer is success,
+// 404 and 410 are ErrSubscriptionGone, any other status a *StatusError.
+func (p *VAPIDPusher) Send(ctx context.Context, sub Subscription, payload []byte, opts SendOptions) error {
+	if p == nil || !p.cfg.Enabled {
+		return ErrPushDisabled
+	}
+	if err := p.checkEndpoint(sub.Endpoint); err != nil {
+		return err
+	}
+	body, err := Encrypt(payload, sub)
+	if err != nil {
+		return err
+	}
+	auth, err := VAPIDHeader(sub.Endpoint, p.cfg.Subject, p.cfg.Keys(), p.now())
+	if err != nil {
+		return err
+	}
+	if ctx == nil {
+		ctx = context.Background()
+	}
+	req, err := http.NewRequestWithContext(ctx, http.MethodPost, sub.Endpoint, bytes.NewReader(body))
+	if err != nil {
+		return fmt.Errorf("flare: build request: %w", err)
+	}
+	ttl := opts.TTL
+	if ttl <= 0 {
+		ttl = p.cfg.TTL
+	}
+	req.Header.Set("TTL", strconv.FormatInt(int64(ttl/time.Second), 10))
+	req.Header.Set("Content-Encoding", ContentEncoding)
+	req.Header.Set("Content-Type", "application/octet-stream")
+	req.Header.Set("Authorization", auth)
+	if opts.Urgency != "" {
+		req.Header.Set("Urgency", opts.Urgency)
+	}
+	if opts.Topic != "" {
+		req.Header.Set("Topic", opts.Topic)
+	}
+	resp, err := p.hc.Do(req)
+	if err != nil {
+		// *url.Error repeats the full endpoint; keep only the cause.
+		var uerr *url.Error
+		if errors.As(err, &uerr) && uerr.Err != nil {
+			err = uerr.Err
+		}
+		return fmt.Errorf("flare: push to %s: %w", hostOf(sub.Endpoint), err)
+	}
+	defer resp.Body.Close()
+	_, _ = io.Copy(io.Discard, io.LimitReader(resp.Body, 64<<10))
+	switch {
+	case resp.StatusCode >= 200 && resp.StatusCode <= 299:
+		return nil
+	case resp.StatusCode == http.StatusNotFound || resp.StatusCode == http.StatusGone:
+		return ErrSubscriptionGone
+	default:
+		return &StatusError{Code: resp.StatusCode}
+	}
+}
+
+// checkEndpoint allows only https URLs, without user info, whose host is in
+// push.allowed_hosts. The error names the host, never the full endpoint,
+// whose path is a capability.
+func (p *VAPIDPusher) checkEndpoint(endpoint string) error {
+	u, err := url.Parse(endpoint)
+	if err != nil || u.Host == "" {
+		return fmt.Errorf("%w: not an absolute URL", ErrEndpointNotAllowed)
+	}
+	if !strings.EqualFold(u.Scheme, "https") {
+		return fmt.Errorf("%w: %s is not https", ErrEndpointNotAllowed, strings.ToLower(u.Scheme))
+	}
+	if u.User != nil {
+		return fmt.Errorf("%w: user info in URL", ErrEndpointNotAllowed)
+	}
+	host := strings.ToLower(u.Hostname())
+	if !HostAllowed(host, p.cfg.AllowedHosts) {
+		return fmt.Errorf("%w: host %s is not in push.allowed_hosts", ErrEndpointNotAllowed, host)
+	}
+	return nil
+}
+
+// HostAllowed reports whether host matches an entry of allowed. An entry
+// "*.example.com" matches any subdomain of example.com but not example.com
+// itself; any other entry must equal host. Matching is case-insensitive.
+func HostAllowed(host string, allowed []string) bool {
+	host = strings.TrimSuffix(strings.ToLower(host), ".")
+	if host == "" {
+		return false
+	}
+	for _, entry := range allowed {
+		entry = strings.ToLower(strings.TrimSpace(entry))
+		if suffix, ok := strings.CutPrefix(entry, "*."); ok {
+			if suffix != "" && strings.HasSuffix(host, "."+suffix) {
+				return true
+			}
+			continue
+		}
+		if entry != "" && host == entry {
+			return true
+		}
+	}
+	return false
+}
+
+func hostOf(endpoint string) string {
+	if u, err := url.Parse(endpoint); err == nil {
+		return u.Host
+	}
+	return "endpoint"
+}
+
+func loggerFromApp(app *backpack.App) *slog.Logger {
+	if app != nil {
+		if log, ok := app.Lookup[*slog.Logger](); ok && log != nil {
+			return log
+		}
+	}
+	return slog.Default()
+}
--- a/modules/flare/send_test.go
+++ b/modules/flare/send_test.go
@@ -0,0 +1,371 @@
+package flare
+
+import (
+	"context"
+	"crypto/aes"
+	"crypto/cipher"
+	"crypto/ecdh"
+	"crypto/ecdsa"
+	"crypto/elliptic"
+	"crypto/rand"
+	"encoding/base64"
+	"encoding/binary"
+	"errors"
+	"fmt"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"sync/atomic"
+	"testing"
+	"time"
+
+	"github.com/golang-jwt/jwt/v5"
+)
+
+// decryptForTest is the receiving user agent's side of RFC 8291: it parses
+// the aes128gcm header, agrees on the key with the sender's key id, derives
+// CEK and nonce, opens the record and checks the 0x02 padding delimiter.
+func decryptForTest(body []byte, uaPrivate, authSecret string) ([]byte, error) {
+	if len(body) < 21 {
+		return nil, errors.New("body shorter than the header")
+	}
+	salt := body[:16]
+	if rs := binary.BigEndian.Uint32(body[16:20]); rs < 18 {
+		return nil, fmt.Errorf("record size %d", rs)
+	}
+	idlen := int(body[20])
+	if len(body) < 21+idlen {
+		return nil, errors.New("truncated key id")
+	}
+	keyID := body[21 : 21+idlen]
+	ciphertext := body[21+idlen:]
+	privRaw, err := base64.RawURLEncoding.DecodeString(uaPrivate)
+	if err != nil {
+		return nil, err
+	}
+	uaKey, err := ecdh.P256().NewPrivateKey(privRaw)
+	if err != nil {
+		return nil, err
+	}
+	asPublic, err := ecdh.P256().NewPublicKey(keyID)
+	if err != nil {
+		return nil, fmt.Errorf("key id is not a P-256 point: %w", err)
+	}
+	secret, err := uaKey.ECDH(asPublic)
+	if err != nil {
+		return nil, err
+	}
+	auth, err := base64.RawURLEncoding.DecodeString(authSecret)
+	if err != nil {
+		return nil, err
+	}
+	cek, nonce, err := deriveKeys(secret, auth, salt, uaKey.PublicKey().Bytes(), keyID)
+	if err != nil {
+		return nil, err
+	}
+	block, err := aes.NewCipher(cek)
+	if err != nil {
+		return nil, err
+	}
+	gcm, err := cipher.NewGCM(block)
+	if err != nil {
+		return nil, err
+	}
+	plain, err := gcm.Open(nil, nonce, ciphertext, nil)
+	if err != nil {
+		return nil, err
+	}
+	i := len(plain) - 1
+	for i >= 0 && plain[i] == 0 {
+		i--
+	}
+	if i < 0 || plain[i] != 0x02 {
+		return nil, errors.New("missing 0x02 padding delimiter")
+	}
+	return plain[:i], nil
+}
+
+// testSubscriber is a browser-side subscription with its private key.
+type testSubscriber struct {
+	sub     Subscription
+	private string
+}
+
+func newTestSubscriber(t *testing.T, endpoint string) testSubscriber {
+	t.Helper()
+	key, err := ecdh.P256().GenerateKey(rand.Reader)
+	if err != nil {
+		t.Fatal(err)
+	}
+	auth := make([]byte, 16)
+	if _, err := rand.Read(auth); err != nil {
+		t.Fatal(err)
+	}
+	return testSubscriber{
+		sub: Subscription{
+			Endpoint: endpoint,
+			P256dh:   base64.RawURLEncoding.EncodeToString(key.PublicKey().Bytes()),
+			Auth:     base64.RawURLEncoding.EncodeToString(auth),
+		},
+		private: base64.RawURLEncoding.EncodeToString(key.Bytes()),
+	}
+}
+
+// receivedPush is what the fake push service saw and verified.
+type receivedPush struct {
+	header    http.Header
+	claims    jwt.MapClaims
+	jwtHeader map[string]any
+	k         string
+	payload   []byte
+	err       error
+}
+
+// verifyVAPIDRequest checks the Authorization header the way a push service
+// does: the JWT must verify with the ES256 key from k=.
+func verifyVAPIDRequest(r *http.Request, sub testSubscriber) receivedPush {
+	got := receivedPush{header: r.Header.Clone()}
+	auth := r.Header.Get("Authorization")
+	rest, ok := strings.CutPrefix(auth, "vapid t=")
+	if !ok {
+		got.err = fmt.Errorf("authorization %q is not vapid t=", auth)
+		return got
+	}
+	token, k, ok := strings.Cut(rest, ", k=")
+	if !ok {
+		got.err = errors.New("authorization lacks k=")
+		return got
+	}
+	got.k = k
+	rawKey, err := base64.RawURLEncoding.DecodeString(k)
+	if err != nil {
+		got.err = fmt.Errorf("k is not unpadded base64url: %w", err)
+		return got
+	}
+	pub, err := ecdsa.ParseUncompressedPublicKey(elliptic.P256(), rawKey)
+	if err != nil {
+		got.err = fmt.Errorf("k is not a P-256 point: %w", err)
+		return got
+	}
+	claims := jwt.MapClaims{}
+	parsed, err := jwt.ParseWithClaims(token, claims, func(*jwt.Token) (any, error) { return pub, nil },
+		jwt.WithValidMethods([]string{"ES256"}), jwt.WithExpirationRequired())
+	if err != nil {
+		got.err = fmt.Errorf("JWT does not verify: %w", err)
+		return got
+	}
+	got.claims = claims
+	got.jwtHeader = parsed.Header
+	body, err := io.ReadAll(r.Body)
+	if err != nil {
+		got.err = err
+		return got
+	}
+	got.payload, got.err = decryptForTest(body, sub.private, sub.sub.Auth)
+	return got
+}
+
+func testConfig(keys VAPIDKeys, hosts ...string) Config {
+	return Config{
+		Enabled:      true,
+		PublicKey:    keys.PublicKey,
+		PrivateKey:   keys.PrivateKey,
+		Subject:      "mailto:ops@example.com",
+		TTL:          DefaultTTL,
+		AllowedHosts: hosts,
+	}
+}
+
+func TestVAPIDSendRoundTrip(t *testing.T) {
+	keys, err := GenerateVAPIDKeys()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(keys.PublicKey) != PublicKeyLength || len(keys.PrivateKey) != PrivateKeyLength {
+		t.Fatalf("key lengths %d/%d", len(keys.PublicKey), len(keys.PrivateKey))
+	}
+	if strings.Contains(fmt.Sprintf("%v %#v %s", keys, keys, testConfig(keys)), keys.PrivateKey) {
+		t.Fatal("formatting VAPIDKeys or Config printed the private key")
+	}
+
+	var sub testSubscriber
+	received := make(chan receivedPush, 4)
+	srv := httptest.NewTLSServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		switch r.URL.Path {
+		case "/gone":
+			w.WriteHeader(http.StatusGone)
+			return
+		case "/missing":
+			w.WriteHeader(http.StatusNotFound)
+			return
+		case "/boom":
+			w.WriteHeader(http.StatusInternalServerError)
+			_, _ = io.WriteString(w, "internal detail")
+			return
+		}
+		received <- verifyVAPIDRequest(r, sub)
+		w.WriteHeader(http.StatusCreated)
+	}))
+	defer srv.Close()
+	sub = newTestSubscriber(t, srv.URL+"/push/JzLQ3raZJfFBR0aqvOMsLrt54w4rJUsV")
+
+	pusher := NewVAPIDPusher(testConfig(keys, "127.0.0.1"), srv.Client())
+	now := time.Now()
+	payload := []byte(`{"title":"acme test","body":"hello"}`)
+	if err := pusher.Send(context.Background(), sub.sub, payload, SendOptions{Urgency: "high", Topic: "acme"}); err != nil {
+		t.Fatalf("Send: %v", err)
+	}
+	got := <-received
+	if got.err != nil {
+		t.Fatalf("push service rejected the request: %v", got.err)
+	}
+	if string(got.payload) != string(payload) {
+		t.Fatalf("decrypted payload %q", got.payload)
+	}
+	for name, want := range map[string]string{
+		"Content-Encoding": "aes128gcm",
+		"Content-Type":     "application/octet-stream",
+		"TTL":              "2419200",
+		"Urgency":          "high",
+		"Topic":            "acme",
+	} {
+		if v := got.header.Get(name); v != want {
+			t.Errorf("header %s = %q, want %q", name, v, want)
+		}
+	}
+	if got.k != keys.PublicKey {
+		t.Errorf("k = %q, want the configured public key", got.k)
+	}
+	if got.jwtHeader["typ"] != "JWT" || got.jwtHeader["alg"] != "ES256" {
+		t.Errorf("JWT header %v", got.jwtHeader)
+	}
+	if got.claims["aud"] != srv.URL {
+		t.Errorf("aud = %v, want %s", got.claims["aud"], srv.URL)
+	}
+	if got.claims["sub"] != "mailto:ops@example.com" {
+		t.Errorf("sub = %v", got.claims["sub"])
+	}
+	exp, err := got.claims.GetExpirationTime()
+	if err != nil || exp == nil {
+		t.Fatalf("exp: %v", err)
+	}
+	if ahead := exp.Sub(now); ahead <= 0 || ahead > 24*time.Hour {
+		t.Errorf("exp is %s ahead, want within 24h", ahead)
+	}
+
+	// A per-send TTL replaces push.ttl; Urgency and Topic are omitted
+	// when unset.
+	if err := pusher.Send(context.Background(), sub.sub, []byte("x"), SendOptions{TTL: 90 * time.Second}); err != nil {
+		t.Fatalf("Send: %v", err)
+	}
+	got = <-received
+	if got.err != nil || got.header.Get("TTL") != "90" || got.header.Get("Urgency") != "" || got.header.Get("Topic") != "" {
+		t.Fatalf("second push: err=%v headers=%v", got.err, got.header)
+	}
+
+	// Status mapping.
+	for path, check := range map[string]func(error) bool{
+		"/gone":    func(err error) bool { return errors.Is(err, ErrSubscriptionGone) },
+		"/missing": func(err error) bool { return errors.Is(err, ErrSubscriptionGone) },
+		"/boom": func(err error) bool {
+			var se *StatusError
+			return errors.As(err, &se) && se.Code == 500 && !strings.Contains(err.Error(), "internal detail")
+		},
+	} {
+		s := sub.sub
+		s.Endpoint = srv.URL + path
+		if err := pusher.Send(context.Background(), s, []byte("x"), SendOptions{}); !check(err) {
+			t.Errorf("%s: unexpected error %v", path, err)
+		}
+	}
+}
+
+func TestSendRefusesDisallowedEndpoint(t *testing.T) {
+	keys, err := GenerateVAPIDKeys()
+	if err != nil {
+		t.Fatal(err)
+	}
+	var hits atomic.Int32
+	count := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		hits.Add(1)
+		w.WriteHeader(http.StatusCreated)
+	})
+	plain := httptest.NewServer(count)
+	defer plain.Close()
+	tlsSrv := httptest.NewTLSServer(count)
+	defer tlsSrv.Close()
+	// A push service that redirects elsewhere: the redirect is not
+	// followed.
+	redirect := httptest.NewTLSServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		http.Redirect(w, r, tlsSrv.URL+"/internal", http.StatusFound)
+	}))
+	defer redirect.Close()
+
+	sub := newTestSubscriber(t, "")
+	pusher := NewVAPIDPusher(testConfig(keys, "127.0.0.1"), tlsSrv.Client())
+	port := tlsSrv.URL[strings.LastIndex(tlsSrv.URL, ":")+1:]
+	for name, endpoint := range map[string]string{
+		"plain http":      plain.URL + "/push/abc",
+		"unlisted host":   "https://localhost:" + port + "/push/abc",
+		"foreign host":    "https://evil.example.com/push/abc",
+		"user info":       "https://user:secret@127.0.0.1:" + port + "/push/abc",
+		"relative URL":    "/push/abc",
+		"other scheme":    "ftp://127.0.0.1:" + port + "/push/abc",
+		"no host":         "https:///push/abc",
+		"host suffix hit": "https://127.0.0.1.evil.example.com/push/abc",
+	} {
+		s := sub.sub
+		s.Endpoint = endpoint
+		err := pusher.Send(context.Background(), s, []byte("x"), SendOptions{})
+		if !errors.Is(err, ErrEndpointNotAllowed) {
+			t.Errorf("%s: err = %v, want ErrEndpointNotAllowed", name, err)
+		}
+		if err != nil && strings.Contains(err.Error(), "/push/abc") {
+			t.Errorf("%s: error exposes the endpoint path: %v", name, err)
+		}
+	}
+	if n := hits.Load(); n != 0 {
+		t.Fatalf("%d requests reached a server for refused endpoints", n)
+	}
+
+	s := sub.sub
+	s.Endpoint = redirect.URL + "/push/abc"
+	var se *StatusError
+	if err := pusher.Send(context.Background(), s, []byte("x"), SendOptions{}); !errors.As(err, &se) || se.Code != http.StatusFound {
+		t.Fatalf("redirect: err = %v, want StatusError 302", err)
+	}
+	if n := hits.Load(); n != 0 {
+		t.Fatal("the redirect was followed")
+	}
+
+	disabled := testConfig(keys, "127.0.0.1")
+	disabled.Enabled = false
+	s.Endpoint = tlsSrv.URL + "/push/abc"
+	if err := NewVAPIDPusher(disabled, tlsSrv.Client()).Send(context.Background(), s, []byte("x"), SendOptions{}); !errors.Is(err, ErrPushDisabled) {
+		t.Fatalf("disabled: err = %v", err)
+	}
+	if n := hits.Load(); n != 0 {
+		t.Fatal("a disabled pusher sent a request")
+	}
+
+	defaults := DefaultAllowedHosts()
+	for host, want := range map[string]bool{
+		"fcm.googleapis.com":                     true,
+		"FCM.googleapis.com.":                    true,
+		"updates.push.services.mozilla.com":      true,
+		"web.push.apple.com":                     true,
+		"wns2-by3p.notify.windows.com":           true,
+		"push.apple.com":                         false,
+		"fcm.googleapis.com.evil.example":        false,
+		"evilpush.apple.com":                     false,
+		"notify.windows.com":                     false,
+		"android.googleapis.com":                 false,
+		"127.0.0.1":                              false,
+		"updates.push.services.mozilla.com.evil": false,
+	} {
+		if got := HostAllowed(host, defaults); got != want {
+			t.Errorf("HostAllowed(%q) = %t, want %t", host, got, want)
+		}
+	}
+}
--- a/modules/flare/vapid.go
+++ b/modules/flare/vapid.go
@@ -0,0 +1,140 @@
+package flare
+
+import (
+	"crypto/ecdh"
+	"crypto/ecdsa"
+	"crypto/elliptic"
+	"crypto/rand"
+	"crypto/subtle"
+	"encoding/base64"
+	"errors"
+	"fmt"
+	"net/url"
+	"strings"
+	"time"
+
+	"github.com/golang-jwt/jwt/v5"
+)
+
+// VAPIDTokenLifetime is how far ahead of the request the exp claim of a
+// VAPID token lies. RFC 8292 allows at most 24 hours.
+const VAPIDTokenLifetime = 12 * time.Hour
+
+// Lengths of unpadded base64url VAPID keys: a 65-byte uncompressed P-256
+// point and a 32-byte scalar.
+const (
+	PublicKeyLength  = 87
+	PrivateKeyLength = 43
+)
+
+// ErrInvalidVAPIDKeys is returned when the configured key pair cannot be
+// parsed or does not match. It never carries key material.
+var ErrInvalidVAPIDKeys = errors.New("flare: invalid VAPID key pair")
+
+// ErrInvalidSubject is returned when the VAPID subject is not a mailto: or
+// https: URI.
+var ErrInvalidSubject = errors.New("flare: VAPID subject must start with mailto: or https:")
+
+// VAPIDKeys is an application server key pair (RFC 8292) as unpadded
+// base64url: PublicKey is the 65-byte uncompressed P-256 point, PrivateKey
+// the 32-byte private scalar. String and GoString never print PrivateKey.
+type VAPIDKeys struct {
+	PublicKey  string
+	PrivateKey string
+}
+
+// String prints the public key and hides the private key.
+func (k VAPIDKeys) String() string {
+	return "flare.VAPIDKeys{PublicKey: " + k.PublicKey + ", PrivateKey: [redacted]}"
+}
+
+// GoString is String, so %#v does not print the private key either.
+func (k VAPIDKeys) GoString() string { return k.String() }
+
+// GenerateVAPIDKeys returns a fresh P-256 key pair.
+func GenerateVAPIDKeys() (VAPIDKeys, error) {
+	priv, err := ecdh.P256().GenerateKey(rand.Reader)
+	if err != nil {
+		return VAPIDKeys{}, fmt.Errorf("flare: generate VAPID keys: %w", err)
+	}
+	return VAPIDKeys{
+		PublicKey:  base64.RawURLEncoding.EncodeToString(priv.PublicKey().Bytes()),
+		PrivateKey: base64.RawURLEncoding.EncodeToString(priv.Bytes()),
+	}, nil
+}
+
+// ParseVAPIDKeys decodes a key pair given as base64url, with or without
+// padding, and checks that the public key belongs to the private key.
+func ParseVAPIDKeys(public, private string) (*ecdh.PrivateKey, error) {
+	privRaw, err := decodeBase64URL(private)
+	if err != nil || len(privRaw) != 32 {
+		return nil, fmt.Errorf("%w: the private key must be 32 bytes of base64url", ErrInvalidVAPIDKeys)
+	}
+	priv, err := ecdh.P256().NewPrivateKey(privRaw)
+	if err != nil {
+		return nil, fmt.Errorf("%w: the private key is not a P-256 scalar", ErrInvalidVAPIDKeys)
+	}
+	pubRaw, err := decodeBase64URL(public)
+	if err != nil || len(pubRaw) != publicKeySize {
+		return nil, fmt.Errorf("%w: the public key must be 65 bytes of base64url", ErrInvalidVAPIDKeys)
+	}
+	if subtle.ConstantTimeCompare(pubRaw, priv.PublicKey().Bytes()) != 1 {
+		return nil, fmt.Errorf("%w: the public key does not match the private key", ErrInvalidVAPIDKeys)
+	}
+	return priv, nil
+}
+
+// VAPIDHeader returns the RFC 8292 Authorization header value for a push to
+// endpoint: "vapid t=<JWT>, k=<public key>". The ES256 token carries aud
+// (the endpoint's origin), exp (now + VAPIDTokenLifetime) and sub.
+func VAPIDHeader(endpoint, subject string, keys VAPIDKeys, now time.Time) (string, error) {
+	if !validSubject(subject) {
+		return "", ErrInvalidSubject
+	}
+	aud, err := origin(endpoint)
+	if err != nil {
+		return "", err
+	}
+	priv, err := ParseVAPIDKeys(keys.PublicKey, keys.PrivateKey)
+	if err != nil {
+		return "", err
+	}
+	signer, err := ecdsa.ParseRawPrivateKey(elliptic.P256(), priv.Bytes())
+	if err != nil {
+		return "", ErrInvalidVAPIDKeys
+	}
+	token := jwt.NewWithClaims(jwt.SigningMethodES256, jwt.MapClaims{
+		"aud": aud,
+		"exp": now.Add(VAPIDTokenLifetime).Unix(),
+		"sub": subject,
+	})
+	signed, err := token.SignedString(signer)
+	if err != nil {
+		return "", fmt.Errorf("flare: sign VAPID token: %w", err)
+	}
+	k := base64.RawURLEncoding.EncodeToString(priv.PublicKey().Bytes())
+	return "vapid t=" + signed + ", k=" + k, nil
+}
+
+func validSubject(subject string) bool {
+	return strings.HasPrefix(subject, "mailto:") || strings.HasPrefix(subject, "https:")
+}
+
+// origin is the RFC 6454 serialization of the endpoint's origin: scheme and
+// host, with the port only when it is not the scheme's default.
+func origin(endpoint string) (string, error) {
+	u, err := url.Parse(endpoint)
+	if err != nil || u.Scheme == "" || u.Host == "" {
+		return "", fmt.Errorf("%w: not an absolute URL", ErrEndpointNotAllowed)
+	}
+	scheme := strings.ToLower(u.Scheme)
+	host := strings.ToLower(u.Hostname())
+	if strings.Contains(host, ":") {
+		host = "[" + host + "]"
+	}
+	port := u.Port()
+	if port != "" && !(scheme == "https" && port == "443") && !(scheme == "http" && port == "80") {
+		host += ":" + port
+	}
+	return scheme + "://" + host, nil
+}