feat(account): VK ID web login to link a VK identity from a browser

A browser has no signed VK Mini App launch params, so linking VK on the web uses
VK ID's raw OAuth 2.1 flow (PKCE, no @vkid/sdk): the SPA redirects to VK's hosted
login and returns with an authorization code, which the gateway exchanges
server-side (confidential, under the VK "Web" app's protected key) for the trusted
vk user id — then the existing link/merge machinery attaches or merges it.

- fbs LinkVKRequest{code, device_id, code_verifier}; codec + TS bindings.
- backend link.Service ConfirmVK/MergeVK/attachVK (KindVK, mirror Telegram),
  handleLinkVK[Merge], routes /user/link/vk[/merge], backendclient LinkVK[Merge].
- gateway internal/vkid confidential code exchange (id.vk.com/oauth2/auth);
  transcode link.vk.confirm/merge (registered only when configured) + config
  GATEWAY_VK_ID_{APP_ID,CLIENT_SECRET,REDIRECT_URL} + main wiring.
- UI lib/vkid (PKCE authorize redirect + callback), Profile "Link VK" control,
  boot callback handling; a merge re-authorizes for a fresh code (VK codes are
  single-use). Web-only (a redirect strands a Mini App webview).
- Deploy: VITE_VK_APP_ID + VITE_VK_ID_REDIRECT_URL build args + gateway env,
  ci.yaml/prod-deploy TEST_/PROD_ vars, compose/Dockerfile/.env.example/README.
- Tests: vkid exchange unit (string/number user_id, id_token fallback, errors),
  transcode link.vk, backend ConfirmVK/MergeVK inttest, codec encodeLinkVK.
- Docs: ARCHITECTURE §4, FUNCTIONAL(+ru), gateway README.

gateway/Dockerfile

@@ -25,12 +25,16 @@ ARG VITE_TELEGRAM_BOT_ID=
 ARG VITE_TELEGRAM_LINK=
 ARG VITE_TELEGRAM_GAME_CHANNEL_NAME=
 ARG VITE_VK_APP_LINK=
+ARG VITE_VK_APP_ID=
+ARG VITE_VK_ID_REDIRECT_URL=
 ARG VITE_GATEWAY_URL=
 ARG VITE_APP_VERSION=
 ENV VITE_TELEGRAM_BOT_ID=$VITE_TELEGRAM_BOT_ID \
     VITE_TELEGRAM_LINK=$VITE_TELEGRAM_LINK \
     VITE_TELEGRAM_GAME_CHANNEL_NAME=$VITE_TELEGRAM_GAME_CHANNEL_NAME \
     VITE_VK_APP_LINK=$VITE_VK_APP_LINK \
+    VITE_VK_APP_ID=$VITE_VK_APP_ID \
+    VITE_VK_ID_REDIRECT_URL=$VITE_VK_ID_REDIRECT_URL \
     VITE_GATEWAY_URL=$VITE_GATEWAY_URL \
     VITE_APP_VERSION=$VITE_APP_VERSION
 

gateway/README.md

@@ -61,6 +61,12 @@ round-trip, then forwards the trusted `vk_user_id` (and the client-read display
 from the signed params) to the backend. When `GATEWAY_VK_APP_SECRET` is unset VK auth is disabled
 (`auth.vk` is unregistered).
 
+`link.vk.confirm`/`merge` link a VK identity from a **browser** (not a Mini App) via **VK ID web
+login** (`internal/vkid`): the SPA runs the VK ID raw OAuth 2.1 flow (PKCE, no `@vkid/sdk`) and
+the gateway completes the **confidential code exchange** at `id.vk.com` under a SEPARATE VK "Web"
+app's protected key — the only op here that calls VK (the Mini App path above is offline). All
+three `GATEWAY_VK_ID_*` unset leaves the ops unregistered.
+
 The message-type catalog: `auth.telegram`, `auth.vk`, `auth.guest`,
 `auth.email.request`, `auth.email.login`, `profile.get`, `game.submit_play`,
 `game.state`, `lobby.enqueue`, `lobby.poll`, `chat.post`, `chat.read` and the play-loop ops;
@@ -72,9 +78,10 @@ refetch). The social/account/history ops —
 `blocks.*`, `invitation.*` (list/create/accept/decline/cancel), `profile.update`,
 `stats.get`, `game.gcg`, and the `notify` live event — go through the identical
 transcode pattern (`transcode_social.go`). Account linking & merge
-— `link.email.request/confirm/merge` and `link.telegram.confirm/merge`
-(`transcode_link.go`); the telegram ops validate the **Login Widget** payload via the
-validator (`ValidateLoginWidget`) and forward the trusted `external_id`. These
+— `link.email.request/confirm/merge`, `link.telegram.confirm/merge` and
+`link.vk.confirm/merge` (`transcode_link.go`); the telegram ops validate the **Login
+Widget** payload via the validator (`ValidateLoginWidget`), the vk ops complete the VK ID
+web code exchange via `internal/vkid`, and both forward the trusted `external_id`. These
 **superseded** the former `email.bind.*` ops, which were removed.
 
 ## Configuration
@@ -89,6 +96,7 @@ validator (`ValidateLoginWidget`) and forward the trusted `external_id`. These
 | `GATEWAY_ADMIN_USER` / `GATEWAY_ADMIN_PASSWORD` | unset | enable + guard the admin console at `/_gm` |
 | `GATEWAY_VALIDATOR_ADDR` | unset | Telegram validator gRPC address (enables initData / Login Widget validation) |
 | `GATEWAY_VK_APP_SECRET` | unset | VK app protected key; enables in-process VK Mini App launch-signature verification (`auth.vk`) |
+| `GATEWAY_VK_ID_APP_ID` / `GATEWAY_VK_ID_CLIENT_SECRET` / `GATEWAY_VK_ID_REDIRECT_URL` | unset | VK ID "Web" app credentials for VK web-login linking (`link.vk.*`): the server-side confidential OAuth 2.1 code exchange. A separate VK app from `GATEWAY_VK_APP_SECRET`; all three required to enable the ops |
 | `GATEWAY_BOTLINK_ADDR` | unset | reverse mTLS bot-link listener the remote bot dials (enables out-of-app push + admin relay) |
 | `GATEWAY_BOTLINK_RELAY_ADDR` | unset | plaintext internal listener serving the backend admin `SendToUser`/`SendToGameChannel` relay |
 | `GATEWAY_BOTLINK_TLS_CERT` / `_KEY` / `_CA` | unset | gateway server cert, its key, and the CA that signs accepted bot client certs (required when `GATEWAY_BOTLINK_ADDR` is set) |

gateway/cmd/gateway/main.go

@@ -33,6 +33,7 @@ import (
 	"scrabble/gateway/internal/ratelimit"
 	"scrabble/gateway/internal/session"
 	"scrabble/gateway/internal/transcode"
+	"scrabble/gateway/internal/vkid"
 	"scrabble/pkg/mtls"
 	botlinkv1 "scrabble/pkg/proto/botlink/v1"
 	telegramv1 "scrabble/pkg/proto/telegram/v1"
@@ -190,7 +191,15 @@ func run(ctx context.Context, cfg config.Config, logger *zap.Logger) error {
 		logger.Info("admin console disabled (set GATEWAY_ADMIN_USER and GATEWAY_ADMIN_PASSWORD)")
 	}
 
-	registry := transcode.NewRegistry(backend, validator, transcode.WithVKAuth(cfg.VKAppSecret))
+	// VK ID web login (browser VK-identity linking) is optional: build the confidential
+	// code-exchanger only when fully configured, else leave the interface nil so the
+	// link.vk.* ops stay unregistered.
+	var vkidExchanger transcode.VKIDExchanger
+	if cfg.VKID.Enabled() {
+		vkidExchanger = vkid.New(cfg.VKID.AppID, cfg.VKID.ClientSecret, cfg.VKID.RedirectURI)
+		logger.Info("vk id web login enabled")
+	}
+	registry := transcode.NewRegistry(backend, validator, transcode.WithVKAuth(cfg.VKAppSecret), transcode.WithVKLink(vkidExchanger))
 	edge := connectsrv.NewServer(connectsrv.Deps{
 		Registry:     registry,
 		Sessions:     sessions,

gateway/internal/backendclient/api_social.go

@@ -335,6 +335,24 @@ func (c *Client) LinkTelegramMerge(ctx context.Context, userID, externalID strin
 	return out, err
 }
 
+// LinkVK attaches a gateway-validated VK identity to the caller or reports a required
+// merge. externalID is the trusted vk user id resolved from the VK ID code exchange.
+func (c *Client) LinkVK(ctx context.Context, userID, externalID string) (LinkResultResp, error) {
+	var out LinkResultResp
+	err := c.do(ctx, http.MethodPost, "/api/v1/user/link/vk", userID, "",
+		map[string]string{"external_id": externalID}, &out)
+	return out, err
+}
+
+// LinkVKMerge merges the account owning a gateway-validated VK identity into the
+// caller's.
+func (c *Client) LinkVKMerge(ctx context.Context, userID, externalID string) (LinkResultResp, error) {
+	var out LinkResultResp
+	err := c.do(ctx, http.MethodPost, "/api/v1/user/link/vk/merge", userID, "",
+		map[string]string{"external_id": externalID}, &out)
+	return out, err
+}
+
 // ChangeEmailRequest asks the backend to mail a confirm-code to a new address for an
 // authenticated email change.
 func (c *Client) ChangeEmailRequest(ctx context.Context, userID, email string) error {

gateway/internal/config/config.go

@@ -36,6 +36,11 @@ type Config struct {
 	// VK launch-parameter signature in-process under it (a pure offline HMAC, no VK API
 	// round-trip). Empty disables the VK auth path (auth.vk is then unregistered).
 	VKAppSecret string
+	// VKID configures the VK ID web login used to link a VK identity from a browser
+	// (the confidential OAuth 2.1 code exchange against id.vk.com). It belongs to a
+	// separate VK "Web" app from VKAppSecret's Mini App, so its credentials are distinct.
+	// Any field empty disables the VK web-link ops (link.vk.*).
+	VKID VKIDConfig
 	// BotLink configures the reverse mTLS channel to the remote Telegram bot. An
 	// empty BotLink.Addr disables the bot channel (out-of-app push and admin relay).
 	BotLink BotLinkConfig
@@ -161,6 +166,22 @@ func DefaultAbuse() AbuseConfig {
 	}
 }
 
+// VKIDConfig holds the VK ID web-login credentials for the confidential
+// authorization-code exchange. AppID is the VK "Web" app's client id; ClientSecret is
+// its protected key; RedirectURI must exactly match the trusted redirect URL registered
+// with the app and the one the frontend uses. All three are required to enable the flow.
+type VKIDConfig struct {
+	AppID        string
+	ClientSecret string
+	RedirectURI  string
+}
+
+// Enabled reports whether VK ID web login is fully configured. When false the gateway
+// leaves the VK web-link ops (link.vk.*) unregistered.
+func (c VKIDConfig) Enabled() bool {
+	return c.AppID != "" && c.ClientSecret != "" && c.RedirectURI != ""
+}
+
 // Load reads the configuration from the environment, applies defaults, and
 // validates the result.
 func Load() (Config, error) {
@@ -174,6 +195,11 @@ func Load() (Config, error) {
 		AdminPassword:   os.Getenv("GATEWAY_ADMIN_PASSWORD"),
 		ValidatorAddr:   os.Getenv("GATEWAY_VALIDATOR_ADDR"),
 		VKAppSecret:     os.Getenv("GATEWAY_VK_APP_SECRET"),
+		VKID: VKIDConfig{
+			AppID:        os.Getenv("GATEWAY_VK_ID_APP_ID"),
+			ClientSecret: os.Getenv("GATEWAY_VK_ID_CLIENT_SECRET"),
+			RedirectURI:  os.Getenv("GATEWAY_VK_ID_REDIRECT_URL"),
+		},
 		SessionCacheMax: defaultSessionCacheMax,
 		RateLimit:       DefaultRateLimit(),
 		Abuse:           DefaultAbuse(),

gateway/internal/transcode/transcode.go

@@ -14,6 +14,7 @@ import (
 	"scrabble/gateway/internal/backendclient"
 	"scrabble/gateway/internal/connector"
 	"scrabble/gateway/internal/vkauth"
+	"scrabble/gateway/internal/vkid"
 	fb "scrabble/pkg/fbs/scrabblefb"
 )
 
@@ -152,6 +153,15 @@ func WithVKAuth(secret string) Option {
 	}
 }
 
+// WithVKLink registers the VK web-link ops (link.vk.confirm/merge), which complete a
+// browser VK ID authorization via the server-side code exchange. A nil exchanger leaves
+// them unregistered, so the ops are unknown wherever VK ID web login is not configured.
+func WithVKLink(ex VKIDExchanger) Option {
+	return func(r *Registry, backend *backendclient.Client) {
+		registerVKLinkOps(r, backend, ex)
+	}
+}
+
 // Lookup returns the operation for messageType, and whether it is registered.
 func (r *Registry) Lookup(messageType string) (Op, bool) {
 	op, ok := r.ops[messageType]
@@ -172,6 +182,9 @@ func DomainCode(err error) (string, bool) {
 	if errors.Is(err, vkauth.ErrInvalid) {
 		return "invalid_vk_params", true
 	}
+	if errors.Is(err, vkid.ErrInvalid) {
+		return "invalid_vk_id", true
+	}
 	if errors.Is(err, connector.ErrInvalidLoginWidget) {
 		return "invalid_login_widget", true
 	}

gateway/internal/transcode/transcode_link.go

@@ -4,6 +4,7 @@ import (
 	"context"
 
 	"scrabble/gateway/internal/backendclient"
+	"scrabble/gateway/internal/vkid"
 	fb "scrabble/pkg/fbs/scrabblefb"
 )
 
@@ -18,6 +19,8 @@ const (
 	MsgLinkEmailMerge     = "link.email.merge"
 	MsgLinkTelegram       = "link.telegram.confirm"
 	MsgLinkTelegramMerge  = "link.telegram.merge"
+	MsgLinkVK             = "link.vk.confirm"
+	MsgLinkVKMerge        = "link.vk.merge"
 	MsgLinkUnlink         = "link.unlink"
 	MsgEmailChangeRequest = "link.email.change.request"
 	MsgEmailChangeConfirm = "link.email.change.confirm"
@@ -154,3 +157,44 @@ func linkTelegramHandler(backend *backendclient.Client, tg TelegramValidator, me
 		return encodeLinkResult(res), nil
 	}
 }
+
+// VKIDExchanger completes a VK ID web authorization-code exchange, returning the
+// launching user's trusted VK identity. It is satisfied by *vkid.Exchanger and lets the
+// VK web-link ops resolve a browser VK login, which — unlike the Mini App path — has no
+// offline launch signature to verify.
+type VKIDExchanger interface {
+	Exchange(ctx context.Context, code, deviceID, codeVerifier string) (vkid.Identity, error)
+}
+
+// registerVKLinkOps adds the VK web-link ops when a VK ID exchanger is configured; a nil
+// exchanger leaves them unregistered (VK ID web login not configured).
+func registerVKLinkOps(r *Registry, backend *backendclient.Client, ex VKIDExchanger) {
+	if ex == nil {
+		return
+	}
+	r.ops[MsgLinkVK] = Op{Handler: linkVKHandler(backend, ex, false), Auth: true}
+	r.ops[MsgLinkVKMerge] = Op{Handler: linkVKHandler(backend, ex, true), Auth: true}
+}
+
+// linkVKHandler completes the VK ID code exchange (server-side, under the app's
+// protected key) and then calls the backend's link or merge endpoint with the trusted
+// VK external id.
+func linkVKHandler(backend *backendclient.Client, ex VKIDExchanger, merge bool) Handler {
+	return func(ctx context.Context, req Request) ([]byte, error) {
+		in := fb.GetRootAsLinkVKRequest(req.Payload, 0)
+		user, err := ex.Exchange(ctx, string(in.Code()), string(in.DeviceId()), string(in.CodeVerifier()))
+		if err != nil {
+			return nil, err
+		}
+		var res backendclient.LinkResultResp
+		if merge {
+			res, err = backend.LinkVKMerge(ctx, req.UserID, user.ExternalID)
+		} else {
+			res, err = backend.LinkVK(ctx, req.UserID, user.ExternalID)
+		}
+		if err != nil {
+			return nil, err
+		}
+		return encodeLinkResult(res), nil
+	}
+}

gateway/internal/transcode/transcode_vk_link_test.go

@@ -0,0 +1,90 @@
+package transcode_test
+
+import (
+	"context"
+	"encoding/json"
+	"net/http"
+	"testing"
+
+	flatbuffers "github.com/google/flatbuffers/go"
+
+	"scrabble/gateway/internal/transcode"
+	"scrabble/gateway/internal/vkid"
+	fb "scrabble/pkg/fbs/scrabblefb"
+)
+
+func linkVKPayload(code, deviceID, verifier string) []byte {
+	b := flatbuffers.NewBuilder(64)
+	c := b.CreateString(code)
+	d := b.CreateString(deviceID)
+	v := b.CreateString(verifier)
+	fb.LinkVKRequestStart(b)
+	fb.LinkVKRequestAddCode(b, c)
+	fb.LinkVKRequestAddDeviceId(b, d)
+	fb.LinkVKRequestAddCodeVerifier(b, v)
+	b.Finish(fb.LinkVKRequestEnd(b))
+	return b.FinishedBytes()
+}
+
+// fakeVKIDExchanger records the exchange inputs and returns a canned identity.
+type fakeVKIDExchanger struct {
+	id                              vkid.Identity
+	err                             error
+	gotCode, gotDevice, gotVerifier string
+}
+
+func (f *fakeVKIDExchanger) Exchange(_ context.Context, code, deviceID, codeVerifier string) (vkid.Identity, error) {
+	f.gotCode, f.gotDevice, f.gotVerifier = code, deviceID, codeVerifier
+	return f.id, f.err
+}
+
+func TestLinkVKExchangesAndForwards(t *testing.T) {
+	var gotExternalID string
+	backend, cleanup := fakeBackend(t, func(w http.ResponseWriter, r *http.Request) {
+		if r.URL.Path != "/api/v1/user/link/vk" {
+			t.Errorf("path = %q", r.URL.Path)
+		}
+		var body struct {
+			ExternalID string `json:"external_id"`
+		}
+		_ = json.NewDecoder(r.Body).Decode(&body)
+		gotExternalID = body.ExternalID
+		_, _ = w.Write([]byte(`{"status":"linked"}`))
+	})
+	defer cleanup()
+
+	ex := &fakeVKIDExchanger{id: vkid.Identity{ExternalID: "777"}}
+	reg := transcode.NewRegistry(backend, nil, transcode.WithVKLink(ex))
+	op, ok := reg.Lookup(transcode.MsgLinkVK)
+	if !ok {
+		t.Fatal("link.vk.confirm not registered")
+	}
+	payload, err := op.Handler(context.Background(), transcode.Request{UserID: "u-1", Payload: linkVKPayload("the-code", "dev-1", "verifier-1")})
+	if err != nil {
+		t.Fatalf("handler: %v", err)
+	}
+	// The PKCE inputs from the wire must reach the exchanger verbatim...
+	if ex.gotCode != "the-code" || ex.gotDevice != "dev-1" || ex.gotVerifier != "verifier-1" {
+		t.Errorf("exchange got %q/%q/%q", ex.gotCode, ex.gotDevice, ex.gotVerifier)
+	}
+	// ...and the resolved vk id must be the one forwarded to the backend.
+	if gotExternalID != "777" {
+		t.Errorf("backend external_id = %q, want 777", gotExternalID)
+	}
+	if string(fb.GetRootAsLinkResult(payload, 0).Status()) != "linked" {
+		t.Error("expected a linked result")
+	}
+}
+
+func TestLinkVKUnregisteredWithoutExchanger(t *testing.T) {
+	backend, cleanup := fakeBackend(t, func(http.ResponseWriter, *http.Request) {})
+	defer cleanup()
+
+	reg := transcode.NewRegistry(backend, nil)
+	if _, ok := reg.Lookup(transcode.MsgLinkVK); ok {
+		t.Error("link.vk.confirm must be unregistered when no VK ID exchanger is configured")
+	}
+	if _, ok := reg.Lookup(transcode.MsgLinkVKMerge); ok {
+		t.Error("link.vk.merge must be unregistered when no VK ID exchanger is configured")
+	}
+}

gateway/internal/vkid/vkid.go

@@ -0,0 +1,163 @@
+// Package vkid completes VK ID web authorization on the server. A browser linking a
+// VK identity has no signed Mini App launch parameters (that offline HMAC path is
+// vkauth); instead the frontend runs the VK ID raw OAuth 2.1 flow (PKCE) and hands the
+// gateway the authorization code, which the gateway exchanges — confidentially, under
+// the app's protected key — for the launching user's trusted VK id. Unlike the Mini App
+// path this makes an outbound call to VK (there is no offline verification for the web
+// flow). See docs/ARCHITECTURE.md §12.
+package vkid
+
+import (
+	"context"
+	"encoding/base64"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"net/http"
+	"net/url"
+	"strings"
+	"time"
+)
+
+const (
+	// tokenEndpoint is VK ID's OAuth 2.1 token endpoint. The frontend obtains the
+	// authorization code against the same host, so the confidential exchange targets it
+	// too. It is a fixed constant (not user input), so the outbound request carries no
+	// SSRF risk.
+	tokenEndpoint = "https://id.vk.com/oauth2/auth"
+	// exchangeTimeout bounds one code-for-token exchange. Linking is interactive, so an
+	// unreachable VK must fail fast rather than hold the request open.
+	exchangeTimeout = 10 * time.Second
+	// maxResponseBytes caps the token-response read to bound memory on an oversized body.
+	maxResponseBytes = 1 << 16
+)
+
+// ErrInvalid reports an authorization fault: the exchange was rejected or yielded no vk
+// user id (a bad or expired code, a mismatched verifier or redirect). It is distinct
+// from a transport failure reaching VK, which surfaces as a wrapped error.
+var ErrInvalid = errors.New("vkid: vk id authorization exchange failed")
+
+// Identity is the user resolved from a completed VK ID exchange. ExternalID is the vk
+// user id, used as the identities external_id.
+type Identity struct {
+	ExternalID string
+}
+
+// numericID accepts a VK user id that the token endpoint returns inconsistently as a
+// JSON string or a JSON number, normalising both to their decimal string form (empty
+// for a null or absent field).
+type numericID string
+
+func (n *numericID) UnmarshalJSON(b []byte) error {
+	s := strings.Trim(string(b), `"`)
+	if s == "null" {
+		s = ""
+	}
+	*n = numericID(s)
+	return nil
+}
+
+// Exchanger completes VK ID confidential authorization-code exchanges for one app.
+type Exchanger struct {
+	appID        string
+	clientSecret string
+	redirectURI  string
+	endpoint     string
+	httpClient   *http.Client
+}
+
+// New constructs an Exchanger for the app credentials. redirectURI must equal the
+// trusted redirect URL registered with the app and the one the frontend used, or VK
+// rejects the exchange.
+func New(appID, clientSecret, redirectURI string) *Exchanger {
+	return &Exchanger{
+		appID:        appID,
+		clientSecret: clientSecret,
+		redirectURI:  redirectURI,
+		endpoint:     tokenEndpoint,
+		httpClient:   &http.Client{Timeout: exchangeTimeout},
+	}
+}
+
+// Exchange completes the authorization-code grant and returns the trusted vk user id.
+// code, deviceID and codeVerifier are the PKCE inputs the frontend obtained from the VK
+// ID authorization redirect; the exchange authenticates with the app's protected key.
+func (e *Exchanger) Exchange(ctx context.Context, code, deviceID, codeVerifier string) (Identity, error) {
+	if code == "" || deviceID == "" || codeVerifier == "" {
+		return Identity{}, ErrInvalid
+	}
+	form := url.Values{
+		"grant_type":    {"authorization_code"},
+		"code":          {code},
+		"code_verifier": {codeVerifier},
+		"device_id":     {deviceID},
+		"client_id":     {e.appID},
+		"client_secret": {e.clientSecret},
+		"redirect_uri":  {e.redirectURI},
+	}
+	req, err := http.NewRequestWithContext(ctx, http.MethodPost, e.endpoint, strings.NewReader(form.Encode()))
+	if err != nil {
+		return Identity{}, fmt.Errorf("vkid: build exchange request: %w", err)
+	}
+	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
+	req.Header.Set("Accept", "application/json")
+	resp, err := e.httpClient.Do(req)
+	if err != nil {
+		return Identity{}, fmt.Errorf("vkid: exchange request: %w", err)
+	}
+	defer resp.Body.Close()
+	body, err := io.ReadAll(io.LimitReader(resp.Body, maxResponseBytes))
+	if err != nil {
+		return Identity{}, fmt.Errorf("vkid: read exchange response: %w", err)
+	}
+	if resp.StatusCode != http.StatusOK {
+		// VK answers 400 with {error, error_description} on a bad/expired code or a
+		// verifier/redirect mismatch — an authorization fault, not a transport error.
+		return Identity{}, ErrInvalid
+	}
+	var out struct {
+		UserID  numericID `json:"user_id"`
+		IDToken string    `json:"id_token"`
+		Error   string    `json:"error"`
+	}
+	if err := json.Unmarshal(body, &out); err != nil {
+		return Identity{}, ErrInvalid
+	}
+	if out.Error != "" {
+		return Identity{}, ErrInvalid
+	}
+	uid := string(out.UserID)
+	if uid == "" || uid == "0" {
+		uid = subjectFromIDToken(out.IDToken)
+	}
+	if uid == "" {
+		return Identity{}, ErrInvalid
+	}
+	return Identity{ExternalID: uid}, nil
+}
+
+// subjectFromIDToken extracts the OIDC subject (the vk user id) from the payload of a
+// VK ID id_token. The token arrives inside a direct TLS response from VK, so its
+// signature is not re-verified here; the claim is only a fallback for a response that
+// omits an explicit user_id.
+func subjectFromIDToken(idToken string) string {
+	parts := strings.Split(idToken, ".")
+	if len(parts) != 3 {
+		return ""
+	}
+	payload, err := base64.RawURLEncoding.DecodeString(parts[1])
+	if err != nil {
+		return ""
+	}
+	var claims struct {
+		Sub numericID `json:"sub"`
+	}
+	if err := json.Unmarshal(payload, &claims); err != nil {
+		return ""
+	}
+	if s := string(claims.Sub); s != "0" {
+		return s
+	}
+	return ""
+}

gateway/internal/vkid/vkid_test.go

@@ -0,0 +1,131 @@
+package vkid
+
+import (
+	"context"
+	"encoding/base64"
+	"errors"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"net/url"
+	"testing"
+)
+
+// testExchanger builds an Exchanger pointed at a test server.
+func testExchanger(endpoint string) *Exchanger {
+	return &Exchanger{
+		appID:        "app-1",
+		clientSecret: "secret-1",
+		redirectURI:  "https://example.test/app/",
+		endpoint:     endpoint,
+		httpClient:   &http.Client{},
+	}
+}
+
+func TestExchangeSuccessSendsConfidentialPKCE(t *testing.T) {
+	var gotForm url.Values
+	var gotContentType string
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		gotContentType = r.Header.Get("Content-Type")
+		body, _ := io.ReadAll(r.Body)
+		gotForm, _ = url.ParseQuery(string(body))
+		w.Header().Set("Content-Type", "application/json")
+		_, _ = io.WriteString(w, `{"user_id":"12345","access_token":"a","id_token":"h.e.s"}`)
+	}))
+	defer srv.Close()
+
+	id, err := testExchanger(srv.URL).Exchange(context.Background(), "the-code", "dev-9", "verifier-xyz")
+	if err != nil {
+		t.Fatalf("Exchange: %v", err)
+	}
+	if id.ExternalID != "12345" {
+		t.Fatalf("ExternalID = %q, want 12345", id.ExternalID)
+	}
+	if gotContentType != "application/x-www-form-urlencoded" {
+		t.Errorf("Content-Type = %q", gotContentType)
+	}
+	// The confidential exchange must carry the PKCE inputs and the app credentials.
+	want := map[string]string{
+		"grant_type":    "authorization_code",
+		"code":          "the-code",
+		"code_verifier": "verifier-xyz",
+		"device_id":     "dev-9",
+		"client_id":     "app-1",
+		"client_secret": "secret-1",
+		"redirect_uri":  "https://example.test/app/",
+	}
+	for k, v := range want {
+		if gotForm.Get(k) != v {
+			t.Errorf("form[%s] = %q, want %q", k, gotForm.Get(k), v)
+		}
+	}
+}
+
+func TestExchangeAcceptsNumericUserID(t *testing.T) {
+	// VK returns user_id as a bare JSON number in some responses; it must parse too.
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		_, _ = io.WriteString(w, `{"user_id":1234567890,"access_token":"a"}`)
+	}))
+	defer srv.Close()
+
+	id, err := testExchanger(srv.URL).Exchange(context.Background(), "c", "d", "v")
+	if err != nil {
+		t.Fatalf("Exchange: %v", err)
+	}
+	if id.ExternalID != "1234567890" {
+		t.Fatalf("ExternalID = %q, want 1234567890", id.ExternalID)
+	}
+}
+
+func TestExchangeFallsBackToIDTokenSub(t *testing.T) {
+	sub := "987654321"
+	payload := base64.RawURLEncoding.EncodeToString([]byte(`{"sub":"` + sub + `"}`))
+	idToken := "header." + payload + ".sig"
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		_, _ = io.WriteString(w, `{"access_token":"a","id_token":"`+idToken+`"}`)
+	}))
+	defer srv.Close()
+
+	id, err := testExchanger(srv.URL).Exchange(context.Background(), "c", "d", "v")
+	if err != nil {
+		t.Fatalf("Exchange: %v", err)
+	}
+	if id.ExternalID != sub {
+		t.Fatalf("ExternalID = %q, want %q (from id_token sub)", id.ExternalID, sub)
+	}
+}
+
+func TestExchangeRejectedIsErrInvalid(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusBadRequest)
+		_, _ = io.WriteString(w, `{"error":"invalid_grant","error_description":"code expired"}`)
+	}))
+	defer srv.Close()
+
+	_, err := testExchanger(srv.URL).Exchange(context.Background(), "c", "d", "v")
+	if !errors.Is(err, ErrInvalid) {
+		t.Fatalf("err = %v, want ErrInvalid", err)
+	}
+}
+
+func TestExchangeEmptyInputsFailFast(t *testing.T) {
+	// Missing PKCE inputs are rejected without any network call.
+	ex := testExchanger("http://127.0.0.1:0/never")
+	for _, args := range [][3]string{{"", "d", "v"}, {"c", "", "v"}, {"c", "d", ""}} {
+		if _, err := ex.Exchange(context.Background(), args[0], args[1], args[2]); !errors.Is(err, ErrInvalid) {
+			t.Errorf("Exchange%v err = %v, want ErrInvalid", args, err)
+		}
+	}
+}
+
+func TestExchangeTransportErrorIsNotErrInvalid(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(http.ResponseWriter, *http.Request) {}))
+	srv.Close() // closed listener → connection refused
+	_, err := testExchanger(srv.URL).Exchange(context.Background(), "c", "d", "v")
+	if err == nil {
+		t.Fatal("want a transport error")
+	}
+	if errors.Is(err, ErrInvalid) {
+		t.Fatalf("transport error must not be ErrInvalid: %v", err)
+	}
+}