backend/internal/payments/payments.go

@@ -147,12 +147,13 @@ func (m Money) Cmp(o Money) (int, error) {
 	}
 }
 
-// String renders the amount as "<value> <currency>", with the currency's
-// fractional digits and no floating point (e.g. "149.50 RUB", "250 XTR").
-func (m Money) String() string {
+// Major renders the amount as a decimal string without the currency, with the currency's
+// fractional digits and no floating point (e.g. "149.50", "250") — the form a provider's amount
+// field (Robokassa OutSum) takes.
+func (m Money) Major() string {
 	scale := m.currency.minorPerUnit()
 	if scale == 1 {
-		return fmt.Sprintf("%d %s", m.minor, m.currency)
+		return fmt.Sprintf("%d", m.minor)
 	}
 	neg := m.minor < 0
 	abs := m.minor
@@ -167,5 +168,10 @@ func (m Money) String() string {
 	if neg {
 		sign = "-"
 	}
-	return fmt.Sprintf("%s%d.%0*d %s", sign, abs/scale, width, abs%scale, m.currency)
+	return fmt.Sprintf("%s%d.%0*d", sign, abs/scale, width, abs%scale)
+}
+
+// String renders the amount as "<value> <currency>" (e.g. "149.50 RUB", "250 XTR").
+func (m Money) String() string {
+	return m.Major() + " " + string(m.currency)
 }

backend/internal/payments/service_intake.go

@@ -0,0 +1,73 @@
+package payments
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/google/uuid"
+)
+
+// OrderResult is what CreateOrder returns to the transport: the created order id and the details a
+// provider launch payload needs — the amount to charge and a human title for the payment.
+type OrderResult struct {
+	OrderID uuid.UUID
+	Amount  Money
+	Title   string
+}
+
+// CreateOrder opens a pending order to fund a chip pack in the execution context's payment method,
+// tagged with the provider that will settle it. It gate-checks the context (trusted, not the
+// VK-iOS spend freeze) and that the method's funding segment is attached, prices the pack in the
+// method's currency, then writes the order. The caller enforces any account-level precondition
+// (e.g. the direct email anchor, D36) before calling — payments holds no cross-schema identity
+// knowledge.
+func (s *Service) CreateOrder(ctx context.Context, accountID uuid.UUID, cxt Context, present []Source, productID uuid.UUID, provider string) (OrderResult, error) {
+	if !cxt.Trusted() || cxt.vkFrozen() {
+		return OrderResult{}, ErrUntrusted
+	}
+	method := cxt.Kind
+	if !has(present, method) {
+		return OrderResult{}, ErrUntrusted // the funding segment is not attached to the account
+	}
+	pack, err := s.store.loadPackForOrder(ctx, productID, method)
+	if err != nil {
+		return OrderResult{}, err
+	}
+	orderID, err := uuid.NewV7()
+	if err != nil {
+		return OrderResult{}, fmt.Errorf("payments: order id: %w", err)
+	}
+	o := newOrder{
+		orderID:   orderID,
+		accountID: accountID,
+		platform:  string(method),
+		productID: productID,
+		amount:    pack.price,
+		origin:    method,
+		provider:  provider,
+	}
+	if err := s.store.createOrder(ctx, o, s.clock()); err != nil {
+		return OrderResult{}, err
+	}
+	return OrderResult{OrderID: orderID, Amount: pack.price, Title: pack.title}, nil
+}
+
+// Fund credits a paid order into its funded segment exactly once, from a verified provider callback
+// — the single writer for every rail. It matches the order, verifies the paid amount, appends the
+// fund ledger row (idempotent on (provider, provider_payment_id)), credits the balance and marks
+// the order paid. A duplicate callback returns AlreadyCredited without a second credit; a valid
+// callback is honoured even on an expired order (§9/D23).
+func (s *Service) Fund(ctx context.Context, orderID uuid.UUID, provider, providerPaymentID string, paid Money) (FundOutcome, error) {
+	return s.store.fund(ctx, orderID, provider, providerPaymentID, paid, s.clock())
+}
+
+// ExpireOrders marks pending orders older than the configured lifetime as expired, returning how
+// many were swept. It backs the periodic pending reaper; expiry is cosmetic (a late valid callback
+// still credits — see Fund).
+func (s *Service) ExpireOrders(ctx context.Context) (int, error) {
+	ttl, err := s.store.orderTTL(ctx)
+	if err != nil {
+		return 0, err
+	}
+	return s.store.expirePending(ctx, ttl, s.clock())
+}

backend/internal/payments/service_intake_test.go

@@ -0,0 +1,42 @@
+package payments
+
+import (
+	"context"
+	"errors"
+	"testing"
+	"time"
+
+	"github.com/google/uuid"
+)
+
+// gateOnlyService builds a Service with a fixed clock and no store, usable only for the CreateOrder
+// gate rejections that return before any store access.
+func gateOnlyService() *Service {
+	return &Service{clock: func() time.Time { return time.Unix(0, 0).UTC() }}
+}
+
+// TestCreateOrderGateRejections checks that CreateOrder fails closed — before touching the store —
+// on an untrusted platform, the VK-iOS spend freeze, and a method whose funding segment the account
+// does not hold.
+func TestCreateOrderGateRejections(t *testing.T) {
+	ctx := context.Background()
+	acc, prod := uuid.New(), uuid.New()
+	present := []Source{SourceDirect, SourceVK}
+
+	cases := []struct {
+		name string
+		cxt  Context
+	}{
+		{"untrusted context", Context{}},
+		{"vk-ios spend freeze", Context{Kind: SourceVK, Subtype: SubtypeIOS}},
+		{"method segment not attached", Context{Kind: SourceTelegram}},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			_, err := gateOnlyService().CreateOrder(ctx, acc, tc.cxt, present, prod, "robokassa")
+			if !errors.Is(err, ErrUntrusted) {
+				t.Fatalf("CreateOrder(%s) = %v, want ErrUntrusted", tc.name, err)
+			}
+		})
+	}
+}

backend/internal/payments/store_intake.go

@@ -0,0 +1,324 @@
+package payments
+
+import (
+	"context"
+	"database/sql"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"time"
+
+	"github.com/go-jet/jet/v2/postgres"
+	"github.com/go-jet/jet/v2/qrm"
+	"github.com/google/uuid"
+	"github.com/jackc/pgx/v5/pgconn"
+
+	"scrabble/backend/internal/postgres/jet/payments/model"
+	"scrabble/backend/internal/postgres/jet/payments/table"
+)
+
+// Intake errors surfaced by the order-flow and external-credit (fund) path.
+var (
+	// ErrNotAPack means the product is not a fundable chip pack in the requested method: it
+	// carries no chips atom, or no price for that payment method.
+	ErrNotAPack = errors.New("payments: product is not a chip pack for this method")
+	// ErrOrderNotFound means no order matches the id (an unknown or forged callback reference).
+	ErrOrderNotFound = errors.New("payments: order not found")
+	// ErrAmountMismatch means the callback's paid amount or currency does not match the order's
+	// expected amount — the credit is refused (§9: verify amount after matching by order id).
+	ErrAmountMismatch = errors.New("payments: paid amount does not match the order")
+)
+
+// errAlreadyCredited is the internal sentinel that unwinds the fund transaction when the ledger's
+// (provider, provider_payment_id) unique index rejects a duplicate callback. It is not surfaced:
+// a replayed callback is a success that credits nothing.
+var errAlreadyCredited = errors.New("payments: already credited")
+
+// packInfo is a chip pack resolved for an order: the product, the chips it funds and its price in
+// the requested payment method's currency.
+type packInfo struct {
+	productID uuid.UUID
+	title     string
+	chips     int
+	price     Money
+}
+
+// packChips returns the quantity of the chips atom a product carries, or 0 if it has none (which
+// marks it as a value, not a fundable pack).
+func (s *Store) packChips(ctx context.Context, productID uuid.UUID) (int, error) {
+	var item model.ProductItem
+	err := postgres.SELECT(table.ProductItem.AllColumns).
+		FROM(table.ProductItem).
+		WHERE(table.ProductItem.ProductID.EQ(postgres.UUID(productID)).
+			AND(table.ProductItem.AtomType.EQ(postgres.String(atomChips)))).
+		LIMIT(1).
+		QueryContext(ctx, s.db, &item)
+	if errors.Is(err, qrm.ErrNoRows) {
+		return 0, nil
+	}
+	if err != nil {
+		return 0, fmt.Errorf("payments: load pack chips %s: %w", productID, err)
+	}
+	return int(item.Quantity), nil
+}
+
+// loadPackForOrder resolves an active chip pack for a new order in the given payment method: an
+// active product carrying a chips atom and a price row for the method (its currency and amount are
+// the order's expected amount). It rejects a missing/deactivated product (ErrProductNotFound) and
+// a product that is not a pack for the method (ErrNotAPack).
+func (s *Store) loadPackForOrder(ctx context.Context, productID uuid.UUID, method Source) (packInfo, error) {
+	var p model.Product
+	err := postgres.SELECT(table.Product.AllColumns).
+		FROM(table.Product).
+		WHERE(table.Product.ProductID.EQ(postgres.UUID(productID))).
+		LIMIT(1).
+		QueryContext(ctx, s.db, &p)
+	if errors.Is(err, qrm.ErrNoRows) || (err == nil && !p.Active) {
+		return packInfo{}, ErrProductNotFound
+	}
+	if err != nil {
+		return packInfo{}, fmt.Errorf("payments: load product %s: %w", productID, err)
+	}
+
+	chips, err := s.packChips(ctx, productID)
+	if err != nil {
+		return packInfo{}, err
+	}
+	if chips <= 0 {
+		return packInfo{}, ErrNotAPack
+	}
+
+	var price model.ProductPrice
+	err = postgres.SELECT(table.ProductPrice.AllColumns).
+		FROM(table.ProductPrice).
+		WHERE(table.ProductPrice.ProductID.EQ(postgres.UUID(productID)).
+			AND(table.ProductPrice.Method.EQ(postgres.String(string(method))))).
+		LIMIT(1).
+		QueryContext(ctx, s.db, &price)
+	if errors.Is(err, qrm.ErrNoRows) {
+		return packInfo{}, ErrNotAPack
+	}
+	if err != nil {
+		return packInfo{}, fmt.Errorf("payments: load pack price %s: %w", productID, err)
+	}
+	money, err := MoneyFromMinor(price.Amount, Currency(price.Currency))
+	if err != nil {
+		return packInfo{}, err
+	}
+	return packInfo{productID: productID, title: p.Title, chips: chips, price: money}, nil
+}
+
+// packForCredit resolves the chips and title of an ordered pack at credit time, ignoring the
+// product's active flag: the money is real, so an order is honoured even if the pack was
+// deactivated after it was placed (§9/D23).
+func (s *Store) packForCredit(ctx context.Context, productID uuid.UUID) (chips int, title string, err error) {
+	var p model.Product
+	e := postgres.SELECT(table.Product.Title).
+		FROM(table.Product).
+		WHERE(table.Product.ProductID.EQ(postgres.UUID(productID))).
+		LIMIT(1).
+		QueryContext(ctx, s.db, &p)
+	if errors.Is(e, qrm.ErrNoRows) {
+		return 0, "", ErrProductNotFound
+	}
+	if e != nil {
+		return 0, "", fmt.Errorf("payments: load product %s: %w", productID, e)
+	}
+	chips, err = s.packChips(ctx, productID)
+	if err != nil {
+		return 0, "", err
+	}
+	if chips <= 0 {
+		return 0, "", ErrNotAPack
+	}
+	return chips, p.Title, nil
+}
+
+// newOrder is the intent a CreateOrder writes: a pending order for a pack, priced in the method's
+// currency, tagged with the provider that will settle it.
+type newOrder struct {
+	orderID   uuid.UUID
+	accountID uuid.UUID
+	platform  string
+	productID uuid.UUID
+	amount    Money
+	origin    Source
+	provider  string
+}
+
+// createOrder inserts a pending order.
+func (s *Store) createOrder(ctx context.Context, o newOrder, now time.Time) error {
+	stmt := table.Orders.INSERT(
+		table.Orders.OrderID, table.Orders.AccountID, table.Orders.Platform,
+		table.Orders.ProductID, table.Orders.ExpectedAmount, table.Orders.Currency,
+		table.Orders.Origin, table.Orders.Status, table.Orders.Provider,
+		table.Orders.CreatedAt, table.Orders.UpdatedAt,
+	).VALUES(
+		o.orderID, o.accountID, o.platform,
+		o.productID, o.amount.Minor(), string(o.amount.Currency()),
+		string(o.origin), "pending", o.provider,
+		now, now,
+	)
+	if _, err := stmt.ExecContext(ctx, s.db); err != nil {
+		return fmt.Errorf("payments: create order: %w", err)
+	}
+	return nil
+}
+
+// orderRow is a stored order read back for the intake path.
+type orderRow struct {
+	orderID        uuid.UUID
+	accountID      uuid.UUID
+	productID      uuid.UUID
+	expectedAmount int64
+	currency       string
+	origin         string
+	status         string
+}
+
+// orderByID reads an order, or ErrOrderNotFound.
+func (s *Store) orderByID(ctx context.Context, orderID uuid.UUID) (orderRow, error) {
+	var o model.Orders
+	err := postgres.SELECT(table.Orders.AllColumns).
+		FROM(table.Orders).
+		WHERE(table.Orders.OrderID.EQ(postgres.UUID(orderID))).
+		LIMIT(1).
+		QueryContext(ctx, s.db, &o)
+	if errors.Is(err, qrm.ErrNoRows) {
+		return orderRow{}, ErrOrderNotFound
+	}
+	if err != nil {
+		return orderRow{}, fmt.Errorf("payments: load order %s: %w", orderID, err)
+	}
+	return orderRow{
+		orderID:        o.OrderID,
+		accountID:      o.AccountID,
+		productID:      o.ProductID,
+		expectedAmount: o.ExpectedAmount,
+		currency:       o.Currency,
+		origin:         o.Origin,
+		status:         o.Status,
+	}, nil
+}
+
+// FundOutcome reports the result of an intake credit: whose balance, which segment and how many
+// chips were credited, and whether the callback was a duplicate that credited nothing.
+type FundOutcome struct {
+	AccountID       uuid.UUID
+	Source          Source
+	Chips           int
+	AlreadyCredited bool
+}
+
+// fund credits a paid order exactly once: it matches the order, verifies the amount, then in one
+// transaction appends a fund ledger row (idempotent on the (provider, provider_payment_id) unique
+// index), credits the funded segment's balance and marks the order paid. A duplicate callback is
+// rejected by the unique index and returns AlreadyCredited with no error and no second credit. A
+// valid callback is honoured even on an expired order (§9/D23). The read cache is invalidated after
+// the commit, since the credit runs outside any request the payments package owns.
+func (s *Store) fund(ctx context.Context, orderID uuid.UUID, provider, providerPaymentID string, paid Money, now time.Time) (FundOutcome, error) {
+	ord, err := s.orderByID(ctx, orderID)
+	if err != nil {
+		return FundOutcome{}, err
+	}
+	if paid.Currency() != Currency(ord.currency) || paid.Minor() != ord.expectedAmount {
+		return FundOutcome{}, ErrAmountMismatch
+	}
+	chips, title, err := s.packForCredit(ctx, ord.productID)
+	if err != nil {
+		return FundOutcome{}, err
+	}
+	snapshot, err := marshalFundSnapshot(ord.productID, title, chips, paid)
+	if err != nil {
+		return FundOutcome{}, err
+	}
+
+	src := Source(ord.origin)
+	outcome := FundOutcome{AccountID: ord.accountID, Source: src, Chips: chips}
+	pv, pp := provider, providerPaymentID
+	productID := ord.productID
+	err = withTx(ctx, s.db, func(tx *sql.Tx) error {
+		if e := insertLedgerTx(ctx, tx, ord.accountID, "fund", &src, &src, chips, &productID, &orderID, &pv, &pp, snapshot, now); e != nil {
+			if isUniqueViolation(e) {
+				outcome.AlreadyCredited = true
+				return errAlreadyCredited
+			}
+			return e
+		}
+		if _, e := tx.ExecContext(ctx,
+			`INSERT INTO payments.balances (account_id, source, chips, updated_at)
+			 VALUES ($1, $2, $3, now())
+			 ON CONFLICT (account_id, source) DO UPDATE
+			   SET chips = payments.balances.chips + EXCLUDED.chips, updated_at = now()`,
+			ord.accountID, string(src), chips); e != nil {
+			return fmt.Errorf("payments: credit balance %s: %w", src, e)
+		}
+		if _, e := tx.ExecContext(ctx,
+			`UPDATE payments.orders SET status = 'paid', provider = $2, provider_payment_id = $3, updated_at = now()
+			 WHERE order_id = $1`,
+			orderID, provider, providerPaymentID); e != nil {
+			return fmt.Errorf("payments: mark order paid: %w", e)
+		}
+		return nil
+	})
+	if err != nil {
+		if errors.Is(err, errAlreadyCredited) {
+			return outcome, nil
+		}
+		return FundOutcome{}, err
+	}
+	s.cache.invalidate(ord.accountID)
+	return outcome, nil
+}
+
+// orderTTL reads the configured pending-order lifetime in whole seconds.
+func (s *Store) orderTTL(ctx context.Context) (int, error) {
+	var cfg model.Config
+	if err := postgres.SELECT(table.Config.OrderTTLSeconds).
+		FROM(table.Config).
+		LIMIT(1).
+		QueryContext(ctx, s.db, &cfg); err != nil {
+		return 0, fmt.Errorf("payments: read order ttl: %w", err)
+	}
+	return int(cfg.OrderTTLSeconds), nil
+}
+
+// expirePending marks every pending order older than ttlSeconds as expired, returning how many.
+// Expiry is cosmetic DB hygiene: a later valid callback still credits an expired order (§9/D23).
+func (s *Store) expirePending(ctx context.Context, ttlSeconds int, now time.Time) (int, error) {
+	cutoff := now.Add(-time.Duration(ttlSeconds) * time.Second)
+	res, err := table.Orders.
+		UPDATE(table.Orders.Status, table.Orders.UpdatedAt).
+		SET(postgres.String("expired"), postgres.TimestampzT(now)).
+		WHERE(table.Orders.Status.EQ(postgres.String("pending")).
+			AND(table.Orders.CreatedAt.LT(postgres.TimestampzT(cutoff)))).
+		ExecContext(ctx, s.db)
+	if err != nil {
+		return 0, fmt.Errorf("payments: expire pending orders: %w", err)
+	}
+	n, _ := res.RowsAffected()
+	return int(n), nil
+}
+
+// marshalFundSnapshot records what a fund credited (the pack, chips and paid amount) on the ledger
+// row, so history stays independent of later catalog edits (§7/D34).
+func marshalFundSnapshot(productID uuid.UUID, title string, chips int, paid Money) ([]byte, error) {
+	b, err := json.Marshal(struct {
+		ProductID string `json:"product_id"`
+		Title     string `json:"title,omitempty"`
+		Chips     int    `json:"chips"`
+		Amount    int64  `json:"amount_minor"`
+		Currency  string `json:"currency"`
+	}{productID.String(), title, chips, paid.Minor(), string(paid.Currency())})
+	if err != nil {
+		return nil, fmt.Errorf("payments: marshal fund snapshot: %w", err)
+	}
+	return b, nil
+}
+
+// isUniqueViolation reports whether err is a PostgreSQL unique-constraint violation (SQLSTATE
+// 23505) — here, a duplicate provider callback hitting the ledger idempotency index.
+func isUniqueViolation(err error) bool {
+	var pgErr *pgconn.PgError
+	return errors.As(err, &pgErr) && pgErr.Code == "23505"
+}

backend/internal/payments/store_wallet.go

@@ -230,8 +230,11 @@ func applyBenefitTx(ctx context.Context, tx *sql.Tx, accountID uuid.UUID, origin
 	return nil
 }
 
-// insertLedgerTx appends one append-only ledger row inside tx.
-func insertLedgerTx(ctx context.Context, tx *sql.Tx, accountID uuid.UUID, kind string, source, origin *Source, chipsDelta int, productID *uuid.UUID, snapshot []byte, now time.Time) error {
+// insertLedgerTx appends one append-only ledger row inside tx. orderID, provider and
+// providerPaymentID are set only on an intake credit (fund/refund) and are nil for a
+// spend/admin_grant; a non-nil (provider, providerPaymentID) pair is guarded by the partial
+// unique index, so a duplicate provider callback fails here (the idempotency key).
+func insertLedgerTx(ctx context.Context, tx *sql.Tx, accountID uuid.UUID, kind string, source, origin *Source, chipsDelta int, productID, orderID *uuid.UUID, provider, providerPaymentID *string, snapshot []byte, now time.Time) error {
 	id, err := uuid.NewV7()
 	if err != nil {
 		return fmt.Errorf("payments: ledger id: %w", err)
@@ -246,11 +249,13 @@ func insertLedgerTx(ctx context.Context, tx *sql.Tx, accountID uuid.UUID, kind s
 	stmt := table.Ledger.INSERT(
 		table.Ledger.LedgerID, table.Ledger.AccountID, table.Ledger.Kind,
 		table.Ledger.Source, table.Ledger.Origin, table.Ledger.ChipsDelta,
-		table.Ledger.ProductID, table.Ledger.Snapshot, table.Ledger.CreatedAt,
+		table.Ledger.ProductID, table.Ledger.OrderID, table.Ledger.Provider,
+		table.Ledger.ProviderPaymentID, table.Ledger.Snapshot, table.Ledger.CreatedAt,
 	).VALUES(
 		id, accountID, kind,
 		sourceOrNull(source), sourceOrNull(origin), int32(chipsDelta),
-		uuidOrNull(productID), snap, now,
+		uuidOrNull(productID), uuidOrNull(orderID), stringOrNull(provider),
+		stringOrNull(providerPaymentID), snap, now,
 	)
 	if _, err := stmt.ExecContext(ctx, tx); err != nil {
 		return fmt.Errorf("payments: insert %s ledger: %w", kind, err)
@@ -278,7 +283,7 @@ func (s *Store) spend(ctx context.Context, accountID uuid.UUID, draws []sourceAm
 				return ErrInsufficientChips
 			}
 			src := dr.source
-			if err := insertLedgerTx(ctx, tx, accountID, "spend", &src, &origin, -dr.amount, &productID, snapshot, now); err != nil {
+			if err := insertLedgerTx(ctx, tx, accountID, "spend", &src, &origin, -dr.amount, &productID, nil, nil, nil, snapshot, now); err != nil {
 				return err
 			}
 		}
@@ -295,7 +300,7 @@ func (s *Store) spend(ctx context.Context, accountID uuid.UUID, draws []sourceAm
 // the chosen origin, in one transaction — a zero-price sale of a value.
 func (s *Store) grant(ctx context.Context, accountID uuid.UUID, origin Source, d benefitDelta, snapshot []byte, now time.Time) error {
 	err := withTx(ctx, s.db, func(tx *sql.Tx) error {
-		if err := insertLedgerTx(ctx, tx, accountID, "admin_grant", nil, &origin, 0, nil, snapshot, now); err != nil {
+		if err := insertLedgerTx(ctx, tx, accountID, "admin_grant", nil, &origin, 0, nil, nil, nil, nil, snapshot, now); err != nil {
 			return err
 		}
 		return applyBenefitTx(ctx, tx, accountID, origin, d, now)
@@ -419,3 +424,11 @@ func uuidOrNull(id *uuid.UUID) postgres.Expression {
 	}
 	return postgres.UUID(*id)
 }
+
+// stringOrNull renders an optional string as a SQL string or NULL.
+func stringOrNull(s *string) postgres.Expression {
+	if s == nil {
+		return postgres.NULL
+	}
+	return postgres.String(*s)
+}

backend/internal/robokassa/robokassa.go

@@ -0,0 +1,104 @@
+// Package robokassa builds and verifies Robokassa direct-rail (RUB) payments: it forms the signed
+// hosted-payment URL a client is sent to, and verifies the Result-URL server callback that credits
+// an order. It is pure provider glue — no database, no payments-domain coupling — so the payments
+// domain stays provider-agnostic and this layer is unit-testable in isolation.
+//
+// The order is threaded through Robokassa's custom-parameter channel as Shp_order=<order id> (echoed
+// back in the callback and bound into the signature), not the numeric InvId, because an order id is
+// a uuid; InvId is sent as 0. Idempotency is therefore keyed on the order id at the credit site.
+// Signatures use SHA-256 (configured to match the shop's technical settings); the shop's test mode
+// is carried by IsTest.
+package robokassa
+
+import (
+	"crypto/sha256"
+	"encoding/hex"
+	"net/url"
+	"sort"
+	"strings"
+
+	"github.com/google/uuid"
+)
+
+// payEndpoint is Robokassa's hosted payment page; the signed query sends the client there.
+const payEndpoint = "https://auth.robokassa.ru/Merchant/Index.aspx"
+
+// Config is a Robokassa shop's credentials. Password1 signs the outgoing payment request;
+// Password2 signs (and so verifies) the incoming Result callback. IsTest adds IsTest=1 so the shop's
+// test mode simulates payments without money movement.
+type Config struct {
+	MerchantLogin string
+	Password1     string
+	Password2     string
+	IsTest        bool
+}
+
+// PaymentURL builds the signed hosted-payment URL for an order: amount is the OutSum decimal string
+// (roubles, e.g. "149.00"), description is the human payment purpose. The order id rides as
+// Shp_order and is bound into the SHA-256 signature; InvId is 0 (unused).
+func (c Config) PaymentURL(orderID uuid.UUID, amount, description string) string {
+	q := url.Values{}
+	q.Set("Shp_order", orderID.String())
+	// Signature base: MerchantLogin:OutSum:InvId:Password1[:Shp_key=value...sorted].
+	base := c.MerchantLogin + ":" + amount + ":0:" + c.Password1 + shpSuffix(q)
+
+	q.Set("MerchantLogin", c.MerchantLogin)
+	q.Set("OutSum", amount)
+	q.Set("InvId", "0")
+	q.Set("Description", description)
+	q.Set("SignatureValue", sign(base))
+	if c.IsTest {
+		q.Set("IsTest", "1")
+	}
+	return payEndpoint + "?" + q.Encode()
+}
+
+// VerifyResult verifies a Result-URL callback and extracts the order it credits. It recomputes the
+// SHA-256 signature OutSum:InvId:Password2[:Shp_...sorted] over the callback's own fields and
+// compares it (case-insensitively) with SignatureValue. On success it returns the order id (from
+// Shp_order) and the raw OutSum string the caller re-checks against the order amount; on any
+// missing field, a signature mismatch or an unparseable order id it returns ok=false.
+func (c Config) VerifyResult(v url.Values) (orderID uuid.UUID, outSum string, ok bool) {
+	outSum = v.Get("OutSum")
+	sig := v.Get("SignatureValue")
+	order := v.Get("Shp_order")
+	if outSum == "" || sig == "" || order == "" {
+		return uuid.Nil, "", false
+	}
+	// Robokassa signs with the InvId it returns in the callback; recompute with that same value.
+	base := outSum + ":" + v.Get("InvId") + ":" + c.Password2 + shpSuffix(v)
+	if !strings.EqualFold(sign(base), sig) {
+		return uuid.Nil, "", false
+	}
+	id, err := uuid.Parse(order)
+	if err != nil {
+		return uuid.Nil, "", false
+	}
+	return id, outSum, true
+}
+
+// shpSuffix renders the Shp_ custom parameters of v as Robokassa binds them into a signature:
+// every Shp_-prefixed key, sorted alphabetically, appended as ":key=value".
+func shpSuffix(v url.Values) string {
+	var keys []string
+	for k := range v {
+		if strings.HasPrefix(k, "Shp_") {
+			keys = append(keys, k)
+		}
+	}
+	sort.Strings(keys)
+	var b strings.Builder
+	for _, k := range keys {
+		b.WriteString(":")
+		b.WriteString(k)
+		b.WriteString("=")
+		b.WriteString(v.Get(k))
+	}
+	return b.String()
+}
+
+// sign returns the lowercase hex SHA-256 of s, the hash Robokassa compares against SignatureValue.
+func sign(s string) string {
+	sum := sha256.Sum256([]byte(s))
+	return hex.EncodeToString(sum[:])
+}

backend/internal/robokassa/robokassa_test.go

@@ -0,0 +1,85 @@
+package robokassa
+
+import (
+	"net/url"
+	"strings"
+	"testing"
+
+	"github.com/google/uuid"
+)
+
+func testCfg() Config {
+	return Config{MerchantLogin: "shop", Password1: "p1", Password2: "p2", IsTest: true}
+}
+
+// resultSig computes the Pass2 Result signature Robokassa would send for a callback carrying only
+// Shp_order — the fixture the verifier must accept.
+func resultSig(pass2, outSum, invID string, orderID uuid.UUID) string {
+	return sign(outSum + ":" + invID + ":" + pass2 + ":Shp_order=" + orderID.String())
+}
+
+func TestPaymentURL(t *testing.T) {
+	cfg := testCfg()
+	id := uuid.New()
+	u, err := url.Parse(cfg.PaymentURL(id, "149.00", "10 chips"))
+	if err != nil {
+		t.Fatalf("parse payment url: %v", err)
+	}
+	q := u.Query()
+	if got := q.Get("OutSum"); got != "149.00" {
+		t.Errorf("OutSum = %q, want 149.00", got)
+	}
+	if got := q.Get("InvId"); got != "0" {
+		t.Errorf("InvId = %q, want 0 (unused)", got)
+	}
+	if got := q.Get("Shp_order"); got != id.String() {
+		t.Errorf("Shp_order = %q, want the order id", got)
+	}
+	if got := q.Get("IsTest"); got != "1" {
+		t.Errorf("IsTest = %q, want 1 (test shop)", got)
+	}
+	want := sign("shop:149.00:0:p1:Shp_order=" + id.String())
+	if got := q.Get("SignatureValue"); got != want {
+		t.Errorf("SignatureValue = %q, want %q", got, want)
+	}
+}
+
+func TestVerifyResult(t *testing.T) {
+	cfg := testCfg()
+	id := uuid.New()
+
+	valid := func() url.Values {
+		v := url.Values{}
+		v.Set("OutSum", "149.00")
+		v.Set("InvId", "0")
+		v.Set("Shp_order", id.String())
+		// Robokassa sends the hash uppercase; the verifier must be case-insensitive.
+		v.Set("SignatureValue", strings.ToUpper(resultSig("p2", "149.00", "0", id)))
+		return v
+	}
+
+	gotID, gotSum, ok := cfg.VerifyResult(valid())
+	if !ok || gotID != id || gotSum != "149.00" {
+		t.Fatalf("VerifyResult(valid) = %v/%q/%v, want %v/149.00/true", gotID, gotSum, ok, id)
+	}
+
+	// A tampered amount breaks the signature.
+	tampered := valid()
+	tampered.Set("OutSum", "1.00")
+	if _, _, ok := cfg.VerifyResult(tampered); ok {
+		t.Error("VerifyResult accepted a tampered amount")
+	}
+
+	// The wrong Password2 (a forged callback) does not verify.
+	wrongPass := Config{MerchantLogin: "shop", Password1: "p1", Password2: "other"}
+	if _, _, ok := wrongPass.VerifyResult(valid()); ok {
+		t.Error("VerifyResult accepted a signature under the wrong password")
+	}
+
+	// A missing Shp_order is rejected (no order to credit).
+	noOrder := valid()
+	noOrder.Del("Shp_order")
+	if _, _, ok := cfg.VerifyResult(noOrder); ok {
+		t.Error("VerifyResult accepted a callback with no order")
+	}
+}