Add background update for middle proxy

mtproto/rpc/nonce_request.go

@@ -0,0 +1,51 @@
+package rpc
+
+import (
+	"bytes"
+	"crypto/rand"
+	"encoding/binary"
+	"fmt"
+	"time"
+
+	"github.com/9seconds/mtg/mtproto"
+)
+
+type NonceRequest struct {
+	KeySelector []byte
+	CryptoTS    []byte
+	Nonce       []byte
+}
+
+// Bytes returns serialized nonce request.
+func (r *NonceRequest) Bytes() []byte {
+	buf := &bytes.Buffer{}
+
+	buf.Write(mtproto.TagNonce)       // nolint: gosec
+	buf.Write(r.KeySelector)          // nolint: gosec
+	buf.Write(mtproto.NonceCryptoAES) // nolint: gosec
+	buf.Write(r.CryptoTS)             // nolint: gosec
+	buf.Write(r.Nonce)                // nolint: gosec
+
+	return buf.Bytes()
+}
+
+// NewNonceRequest builds new none request based on proxy secret.
+func NewNonceRequest(proxySecret []byte) (*NonceRequest, error) {
+	nonce := make([]byte, 16)
+	keySelector := make([]byte, 4)
+	cryptoTS := make([]byte, 4)
+
+	if _, err := rand.Read(nonce); err != nil {
+		return nil, fmt.Errorf("cannot generate nonce: %w", err)
+	}
+	copy(keySelector, proxySecret)
+
+	timestamp := time.Now().Truncate(time.Second).Unix() % 4294967296 // 256 ^ 4 - do not know how to name
+	binary.LittleEndian.PutUint32(cryptoTS, uint32(timestamp))
+
+	return &NonceRequest{
+		KeySelector: keySelector,
+		CryptoTS:    cryptoTS,
+		Nonce:       nonce,
+	}, nil
+}