move gotd fork into repo. (#111)

- update to latest telegram layer
- remove some references to fields in tg.Entities that don't exist in
the schema
- originally added here:
https://github.com/beeper/td/commit/820929062a2ba0104397bc01235ab58a9cff780e
  - referenced here
-
https://github.com/mautrix/telegramgo/commit/124f0967ed195b5a380c9bd02e170ada9710dde3
-
https://github.com/mautrix/telegramgo/commit/4205047aab2e0639217148b5d125bfaab668bd8e

pkg/gotd/mtproxy/faketls/server_hello.go

@@ -0,0 +1,65 @@
+package faketls
+
+import (
+	"bytes"
+	"crypto/hmac"
+	"crypto/sha256"
+	"io"
+
+	"github.com/go-faster/errors"
+)
+
+// readServerHello reads faketls ServerHello.
+func readServerHello(r io.Reader, clientRandom [32]byte, secret []byte) error {
+	packetBuf := bytes.NewBuffer(nil)
+	r = io.TeeReader(r, packetBuf)
+
+	handshake, err := readRecord(r)
+	if err != nil {
+		return errors.Wrap(err, "handshake record")
+	}
+	if handshake.Type != RecordTypeHandshake {
+		return errors.Wrap(err, "unexpected record type")
+	}
+
+	changeCipher, err := readRecord(r)
+	if err != nil {
+		return errors.Wrap(err, "change cipher record")
+	}
+	if changeCipher.Type != RecordTypeChangeCipherSpec {
+		return errors.Wrap(err, "unexpected record type")
+	}
+
+	cert, err := readRecord(r)
+	if err != nil {
+		return errors.Wrap(err, "cert record")
+	}
+	if cert.Type != RecordTypeApplication {
+		return errors.Wrap(err, "unexpected record type")
+	}
+
+	// `$record_header = type 1 byte + version 2 bytes + payload_length 2 bytes = 5 bytes`
+	// `$server_hello_header = type 1 bytes + version 2 bytes + length 3 bytes = 6 bytes`
+	// `$offset = $record_header + $server_hello_header = 11 bytes`
+	const serverRandomOffset = 11
+	packet := packetBuf.Bytes()
+	// Copy original digest.
+	var originalDigest [32]byte
+	copy(originalDigest[:], packet[serverRandomOffset:serverRandomOffset+32])
+	// Fill original digest by zeros.
+	var zeros [32]byte
+	copy(packet[serverRandomOffset:serverRandomOffset+32], zeros[:])
+
+	mac := hmac.New(sha256.New, secret)
+	if _, err := mac.Write(clientRandom[:]); err != nil {
+		return errors.Wrap(err, "hmac write")
+	}
+	if _, err := mac.Write(packet); err != nil {
+		return errors.Wrap(err, "hmac write")
+	}
+	if !bytes.Equal(mac.Sum(nil), originalDigest[:]) {
+		return errors.New("hmac digest mismatch")
+	}
+
+	return nil
+}