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/obfuscated2/keys_util.go

@@ -0,0 +1,63 @@
+package obfuscated2
+
+import (
+	"crypto/aes"
+	"crypto/cipher"
+	"encoding/binary"
+	"io"
+)
+
+func createCTR(key, iv []byte) (cipher.Stream, error) {
+	block, err := aes.NewCipher(key)
+	if err != nil {
+		return nil, err
+	}
+	return cipher.NewCTR(block, iv), nil
+}
+
+func getDecryptInit(init []byte) (initRev [48]byte) {
+	copy(initRev[:], init[8:56])
+	// https://github.com/golang/go/wiki/SliceTricks#reversing
+	for left, right := 0, len(initRev)-1; left < right; left, right = left+1, right-1 {
+		initRev[left], initRev[right] = initRev[right], initRev[left]
+	}
+
+	return
+}
+
+// function from https://core.telegram.org/mtproto/mtproto-transports#transport-obfuscation
+func generateInit(randSource io.Reader) (init [64]byte, err error) {
+	// init := (56 random bytes) + protocol + dc + (2 random bytes)
+	for {
+		_, err = io.ReadFull(randSource, init[:])
+		if err != nil {
+			return [64]byte{}, err
+		}
+
+		// Filter some start sequences
+		// See https://github.com/DrKLO/Telegram/blob/master/TMessagesProj/jni/tgnet/Connection.cpp#L531.
+		// See https://github.com/tdlib/td/blob/master/td/mtproto/TcpTransport.cpp#L157-L158.
+		if init[0] == 0xef { // Abridged header
+			continue
+		}
+
+		firstInt := binary.LittleEndian.Uint32(init[0:4])
+		if firstInt == 0x44414548 || // HEAD
+			firstInt == 0x54534f50 || // POST
+			firstInt == 0x20544547 || // GET
+			firstInt == 0x4954504f || // OPTI
+			firstInt == 0x02010316 || // ????
+			firstInt == 0xdddddddd || // PaddedIntermediate header
+			firstInt == 0xeeeeeeee /* Intermediate header */ {
+			continue
+		}
+
+		if secondInt := binary.LittleEndian.Uint32(init[4:8]); secondInt == 0 {
+			continue
+		}
+
+		break
+	}
+
+	return init, nil
+}