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.go

@@ -0,0 +1,75 @@
+package obfuscated2
+
+import (
+	"crypto/cipher"
+	"encoding/binary"
+	"io"
+
+	"github.com/go-faster/errors"
+
+	"go.mau.fi/mautrix-telegram/pkg/gotd/crypto"
+)
+
+type keys struct {
+	header  []byte
+	encrypt cipher.Stream
+	decrypt cipher.Stream
+}
+
+func (k *keys) createStreams(init, secret []byte) error {
+	// preallocate 256 bit key + 16 byte secret
+	const keyLength = 32 + 16
+
+	encryptKey := append(make([]byte, 0, keyLength), init[8:40]...)
+	encryptIV := append(make([]byte, 0, 16), init[40:56]...)
+
+	initRev := getDecryptInit(init)
+	decryptKey := append(make([]byte, 0, keyLength), initRev[:32]...)
+	decryptIV := append(make([]byte, 0, 16), initRev[32:48]...)
+
+	if len(secret) > 0 {
+		if len(secret) < 16 {
+			return errors.Errorf("invalid secret size %d", len(secret))
+		}
+		secret = secret[0:16]
+
+		encryptKey = crypto.SHA256(encryptKey, secret)
+		decryptKey = crypto.SHA256(decryptKey, secret)
+	}
+
+	var err error
+	k.encrypt, err = createCTR(encryptKey, encryptIV)
+	if err != nil {
+		return err
+	}
+
+	k.decrypt, err = createCTR(decryptKey, decryptIV)
+	if err != nil {
+		return err
+	}
+
+	return nil
+}
+
+func generateKeys(randSource io.Reader, protocol [4]byte, secret []byte, dc int) (keys, error) {
+	init, err := generateInit(randSource)
+	if err != nil {
+		return keys{}, err
+	}
+
+	var k keys
+	if err := k.createStreams(init[:], secret); err != nil {
+		return keys{}, err
+	}
+
+	copy(init[56:60], protocol[:])
+	binary.LittleEndian.PutUint16(init[60:62], uint16(dc))
+
+	var encryptedInit [64]byte
+	k.encrypt.XORKeyStream(encryptedInit[:], init[:])
+	k.header = make([]byte, 64)
+	copy(k.header, init[0:56])
+	copy(k.header[56:], encryptedInit[56:56+8])
+
+	return k, nil
+}