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/secret.go

@@ -0,0 +1,84 @@
+package mtproxy
+
+import (
+	"github.com/go-faster/errors"
+
+	"go.mau.fi/mautrix-telegram/pkg/gotd/proto/codec"
+)
+
+// SecretType represents MTProxy secret type.
+type SecretType int
+
+const (
+	// Simple is a basic MTProxy secret.
+	Simple SecretType = iota + 1
+	// Secured is dd-secret.
+	Secured
+	// TLS is fakeTLS MTProxy secret.
+	// First byte should be ee.
+	TLS
+)
+
+// Secret represents MTProxy secret.
+type Secret struct {
+	Secret    []byte
+	Tag       byte
+	CloakHost string
+	Type      SecretType
+}
+
+// ExpectedCodec returns codec from secret tag if it exists.
+func (s Secret) ExpectedCodec() (cdc codec.Codec, _ bool) {
+	switch s.Tag {
+	case codec.AbridgedClientStart[0]:
+		cdc = codec.Abridged{}
+	case codec.IntermediateClientStart[0]:
+		cdc = codec.Intermediate{}
+	case codec.PaddedIntermediateClientStart[0]:
+		cdc = codec.PaddedIntermediate{}
+	default:
+		return nil, false
+	}
+
+	return cdc, true
+}
+
+// ParseSecret checks and parses secret.
+func ParseSecret(secret []byte) (Secret, error) {
+	r := Secret{
+		Secret: secret,
+	}
+	const simpleLength = 16
+
+	switch {
+	case len(secret) == 1+simpleLength:
+		r.Type = Secured
+
+		r.Tag = secret[0]
+		secret = secret[1:]
+		r.Secret = secret[:simpleLength]
+	case len(secret) > simpleLength:
+		r.Type = TLS
+
+		r.Tag = secret[0]
+		secret = secret[1:]
+		r.Secret = secret[:simpleLength]
+		r.CloakHost = string(secret[simpleLength:])
+	case len(secret) == simpleLength:
+		r.Type = Simple
+	default:
+		return Secret{}, errors.Errorf("invalid secret %q", string(secret))
+	}
+
+	if r.Type != Simple {
+		switch r.Tag {
+		case codec.AbridgedClientStart[0],
+			codec.IntermediateClientStart[0],
+			codec.PaddedIntermediateClientStart[0]:
+		default:
+			return Secret{}, errors.Errorf("unknown tag %+x", r.Tag)
+		}
+	}
+
+	return r, nil
+}