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/session/telethon.go

@@ -0,0 +1,87 @@
+package session
+
+import (
+	"encoding/base64"
+	"encoding/binary"
+	"net"
+	"strconv"
+
+	"github.com/go-faster/errors"
+
+	"go.mau.fi/mautrix-telegram/pkg/gotd/crypto"
+)
+
+// https://github.com/LonamiWebs/Telethon/blob/master/telethon/sessions/string.py#L11
+const latestTelethonVersion byte = '1'
+
+// TelethonSession decodes Telethon's StringSession string to the Data.
+// Notice that Telethon does not save tg.Config and server salt.
+//
+// See https://docs.telethon.dev/en/latest/modules/sessions.html#telethon.sessions.string.StringSession.
+//
+// See https://github.com/LonamiWebs/Telethon/blob/master/telethon/sessions/string.py#L29-L46.
+func TelethonSession(hx string) (*Data, error) {
+	if len(hx) < 1 {
+		return nil, errors.Errorf("given string too small: %d", len(hx))
+	}
+	version := hx[0]
+	if version != latestTelethonVersion {
+		return nil, errors.Errorf("unexpected version %q, latest supported is %q",
+			version,
+			latestTelethonVersion,
+		)
+	}
+
+	data, err := base64.URLEncoding.DecodeString(hx[1:])
+	if err != nil {
+		return nil, errors.Wrap(err, "decode hex")
+	}
+
+	return decodeStringSession(data)
+}
+
+func decodeStringSession(data []byte) (*Data, error) {
+	// Given parameter should contain version + data
+	// where data encoded using pack as '>B4sH256s' or '>B16sH256s'
+	// depending on IP type.
+	//
+	// Table:
+	//
+	// | Size |  Type  | Description |
+	// |------|--------|-------------|
+	// | 1    | byte   | DC ID       |
+	// | 4/16 | bytes  | IP address  |
+	// | 2    | uint16 | Port        |
+	// | 256  | bytes  | Auth key    |
+	var ipLength int
+	switch len(data) {
+	case 263:
+		ipLength = 4
+	case 275:
+		ipLength = 16
+	default:
+		return nil, errors.Errorf("decoded hex has invalid length: %d", len(data))
+	}
+
+	// | 1    | byte   | DC ID       |
+	dcID := data[0]
+
+	// | 4/16 | bytes  | IP address  |
+	addr := make(net.IP, 0, 16)
+	addr = append(addr, data[1:1+ipLength]...)
+
+	// | 2    | uint16 | Port        |
+	port := binary.BigEndian.Uint16(data[1+ipLength : 3+ipLength])
+
+	// | 256  | bytes  | Auth key    |
+	var key crypto.Key
+	copy(key[:], data[3+ipLength:])
+	id := key.WithID().ID
+
+	return &Data{
+		DC:        int(dcID),
+		Addr:      net.JoinHostPort(addr.String(), strconv.Itoa(int(port))),
+		AuthKey:   key[:],
+		AuthKeyID: id[:],
+	}, nil
+}