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/crypto/rsa_fingerprint.go

@@ -0,0 +1,27 @@
+package crypto
+
+import (
+	"crypto/rsa"
+	"crypto/sha1" // #nosec
+	"encoding/binary"
+	"math/big"
+
+	"go.mau.fi/mautrix-telegram/pkg/gotd/bin"
+)
+
+// RSAFingerprint returns fingerprint of RSA public key as defined in MTProto.
+func RSAFingerprint(key *rsa.PublicKey) int64 {
+	e := big.NewInt(int64(key.E))
+
+	// See "Creating an Authorization Key" for reference:
+	// * https://core.telegram.org/mtproto/auth_key#dh-exchange-initiation
+	// rsa_public_key n:string e:string = RSAPublicKey
+	buf := new(bin.Buffer)
+	buf.PutBytes(key.N.Bytes())
+	buf.PutBytes(e.Bytes())
+
+	h := sha1.New() // #nosec
+	_, _ = h.Write(buf.Buf)
+	result := h.Sum(nil)[12:sha1.Size]
+	return int64(binary.LittleEndian.Uint64(result))
+}