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.go

@@ -0,0 +1,53 @@
+package crypto
+
+import (
+	"crypto/rsa"
+	// #nosec
+	//
+	// Allowing sha1 because it is used in MTProto itself.
+	"crypto/sha1"
+	"math/big"
+)
+
+// RSAKeyBits is RSA key size.
+//
+// Can be used as rsa.GenerateKey(src, RSAKeyBits).
+const RSAKeyBits = 2048
+
+const (
+	rsaLen         = 256
+	rsaWithHashLen = 255
+	rsaDataLen     = rsaWithHashLen - sha1.Size
+)
+
+// RSAPublicDecrypt recovers the message digest from the raw signature
+// using the signer’s RSA public key.
+//
+// See also OpenSSL’s RSA_public_decrypt with RSA_NO_PADDING.
+func RSAPublicDecrypt(pub *rsa.PublicKey, sig []byte) ([]byte, error) {
+	k := pub.Size()
+	if k < 11 || k != len(sig) {
+		return nil, rsa.ErrVerification
+	}
+
+	c := new(big.Int).SetBytes(sig)
+	e := big.NewInt(int64(pub.E))
+	m := new(big.Int).Exp(c, e, pub.N)
+
+	return m.Bytes(), nil
+}
+
+func rsaEncrypt(data []byte, key *rsa.PublicKey) []byte {
+	z := new(big.Int).SetBytes(data)
+	e := big.NewInt(int64(key.E))
+	c := new(big.Int).Exp(z, e, key.N)
+	res := make([]byte, rsaLen)
+	c.FillBytes(res)
+	return res
+}
+
+func rsaDecrypt(data []byte, key *rsa.PrivateKey, to []byte) bool {
+	c := new(big.Int).SetBytes(data)
+	m := new(big.Int).Exp(c, key.D, key.N)
+	return FillBytes(m, to)
+}