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/srp/new_hash.go

@@ -0,0 +1,54 @@
+package srp
+
+import (
+	"io"
+	"math/big"
+
+	"github.com/go-faster/errors"
+)
+
+// computeXV computes following numbers
+//
+// `x = PH2(password, salt1, salt2)`
+// `v = pow(g, x) mod p`
+//
+// TDLib uses terms `clientSalt` for `salt1` and `serverSalt` for `salt2`.
+func (s SRP) computeXV(password, clientSalt, serverSalt []byte, g, p *big.Int) (x, v *big.Int) {
+	// `x = PH2(password, salt1, salt2)`
+	x = new(big.Int).SetBytes(s.secondary(password, clientSalt, serverSalt))
+	// `v = pow(g, x) mod p`
+	v = new(big.Int).Exp(g, x, p)
+	return x, v
+}
+
+// NewHash computes new user password hash using parameters from server.
+//
+// See https://core.telegram.org/api/srp#setting-a-new-2fa-password.
+//
+// TDLib implementation:
+// See https://github.com/tdlib/td/blob/fa8feefed70d64271945e9d5fd010b957d93c8cd/td/telegram/PasswordManager.cpp#L57.
+//
+// TDesktop implementation:
+// See https://github.com/telegramdesktop/tdesktop/blob/v3.4.8/Telegram/SourceFiles/core/core_cloud_password.cpp#L68.
+func (s SRP) NewHash(password []byte, i Input) (hash, newSalt []byte, _ error) {
+	// Generate a new new_password_hash using the KDF algorithm specified in the new_settings,
+	// just append 32 sufficiently random bytes to the salt1, first. Proceed as for checking passwords with SRP,
+	// just stop at the generation of the v parameter, and use it as new_password_hash:
+	p := new(big.Int).SetBytes(i.P)
+	if err := checkInput(i.G, p); err != nil {
+		return nil, nil, errors.Wrap(err, "validate algo")
+	}
+
+	// Make a copy.
+	newClientSalt := append([]byte(nil), i.Salt1...)
+	newClientSalt = append(newClientSalt, make([]byte, 32)...)
+	// ... append 32 sufficiently random bytes to the salt1 ...
+	if _, err := io.ReadFull(s.random, newClientSalt[len(newClientSalt)-32:]); err != nil {
+		return nil, nil, err
+	}
+
+	_, v := s.computeXV(password, newClientSalt, i.Salt2, big.NewInt(int64(i.G)), p)
+	// As usual in big endian form, padded to 2048 bits.
+	padded, _ := s.pad256FromBig(v)
+	return padded[:], newClientSalt, nil
+}